Results of L Band Multipath Measurements at Operational ... · Microwave Landing System (MLS) i...
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DOT/FAA/RD-81/63 Project Report ATC-109 Results of L Band Multipath Measurements at Operational United States (U.S.) Airports in Support of the Microwave Landing System (MLS) Precision Distance Measuring Equipment (DME/P) J. E. Evans P. H. Swett 23 July 1981 Lincoln Laboratory MASSACHUSETTS INSTITUTE OF TECHNOLOGY LEXINGTON, MASSACHUSETTS Prepared for the Federal Aviation Administration, Washington, D.C. 20591 This document is available to the public through the National Technical Information Service, Springfield, VA 22161
Transcript of Results of L Band Multipath Measurements at Operational ... · Microwave Landing System (MLS) i...
DOT/FAA/RD-81/63
Project ReportATC-109
Results of L Band Multipath Measurements at Operational United States (U.S.) Airports in
Support of the Microwave Landing System (MLS) Precision Distance Measuring Equipment
(DME/P)
J. E. EvansP. H. Swett
23 July 1981
Lincoln Laboratory MASSACHUSETTS INSTITUTE OF TECHNOLOGY
LEXINGTON, MASSACHUSETTS
Prepared for the Federal Aviation Administration, Washington, D.C. 20591
This document is available to the public through
the National Technical Information Service, Springfield, VA 22161
This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof.
..
•
Technical Report Documentation Page
1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.
DOT/FAA/RD-81/63
4. T;tle Qnd Subtdle 5. Report Date
Results of L Band Multipath Measurements at Operational United States 23 July 1981(U. S.) Airports in Support of the Microwave Landing System (MLS) 6. Performing Orgoni zatian CodePrecision Distance Measuring Equipment (DME/P)
7. Author( ,} 8. Performing Orgoni zatian Report No,
James E. Evans and Paul H. Swett ATC-109
9. Performing Orgoni zatian Nome and Address 10. Work Unit No. (TEAlS)
Massachusetts Institute of TechnologyLincoln Laboratory 11. Contract or Grant No.
P.O. Box 73 OOT-FA74-WAI-461Lexington, MA 02173 13. Type of Report and Period Covered
12 . Sponsoring Agency Name and Address
Department of Transportation Project Report
Federal Aviation AdministrationSystem Research and Development Service 14. Sponsoring Agency CodeWashington, DC 20591 ARD-31O
15. Supplementary Notes
The work reported in this document was performed at Lincoln Laboratory, a center for research operatedby Massachusetts Institute of Technology under Air Force Contract F19628-80-C-0002.
16. Abstract
This report presents the results of a short dUl"ation L band multipath measurement program at five majorU. S. airports (St. LOUis, Tulsa, Wright Patterson Air Force Base, Philadelphia and Washington National) andone smaller airport (Quonset, I"ZI) to better quantify the expected muLtipath environment for the MicrowaveLanding System (MLS) Precision Distance Measuring Equipment (DME/P). Specific objectives included:
(1) Measurements of the principal multipath parameters (amplitude and time delay) with realistic aircraft/ground site locations at runways which had the major DME/P multipath sources (large buildings) idEm-tified in previous analytical (simulation) studies.
(2) Determination of whether significant DME/P multipath SOLlrces exist which had not been considered to
and date.
(3) Comparison of the measured results \\~ th computer simulation results obtained with simplified ai rportmodels (such as have been used for lJME/P system design to date).
Particular emphasis was placed on the final approach region including the flare and rollout regions since theseareas correspond to the most stringent lJME/P accuracy requirements amI, have not been utilized operation-ally with the current L band DME.
All of the above objectives were achieved although in some cases the experimental data in the flare/rolloutregion was of poor quality due to low signal to noise catio. The spatial region and time delay of specular multi-path generally correlated well with expectations based on simple ray tracing. With the exception of WashingtonNational, 110 significant [multipath to direct signal ratio (M/D) >-lO dB] multipath was encountered in opera-tionally relevant areas which was not predicted. The quantitative predictions of the simple airport: modelsagreed with the experimental data, although in some cases, (especially, near threshold) the measured M/ [)values were considerably higher than predictions.
17. Key Words1
18 . Distribution Statement
Microwave Landing System (MLS) iDocument is available to the public throughDistance Measuring Equipment (DME) the National Technical Information SerVice,
DME Based Landing System (DLS)Springfield, VA 22161
Multipath
19. Security Classif. (Qf this ".port) 20. Security CIQssi/. (of thi S pQgel 21. No. of Pages 22. Price
Unclassified Unclassified 194
Form DOT F HOO.7 (I) - (2) "eproduction of completed p"gc 'Iutilori~ed
ABSTRACT
This report presents the results of a short duration L band multipath
measurement program at five maj or U. S. airports (St. Louis, Tulsa, Wright
Patterson Air Force Base, Philadelphia and Washington National) and one
smaller airport (Quonset, iU) to better quantify the expected mul tipath
environment for the Microwave Landing System (MLS) Precision Distance
Measuring Equipment (DME/P). Specific objectives included:
•
..
1) measurements of the principal multipath parameters (amplitude and
time delay) with realistic aircraft/ground site locations at runways
which had the major UME/P multipath sources (large buildings) identi
fied in previous analytical (simUlation) studies •
•
2) determination of whether significant DME/P multipath sources exist
which had not been considered to date.
and
3) comparison of the measured results with computer simulation results
obtained with simplified airport models (such as have been used for
DME/P system design to date).
Particular emphasis was placed on the final approach region including the
flare and rollout regions since these areas correspond to the most stringent
Dr1E/P accuracy requirements and, have not been utilized operationally with the
current L band DME.
All of the above objectives were achieved although in some cases the
experimental data in the flare/rollout region was of poor quality due to low
signal to noise ratio. The spatial region and time delay of specular multi
path generally correlated well with expectations based on simple ray
tracing. With the exception of \-lashington National, no significant [multipath
to direct signal ratio (M/D) > -10 dBJ multipath was encountered in
operationally relevant areas which was not predicted. The quantitative
predictions of the simple airport models generally agreed with the
experimental data, although in some cases, (especially, near threshold) the
measured M/D values were considerably higher than predictions.
iii
•
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ACXNOWLEDGEMENTS
A number of people made substantive contributions to the work described
in this report. A. Vierstra and R. Parr designed the key elements of the
airborne system and the data acquisition system, respectively. R. Close and
A. Gregory assembled much of the electronics, while C. Catalano developed the
real time display software as well as a waveform playback and analysis
capability on the laboratory Eclipse minicomputer.
J. Yaeger-Charriere developed much of the software used for waveform
analysis and data summaries as well as processing the data and running the
corresponding simulation studies.
The actual measurements a t the va rious maj or airports were accomplished
in a total of 2 1/2 weeks through the hard work of A. Gregory, J. Kalil and
the project pilots (T. Magnan, C. Magnarelli, J. Dufort, and D. Cassalia).
The ready cooperation of the airport authorities at the various airports was
essential to ttle program pace. Particular help was extended by W. Melnick at
WPAF Band T. David son at Lambert-St. Louis.
The measurements a t Quonset Point, RI, were carried out by A. Vierstra,
while the data reduction and analysis was accomplished by D. Easterday.
K. Eastburn assisted in developing and running many of the simulation
scenarios, while D. Young and N. Campbell ably typed the text and figures in a
very short time period •
v
TABLE OF CONTENTS
Abstract
Acknowledgements
I. Introduction
II. Equipment Configuration
III. Data Analysis Procedure
IV. LaQbert-St. Louis International Airport
V. Tulsa International Airport
VI. Wrignt-Patterson Air force Base
VII. Philadelphia International Airport
VIII. Wasnington National Airport
IX. Swmnary and Conclusions
~eferences
iii
v
1-1
•2-1
3-1
4-1 •5-1
6-1
7-1
~-1
9-1
R-l
Appendix A Pilot Scale L-Hand Multipath Aeasurements
at ~uonset State Airport, Rhode Island
vi
A-I
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I. INTRODUCTION
The characteristics of L band Distance Measuring Equipment (DME)
multipath in the approach and landing region have assumed increased attention
as a consequence of the Federal Aviation Administration (FAA) /International
Civil Aviation Organization (IeAO) program to develop a precision DME (DME/P)
for the Nicrowave Landing System (MLS). In an earlier study of multipath
•
effects on the DHE/P [1 J, it was concluded that experimental measurements of
the pertinant mul tipath parameters at represet'ltative operational airports
would be very useful in:
(1\ verifying that the principal mul tipath challenges identified in the analytical/simulation studies (specularreflections from buildings coupled witn specular terrainreflections) are, in fact, the major contributions ofsignificant DNE/P multipath
and(2) acertaining to what extent the simplified building and
terrain models used to date can yield good quantitativeprediction of multipath at a given location.
This report summarizes the results of experimental L band measurements at five
major operational US airports (Philadelphia, Washington National, Wright
Patterson AFB, St. Louis, and Tulsa) as well as a preliminary test at Quonset
Point, RIo
The digital multipattl measurement equipment is described in Section II.
A highly mobile equipment was desired which could measure the multipath para
meters of greatest interest. This was accomplished oy transmitting a narrow
(100 nsec - 200 nsec wide) L band pulse from an aircraft and (digitally)
recording the received signal envelope at a ground antenna as a function of
time as shown in Figs. 1-1 and 1-2. By examination of the digitized envelope,
it was then possible to determine the pertinent mul tipath characteristics
(amplitude and time delay relative to the direct signal level) on a given
signal reception. The aircraft transmitted signals at a 10 Hz rate, corre-
sponding to approximately 18 feet of aircraft displacement between successive
measurements. This relatively dense spatial sampling of the multipath envi-
ronment allowed us to use correlation between adjacent measurements to reject
1-1
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erroneous data due to cochannel interference and/or low signal to noise ratio
(SNK).
Aircraft range information was obtained by having the narrow pulse trans
mission times controlled by a standard Air Traffic Control Radar Beacon
(ATClZBS) transponder which was being interrogated by the ground measurement
system. In this way, the delay time between the interrogation and received
ATClZBS reply yielded the aircraft range. The flight profiles were such (e.g.,
centerline approaches using an ILS localizer to furnish lateral position and
the ILS glideslope to furnish vertical position) that knowledge of the range
generally would permit one to determine the aircraft position. •
Section III describes the data reduction method. Each (digitized) re-
ceived waveform was examined to locate discrete pulsea according to criteria
based on pulse widths and magnitude. Cochannel interference due to asynchron
ous replies from other ATCKBS transponders was rejected by measurement of the
pulse widths. The reduced pulse parameter data are then displayed in plots of
multipath level and time delay versus distance along the flight path. By
considering ttle nature of adjacent multipath environment estimates and repeat
ability of the phenomena between successive (nominally) identical runs, it was
possible to identify questionable data whicll required hand analysis of the
waveforms.
Section IV to VIII describe the measurement results at Philadelphia,
Washington National, Wright Patterson AFB, St. Louis (Lambert), and Tulsa
International Airport. Each section begins with a description of the airport
geometry, major building reflection multipath threats, and expected multipath
regions. Next, the results of the received waveform analysis along the vari
ous flight paths are shown. Finally, the field data is compared with the
computed multipath characteristics using a simple model of the airport envi-
ronment whereby building walls are modeled by flat rectangular plates and the
terrain as a flat dielectric half plane.
Section IX summarizes the preliminary results of this measurement program
and makes some suggestions for further work in this area. Appendix A de-
1-4
•
•
•
scribes a preliminary DME/P measurement at Quonset Point, RI, where in-scope
photographs were used to record the received waveform at various discrete
points along the airport main runway •
1-5
•
II. EQUIPMENT CONFIGURATION
The need to obtain multipath data in a time schedule compatible with the
ICAO program and consistent with the overall program level of effort made it
imperative that the technique chosen be relatively straightforward and that
the required hardware components be readily available or easily and quickly
fabricated. The method that was finally adopted is shown in the block diagram
of Fig. 2-1. The bulk of the hardware used was that which had previously been
deployed to make terrain reflection multipath measurements [2].
A. Ground Equipment
The ~~S terrain multipath measurement system has been described in detail
previously [2], so our discussion here will only concern itself with those
features which were unique to the DME multipath measurement program.
1. Interrogation Subsystem
A measurement is initiated by the ground transmitting an ATCRBS "B" mode
interrogation at 1030 MHz through a 6-foot dish antenna. This mode is used
rather than the nor,ual A and C ATCRBS modes, since the B mode has not been
used for many years and thus will not elicit replies from normal ATCRBS tran-
sponders. Thus, we avoid "synchronous garble" from other aircraft which are
approximately at the same range as our test aircraft. The dish antenna was
.,
•
used in all cases save one site at Tulsa International Airport to
(1) provide a better SNR for our uplinkand
(2) minimize the likelihood of uplink multipath with a delaytime of approximately 2 ~ec from suppressing the test
*aircraft transponder •
*Tne mode B interrogation consists of two 400 nsec pulses (PI and P3) whichare separated in time by 9 ~ec. However, normal ATCRBS ground stations alsotransmit a third pulse (delayed 2 jlsec after the PI pulse) through an omniantenna to indicate situations in which the aircraft is in the sidelobes ofthe interrogator antenna. If an ATCR8S transponder measures a received signallevel in the SLS pulse position which is comparable to the PI pulse positionlevel, it is not supposed to reply. Consequently, high level reflections ofthe PI interrogation pulse can suppress the ATCRBS transponder if they have amultipath delay of approximately 2 \lsec.
2-1
•
•DME PulseGenerator100 ns/200 ns
Airborne Electronics
AircraftTransponder
AircraftAntenna
L BandReceivingAntenna
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DME Pulse
Fig. 2-1. DME hardware block diagram.
2-2
The size of this dish made it awkward to use in situations where the interro
gator antenna had to be elevated to avoid shadowing by terrain and/or runway
humps. At one Tulsa site, it was found necessary to use a smaller interro-
..
gation antenna which could be mounted high upon a pole attached to the roof of
the van.
