Getting a Bearing on ASF Getting a Bearing on ASF Directional CorrectionsDirectional Corrections
3232ndnd Annual Technical Symposium Annual Technical Symposium International Loran AssociationInternational Loran Association
5 Nov 20035 Nov 2003
Boulder, COBoulder, CO
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AuthorsAuthors CAPT Richard HartnettCAPT Richard Hartnett
US Coast Guard AcademyUS Coast Guard Academy
Gregory JohnsonGregory Johnson John J. McMullen Assoc. Inc.John J. McMullen Assoc. Inc.
Dr. Peter SwaszekDr. Peter Swaszek University of Rhode IslandUniversity of Rhode Island
Ken DykstraKen Dykstra John J. McMullen Assoc. Inc.John J. McMullen Assoc. Inc.
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OutlineOutline
IntroductionIntroduction Problem identificationProblem identification Error Sources and SolutionsError Sources and Solutions SummarySummary
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IntroductionIntroduction
LORAN-C as “the” backup for GPSLORAN-C as “the” backup for GPS Accuracy, availability, integrity, continuity Accuracy, availability, integrity, continuity
Accuracy:Accuracy: Needs:Needs:
Aviation: NPA – RNP 0.3 (309 meters) Maritime: HEA (8-20 meters)
Limitations: Limitations: Spatial and temporal variations in TOA (ASF)
observed by the receiver and presented to the position solution algorithm
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Loran 2003+Loran 2003+
Multi-station, multi-chain, all-in-view Multi-station, multi-chain, all-in-view receiverreceiver
TOA vs. TD measurementsTOA vs. TD measurements Separate “errors” into spatial, temporal, Separate “errors” into spatial, temporal,
and directional componentsand directional components DSP-based receiversDSP-based receivers H-field vs. E-field antennasH-field vs. E-field antennas Grid approach vs. waypointsGrid approach vs. waypoints
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Current ResearchCurrent Research
USCGA/JJMA activities: USCGA/JJMA activities: Explore ways to mitigate TOA variationExplore ways to mitigate TOA variation Current approach:Current approach:
Model the ASF sources Spatial effectsSpatial effects Temporal effectsTemporal effects
Model the directional effects (H-field) Estimate parameters/values for the models “Correct” the TOA observations using the models
Of interest today
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ASF Variation ModelASF Variation Model
Assume that the variation in the observed TOAs Assume that the variation in the observed TOAs can be decomposed into 3 independent, additive can be decomposed into 3 independent, additive terms:terms: Spatial term:Spatial term:
Due to differences in the Loran signal propagation path
Temporal term:Temporal term: For short term TOA variability on a local region
Directional term:Directional term: Based on the relative bearing of the Loran tower to
the vessel
Key assumptions
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Block diagram of approachBlock diagram of approach
TOAs From Rcvr
TOAs To Position Algorithm
ASF Temporal Correction
ASF Spatial Correction
Antenna Direction Correction
Live MonitorUpdate
Recent Position
Current Heading
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Problem identificationProblem identification First noticed directional TOA in our data sets over a year First noticed directional TOA in our data sets over a year
ago, started investigating:ago, started investigating: Have numerous examples of this:Have numerous examples of this:
Airplane rotation data on groundAirplane rotation data on ground Convair 580 Ohio University King Air Locus antenna/Satmate rcvr Megapulse antenna/DDC rcvr
Airplane rotation data in airAirplane rotation data in air Convair in CA both rcvrs/antennas Rockwell data – Locus antenna/satmate
Boat rotationsBoat rotations T-boats Both Megapulse antenna/DDC and Locus antenna/satmate
Field data, various antennasField data, various antennas DDC, aircraft antenna, small antennas, split loop antennas
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Graphs of TOA variationGraphs of TOA variation
10 15 20 25 30 35 40 45 50 55-2
-1
0
1
2
3
4
5
6Antenna rotation 11 Sept 02; ASF Corrections for 9960
Mic
rose
cond
s
Time (minutes) Seneca - black Caribou - blue Nantucket - green Carolina-B - read Dana - cyan
0 50 100 150 200 250 300 350-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4SatMate Loop Coupling
TO
A, d
iffe
ren
ce
fro
m m
edia
n,
sec
Heading
9960-Seneca9960-Caribou9960-Nantucket9960-Carolina Beach
Field – DDC – Split Loops
Field – SatMate - Locus
0 50 100 150 200 250 300 3500.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Heading
AS
F,
se
c
9960
SenecaCaribouNantucketCarolina-B
Tboat – DDC - Megapulse
Convair – PC104 - Megapulse
0 50 100 150 200 250 300 350-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
Heading
TO
A c
ha
ng
e,
se
c
Relative TOA Variations 9960 Chain - Split Loops on Lower Field
9960-Seneca9960-Caribou9960-Nantucket9960-Carolina-B
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Expected Position ErrorExpected Position Error
Given that there are TOA variations as the Given that there are TOA variations as the antenna is rotated, what is the effect in the antenna is rotated, what is the effect in the position domain?position domain?