2. Receiving Antenna
To reduce the magnitude of the direct signal fades which can arise near
threshold due to ground lobing, it has been suggested [3] that the DME/P
ground antenna would
(1) have a phase center elevated some 20 - 30 feet above thelocal terrain at most CTOL runways
and(2) have an elevation pattern whose gain decreases rapidly
below the horizon to avoid signal lobing at high elevation angles.
Both of these features were deemed essential for the DME multipath measurement
ground receiving antenna. Figure 2-2 shows the final configuration using one
of the PALM system antenna elements [4] on a light weight tubular mast. The
PALM antenna elements were designed to yield a rapid pattern rolloff near the
horizon as shown in Fig. 2-3. The element mounting bracket was designed so
*that the horizon corresponded to the -6 dB point on the pattern •
The azimuth pattern rolloff at wide angles off centerline is similar to
that suggested by Kelly and LaBerge to mitigate OME/P multipath. The tubular
mast had a removable center section so that the phase center could be at
nominal heights of 20 feet or 30 feet. The hinged base of the array is con
nected to a rectangular base which was secured to the ground by driven
stakes. The mast and element are then erected by a hand cranked winch. Final
alignment is then achieved by adjusting three guy wires.
*i.e., the antenna was tilted downward 2° in elevation.
2-3
Fig. 2-2a. DME measurement equip~ent.
2-4
•
•
• Fig. 2-2b. Measurement equipment at Washington National Airport.
2-5
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Fig. 2-3. Pattern cuts for receiving antenna.
2-6
,
3. RF/IF Electronics
A block diagram of the van electronics hardware is shown in Fig. 2-4.
The basic difference between the normal MLS terrain multipath measure~ent
receiver architecture and the DME multioath measurement architecture were
hardware changes to the trigger circuit, a 10 HHz bandwidth filter in the IF
and the fact that only one receiver channel was used for DME. There were also
changes in the logic to create a 3.2 ~ window in which the received pulse was
sampled.
In DME multipath measurement operation, the 1030 lli{z received signal was
mixed down to a 60 ['1Hz IF by an RHG model 1-2/10B mixer/preamp. The noise
..figure of the front end is 8 dB and the measured sensitivity of the front end
is -78 dBm*. The dynamic range of the log If amp is 80 dB. RF cable loss is
approximately 2 dB, and IF cable loss is 1 dB. The IF filter is a CIR-Q-TEL
model B2-bO/l0-4/S0, which is a 4-pole Butterworth filter with a 10 HHz band-
width. The burst responses of the filter are shown in Figs. 2-Sa and 2-5b.
The IF filter is followed by the PAUl/t1LS log amplifier. From there, the
detected signal goes to an oscilloscope for viewing the pulses by the operator
during the mission and the signal also goes to the salilple/hold and the A/D
converter. The response of the IF stage including the lU HHz BW filter and
the log amp is shown in Figs. 2-Sc and 2-5d.
The procedure cnecklist at the beginning of a mission included a calibra-
tion of the receiver channeL This was to verify the operational status of
the equipment. Figure 2-6 ShOWS an araplitude calibration of the receiver. We
see that the digitized amplitude is essentially logarithmically linear over a
dynamic range of 70 dB.
*The measured sensitivity was approximately 1S dB lower than that implied bythe noise figure and IF bandwidth due to losses in the mixer.
2-7
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Fig. 2-Sa. Gated IF responseof 10 MHz BP IF filter.
Fig. 2-Sc. Gated IF responseof IF filter and log AMP stage.Upper trace is stepped 60 MHzinput. Lower trace is filter/log AMP detected video output.
2-9
Fig. 2-Sb. Gated IF burst responseof 10 MHz BP IF filter.
Fig. 2-Sd. Gated IF burst responseof IF filter. and log AMP stage.Upper trace 1S gated 60 MHz input.Lower trace is filter/log AMPdetected video output.
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..
4. AID Converter
In DME mode, the fast eight bit A/v converter samples the input waveform
in a burst starting about 50 \..IS after the opening of the range gate. Each
window comprises 64 samples spaced 50 ns per sample. The TRW AID converter
was observed to follow the input waveform faithfully most of the time, but it
showed an occasional tendency to glitch for a single sample for input signals
with rapidly rising edges.
a. Time Tracking
Tracking of tne received signal during a mission WCiti done manually by
using a joystick to adjust tlle relative positions of the first ATCBRS reply
pulse (i.e., the F1 pulse) and an internally generated range gate signal,
based on an oscilloscope display of the two signals. Maintaining the F1 pulse
at the leading end of the range gate insured that the received direct DME
lJulse was positioned near the beginning of the 3.2 IlS DME sampling interval.
No further refinement or automation of the range track was used during the
missions. The hardware recorded a range parameter based on the first crossing
within the range gate of a fixed threShold by the input signal; however, this
data was often erratic (see Section Ill).
5. Recording
Data recording for OHE measurements was identical to the MLS terrain
reflection recording described in ATC-~8 Volume I l2]. Digital data is re
corded on the Data General disk and tllen transferred to tape for a permanent
record.
The graphics program, which displays the 3.2 Ilsec wavefocm sampling
window, writes one frame per second, therefore 1 in 10 replies is seen on the
terminal during measurements since there are 10 replies per second. The
pulses can be viewed during the field mission and later they can be seen again
on the van Eclipse computer or on the computers at the Laboratory.
2-11
B. Airborne System
A diagram of the airborne electronics is shown in Fig. 2-7. The DME
reply pulse was generated 50 ).IS after the transponder Fl pulse, and the DME
pulse width was selectable at either 100 ns or 200 ns. The transponder and
the 200 ns selection was added to improve the signal/noise ratio when neces
sary. The DME pulse peak power is approximately 160 watts for the 200 ns
setting.
Photographs of the transponder DME pulse are shown in Fig. 2-8a and 2-8b
for the 100 ns and 200 ns widths, respectively. These photographs were made
using a detector with an approximate square law characteristic.
Several L band antennas were available on the Beechcraft Bonanza which
could be used to receive and transmit. For the bulk of the tests, a bottom
mounted antenna roughly abreast of the wings was utilized. However, a top
mounted antenna just aft of the cockpit was used for the taxiing tests so as
to reduce the signal loss due to ground lobing and have the antenna near the
DME/ P minimum operational height above ground. The pattern of the specific
antenna was not measured; however, it should be quite similar to that of a
Piper Cherokee (both single engine small G/A aircraft). Figure 2-9 shows the
L band pattern for a model Piper Cherokee in the same configuration used in
the flight tests (flaps up, wheels down). The pattern is seen to be reason
ably flat in the horizontal plane.
C. System Power Budgets
The power budgets for the interrogation and reply links under the "nomi
nal" airport conditions assumed to date in DME/P power budgets [3] are shown
in Tables 2-1 and 2-2. The low height of the interrogator antenna
substanti.ally increases the ground reflection lobing loss; however, this is
largely compensated by the high gain of the directive dish. The minimum
triggering level (MTL) shown is a typical value for ATCRBS transponders. The
receiving antenna has a smaller gain than the transmitting dish; however, its
greater height offsets much of the gain differences at low aircraft altitudes.
2-12
r
..
AircraftReceive/TransmitAntenna
TransponderNarco Transmit CommandModel UAT-l ~
r---'1 "'
Fl 50 1J s ......100/200 ns
.... Pulse .... Delay DME Pulse,. ;' ;'
Detector Generator
Fig. 2-1. DME Airborne Electronics
Fig. 2-8a. Detected 100 ns DME pulseat transponder output. 3 dB = 1.3vertical divisions.
II
.. , . ... . ...~. ... . ... . ... . ..~.. i~.... ... . ... .:.1
til
·111 ~~
r:~, rJ t
- r,r ~ rJr .~~ill
11'l< ••• ... . ... . ... . ... . '" . ... . ... . ... . '" .
~ !JV 1~ .:ilS-- -
Fig. 2-80. Detected 200 ns DME pulseat transponder output. 3 dB = 1.7vertical divisions.
2-13
" .I'IOSII
\80 t. I-t-\-HH-1-1-t-t-t~ic+---+---+-+-+-+-+-+--jf---iI,T
'"
Fig. 2-9. Horizontal plane antenna pattern ofPiper Cherokee model aircraft, flaps up, wheelsdown (from [10]).
2-14
r
•
•
•
•
TABLE 2-1
DME MULTIPATH MEASUREMENT SYSTEM UPLINK POWER BUDGET
Aircraft LocationCat II DR Threshold Rollout
Aircraft Altitude (ft. ) 50 50 15
Range (ft. ) 14,500 12,000 10,000
EtZP 74 dBm 74 dBm 74 dBm
Path Loss -105 dB -104 dB -102 dB
Ground Reflection Lobing Loss -7 dB -11 dB -21 dB-----
Received Signal at alc Antenna -38 dBm -41 dBm -49 dBm
Aircraft Antenna Gain 0 dB 0 dB 0 dB
Coupling Loss -4 dB -4 dB -4 dBxpdr ----- -----
to antenna
Received Power -42 dBm -45 dBm -53 dBm
Margin (HTL = -70 dBm) +28 dB +25 dB +17 dB
Assumptions:
Interrogator dish height 5 feetDME site 2000 ft. behind stop end of 1U,000 ft. runwayEffective Radiated Power (ERP):
transmitter output 200 watts +53 dUmground coupling and cable loss -3 dBground antenna gain (6 foot dish) +24 dtl
+74 dUm
2-15
TABLE 2-2
DME MULTIPATH MEASUREMENT SYSTEM REPLY POWER BUDGET
Aircraft LocationCat II DR Threshold Rollout
(l 00' ) (50' ) (l5' )
ERP +46 d Bm +46 d Rn +46 d Bin
Path Loss -105dB -104 dB -102 dB
Ground Multipath Loss* o dB o dB -4 dR
Receiving Antenna Peak Gain 13 dB 13 dB 13 dB
Elevation Nea r HorizonPattern Rolloff -6 dB -6 dB -6 dB
------
Received Power -52 d:&n -51 d:&n -53 d Bin
Ca b1e Lo s s* * (30 feet) -3 dB -3 dB -3 dB
Receiver MTLt -78 d:&n -78 d:&n -78 dBm
SNR (video) +23 dB + 24 dB +22 dB
Assumptions:
*Receiver height = 30 feetReceiver antenna 2000 ft. behind stop end of a 10,000 ft. runway
**Front end at bottom of array as opposed to behind receiving element.
tMeasured front end sensitivity.
Effective Radiated Power (ERP)transmitter power (laO watts)coupling and cable lossaircraft antenna gain
ERP
2-16
50 dBm-4 dBo dB
+46 d Bin
•
..
•
.1
ATTN2 = 40 dB ll10219-N LATTNl ADJUSTED UNTIL AIC XPDR STOPS RESPONDING
ATTN3 ADJUSTED UNTIL RECEIVED PULSE SNR IS TOO LOW TO IDENTIFYPULSES IN THE RECEIVED WAVEFORM
1030 MHz
; 2.42 ft
)/
100 ft
1090 MHz
Fig. 2-10. Taxiway test of measurement equipment power budgets.
2-17
To validate the power budgets of Tables 2-1 and 2-2, tests were carried
out on a taxiway adjacent to the Lincoln Laboratory Flight Facility at Hanscom
AFB. For the uplink test, the transmitting antenna heights were chosen to
yield a ground reflection in quadratuare with the direct signal (see Fig. 2
10). Table 2-3 shows the corresponding power budget using a simple Alford
dipole element as the transmitting antenna. The observed attenuation required
in this case to reach MTL was 65 dB, which is within 1 dB of the attenuation
called for in Table 2-3.
When the Alford element was used as the receiving antenna for the tran
sponder reply, the required attenuation to cause loss of signal was 73 dB,
which agrees quite well with the budget indicated in Table 2-4. The PALM
element used as a receiving antenna in the same location required 66 dB atten
uation to cause loss of transponder reply which was initially surprising,
since the PALM eleQent peak gain is 5 dB higher than the Alford antenna peak
gain.
However, the PALM element phase center height was 8 feet so that the
aircraft antenna was at an elevation angle of -1.43° with respect to the
element boresighted pattern. This corresponds to a point near the first null
of the PALM elevation pattern in which the pattern is 20 dB down with respect
to ttle peak gain. Consequently, we would expect the rel\uired attenuation to
reach 15 dB SNR witn the PALl1 element to be as much as 15 dB less than that
with the Alford. The actual difference was 7 dB, which could be explained by
assuming that the array was tilted back 2° for the test. This could easily
have been the case, since the normal alignment procedure was not utilized on
the taxi test.
The conclusion reached then was that the taxiway test experimental re
sults basically confirmed many of the key features in the power budgets of
Tables 2-1 and 2-2. This is an important issue, since if the experimental
losses during tests are greater than predicted using a simple runway reflec
tion model, this would need to be incorporated in the DME/P decision-making
process.
2-18
•
t
TABLE 2-3
UPLINK POWER BUDGET FOR TAXIWAY TEST
•
Transmitter Output (200 watts)Coupling, Cable LossAlford Ga in
Path Loss (100 feet)
Ground Multipath GainDue to Lobing*
Received Signal Levela t Ale Antenna
Cabling, Coupling Lossto Transponder
Signa I a t Transpond er inAbsence of any Attenuation
Transponder MTL
Required Attenuation toreach MTL
+53 d Bm-3 dB
+8.8 dB+5R.8 clEm
-62 dB
+3 dB
a dBm
-4 iI B
-4 dRm
-70 dRm
66 dB
*The geometry parameters of Fig. 2-10 are such that the ground reflectionshould be in quadrature with the direct signal.
2-19
TABLE 2-4
DOWNLINK POWER BUDGET FOR TAXIWAY TEST
Transponder Output (100 watts)COupling, cable Loss (estimated)An tenna Ga inEffective Radiated Power
Pa th Loss (loa feet)
Ground Multipath C~in Dueto Lobing (approx.)
Received Signal at GroundAntenna
Receiving Antenna (AlfordDipole) Ga in
cable Loss (30 feet) (Usingsame ca ble a s with experiment)
Signal at Front End
Receiver Sensitivity
SNR at Input wlo Attenuation
Attenuation to Reach Threshold
2-20
+50 dBm-4 dBo dB
-+-4'""""'6"'-d Bm
-62 dB
+3 dB
-13 d Bm
+8.8 dB
-3 dB-7.2 dBm
-78 dBm
+71 dB
~73 dB
•
,
III. DATA ANALYSIS PROCEDURE
In this section, we describe the algorithms used in analyzing the digi
tized waveforms to ascertain the multipath environment.
The principal focus for the automated analysis has been specular reflections
which are manifested by large pulses which are well separated from the d irec t
signal as shown in Fig. 3-1. In these cases, fairly simple criteria are used
to identify the pulses:
(l) peak amp 1 itud e correspond ing to an SNR ) approxima tely+10 dB or the minimum MID ratio of concern (typically -20dB)
(2) pulse width between -6 dB points which lies in the interval (W -50 jJsec, W +100 jJsec) where W = expected pulsewid th in Ilsec. (W = l50llsec for the narrowest pulsesused)
The first pulse encountered in the digitized time interval which meets the
above criteria is assumed to be the direct signal. The peak level of the
pulse is taken to be the direct signal amplitude and the point midway between
the first leading and trailing edge digitized amplitudes which are at least fi
dB down from the peak level is taken to be the centroid.
If no pulses meeting the above criteria were encountered in the digitized
waveform, an "M" is placed on the MID plot at the -25 dB MID level and no
symbol is placed in the corresponding time delay (T) plot. If only a "direct"
pulse is encountered, an "X" is plotted at -20 dB on the MID plot with no
corresponding symbol on the T plot.
Any add itional pulses meeting criteria (1) and (2) are assumed to be
multipath. Their peak amplitude and centroid are computed as for the direct
signal. The displayed MID ratio represents the ratio of peak amplitudes while
the relative time delay is computed as the time between the respective pulse
centroid s.
3-1
direct multipath
lolASHIHGTOH D.C.
f
IoH\SHIHCTOH D.C.
JOYRANGES 2.6
lolASIHHGTOH D.C.
JOY" 2.3
ME21&C
JOYR~QES 2.2 JOYRAHGES 2.2
time ----
Fig. 3-1. Example of received waveforms with multipath (Washington, D.C.at 1 nm from threshold).
3-2
..
The first multipath signal encountered after the direct signal is denoted
by an "X" in the l1/D level and T plots. Succeeding multipath signals, if any,
are denoted by the letters Y, Z, A, and B, respectively, on both plots.
Several special cases deserve mention. The replies of other ATCRBS
transponders to other ATCRBS interroga tors may lie within the da ta record ing
intervaL Tnese "fruit" pulses are readily identified since tneir pulse width
is approximately 450 nsec (see Fig. 3-2). Measurements with fruit present are
flagged in the summary plots with an "F" at an MID level of -30 dB as a warn
ing tha t the da ta on tha t ind ividual measurement may have been corrupted by
the fruit.
If the amplitude at tne beginning of the digitized interval is above the
threshold, but decreasing, the data is ignored until the amplitudes begin to
arise. This is done to avoid declaring the end of a "fruit" pulse as the
direct signaL
Occasionally, t~le AID converters would fail to properly track a rapidly
rising signal for one sample on the leading edge. This gave rise to sharp
"glitches" in the digitized waveform (see Fig. 3-3), which was completely
inconsistent with valid pulses passing through the IF filters. Since these
artifacts arose only on the leading edge of high level pulses, it was
straightforward to recognize them and reduce their effects by replacing the
"glitch" level by the average of the adjacent samples (equivalent to inter
polation between the adjacent samples).
The manual method of setting the 3.2 ~s waveform sampling aperture is, of
course, vulnerable to loss of data if the sampling gate drifted too far away
from the n pulse. To a significant degree, situations in which this arose
could be identified by erratic time behavior in· the tracking gate range versus
time plot since the actual flight profiles along runway centerline were flown
at roughly constant velocity. Time intervals where erratic behavior arose
(see, e.g., Fig. 3-4) were generally excluded from analysis.
3-3
w I .po
9
••••
64.'
4'.'
32
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A "1
6.'
,. L I•.•
T U D-1
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E
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lr~
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-11
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Was
hin
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ati
on
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2.5
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resh
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vert
ical
scale
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ith
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it
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as.
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•••
Fig
.3
-2.
Ex
amp
leo
ffru
itin
terf
ere
nce.
Tu
lsa
Sit
e2
at
Th
resh
old
of
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way
17L
ho
rizo
nta
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ple
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ical
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un
it=
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mu
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omAA
han
ger
dir
ect
AID
co
nv
ert
er
/.1
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e,
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EI VI
-3i!••
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Fig
.3
-3.
Ex
amp
leo
fir
reg
ula
rA
IDco
nv
ert
er
acti
on
.
~
g.'
/;J
,
8.9
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ack
ing
air
craft
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uir
ing
.~
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craft
~- ~
6.•
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L!)
~U
J:z
:I
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cae
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~ <->
I--
Ph
ilad
elp
hia
en
>-
Data
Fil
e20
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>....
....,
3.1
2.8 1.'
Fig
.3
-4.
Ex
aftp
leo
ftr
ack
ing
gate
eff
ecti
ve
ran
ge.
•
It had been hoped that the PALM range output [5J, which determines the
round trip time to the F1 pulse arrival using a simple amplitude comparison
within the manual tracking gate, would also be useful in delineating loss of
track. However, it gave very erratic behavior in many cases which cannot be
explained in terms of tracking gate misalignment (see, e.g., Fig.3-5). Our
current hypothesis is tha t either the severe multipa th environment may have
caused erratic triggering and/or the triggering circuit may have been defec
tive.
The other function of the PALM ranging mode was to have been to furnish a
distance reference for correlation of different approaches and correlation
with simulation results. The raw sampling window position (Le., "joystick"
range) was not satisfactory in this respect due to its coarse quantization
(0.1 nmi). However, by fitting a piecewise linea r function to the measured
joystick range versus time, it was possible to generate a smoothed (Le.,
interpola ted) range versus time plot which would yield the desired range
quantization. Such a smoothed range function was created for each approach
*and used as the ordinate for both M/D level and T plots.
Diffuse reflections may be viewed as specular reflections from many small
reflectors, especially irregular terrain features. Theoretical treatments of
diffuse scattering from terrain [7 J suggest that substantial contributions
would come from the so called "glistening surface" which gives rise to a
received waveform resembling a Gaussian random process. The validity/utility
of such models for terrain reflections at L band has not yet been established
experimentally; thus, it was not clear what criteria should be utilized in
estima ting the diffuse component. One possible criteria is to compare tile
•
general background level prior to and follOWing the direct signal. However,
'/(
We considered plotting all runs versus time to threshold, but this would haveyielded problems in data comparison between approaches made at differentground speeds and/or in cases where the ground speed varied significantlyduring an approach.
3-7
35
M.
25
88
.2
8.1
.1
5M
.1....
SM
.
Ph
ilad
elp
hia
Dat
aF
ile
204M
g••
8.'
1.e
~6
.'w
l..L
II
t.!)
00
;z: -Q
I:s.
•0
..
.... 3.e
2.e
1.• •••
e.
HU
ftlER
OF
RE
PLIE
S
Fig
.3
-5.
Ex
amp
leo
fm
easu
red
PALM
ran
ge
ou
tpu
t
..
this did not yield a clear indication of the diffuse multipath level on the
cases attempted due to
(1) insufficient waveform data prior to the direct signaland
(2) corruption of the metric by specular pulses
Additionally, the diffuse multipath delays between 0 and 200 nsec are of the
greatest practical interest and these would be obscured in all cases by the
direct signal pulse •
3-9
IV. LAMBERT-ST. LOUIS INTERNATIONAL AIRPORT
A. Multipath Environment
Figure 4-1 is an aerial photograph of Lambert-St. Louis International
Airport which shows the reflection rays corresponding to the major anticipated
threats for an aircraft landing on runway 12R (currently a category IlLS
runway). Fig. 4-2 shows additional airport geometry details. The major
multipath threats were expected to be the large McDonnell Douglas (M-D)
aircraft factory buildings shown in Figs. 4-3 to 4-4. The main factory
" building has predominantly masonry fronts (Gunite) punctuated by windows,
while hangars 42 and 45 have several large vertical doors with windows.
The various low buildings (e.g., aircraft shelters) and a parking lot
between the buildings and the threshold of runway 12R are expected to prevent
some of the building reflections from reaching a receiver, especially if the
receiver is at a low altitude. Similar circumstances arise for reflections
from the terminal buildings shown in Fig. 4-5.
The expected multipath relative time delays for a centerline approach are
as follows:
BuildingMcDonnell-Douglas main factoryMcDonnell-Douglas Hangar 42McDonnell-Douglas Hangar 45Terminal buildings
T (nsec)144024403300
310
The runway contour (shown in Fig. 4-6) slopes downward from the localizer
site toward the approach end of the runway. Thus, it was not deemed necessary
to erect the receiver antenna to its full height*. The transmitter antenna
was sited to the immediate left of the ILS localizer as shown in Figs. 4-1 and
4-2 with a phase center height of approximately 10 feet. Figure 4-7 shows the
equipment at the measurement site. At the time the measurements were made,
*Obstruction clearance criteria also entered in this decision.
4-1
~ I N
l087
95-R
Fig
.4
-1.
Lam
ber
t-S
t.L
ou
isIn
tern
ati
on
al,
St.
Lo
uis
,M
isso
uri
,sh
owin
gaz
imu
thre
flecti
on
path
s.
~ I W
Fig
.4
-2.
St.
Lo
uis
-L
amb
ert
Air
po
rtL
ayo
ut.
hanq
ar\
hanq
ar42
bu
ild
ing
Fig
.4
-3.
McDonnel~Douglas
bu
ild
ing
sa
sse
enfr
omIL
Slo
ca
lizer
sit
e.
,
-
Fig. 4-4. ILS localizer and McDonnell-Douglas buildings from St. Louismultipath measurement site.
4.....5
fJ• - ..
Fig. 4-5. St. Louis (Lambert) terminal area buildings as seenfrom measurement site.
4-6
ii
OP
lRA
T1
QN
Al
DA
.1A
\• l
(44
-18
12
L
-
!J~
/
@@
II
-@
r--
l®@
@Gf
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i--
-~
[J~}l
-,j1
110'
~-t
-r-
IJ"'I
"""
IJ\f!\l
lI
""",.
.,,,..
II
\\
\
-t-~
/I
I\
-,
,,-I
I
Fig
.4
-6.
Ru
nw
ayl2
R-3
0L
co
nto
ur
at
La
mb
ert
Fie
ld-
St.
Lo
uis
.
--
. Fig. 4-7. Multipath measur~ent equipment being erected at St. Louis(Lambe1:tl.
4-8
•
•
the ground was covered by approximately one foot of snow with two to three
foot deep drifts in the grass covered areas, but the runway was clear.
B. Measurements Made
Table 4-1 summarizes the measurements made at St. Louis. A total of two
taxi tests and 14 centerline approaches at approximately 110 knots were made
between the hours of 12:32 a.m. and 3:30 a.m. February 13. This late hour was
necessitated by the air traffic activity at the airport.
c. Waveform Analysis Results
All of the approaches accomplished at St. Louis have been analyzed and
representative results will be described in this section.
The taxiway tests were inconclusive due to low SNR caused by a depression
in the taxiway. By increasing the narrow pulse width to 200 nsec, the tran
sponder output power was increased enough to permit reply tracking on the
approaches.
Figure 4-8 shows the multipath levels and corresponding time delays for a
centerline approach with a 50 foot threshold crossing height. Figures 4-9 and
4-10 show the corresponding results for two other tests with the same nominal
:light profile. Figures 4-11 to 4-14 show representative waveforms correspon
ding to high MID levels in the region near threshold and when over the runway.
None of the approach data summaries indicated any specular multipath in
the region of 2.3 nm! to 0.3 nmi from the threshold*. The region near thresh
old where multipath is expected (recall Fig. 4-1) corresponds to 1.7 nmi in
joystick range.
We see that moderate co low level multipath is indeed encountered in that
*rsolated multipath declarations which do not correlate with adjacent samplesas far as MID level T are concerned and, do not appear on other nominallyidentical profiles are assumed to be due to fruit and lor low SNR.
4-9
TABLE 4-1
PDME MULTIPATH MEASUREMENTS AT LAMBERT (ST. LOUIS) INTERNATIONAL AIRPORTFebruary 13, 1981
Taxiway Test s:
2 runs transmitting/receiving through top antenna on aircraft (runs 213A,213B)
Flight Tests:
1. Profile 1 (30 gHdeslope, 50-foot threshold crossing height, IS-footheight along runway)
10 runs [runs 213C through 213K, 213P, 2130, one run aborted (213C)]
2. Profile 2 (3° glideslope, 25-foot threshold crossing height, la-footheight along runway)
~ runs (runs 213L through 2130)
4-10
•
Dti'v· ..... HOUR. 7,"INa 15,$[Ca 35,JOVSTI( RA~CES· iL. 1.e,PAL" RANGES- J.D. 1••2
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ST. LOUIS "ISSIOHa 32 DATAFlU:· 1lf\[!13D
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•
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1.II
•S..OOTH£D Jov SUCIC IlAI'ICE 'HPtI )
ST. LOUIS 1'I1SSI0"- 3~ DollTlIlFIUa Dl"E213D
Fig. 4-8a. Data summary for St. Louis's approach with 50 ft.threshold height.