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Model of TOA VariationsModel of TOA Variations
0 50 100 150 200 250 300 350-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
Bearing
TO
A o
ffs
et
Model of antenna delay
SenecaCaribouNantucketCarolina Beach
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Resulting Position ErrorResulting Position Error
0 50 100 150 200 250 300 3500
10
20
30
40
50
60
70
Bearing
Po
sit
ion
err
or
in m
ete
rs
95% error bound
Position Error as Antenna is Rotating
noise onlycoupling onlyboth
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Position Error relative to centerPosition Error relative to center
-60 -40 -20 0 20 40 60-40
-30
-20
-10
0
10
20
30
40
Error in meters
Err
or
in m
ete
rsPosition Error as Antenna is Rotated
noise onlycoupling onlyboth
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Error SourcesError Sources
Time delayTime delay CouplingCoupling Antenna phase issuesAntenna phase issues Reradiation\Local EffectsReradiation\Local Effects
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Time DelayTime Delay
Unequal phase delay between the two Unequal phase delay between the two channels leads to directional TOA channels leads to directional TOA variationvariation
SourcesSources Amplifier/FiltersAmplifier/Filters CablesCables AntennasAntennas
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Time Delay – Theoretical Time Delay – Theoretical FormulationFormulation
tteA
tteA
sin90cos:2 Channel
sincos:1 Channel
tteA
tteA
sin90cos:2 Channel
sincos:1 Channel
Ideal
Actual
offset phase channel-inter
envelope signal
arrival signal ofdirection
amplitude signal
te
A
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Theoretical Effect of Time Theoretical Effect of Time DifferenceDifference
0 50 100 150 200 250 300 350 400-80
-60
-40
-20
0
20
40
60
80
Angle of Rotation, degrees
TO
A V
ari
ati
on
, ns
ec
TOA error as a function of phase difference (degrees)
012345
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Measured Time DelayMeasured Time DelayLoran Simulator
(ICS660 card)RF Front End
(Freq Devices gain modules and filters)
Ch 1
Ch 2
RF in RF out
Loran Receiver(ICS 650 card)
Ch 1
Ch 2
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S/w compensation for time delayS/w compensation for time delay
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CouplingCoupling
Some coupling seen with combined loops Some coupling seen with combined loops vs. with split loopsvs. with split loops Maximize strong signal on one loopMaximize strong signal on one loop Should be zero on other loopShould be zero on other loop
This is not the case with some H-field This is not the case with some H-field antennasantennas Maximize strong signal on one loopMaximize strong signal on one loop Signal is NOT zero on other loopSignal is NOT zero on other loop
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Single Loop Phase IssueSingle Loop Phase IssueLoop Antenna Phase
-210
-180
-150
-120
-90
-60
-30
0
30
60
90
120
150
180
210
0 30 60 90 120 150 180 210 240 270 300 330 360
angle of rotation
Ph
as
e (d
eg
ree
s)
Ideal
Actual
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Strongest Beam OnlyStrongest Beam Only
0 200 400 600 800 1000 1200 1400 1600 1800 2000
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
Time (seconds)
Rel
ativ
e TO
A (
usec
)TOAs with Stongest Loop
Seneca Nantucket
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Strongest Beam and Delay Strongest Beam and Delay CorrectionCorrection
0 200 400 600 800 1000 1200 1400 1600 1800 2000
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
Time (seconds)
Rel
ativ
e TO
A (
usec
)TOAs with Stongest Loop and 5 degrees phase shift
Seneca Nantucket
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SummarySummary
Split loops (coupling)Split loops (coupling) Works – when strong signal is max. in one Works – when strong signal is max. in one
loop is zero on other looploop is zero on other loop
Time delay correction in softwareTime delay correction in software Works - compensate for unequal delay in Works - compensate for unequal delay in
antenna and/or amplifierantenna and/or amplifier
Loop Phase Shift effectLoop Phase Shift effect Use antenna loop with largest magnitude onlyUse antenna loop with largest magnitude only
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Final RemarksFinal Remarks When faced with tremendous coupled When faced with tremendous coupled
challenges we find the best strategy is to challenges we find the best strategy is to do what most engineers on the team do – do what most engineers on the team do – blame Mitch!blame Mitch!
In the future, for harbor mapping we will In the future, for harbor mapping we will use both E and H-field antennas – use both E and H-field antennas – mappings may not be the same -mappings may not be the same -
We are highlighting “solvable challenges” We are highlighting “solvable challenges” with H-field antennaswith H-field antennas New Locus antennasNew Locus antennas New Reelelektronica antennasNew Reelelektronica antennas
Presentation by Wouter tomorrow
Calibration is clearly an important issue – Calibration is clearly an important issue – Must be done in lab and after installation! Must be done in lab and after installation!
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AcknowledgementsAcknowledgements
Mitch Narins, FAA ANDMitch Narins, FAA AND JJMA TeamJJMA Team
Christian OatesChristian Oates George SandersGeorge Sanders Ruslan ShalaevRuslan Shalaev Tim WaldieTim Waldie
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