4-11
10.
, ', ', '
-Ie.
-...
,,xx
>1',
~)(~ .... )(II()(X~)()( ""''''''''''',,:00:00<: lOOOOOOOOO< lOOOOOOO¢¢( * )(
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1 •••• I.PSI ,.WI •. asl .....SltOOTHED Jov sTle.: RAHCECHI'lII
ST. LOUIS "'15510N .. 32
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,,
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•
5"00T"£» JO'l' STICK IlAHC[t""t l5T. lOUIS "ISSIO"" 32 DATMIU- DN[21JD
:Fig. 4-8b. Data summary for St. Louis's approach with 50 ft.threshold height.
4-12
MY" .<4. HOUR .. 7.1'1". i •• SEC. <41.JOVST( ."I"I(OES- ~ .• 1.t,PALM .""U.£5. 1.11 1.61
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4-13
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Fig. 4-9b. Data summary for St. Louis's approach with 50 ft.threshold height.
4-14
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4-16
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Fig. 4-10c.
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Data summary for St. Louis's approach with 25 ft.threshold height.
4-17
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TABLE 4-2
FLIGHT PROFILE FOR LAMBERT SIMULATION
Distance From Distance AlongThreshold Flight Path
Way Point (nmi) (feet)
1 0.50 0
2 0.00 3000
3 -0.083 3500
4 -0.16 3804
5 -1.32 10854
4-22
Height(ft)
200
50
29
20
20
•
•'"
Mid
coas
tH
gr
=B
9
=B
e
TWA
Cr===========~1
88
Ter
min
al
!t2
,~
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.l: I N Lt.J
vP~S
FT
-25
09
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uil
din
gs
0.
20
00
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POS
FT6
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8g
e0
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DME
pea
kle
vels
OBS
TR
At«
AMP
DIS
TRD
OP
C3
03
82
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85
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4".
0-3
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e7
28
9.0
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.
Fig
.4
-15
.A
irp
ort
map
for
Lam
ber
t-
St.
Lo
uis
sim
pli
fied
scen
ari
o.
region with multipath delays in the expected range of delays 0.4 \.lS to
2.4 \.ls). Further down the runway, high level (> -6 dB MID) is encountered
with delays of approximately 300 - 500 nsec as well as some multipath with
long delays. We attribute the short delay multipath to reflections from the
terminal area buildings while the longer delay multipath probably arises from
the fences and parking lots, etc. in front of the McDonnell-Douglas hangars.
There are some noticeable differences betwen various flights in terms of
precise joystick range at which the threshold multipath is encountered and the
MID levels. The joystick range differences probably reflect the track gate
location relative to the PI pulse, whereas the MID level differences probably
reflect differences in height over runway threshold (the heights shown in
Table 4-1 are necessarily approximate given the time of day at which the tests
were conducted).
D. Simulation Results
Figure 4-15 shows the airport map for a simulation of the DME measurement
geometry. The various McDonnell-Douglas build ing walls were represented by
flat plates whose height corresponded to the physical height except in the
case of the east end of the McDonnell-Douglas main factory which was repre
sented by two separate plates to account for the interspersing of glass
windows with metal and stucco siding. The terminal building wing was
represented by a single 50 foot high fla t metal plate as were the TWA and
Midcoast Aviation hangars. The assumed flight profile is shown in Table 4
2. No account was taken of the runway or terrain contours nor of the various
intervening obstacles which partially block the building reflections [2].
*Figure 4-16 shows the effective MID level and relative time delay for
the various plates as a function of distance along the flight path. We see
that multipath from the M-D building 42 with a level of approximately -8 dB is
*See reference [1] for the definition of effective MID level.
4-24
•
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e5
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11128:5~LA~BERT
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.4
-16
.C
ompu
ted
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1ti
path
ch
ara
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risti
cs
for
Lam
ber
t-
St.
Lo
uis
scen
ari
o.
expected near threshold with a time delay of approximately 1.4 llS to 2.0 llS.
This level agrees reasonably well with the measured results in Figs. 4-8 to 4
10. Similar levels/time delays are predicted from the other M-D buildings in
a 1200 foot region (0.2 nmi) starting 1200 feet (0.2 nmi) after threshold and.
in fact. this appears to be the case although several very high level MID
experimental points occur which are not suggested by the simulation mod el.
These undoubtedly arise from the complicated fine structure of the M-D build
ing surfaces which was not considered in developing the simulation airport
model.
The experimental data multipath after threshold has delays comparable to
those predicted for the terminal building east concourse; however. the levels
and spatial duration are significantly less than suggested by the simulation
result. This dramatic difference arises because the loading gates and parked
aircraft block most of the multipath from the building surface. The
experimental short duration multipath at 1.0 nmi joystick range correlates
with the region predicted for the TWA hangar multipath.
E. Summary
The multipath regions at St. Louis in the approach and flare regions
correlated fairly well with the specular regions associated with the large
buildings which face the runway. The MID levels and time delays predicted
using the MLS propagation model and a very simple airport mod el agree fairly
well for the M-D buildings modeled, although some isolated measurements sug
gested M/D levels much higher than predicted.
The measured M/D levels for the terminal concourse wing were
substantially lower than suggested by the simple airport model. The low
terminal concourse levels are attributed to blockage of the reflection paths
by the parked aircraft and jetways. Similar phenomena were noted in C band
multipath measurements at Logan airport [13]. The general lower levels in the
field data for the TWA hangar reflect blockage by the intervening buildings.
4-26
•
..
•
It is interesting to compare these L band MID levels with the C band
levels measured in a previous program [2] which are shown in Fig. 4-17. We
see that the L-band multipath levels were considerably (e.g., 10 dB) in excess
of those measured at C band in the region approximately 1000 ft. after thresh
old, but similar in the region near threshold •
4-27
Vde
note
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Fig
.4
-17
.Su
mm
ary
of
St.
Lo
uis
(Lam
ber
t)
C-b
and
mu
1ti
pat
hch
ara
cte
rist
ics
(fro
m[2
]).
..
v. TULSA INTERNATIONAL AIRPORT
A. Multipath Environment
Figure 5-1 shows a map of Tulsa International Airport together with
reflection rays from the buildings most likely to cause DME multipath for
aircraft landing on runway 35R (currently a category I ILS runway). These
buildings are the American Airlines (AA) , Rockwell International, and
McDonnell-Douglas (M-D) hangars shown in Figs. 5-2 to 5-4. The large M-D
building parallel to and behind the hangar can cause some multipath, but much
of it is blocked by various small hangars and other buildings. In contrast,
the multipath from the AA hangar is almost totally unblocked by interveninp;
obstacles.
The expected multipath relative time delays for a centerline approach to
runway 35R are as follows:
Build ing
M-D hangar
M-D build ing
AA hangar
T (nsec)
1500
2000
700
•
The runway contour (Fig. 5-5) shows a large hump peaking near the inter
section with runway 8-26. For this reason, at site 1, the transmitter antenna
was placed on the measurement van at a locatio:l adjacent to the building
housing the transmitter (Fig. 5-6) for the runway 35R ILS localizer.
Measurements were also made at a second site to discern the multipath
environment for aircraft land ing on runway 17L. The transmitter site and
reflection rays for this site are shown in Fig. 5-7. Cbnsiderable difficult
ies were encountered at this site in achieving an adequate uplink SNR even
though the transmitting dish was placed on top of the van. It was found that
by using a small dipole antenna mounted on a pole atop the van at a height
above ground level (AGL) of approximately 30 feet, that an adequate uplink
margin was finally obtained.
5-1
PINE STREET
, ROCKWELL INTERNATIONAL
Approach direction
Site 1
)
Urn[~ffi
[Irnu~m millU IJ 00 mill[iTIIJrn~rnrnU
HCNIJAflIO NEEDLES TAMMEN &. IIERGENDOFF
Reflection rays
C44-1799
Fig. 5-1. Tulsa Site 1 measurement geometry and reflection rays.
')-2
.'
Fig
.5
-2.
Am
eric
anA
irli
nes
han
gar
as
seen
fro
mth
resh
old
of
runw
ayI7
L.
fig. 5'"'3. Ameri'Can Airlines and McDonnell Douglas buildings as seen frommeasurement site 1.
5-4
..
r'• ....,cOO..,.ELL DO(JGLAII~
TULSA '-"
Fig. 5~4. McDonnell Douglas building And meaSll'rement equ;i.'PIDent a, t Tulsasite 2.
5-5
PR
OF
ILE
Fig
.5
-5.
Terr
ain
con
tou
ral
on
gT
uls
aru
nw
ay1
4-3
5•
.,•
Fig. 5-6. Measurement equipment at site 1 next to runway 35R ILS localizerbuilding.
5-7
HOINAAD NEEDLES TAMMEN. 8EROENDOFF-------~
PINE
Q«Qa:QClz
j
STREET
C44-1799
..
Ii
Fig. 5-7. Tulpa Site 2 measurement geometry and reflection rays.
5-8
The expected multipath relative time delays for a centerline approach to
runway 17L are as follows:
Build ing
M-D hangar 1
M-D hangar 2
M hangar
B. Measurements Made at Tulsa
T (nsec)
1000
1500
600
Table 5-1 summarizes the measurements made at Tulsa. A total of 9 ap
proaches were made to runway 35R between 4 :00 p.m. and 7 :00 p.m. on February
16 and 8 approaches to runway 17L hetween 3:00 p.m. and 6:00 p.m. on February
15. Add itionally, five flights were flown at right angles to the extended
runway centerline to emulate a turnon from the north at approximately 9 nmi
from the threshold of 17 L as shown in Fig. 5-8. Taxi tests were mad e along
the runway from both of the thresholds to the runway midpoint (i.e., through
out the flare region); however, the signal attenuation due to the runway hump
was so high that no data was obtained in the regions of greatest interest.
c. Waveform Analysis Results
All of the approaches at Tulsa have been analyzeil and representative
results will be described in this section.
1. Site 1 (Approaches to Runway 35R)
The reflection rays shown in Fig. 5-1 suggest that specular multipath
would be found only in the flare region for an approach to runway 35R, and
this was found to be the case. Figures 5-9 and 5-10 show the summary results
for two approaches to this runway. We see that virtually no multipath is
encountered prior to threshold. A similar lack of multipath prior to thresh
old was found on the other 6 approaches to runway 35R.
5-9
TABLE 5-1
FLIGHT TESTS AT TULSA INTERNATIONAL AIRPORT
Threshold Height AboveRunway Crossing Height Runway
Threshold Run Numbers (f eet) (f eet)
35R 215B through 215E 50 20
35R 21SF through 2151 25 10
35R 215J 65 25
17L 216B through 216F 50 15
17L 216G through 216J 20 10
IlL 216K through 2160 Partial orbit at right angles to extendedrunway CL from 5 nmi north of CL to CL.Intersection with CL at 9 nmi from IlLthreshold. Height AGL of 1000 feet.
5-10
end start
t-•
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lIlL
35R
•./Ground Site
Fig. 5-8. Flight profile for partial orbitat Tulsa site 2.
5-11
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Fig. 5-9a. Tulsa site 1 data summary for 25 ft. thresholdcrossing height.
5-12
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Fig. 5-9b. Tulsa site 1 data summary for 25 ft. thresholdcrossing height.
5-D
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5-14
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Fig. 5-10 b. Tulsa site 1 data summary for 50 ft. thresholdcrossing height.
5-15
However, just after threshold (joystick range = 1.81 nmi) , much of the
data was missed due to a combination of severe attentuation from the runway
htnnp and /or suppression of the transponder by high level mul tipath from the
McDonnell-Douglas factory build ing with delay times of approximately 2 ].lsec.
The valid data points measured in the region indicate MID levels between -5 dB
and +10 dB with time delay of 700 nsec - 1000 nsec, presumably corresponding
to the AA hangar. Figures 5-11 and 5-12 show the waveforms corresponding to
the high M/D levels.
To summarize, the experimental data for approaches to runway 35R suggests
that specular multipath with levels ~ -20 dB was encountered only in the
regions where expected from the specular ray considerations indicated in Fig.
5-1. Unfortunately, the expected specular region also coincided with a region
of high direct path attentuation due to the runway hump, so only fragmentary
data was obtained. However, this fragmentary data suggests high level multi
path from the AA hangar.
2. Site 2 (Approaches to Runway 17L)
The reflection rays shown in Fig. 5-6 suggest that the specular multipath
on this approach will be encountered throughout the approach region with
reflections from two hangars in the threshold region. However, the long
duration multipath from the McDonnell-Douglas factory buildings would be
attenuated by the azimuth pattern of the receive antenna (Fig. 2-4).
Figures 5-13 to 5-15 show stnnmary results for approaches with a 50-foot
threshold crossing height. Figure 5-16 shows the summary results for a flight
profile with a threshold crossing height of 20 feet and a height AGL of 15
feet along the runway. The results prior to threshold on the various runs are
seen to be very similar. The M/D levels at threshold and further down the
runway show a greater variation, with higher levels ocurring at the lowest
aircraft heights.
Figures 5-17 and 5-18 show received waveforms correspond ing to high M/n
levels in regions just prior to and after the runway threshold. The waveforms
5-16
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Tulsa site 2 data summary for 50 ft. threshold crossing
5-19
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Tulsa site 2 data summary for 50 ft. threshold crossing
5-20
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Fig. 5~14a. Tulsa site 2 data summary for 50 ft. thresholdcrossing hei'ght.
5-21
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5-22
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Fig. 5-l4c. Tulsa site 2 data summary for 50 ft. thresholdcrossing height.
5-23
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5-24
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5-25
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5-26
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5-27
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5-28
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before threshold were fairly easy to analyze, whereas at and after threshold,
the analysis was difficult due to the low SNR.
The very high level multipath (+5 dB to as high as +15 dB MID ratios) at
threshold with relative time delays in the 400 - 600 nsec range correlates
quite well with the expected time delays and multipath region for the AA
hangar. Further down the runway, high to very high level multipath is encoun
tered with a variety of multipath delays corresponding to reflections from
several of the buildings.
No multipath within 20 dB of the direct signal was encountered on any of
the four off centerline partial orbit flights at a range of approximately 11
nmi. Figure 5-19 shows the summary results from one of the orbit flights.
Although the McDonnell-Douglas factory build ings were oriented to produce
specular reflections in this region at low elevation angles (e.g., < 1°), the
levels were reduced by the 1) partial Fresnel zone spillage (approximately 6
dB), 2)blockage of reflections by intervening buildings, and 3) the pattern of
the receiving antenna (~ -12 d B) such that no appreciable multipath was en
countered.
D. Simulation Results
Both multipath measurement sites were simulated using a simple airport
model in which
(I) build ing walls were represented by rectangular pIa teswhose lateral dimension coincided with the locations onthe airport map. The plate heights and base elevationwere determined from the MLS multipath airport surveyda ta [6]
(2) the runway hump was modeled as indicated in Capon [7].The three points describing the hump were the runway 35Rthreshold, and points 3200 feet and 1500 feet d own therunway from the 35R threshold. The runway heights ofthose points were obtained from Fig. 5-5
(3) the transmitter x,y coordinates were inferred from thelocation of nearby permanent structures (e.g., ILS local-
5-31
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Fig. 5-19. Tulsa site 2 data summary for partial orbital flight.
5-32
•
izer, fences, etc) which appear on airport maps. Theground heights for the measurement site were estimateelfrom Fig. 5-5
and(4) no effort was made to model terrain features other than
the runway hump. Thus, for example, it was assumed thatthe terrain along the building reflection paths was flatand at the same elevation as the DME measurement site.The runway hump, in fact, d iel not extend over to thebuildings: however, the terrain was definitely not uniformly flat along these paths •
Figures 5-20 and 5-21 show the simulation results for an approach to
runway 35R with a 50-foot threshold crossing height and a 25-foot height along
the runway. We see that low level (~ -5 dB) multipath with a l of 700 - 1100
nsec multipath is anticipated in a region approximately 2000 feet prior to the
threshold from the McDonnell-Douglas factory building. This correlates reson
ably well with -15 dB multipath at 2.0 nmi joystick range in Fig. 5-10.
High level multipath is expected in the flare region (approximately 800
feet past threshold to 2000 feet past threshold) from both the AA hangar (800
nsec delay) and McDonnell-Douglas factory building (l000 to 3000 nsec
delays). As noted earlier, these multipath levels and delay values do corre
late with the few data points that were obtained in this region.
Figures 5-22 and 5-23 show the simulation results for an approach to
runway 17L with a 50-foot threshold crossing height and a 25-foot height along
the runway. High level (> 0 dB) multipath with a l of 550 - 650 nsec is
pred icted in a 600 foot region approxima tely 1000 feet prior to threshold
(corresponding to a joystick range of approximately 2 nmi). This prediction
of multipath region and delay correlates quite well with the measured results
in Figs. 5-13 to 5-16; however, the peak measured MID levels are considerably
higher (6 dB to 12 dB) than the simulation results. This discrepancy probably
reflects terrain contour features not considered in the simulation (see Chap
ters III and IV in [1] for a discussion of the effects of terra in height
differences on the MID levels),
5-33
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E. Summary
The measured multipath regions and time delays at Tulsa International
agreed quite well with the predicted characteristics using simple ray
tracing. The measured MID levels agreed reasonably well with the predicted
levels at one site (although a detailed comparison in the flare region was not
possible due to the many missed measurements), while at the other, the observ
ed MID levels were considerably larger (e.g., 6 to 12 dB) than those
predicted. The large differences are felt to arise from the (sizable) differ
ences in terrain contour features along runway centerline and along the paths
to the build ings which were not considered in the simple a irport model.
5-38
VI. WRIGHT PATTERSON AI~ FORCE BASE
A. Multipath Environment
Figure 6-1 shows the airport geometry at runway 5-23 at Wright Patterson
Air Force Base (WPAFB), Ohio. This a irport had been used for tests of the
Doppler MLS and had been shown to have substantial C band multipa th over the
runway [2J. Figure 6-2 shows an aerial view of the airport.
Figure 6-3 shows the various buildings on the runway south side as seen
from the ground van site located approximately 200 feet northwest of the ILS
localizer serving runway 23 as well as a closeup of hanga r 206, which is a
major multipath threat in the flare guidance region. The other build ings
along the same apron as hangar 206 are much lower (typically 10 meters high)
and typically made of wood or corrugated metal. The expected multipath time
delays are as follows:
Building T(nsec)
152 1477
146 1654
206 1713
145 1832
The terrain along the path to t"he buildings is relatively flat; however,
the runway slopes up noticeably as one proceeds from the threshold of runway 5
to the 23 end of - the runway (see Fig. 6-4). Given the long measurement site
to runway distance (5000 m), the receiving Clntenna was elevated to its full
height (10 m).
A taxi test was conducted along taxiway 17 at approximately 5:00 p.m. on
February 9 as indicated in Fig. 6-1, however, very high winds and rain preven
ted carrying out the flight tests until noon on February 11. Snow also fell
during the period, but the ground was basically bare for both types of tests.
6-1
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6-4
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B. Measurements Made
Table 6-1 summarizes the measurements made at WPAFB. The large number of
approaches required was necessitated by difficulties in obtaining adequate SNR
on the transponder reply. The low SNR represented a combination of low output
power from the transponder and high attenuation due to the "focusing" runway
contour. Successful data was finally achieved by keeping the engine rpm up
during the approaches (so as to generate a higher power supply voltage). One
of the runs with profile 2 was hopelessly corrupted with high level fruit
interference t which appeared to be correlated with the transponder replies.
c. Waveform Analysis
All of the approaches with adequate SNR and the taxiway test have been
analyzed and representative results will be described in this section. Figure
6-5 shows the results of the test along the airport taxiway. At the beginning
of the test t strong multipath (MID> -6 dB) is encountered with delays of
approximately 300 nsec t while at the end of the runt a considerable amount of
low level multipath (MID < -10 dB) is encountered with progressively increas
ing delays. The beginning of the taxi test was at the midpoint of taxiway 17
and the test ended when abreast threshold of runway 05. The multipath delays
here correlate with reflections from an elevated steam pipe and trees to the
north of the runway which are shown on Fig. 6-2.
Figures 6-6 and 6-7 show the summa ry results for two approaches with
flight profile 1. We see tha t a region of strong multipa th with a T of 1600
nsec is encountered just prior to threshold (j oystick range 2.56 nmi) t but
that then much of the data is missed for the next nautical mile. There are a
number of isolated measurement points just after threshold with extremely high
MID ratios and a T of approximately 1600 nsec. Much further down the runwaYt
low level multipath is encountered with delays in the 500 nsec - 1000 nsec
range. These shorter T multipath signals are believed to arise from the trees
to the north of runway.
6-7
TABLE 6-1
PDME MULTIPATH MEASUREMENTS AT WPAFB
Taxiway Test (February 6)
1 run along taxiway 17 transmitting/receiving through top antenn;:l onaircraft
Flight Tests (February 9)
Profile 1: 3° glideslope~ 45-foot threshold crossing height~ 10 feet AGLalong runwaybottom a/c antenna: 9 unsuccessful runs~ 4 successful runstop a/c antenna: 5 unsucessful runs
Profile 2: 3° glideslope~ 20 foot threshold crossing height~ 5 feet AGLalong runwaybottom a/c antenna: 2 unsuccessful~ 4 successful*top a/c antenna: 4 unsuccessful
*One of the runs contained synchronous fruit.
6-8
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6-10
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251••
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5MOTHED JO'" STICK RAAG[<HPU)
IJAtCHT PATTERSONI"llSSIOH- 32 DATN" IL[- D"[212_
Fig. 6-7b. Summary results for Flight Profile 1 at WPAFB.
6-15
Figure 6-8 shows summary results for an approach with flight profile 2.
The results here are quite similar to those of Figs. 6-6 and 6-7: no multi
pa th unt:il just a t threshold, a region of very high level multipa th with
delays of approxima tely 1600 nsec and then a loss of valid da ta. Figures 6-9
and 6-10 show the received waveforms corresponding to the high MID levels near
threshold. The direct signal amplitude is seen to be roughly constant (as
would be expected arise since ground lobing would change slowly), whereas
large increases in multipath level occured at a few points.
These results near threshold correlate quite well with the expected
multipath region and time delays expected for buildings 206, 146, and 152.
The loss of data in this region may also reflect transponder suppression due
to high level reflections of the PI pulse with a delay near 2 \-lsec. These
buildings are approximately 1 - 1.5 beamwidths off the interrogation antenna
boresight:, so tha t they should not have caused transpond er suppression if high
level multipath (MID" 0 dB) had been encountered. However, the levels in
this region were considerably in excess of even that and hence may have caused
transponder suppression.
D. Simulation Results
Flight profile 1 wes simulated using a very simple airport model in which
terrain contours were ignored and each building modeled as a single vertical
rectangular plate. Figures 6-11 and 6-12 show the airport map and computed
multipath characteristics for the simulation. The simulation predicts low
level (-12 dB MID) reflections from building 152 at threshold with a delay of
approximately 1.6 \-lsec and high level (-3 dB MID) reflections from hangar 206
with a time delay of 2 \-lS. The multipath regions and time delays correlate
reasona bly well with the field measurements, but the pred icted MID levels are,
in some cases, considerably lower (e.g., 10 15 dB) than the measured
values. This difference could arise from several factors:
1. the terrain contour along the runway and building reflection paths was assumed to be flat in the simulation. Thismay have· understated the amount of differential direct
6-16
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3.888 e.pse 2.vee 2.8S8
S"OOTMED JOY STICK RAHGEOil'l1 )PalRIQHT PATTERSON "ISSION· 32 DATAFlLE- Dt'IE212V
DAV- 36.HOUR- 18."lH- e•• sEc" S".JOVSTIC RANGES- 3.1 2.5.PALI'I RANGES- 2.13 2.88
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SPIOOTHEO JOY STICK RAHG[o.... r)
".fU QHT PATTERSON"155[0"· 32 DATAF.IU- tlfE212\1
Summary results for Flight Profile 2 at WPAFB.
6-17
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signal lobing due to the ground since the off runwayterrain is lower than the along runway terrain (see [1]).
2. the staggering of the doors on building 206 was ignored.In studies of C band reflection behavior along thisrunway. it was found that the reflected signal levelscould oscillate very rapidly in the specular region dueto reinforcement and cancellation of signals fromadjacent doors [2].
E. Summary
The measured data at WPAFB correlated reasonably well with the multipath
regions and time delays expected from ray tracing and computer simulations.
Unfortuna tely. the severe reflection environment (terrain lobing a nd lor build
ing reflections) was such that only fragmentary data was available in the
flare region where the highest MID levels were anticipated. The measured data
available in that region suggests that the actual MID levels were comparable
to and. in many cases in excess of. the simulation results using a simple
airport model. Several factors were identified which could account for much
of these differences.
6-22
•
•
•
VII. PHILADELPHIA INTERNATIONAL AIRPORT
A. Multipath Environment
Figure 7-1 shows the airport layout at Philadelphia (PA) International
Airport, while Figure 7-2 shows the reflection rays for the principal antici
pated multipath threats. Philadelphia was chosen as a site exemplifying
reflections from. relatively low build ings in the fla re/rollout region and
because earlier C band van measurements [9] had shown high level multipath at
low heights on the adjacent taxiway.
The main threats were the car~o buildings along the runway. A photograph
of the United Airlines (UA) building is shown in Fig. 7-3. This building is
approximately 10 meters high, as is the Cargo Unit number 1 building. The
adjacent American Airlines/F~stern Airlines (AA/EA) building is 8 meters high,
as is the Flight Kitchen buildings. The building surfaces are corrugated
metal or concrete [6]. The expected multipath time delays for a centerline
approach to runway 9R as as follows:
Building
Ca rgo Build ing 1
AA/EA Cargo
UA Cargo
l (nsec)
750
850
950
The terrain contour along the runway and prior to the 9R threshold (see
Fig. 7-4) presented some substantial siting difficulties. The original plan
called for siting the van ad jacent to the 1L8 localizer for runway 27L.
However, the sharp drop in height meant that it would have been quite diffi
cult to obtain a clear view of the aircraft for the interrogation antenna.
This problem, together with the swamp-like terrain available in that location,
forced us to utilize an alternate site at the edge of the holding apron for
the 9R threshold. This site placed the measurement van in line with the path
7-1
mu
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Measurement site----~~·
Fig. 7-2. Philadelphia International AirportMeasurement site and reflection rays.
7-3
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19
from the localizer to the principal multipath threats and hence would yield
essentially the same specular region. Since the ground at this location was
15 - 20 feet higher than the ground near the localizer, a 10-foot receiving
antenna height was used to approximate the results with a 30-foot height at
the localizer site.
B. Measurements Made
~~ble 7-1 summarizes the measurements made at Philadelphia. A taxi test
was attempted along runway AA, but there was insufficient SNR to obtain useful
data. A total of 13 runs were carried out between 4:30 p.m. and 7:00 p.m. on
Februa ry 4, 198 1.
c. Waveform Analysis Results
All of the approaches accomplished at Philad elphia have been analyzed and
representative results will be described in this section. Figures 7-5 to 7-7
show summary results for three approaches with flight profile 1, while Fig. 7
8 shows results for an approach with flight profile 2. We see that no multi
path is encountered until approximately 0.5 nmi before threshold (2.2 nmi
joystick range). At this point, low level (e.g., -10 dB MID ratio) multipath
is encountered with a T of 700 nsec. This correlates well with the expected
multipath region and time delay for reflections from the C3rgo Unit number 1
building.
In the vicinity and just after threshold (joystick range of 1. 7 nmi) ,
higher level multipath (e.g, -3 dB) is encountered. The numerous missed
measun~ments (due to low SNR) and scatter in T values make it difficult to
correlate the multipath precisely with specific buildings. However, the bulk
of the T values are consistent with reflections from the UA and AA/EA cargo
build ings.
Figures 7-9 to 7-11 show waveforms at various points where discernable
multipath levels were detected. The direct signal levels are, in many cases,
just at the minimum threshold we utilized (approximately +12 dR SNR) in data
7-6
•
..
•
•
•
TABLE 7-1
PDME MULTIPATH MEASUREMENTS AT PHILADELPHIA
Flight Tests (February 3)
Profile 1: 3° glideslope, 45-foot threshold crossing height 20 feet ACLalong runwaybottom alc antenna: 6 successful approaches
Profile 2: 3° glideslope, 20 foot threshold crossing height, 10 feet AGLalong runwaybottom alc antenna: 7 successful approaches
7-7
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Fig. 7-5a. Summary results for flight profile 1 at Philadelphia.
7-8
..
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S"OOTHED JOV STICK: AANCE<"''H IPHILADELPHIA 'USSJON- 32 DA1AFIlE· OfIE2....
Summary results for flight profile 1 at Philadelphia.
7-9
...
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PHILADElPHIA
2.38 2.1' 2 •••
Fig. 7-Sc. Summary results for flight profile 1 at Philadelphia.
7-10
DAV- 35. HOUR- 04.1'11"- 12.S£C- 22.J(lYSTK RANGES- .2.' 1.t,PAl" RAHCESa 2.8S 1.33
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PHI lADELPt4IA I'IISSI0t4- 32 DATAFILE- O"'E2e04A
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PHilADElPHIA 1'1155101'4· 32 D(IIlTAF'IU" DfllEZI"A
Fig. 7-Sd. Summary results for flight profile 1 at Philadelphia.
7-11
DlllY- 35;HOlJA- ".1'I1M· "2.SEC· J8.JOVSTIC RANeES- a .• I .'. PAL" RANGES ~ 1.78 '.83
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DAY. 35.HOUA- "."IM- .,Z.SEC- JS.JOVSTK RANGES- 2.1 1. •• PAll"l RAI'fCES· 1.78 '.13
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S"OO'I-IEtl JOv STICIC RAI'tC[(HPlI)
PHlLADEl,.PHIA f'!SSIOH- 32 DATAFILE· Df'IU.....
Fig, 7-6. Summary results for flight profile 1 at Philadelphia.7-12
DAIf- :JS.HOUR. 5,''111. al.SEC- "S,Jovsn: RANGES- 2.8 i.1,PALI'! RANGES- Z.2g 1.82
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PHILADELPHIA 'USSIOH· 32 DATAFIU- DM.2H£
DflIV- 3S.HOUfI- S,I'U". 21,SEc- "S,JOlfSTI( RANGES- 2.' 1.e,PAL" RAI1CES- 2.251 1.e2
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.3/2.".1 11'15'225"OOntED JOV STICI( RAHGEC""I I
PHILADELPHIA "15s10H- 32 DATN' I Uoo DPtEZI"'[
Fig. 7.,...7. Summary results for flight profile 1 at Philadelphia.
7-13
1H'l'Y- 3&.HOUA· 5~I'IIM........ SEC· 53.JOVSTK RANCES· 2.8 1.e,PAL" RANGES- 1.78 e.lz
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5f'lOO"HED JOV STICK RANGEl""I l
PHilADELPHIA 1'1I55ION- 32 l)ATM I LE· O"EM"'C
DAY· 35~HOUA. 5~f"IIN· ..... 5EC· S3.JOVSTK RANeES- 2.' 1.,.PAU'I RANGES· 1.78 '.82
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Sf'lOOTHED JOV STICK RAHQ[CN"J)
PHllAOUPMIlIl DATAFlL[· DIIIEae..c;
Fig. 7-8. Summary results for flight profile 2 at Philadelphia.
7-14
vert
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.7
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analysis. On the last group of approaches, pulses similar to those in Fig. 7
9 and 7-·11 are visible, but below the threshold. In such cases, much of the
multipath data was missed.
D. Simulation Results
A simple model of the Philadelphia airport was developed in which the
relevant building walls were represented by flat rectangular plates and ter
rain contour features ignored. The building locations were taken from Fig. 7
1 with wall heights being obtained from the MLS airport survey data [6].
Figures 7-12 and 7-13 show the simulation airport map and computed multi
path characteristics. The predicted MID level of -8 dB for the UA cargo unit
correlates reasonably well with Fig.7-5. It should he noted, however, that no
multipath within -20 dB of the direct signal was detected on any of the other
epproaches. The pred icted peak MID levels of -18 dB and -28 dB for the AA/EA
cargo building and cargo unit III are not inconsistent with Figs. 7-5 to 7-8,
although here again the experimental data shows large variations which are not
suggested by the computer simulation (e.g., in Fig. 7.Sc, the measured MID for
cargo unit III is -10 to -15 dB, whereas on the other runs it was less than -20
dB) •
E. Summary
The Philadelphia measured results correlated reasonably well with the
predictions from ray tracing analysis and computer simulations using a simple
airport model. The measured MID ratios and T values were quantitatively in
reasonable agreement on the approaches with adequate SNR: however, in most
cases, the SNR was so low as to cause significant problems in data interpreta
tion.
7-18
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.7
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VIII. WASHINGTON NATIONAL AIRPORT
Washington National was selected as a multipath test site because 1) tlle
FAA has several MLS systems installed at the airport and 2) the particular
runway chosen provided an opportunity to explore multipath for V/STOL ap
proaches •
• A. Multipath Environment
Figure 8-1 shows a map of Washington National Airport (DCA) in the vicin
ity of runway 15-33. An MLS small community with a conventional (Card ion) L
band DME has been installed at this runway to support STOL operations by a
commuter airline (Ransome). The principal multipath threats are the north
hangar complex buildings which are typically 20m (60 ft) high with smooth
metal doors facing the runway. Figure 8-2 shows the hangar complex from the
multipath measurement site while Fig. 8-3 shows a closeup of the end hangar.
The hangar orientations are much that multipath was expected from approx
imately 3 nmi prior to threshold (reflections from the south edge of hangar 8)
to midway down the runway (reflections from the north edge of hangar 12) if
the aircraft were at a sufficiently low altitude. From the category I deci
sion height (DH) downward, the aircraft elevation angles are such that specu
lar reflections should be encountered more or less continuously. The expected
multipath relative time delays were as follows:
Hangar T (nsec)
•8 1100
9 950
10 850
11 750
12 700
The runway contour and terrain contours at Washington National are quite
8-1
Q:J I N
MLS
sm
all
com
maz
imu
th.
N
Fig. 8-2. MLS small community azImuth and washington National north hangarcQmplex as seen f~om TIME multipath~easurement site.
8-3
Fig. 8-3. Closeup of hangar 12 at Washington National Airport.
8-4
•
..
•
flat (4 foot height variation over the entire runway length) and thus created
no problems in achieving clear line of sight to the aircraft. The use of a
site further off the runway than the small community azimuth was necessitated
by obstruction clearance considerations. However, the specular region that
resulted is fairly similar to tha t which would have occured if the van were
sited immediately adjacent to the MLS small community azimuth •
B. Measurements Made at Washington National
Table 8-1 summarizes the measurements at DCA. A total of 8 centerline
approaches were made to runway 33 between 6:30 a.m. and 8:00 a.m. on February
6, 1981. Due to unfavorable weather and wind direction as well as air traffic
control constraints, we were not able to fly the off centerline portion of the
curved approach to be utilized by the commuter airline.
c. Waveform Analysis Results
All of the approaches accomplished a t DCA have been analyzed and repre
sentative results will be described in this section. Figures 8-4 and 8-5 show
the summary results for approaches at 6° elevation angle, while Figs. 8-6 and
8-7 are the corresponding results for a 3° approach. Figures 8-8 and 8-9 show
representa tive waveforms in the high level multipath regions 1. 5 nmi before
threshold and a t threshold.
The multipath regions and time delays correlate fairly well with the
expected regions using ray optics except in the case of the 800 nsec delay
Illultipath between 5 nmi and 3.5 nmi. The aircraft x-y location here is at the
edge of the specular region for the North Hangar complex, but the elevation
angle of the aircraft is far in excess of the angle subtended by the lower
level buildings (e.g., general aviation terminal and North Terminal complex)
which are south of hangar 8. Thus, if the hangar walls and doors were verti
cal, large specular reflections should not have been encountered in this
region. Actually, the facade above the doors is tilted approximately 3.9°
toward the runway, so that some specular reflections may occur although the
8-5
TABLE 8-1
FLIGHT TESTS AT WASHINGTON NATIONAL AIRPORT
Flight Tests (February 6)
Profile 1:
Profile 2:
6° glideslope, 45-foot threshold crossing height, 20 feetAGL along runwaybottom alc antenna: 4 successful approaches
3° glideslope, 50 foot thresbold crossing height, 20 feetAGL a long runwa ybottom alc antenna: 4 successful approaches
8-6
·.. , I
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UASMINGTOH D.C.P'lISSIOH. 32 DATAF I LE" DM2.6A
Fig. 8-4a. Summary results for 60 approach at DCA.
8-7
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1 •••• •• 015 '.1358 0,925 '.Q00 e.8?S e .8S8 e.825 '.1"
SMOTHEO JO .... STICK RAHCE01P1I)
UASHIHCTOtl D.C. DATAf I LE- DMZ'GA
•. rT"T"T'T"T'T"T"TTT"TT"T'TT"T"l"T"lT1"'T"1""""'T"1"T1,--r-rrrrrT"T'"T"T'T"T'M'""T"T"'T"TTTTT'T'T"T"T""T"'1
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10'" '.U75 I.gs, '.925 '.511'
SI"OOTHED JOV STICI: AAHG[(H1'II)
~ASHl"GiTON D. C. "'ISSI0"'"' 3C! DATAFILE· Df'I[2K1l
Fig. 8-4b. Summary results for 6° approach at DCA.
8-8
DAV· 3$.IIOUR· I.HIH· 5'.SEC· 15.JOYSTK _5- 5.5 5.'.""LH _5· 5.... 5.5'
~ -r- ""T"T""I ,...,. 'I"""I' ,...,...,...., P"'l" .......
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SIlOO_D _ STICK _''''1'-..IN;TOII D.C. H15s10II- l2 .T..ILE- __
/HlV. 3$.IIOUR' I.HIH· 5'.SEC' 15.JOYSTK _5' 5.5 5.'.""UI _5- 5 .... 5.59
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Fig. 8-5a. Summary results for 6° approach at DCA.
8-9
DAY' :IS,HCUI' 1,"1"' SI,S£C' 15.JOYSTC _5- 5.' '.I,PAUI _5' 5.51 5.68
zt "T'"" -.."T'"".-- _ ..
I'.
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X X XX X XX
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...._--_.......... ".....-....-.~. ..... ~ ...."" "" •
-31.
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_IICTOII D.C. "1551011- Je _.-ILE· __
DAY- 3S,HOUR- 1,ftIM- se,Ke- lS"JOVSTJ( IIbVtGES- 5.' 4.1,PAUt RMGES- 5.Ssa 5.68_..
2Stt.
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<Fig. 8...5b, Sunnnary results for 6° approach at DCA.
8-10
DIN- .,HOUR- 1,"1"- 13,SlC- 34,JOYSTK .,.,..- 4.' 3.','1II1." ....,. 4... 3 .••
•• r-r"'T""r-T"~"'T'"T""1""T"1r-T"T"T"T'"rT"""-r'T'""T~rrT"'l''''''''T'"T''''''"'T'"T"T"'T""'''''''''''''''''''''''''''''''''''''''''''''
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_.,..,."'P""I..,..,..,..,....,...,..r-T"T'T"T"'Ir-r-'I"""I'''T'''Ir-r-,...,....,..,''T'"r-T"'T'''T"T"Ir-r-T'''I''.,...,...,..l''''''I'''''P''''I''T'"I''''I''T'T"T""r'''''I
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Fig 8.,..6a. Summary results for 3° approach to DCA
8-11
..,. . ...... • .. 'UM. 12.. KC. 34.JOYI'TI: ....... 3.1 •••• NUI ....... 3." '.31
n. """'T"T~I"'T""""'~I"T"T'T.,..,-r-T"T"T"I-r-I"'T"TT"'T"'1'"T"T""I""T""T-r-T"T''''''''''''''''''''''''''''''''''''''''''''''
1••
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,.
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Fig. 8-6b. Summary results for 3° approach to DCA.
8-12
MY. ....... • ...IN. 13,Me· 34,JOYSTK ..... I.e 1.',I'AUI ..... 1.11 ....
•• rr-rT'"rT'"rT'"M""n-rT'"IT",..,....Min-Min;Mi"'T'1r-n"'T'1-n""'n..,..,~"'T"'T"T"'!"'T"'T""'I""I
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...,. .,.... 8••tlN. 13,1Ie. ~,J""'r1C ...... I.e t .•,""" ...... I ......
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Fig. 8-6c. Summary results for 3° approach at DCA.
8-13
MY- "1'" 13••C- 34.JiOYSTIC 1.1 I.'.~ I.N ..
... ,..,..,..,..,-r""""""r'T"I"'T"rT"I"'T"I"'T'1I"'T"I"'T"'rT"l~~"T"'I"T"'I"T"'T"T'T"'I"'T""""""""""""""""""""'_""''''
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•• • •••" "
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1._ ...... ..- ..- ..- ...... ..- ..- ..- ...,.,. ..- ...,. ...,.
-·I"T'I"'T"'rT'1r"T""rT1r-nrT'1rT'1I"T"1iT1"T'1"'T"l"T1"'T"1'TT"TT"TTTTTT"TTTTTT"""""",-r,..,..I"'T""""'~1""I
_.
...... ..- ..- ..- ...... ..- ..- ..- ...,.,. ..- ' ..,. ...,. •
••• •••
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1_.
......,. . ____ ITICC _1.1._1_ •.e. "IMI.-. _ILI-_
:Fig. 8... 6d, Summary results for 3° approach to DCA.
8-14
1iW- ...... I.IIJN- .".C· JI"JOVSTIC _.- 3.' •••"NI.... ~.. ..n I.,?
... rT'T"'I"T"'rT'T"T"T"'rT'T'T'T"1iT"T'TT'1"T"'T'TT'1"T'1iT"TT'T"1"T"'T"T'T"T"T"'I""T'T'"T"T'1I""T'''''''''''~
1••
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X X .x. X X I X 'loc_.
•
___ ITICC _(...11_1_ ..e. "1111"· •
Fig. 8-7a. Summary results for 3° approach at DCA,
8-15
1ilW. ...... 1."1".....C· _ • .IOY,TIC ........ a.1 1.8.NUI .-.E'. a.8'7 1.83
... 1"T'T'"I"T"'rT''T'''T"T"'r-T'''T''T''''''I'''''I'''''T''T''''l'''''I''''rr"T'''l"T"'r-T"'''''''~I'''''I''''~'''''''''''''''''''''''''''''_''''_''''
•
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>I' .If<
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• •X lICX ~ X
X X XX
•
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.\e.• •
If< •,l'.~.
~.
*••
w
"...
PI " ... " ..""1111
I."
II
I."
",. lOC 'Ie *'It ••
'k l\c ......'If"t-----------------..,..X )(lIC~ • XX. X
II
....
I •.
-31. ,.,..
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~E'
-II.z
'*
1M\f. ....... I."1M· ••lIe. JI• .IO¥'ITIC ..... 1.1 I.I."UI It.-:'- a.S? 1.13
_·I"T'T'"I"T"T"T"T""I'"T",..,"T"T"TT"'1r-T"T"T"T""T"T.,,-rT"T..,....,I'"T"T"T"T""rT.,,"T",.,...,....,I""'I""T"'I..,..I""I"'.,..,
_.y
y_.~~
1_.
~
1_.•
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•
••• U-J..L.l..L.I...L..L..L...L.JL..J...L..l..1..L.l...LJ...L..L...L...L..L.L.L..l-J...L..L..LJ...L.J..L.J...JL.l..L..I...L..L.L..LJU-.L.L..L.I.... t... t.. I~'N I.M I." 1~'" I.. t.. 1.11 I....
-... M¥t nIce _',,"I I_ ..._D.C. "1.1011- • ...,..tU· ___
Summary results for 3° approach at DCA.
8-16
•• rT'rrl"T"rrrT'1n-I"T"rT"I""nI"'T"1I"T"1"'T'l.,..,"T"T"TT'T'T'TTTT'T"T''TTT'''I''",...,.__,...._~~..........
,
a•• ,
___ nleo: _ ....11_1_ I.C. 111.1..•• IIITWIU;· _
_.
_.~~
a_.
~
1_.
_.X xxx••x x x x xxx .M xxx XX
Xx'UCXX x x x x x x x
•• LL.L.L.LL.L.L.u..LJ...I..1..u..u..u.LJ.J.JI..UL..L.IL..L.Iu..L.LL..L;U..L.I.U.~.L.I.LLJ..L.LLJ..L.u..LJ...I..1.u1._ ••171 ••_ ••_ ••_ ••171 ••_ ••_ ••_ ••ns ..... .... • ....
Fig. 8-7c. Summary results for 3° approach at DCA.
8-17
IolA
SHIN
GTO
ND
.C.
DM
E206
CW
ASH
ING
TON
D.C
.
\ \Ai
V~
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ES
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f)~
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ND
.C.
DI'
E20
6C
JOY
RA
HG
ES2
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AH
GES
2.2
time~
Fig
.8
-8.
DC
Aw
avef
orm
so
n6
°ap
pro
ach
at
1.4
nm
ifr
om
thre
sh
old
.
..
IolA
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GTO
HD
.C.
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E20
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ASH
IHG
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IJA
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GT
Oti
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.
Q)
'""d ;:l
+J
'..-1~
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RA
r;GES
@ M o ,...,
1.1
DM
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6C
JOY
RA
tiGE
S1
.1
UA
SHIN
GTO
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.C.
DM
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6C
lost
track
ond
irect
sig
nal
JOY
RA
HG
ES1
.1JO
YR
AtiG
ES
1.e
tim
e
Fig
.8
-9.
DCA
wav
efo
rms
near
thre
sho
ldon
6°ap
pro
ach
.
predicted levels are low (see section D.) Another possibility is that this
multipath arose from the hillside between the public parking area and Thomas
avenue and/or the Washington Metro Station which borders Smith Boulevard t
since the 800 nsec delay is slightly greater than that associated with the
hangar complex at this range.
Sizable oscillations in the M/D levels are evident on either side of
threshold (joystick range = 0.83 nmi). This reflects the influence of multi
path from different buildings as well as oscillations in the multipath level
from individual scatterers as will be discussed in the section on simulation
results. For the most part t the multipath delays in this region are tightly
grouped in the 700 nsec - 1100 nsec region predicted by ray tracing considera
tions.
D. Simulation Results
A simple model of the DCA mesurement site was developed in which hangars
8 through 12 were represented by flat vertical rectangular metal plates and
the terrain contour features ignored. Hangars 8 and 9 were represented by a
single plate as were hangars 11 and 12. The runway facing wall of hangar 10
resembles a half circle and was represented by three rectangula r pIa tes.
Figures 8-10 and 8-11 show the simulation airport map and computed multipath
characteristics for a 6° approach to runway 33 and then flying along the
runway to a 20 foot height. Figure 8-12 shows the computed multipath charac
teristics for a 3° approach to the same runway.
Since the front walls of hangars 8 t 9, 11, and 12 consist of alternating
strips of smooth metal and glass t the simulations were repeated using a model
in which the metallic surfaces alone were represented. Figures 8-13 to 8-15
show the correspond ing results for this alterna tive simple airport mod eL
UnfortunatelYt the various metallic strips for a given building appear as
separate entities in Figs. 8-9 to 8-15. When two strips have a comparable
magnitude t the resultant could be 6 dB larger than the individual levels or
very much less depending on the exact phasing of the signals. Since the top
8-20
..
•"
•
00 I N i-'
0.
VP
eSFT
-10
0e.
ER
UR
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i'
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Fig
.8
-10
.A
irp
ort
map
for
DC
A6
°ap
pro
ach
scen
ari
o.
16
13
IIe~/2~/81
16
'16
'11
DCA
D"E
AT38
FT~DG
GS
1PL
ATE
ILD
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ELS
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5z
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sti
ck
ran
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1.4
2.0
52
.73
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-8
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c:::
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-:z: -32
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98
0.0
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39
8.0
e.0
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ee0
28
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16
00
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20
00
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00
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ee
Fig
.8
-11
.
DIS
TAH
CE
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CFL
IGH
TPA
TH1F
T')
Com
pute
dDM
Em
u1
tip
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cs
on6
(gli
dest
on
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mp
lifi
ed
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ild
ing
mo
del
.
..
16
...
•
1./2~/81
1612~117
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*•
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see.
s
6ee.
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(f> = >-
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Fig
.8
-12
.
ID
ISY
AN
CE
ALO
Ne
FL
IGH
TPA
TH
(FT
)
Com
pute
dDM
Em
u1
tip
ath
ch
ara
cte
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on
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ing
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Fig
.8
-13
.A
irp
ort
map
for
DCA
6cap
pro
ach
scen
ari
ow
ith
alt
ern
ati
ve
bu
ild
ing
mo
del
.
,
'~/2~/Sl
16
1.7
19
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see.
9L
Ll~ = >
-- -s:
.-1
u.:
l=
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II
II
-
Joy
sti
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ran
ge
(NM
i)
Il.
2.0
52
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)
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Fig
.8
-14
.C
om
pu
ted
mu
1ti
path
ch
ara
cte
risti
cs
for
DC
A6
ap
pro
ach
scen
ari
ow
ith
alt
ern
ati
ve
bu
ild
ing
mo
del.
16
18
'~/2~/81
16
1.1
13
3D
CALL
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AT
3eF
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TO
VER
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17
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:
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.8
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IT
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nw
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resh
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sti
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i)1
.42
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2.
71
3.4
4.0
I'II
II
Ir
I
J28
8e
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lage
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I
Fig
.8
-15
.
.....
DIS
TA
HC
EA
LOH
GFL
IGH
TPA
TH(F
T)
Co
mp
ute
dm
ult
ipath
ch
aracte
ris
tics
for
DC
A3~Approach
scen
ari
o.
•
•
,.
and middle metallic strips of the hangars 8/9 model have comparable contribu
tions, it is felt that Figs. 8-10 to 8-12 should give a better approximation
to the measured da ta.
The models suggest very low level (-36 dB MID ratios) multipath commenc
ing 2.15 nmi from threshold (3.0 nmi joystick range) and increasing almost
linearly to -8 dB MID ratios near threshold on a 6° approach. No multipath is
predicted for these buildings at joystick ranges greater than 3.0 nmi. The
peak observed levels in the 2.0 nmi to 1.3 nmi range from threshold are ap
proximately 20 dB higher than the simulation results (-7 dB vs. -27 dB), but
many of the experimental measurements have MID values less than -20 dB. This
variance probably is due to constructive and destructive reinforcement of the
signals from the various staggered hangar doors.
Near the threshold (0.5 nmi from threshold 1.32 nmi joystick range),
the simulation results and experimental data show high level short duration
multipa th regions. Both experimental regions have high levels (-5 to +5 dB
MID ratio), whereas the simulation suggests only one high MID region. Simi
larly, after threshold, the experimental results are significantly higher than
the simulation results (0 dB to +8 dB vs. -9 dB MID ratio).
The measured and simulation levels on a 3° approach increase rapidly near
1.5 nmi from threshold; however, the measured MID ratio values range from -10
dB to 0 dB, whereas the simulation levels are closer to -27 dB. Both levels
decrease sharply and then increase to near 0 dB near threshold. The fairly
high level (-8 dB to 0 dB MID ratio) multipath measured near 3.0 nmi from
threshold cannot be explained by the simple airport model, as was noted in the
discussion of experimental results.
Since terrain contour variations were small and reflection path shadowing
by intervening obstacles not an issue here, a more detailed airport model was
developed in an effort to better understand the origin of high level multipath
8-27
prior to threshold. This involved including a tilt from the vertical of 3.9°*
toward the runway for the plate corresponding to the metal strip above the
doors of hangars 8, 9, 11 and 12. The doors of these hangars were represetned
by single vertical flat plates. Fig. 8-16 shows the airport layout while
figs. 8-17 and 8-18 show the computed multipath characteristics for 6° and 3°
glideslopes respectively. One of the interesting features of the tilted plate
multipath is that the bulk of the multipath reaches the receiver by reflection
from the ground after it has been reflected from the building as shown in fig.
8-19.
The simulation results using this model show no better agreement with the
field data than do the the earlier results. The small vertical extent of this
section (12 feet which is approximately 25% of the first Fresnel zone radius)
makes it unlikely that alternative tilt values for it would yield better
results.
The other possibility is that the effective direct signal may have been
significantly reduced by ground lobing. Raw signal strength records such as
shown in figures 8-20 and 8-21 show 1) a deep null (10 dB - 15 dB) for joy
stick ranges of 1.4 nmi to 1.5 nmi (Le., 0.5 nmi before threshold) with a
lesser null (~ 6 dB) at ranges of 1.1 nmi and 0.9 0.8 nmi, and 2) lesser
nulls near 1.6, 1.3 and 1.0 nmi on the 3° approaches. These nulls are signi
ficantly sharper than were expected «2 dB) given the receiving antenna eleva
tion pattern (fig. 2-3). These may be due to differences in the aircraft
antenna pattern in the elevation plane and/or a ground antenna which was not
aligned properly in the vertical plane. Measurements on a scale model Piper
Cherokee (fig. 8-22) suggest that the aircraft antenna gain in the direction
of the ground reflection could be several dB higher than that in the direction
of the direct signal.
*the 3.9° value was based on measurements by D. Huntington of the BendixCorpora tion.
8-28
•
•
•
sM.
•
e.E-
----
----
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ll:A
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-50
0.
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ors
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11
an
d1
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2=
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ipab
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11an
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4=
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for
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an
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15
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30
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0.
Fig
.8
-16
.A
irp
ort
map
for
DC
ADM
Em
easu
rem
en
tscen
ari
ow
ith
tilt
ed
bu
ild
ing
wall
s.
16
19
~'1'1"1
1.1~1l1
DCA
6D
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ST
ILT
ED
HG
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AT
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I•
81)(
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0,)
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I
Joy
sti
ck
ran
ge
(NM
i)
1.4
2.0
52
.73
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TAH
C£
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FLIG
HT
PATH
1FT
)
DPle.
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EIl
Fig
.8
-17
.
•
Com
pute
dM
IDle
vel
for
60g
lid
esl
op
ew
ith
tilt
ed
pla
teb
uil
din
gm
od
el.
•
"
tS/I
I/1
11
4'5
1'2
;DC
A3
DEC
CSTI
LTED
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PLAT
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h}.O
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:l:= >
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] =jO
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Joy
sti
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ran
ge
(NM
i)
II
II
II
I
28
00
016
eee
e
~IST~CEAL~~
FLI~HT
PATH
(FT
)
Fig
.8
-18
.C
om
pu
ted
MID
lev
el
for
3~glideslope
usi
ng
tilt
ed
pla
teb
uil
din
gm
od
el.
DC
A6
DEC
NI'P
RO
AC
H3
.88
DEC
TIL
T3
8F
TX
ftTR
HT
a:>
c;:
:)
..-
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ce: = --
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Fi3· 8-22. Elevation pattern measurements on Piper Cherokeemodel for bottom mounted antenna, flaps up, wheels down .
8-35
E. Summary
The multipath regions near the threshold of DCA runway 15-33 correlated
fairly well with the specular regions associated with a row of hangars border
ing the runway. The time delays of the multipath near threshold agreed quite
well quantitatively with the predictions using a simple airport model, but the
experimental MID values were in several cases substantially larger than were
predicted. Also, strong multipath was encountered at longer ranges on the
approach (e.g., 3 - 5 nmi from threshold) which could not be explained by
reflections from vertical walls of the hangars which border the runway.
Several possible sources of this long range multipath were suggested, but none
have been able to quantitatively explain the phenomena to date.
8-36
•
IX. SUMMARY AND CONCLUSIONS
In this section t we summarize the principal results of the measurement
program and make some suggestions for additional measurements to clarify
certain issues.
objectives:
Before summarizing t it is worthwhile repeating the program
•
•
1) measurements of the principal multipath parameters(amplitude and time delay) with realistic aircraft/groundsite loca tions a t runways which had the maj or DME/Pmultipath sources (large buildings) identified inprevious analytical (simulation) studies •
2) determination of whether significant DME/P multipathsources exist which had not been considered to da te inthe DME/P studies to date.
and3) comparison of the measured results with computer
simulation results obtained with simplified airportmodels (such as have been used for DME/P system designto date).
The measurements placed particular emphasis on the final approach region
including the flare and rollout regions since these areas correspond to the
most stringent DME/P accuracy requirements and t have not been utilized
operationally with the current L band DME.
All of the above objectives were achieved although in some cases
(especiallYt WPAFB t PHL t Tulsa site #1) the experimental data in the
flare/rollout region was of poor quality due to low signal to noise ratio.
The spatial region and time delay of specular multipath generally correlated
well with expecta tions based on simple ray tracing for these cases in which
adequate airport maps were available. With the exception of Washington
National (DCA) no significant (M/D ratio > -10 dB) specular multipath was
encounte red which wa s no t pred ic ted. In the case of DCA t there is some
question as to whether the multipath encountered at 2-3 nmi from threshold
arose from the identified buildings as opposed to other airport features.
9-1
The a bsence of significant specular multipa th* other than from read ily
identified structures at aircraft altitudes above 100 feet is viewed as
particularly important for the initial implementation of MLS since it
currently is anticipated that the vast majority of MLS installations will
prOVide category 1/11 service only.
When the aircraft antenna was at low altitudes (e.g., 10-20 feet) over
runways and lor taxiways, a variety of multipath signals were encountered which
generally correlated with the principal identified structures. On the other
hand, the large number of potential multipath sources in this region precluded
a detailed quantitative analysis for each of the various sites.
The airport models used for DME/p analyses to date have typically made a
number of simplifying assumptions such as:
\
•
(1) buildings are represented byrectangular plates with acoefficient.
single flat verticalconstant reflection
cn the terrain is assumed to be flat both along and off therunway centerline.
(3) blockage of reflection paths by intervening objects isignored.
The physical fea tures of actual airports differ considerably from each of
these assumptions, but arguments can be ad vanced to support either higher or
lower levels than predicted by the simplified models. Thus, we sought to
determine to what extent simplified airport models could predict the measured
data. The quantita tive pred ictions of the simple airport mod els generally
agreed with the experimental data, although in some cases, (especially near
threshold at WPAFB, DCA, and Tulsa) the measured MID values were considerably
higher than predictions. We attribute the WPAFB and Tulsa higher levels to
terrain contour features. In this context, it should be noted that 4 of the 6
*The possible existence of numerous low level (e.g., diffuse) specularreflections in this region is discussed below.
9-2
•
•
airports had runway contours which differed consid era bly from the nominally
flat model used for DME/P power budget computations.
The cause of higher than expected multipath levels at DCA is unclear.
Possible explanations include:
(1) non verticality of the hanga r doors
(2) excess ground lobing due to the aircraft antenna verticalpattern and/or the runway slope
and(3) greater aircraft antenna gain in the azimuth plane
towards the hangars than towards the ground site (theapproach was flown with the wheels down).
Two potential sources of significant DME/P multipath were not
•
quantitatively assessed in the experiments presented here:
(1) diffuse reflections from an extended region of smallscatterers. The current DI"1E/P error budgets contain asignificant allowance for diffuse multipath based on FAAexperimental mea surements a t Wallops Island, Va.*Unfortunately, the range of diffuse multipath delays ofgreatest significance for DME/P (0-300 ns) is comparableto the pulse widths used in our multipath measurements.This overlap together with the low SNR OIl the bulk of ourflights made it virtually impossible to estimate thediffuse multipath power vs. delay characteristic.
(2) reflections from rough and/or irregular terrain such asencountered in mountainous regions. Several of the U.S.interim MLS installations are located in mountainousregions (e.g., Aspen, Colorado) and it has been suggested[14J that three dimensional aircraft position informationis particularly important in SUell regions. Limited Lband measurements were conducted by the FRG at Salzburg,Austria [15J, but the pulse widths used (2~sec) were toolarge to resolve the multipath of greatest concern toDME/P. Long delay (2~sec to 20 ~sec) diffuse multipathwas observed as well as some discrete specularmultipath. It is unclear from the published resultswhether the high level (e.g., -6 dB) specular multipath
*K. Kelly, personal communication.
9-3
was due to mountains as opposed to the buildings and nearthe airport.
It is suggested that additional measurements be carried out to address both of
these points. It should be noted that the instrumenta tion required to make
analog llleasurements (e.g., scope photos) is fairly minimal.
The degree of ground reflection lobing of the direct signal is important
both from the viewpoint of multipa th sensitivity and power budgets. The
measurements results suggest that ground lobing in cases of that predicted by
the standard flat earth models can occur due to runway contours and/or
strength at the thresholds of representative non
relatively easy to carry out.
nonisotropic aircraft antenna patterns. Calibrated measurements of signal
flat runways should be
Aircraft antenna pattern measuremellts in the vertical plane on scale
models are available for many aircraft [10 - 12] and could be analyzed to
deterllline the implications of increased gain at negative elevation angles on
the differential lobing. It should be noted tha t since the effective direct
signal in a null is the difference of t\<10 signals:
where G(E) elevation pattern as a function of elevation angle
R reflection coefficient
direct signal elevation angle for aircraft
ground reflection •
relatively small differences between G(Ed ) and G(Em) can produce large changes
in Deff •
9-4
REFEKENCES
1. J. Evans and D. Easterday, "L-Band OME Multipath Environment in the MLSApproach and Landing Region," ATC Working Paper 44WP-5058, LincolnLaboratory, M.I.T. (23 September 1980), presented as AWOP WG-M Paper M/4 BIP/18, by J. Edwards (October 1~80).
2. J. Evans, O. Sun, S. Dolinar, and D. Shnidman, "MLS Multipath Studies,Phase 3 Final Report, Volume I: Overview and Propagation ModelValidation/Refinement Studies," Project Report ATC-88, Lincoln Laboratory,M.I.T. (25 April 1979).
3. R. Kelly and E. LaBerge, "Guidance Accuracy Considerations for theHicrowave Landing System L-Band Precision DME," .1. Navigation (May 1980).
4. J. E. Evans, "Synthesis of Equiripple Sector Antenna Patterns," IEEETrans. Antennas Propag., Vol. 24, 347 (May 1976).
5. J. E. Evans, D. Karp, R. R. LaFrey, K. J. McAulay, and I. G. Stiglitz,"Experimental Validation of PALM - A System for Precise AircraftLocation," Technical Note 1':J7S-29, Lincoln Laboratory, M.LT. (3 Harch1975), ODC AD-AOI0112/1.
6. D. A. Shnidman, "Airport Survey for MLS Multipath Issues," Project ReportATC-S8, Lincoln Laboratory, M.I.T., FAA-RD-75-195 (15 Dec. 1975), DDC ADAO'L2937/7 •
7. J. Capon, "Multipath Parameter Computations for the l1L5 SimulationComputer Program," Project Report ATC-68, Lincoln Laboratory, M.LT. (8April 1976), DDC AU-A024350/1.
8. P. Beckman and A. Spizzichino, The Scattering of Electromagnetic Wavesfrom~~~gh Surfaces, (Pergamon Press, New York 1963).
9. D. Shnidman, "The Philadelphia MLS Experiments," ATC Working Paper 44WP5045, Lincoln Laboratory, M.I.T. (28 Oct. 1976).
10. K. J. Keeping and J. C. Sureau, "Scale Model Measurements of Aircraft LBand Beacon Antennas," Project Report ATC-47, Lincoln Laboratory, M.LT.,FAA-RD-75-23 (4 Mar. 1975), DDC AD-AOI0479/4.
11. G. J. Schlieckert, "An Analysis of L-Band Beacon Antenna Patterns,"Project Report ATC-37, Lincoln Laboratory, M.I.T., FAA-RD-74-144 (15 Jan.1975), DDC AD-A005569/9.
12. D. W. Mayweather, "Model Aircraft L-Band Beacon Antenna Pattern GainMaps," Project Report ATC-44, Lincoln Laboratory, M.LT., FAA-RD-75-75 (16May 1975), DOC AD-AOI3184/7.
13. H. Postel, "Precision L-Band DME Tests", Federal Aviation AdministrationTechnical Center Report, FAA-CT-80-25, FAA-RD-BO-74 , (August 1980).
R-l
14. Federal Republic of Germany, "The need for integrated navigation systemsin the T~1A", IGAU All Weather Qperations Divisional Meeting Paper AWO/78 WP/56 (March lY78).
15. Federal Republic of Germany, "Field tests for illultipath propagationmeasurements in mountainous sites," ICAO All Weather Operations DivisionalMeeting Paper AWO/7cl - WP/101 (April 1978).
R 2
...
,
APPENDIX A
PILOT SCALE L-BAND MULTIPATH MEASUREMENTS AT
QUONSET STATE AIRPORT, RHODE ISLAND
A pilot scale of L-band multipath measurements were conducted at the
Quonset State Airport, operated by the Rhode Island Port Authority, on
September 5, 1980. The objectives of this measurement effort were:
(1) to provide experience in operational procedures andevaluate equipment needs for conducting a full-scalemeasurement program,
(2) to obtain confidence in the validity of the LincolnLaboratory multipath simulation model,
(3) demonstrate the existence of high level MID ratios relevant to the DMElp operating environment.
A narrow width ('" 100 nsec) pulse was used to allow resolution of the
building multipath (M), and direct path (plus ground reflection), (D), re
turns. Building multipath reflections from four hangars were observed along
the runway over a range of from 3500 to 6500 feet of separation between trans
mitter and receiver. Photographic data were recorded from an oscilloscope
throughout this region showing MID ratios of up to 6 dB. These (MID) levels
agreed well with the simulation model predictions over that region. The
simulation results also reflected accurately the (MID) variations with trans
mitter elevation.
1. Local Topography and Instrumentation Deployment
The Quonset State Airport was chosen from available sites proximate to
Lincoln Laboratory due to its very light traffic levels and the fact that four
large, uniform hangars dominated the surroundings so far as multipath reflec
tions were concerned. An aerial view of the airfield is shown in Fig. A-I;
note the relatively sparse amounts of equipment in the area surrounding the
hangars. However, at surface level, it became apparent (Figs. A-lb, A-Ie)
that there were numerous parked aircraft which would partially block the
A-I
;I> I N
(a) (b
)
(d)
(c)
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reflections. Also, the hangar walls themselves were a mixture of glass and
..
concrete (Fig. A-I).
The tests were conducted using an interrogator/transponder pair with an
add itional complement of electronic test gea r to produce a wide bandwid th (10
MHz) pulse following the transponder's reply at a delay of about 40 ~sec. The
1090 MHz reply was hetrodyned to 74 MHz, filtered, and displayed on an oscil
loscope. The wid e receiver bandwid th and narrow pulse produced well-resolved
returns of the (building) multipath and the direct (plus ground multipath)
pulses. A diagram of the instrumentation is given in Fig. A-2.
With a fixed height receiving antenna, oscilloscope photographs were
taken at twenty discrete stations as the transmitter travelled along a path
stretching some 3000 feet parallel to the runway. These positions are indi
cated on the drawing of the airfield in Fig. A-3. Data were recorded at two
different heights of the transmitting antenna. The reference line along which
the transmitter travelled was surveyed to be parallel to the (co-linear) faces
of the hangars. The reference point on this line was directly opposite the
left hand corner of hangar 113. A precise reading of the distance of the
transmitter along the reference line was made for each of the points where
photographic data were record ed.
2. Measurement Results Summary
As indicated in Fig. A-3, one expects on the basis of geometric optics to
find four distinct zones where high level multipath interference will exist.
The zones are approximately 500 feet in length, with 500 feet between zones.
A representative set of the data are presented in Figs. A-4 to A-7 repre
senting data stations #7, 8, 14, and 19 (refer to Fig. A-3) •
3. Simulation Model Preliminary Predictions
Although the simulation model test program used for these preliminary
results did not have the ability to model reflections from more than one
A-3
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building at a time, because reflections from a single building will dominate
in any given region, results of reasonable fidelity can be realized by
considering just that pertinent building in each area. Similarly, although
the building walls consist of several discrete scattering surfaces which could
be modeled as discrete rectangular plates, only a single rectangular metallic
plate was used for the model. Figs. A-8 to A-10 show the computed MID ratios
for each. of the first three zones. In each figure, the boundary of the
multipath reflection zone defined by geometric optics considerations has been
indicated. The actual measured levels (from the photographs) are plotted.
The important conclusions to be noted are:
(1) The peak levels of MID as measured generally agree withthe model predictions, and
(2) The variation of peak MID levels with transmitter heighthas also been predicted well (refer to Fig. A-9).
The agreement here is quite good considering the very crude building model
used as well as the fact that blockage by intervening aircraft was ignored.
A-IO
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