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Transcript of Radar Book Chapter3
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Radar level
measurement
Radar level
measurement
The user's guideThe user's guide
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Radar level measurement
- The users guide
Peter Devine
VEGA Controls / P Devine / 2000All rights reseved. No part of this book may reproduced in any way, or by any means, without priorpermissio in writing from the publisher:VEGA Controls Ltd, Kendal House, Victoria Way, Burgess Hill, West Sussex, RH 15 9NF England.
British Library Cataloguing in Publication Data
Devine, PeterRadar level measurement - The user s guide1. Radar
2. Title621.3 848
ISBN 0-9538920-0-X
Cover by LinkDesign, Schramberg.Printed in Great Britain at VIP print, Heathfield, Sussex.
written byPeter Devine
additional informationKarl Griebaum
type setting and layoutLiz Moakes
final drawings and diagramsEvi Brucker
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Foreword ixAcknowledgement xiIntroduction xiii
Part I1. History of radar 12. Physics of radar 133. Types of radar 33
1. CW-radar 332. FM - CW 363. Pulse radar 39
Part II4. Radar level measurement 47
1. FM - CW 482. PULSE radar 543. Choice of frequency 624. Accuracy 685. Power 74
5. Radar antennas 771. Horn antennas 812. Dielectric rod antennas 923. Measuring tube antennas 1014. Parabolic dish antennas 1065. Planar array antennas 108Antenna energy patterns 110
6. Installation 115
A.Mechanical installation 1151. Horn antenna (liquids) 1152. Rod antenna (liquids) 1173. General consideration (liquids) 1204. Stand pipes & measuring tubes 1275. Platic tank tops and windows 1346. Horn antenna (solids) 139
B.Radar level installation cont. 141
1. safe area applications 1412. Hazardous area applications 144
Contents
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In continuous wave or CW Radar, acontinuous unmodulated frequency is
transmitted and echoes are receivedfrom the target object. If the targetobject is stationary, the frequency ofthe return echoes will be the same asthe transmitted frequency. The range ofthe object cannot be measured.
However, the frequency of the returnsignal from a moving object is changeddepending on the speed and direction
of the object. This is the well knowndoppler effect. The doppler effect isapparent when the siren note of anemergency vehicle changes as it speedspast a pedestrian. The pitch of the siren
note is higher as it approaches the lis-tener and lower as it recedes. The
doppler effect is also used byastronomers to monitor the expansionof the Universe. By measuring the redshift of the spectrum of distant starsand galaxies the rate of expansion canbe measured and the age of distantobjects can be estimated.
In the same way, when an object thathas been illuminated by a CW Radar
approaches the transmitter, the frequen-cy of the return signal will be higherthan the transmitted frequency. Theecho frequency will be lower if theobject is moving away.
33
3. Types of radar
Fig 3.1 CW radar uses doppler shift to derive speed measurement
targetvelocity
v
receivedf
requencyft
+fdp
transmitte
dfrequencyft,w
avelength
1a. CW, continuous wave radar
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34
1b. CW wave-interference radar or bistatic CW radar
1c. Multiple frequency CW radar
In Fig 3.1, the aircraft is travellingtowards the CW radar. Therefore the
received frequency is higher thanthe transmitted frequency and the signof fdp is positive. If the aircraft wastravelling away from the radar at the
same speed, the received frequencywould be ft - fdp.
The velocity of the target in thedirection of the radar is calculated byequation 3.1
c is the velocity of microwavesv is the target velocityft is the frequency of the
transmitted signal
fdp is the doppler beat frequencywhich is proportional to velocity
ft+fdp is received frequency. The signof fdp depends upon whether the
target is closing or receding
Standard continuous wave radar is
used for speed measurement and, asalready explained, the distance to a sta-tionary object can not be calculated.However, there will be a phase shiftbetween the transmitted signal and thereturn signal.
If the starting position of the objectis known, CW radar could be used todetect a change in position of up to halfwavelength (/2) of the transmitted
wave by measuring the phase shift ofthe echo signal. Although furthermovement could be detected, the range
would be ambiguous. With microwave
frequencies this means that the usefulmeasuring range would be very limited.If the phase shifts of two slightly
different CW frequencies are measuredthe unambiguous range is equal to thehalf wavelength (/2) of the differencefrequency. This provides a usable dis-tance measurement device.
However, this technique is limited tomeasurement of a single target.
Applications include surveying andautomobile obstacle detection.
We have already mentioned that CWradar was used in early radar detectionexperiments such as the famousDaventry experiment carried out by
Robert Watson - Watt and his col-leagues. In this case, the transmitterand receiver were separated by a con-siderable distance. A moving objectwas detected by the receiver becausethere was interference between the fre-
quency received directly from thetransmitter and the doppler shifted fre-quency reflected off the target object.Although the presence of the object is
detected, the position and speed cannotbe calculated.
In essence, this is what happenswhen a low flying aircraft interfereswith the picture on a television screen.See Fig 3.2.
v2
= =x fdp c x fdp
2 x ft
[Eq. 3.1]
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3. Types of radar
target
transmitter
televisioninterfere
nce
reflectedsignal
(dopplershift)
transmitted
signalindirect
transmittedsignaldirect
Fig3.2Theeffectoflowflyingaircraftontelevisionreceptionissimilartothemethodof
detection
byCWwave-interferencera
dar
35
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Single frequency CW radar cannotbe used for distance measurement
because there is no time reference markto gauge the delay in the return echofrom the target. A time reference markcan be achieved by modulating the fre-quency in a known manner.
If we consider the frequency of thetransmitted signal ramping up in alinear fashion, the difference betweenthe transmitting frequency and the
frequency of the returned signal will beproportional to the distance to thetarget.
If the distance to the target is R,and c is the speed of light, then the
time taken for the return journey is:-
We can see from Fig. 3.3 that ifwe know the linear rate of change ofthe transmitted signal and measure thedifference between the transmitted and
received frequency fd, then we cancalculate the time t and hence derivethe distance R.
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time
fd
trans
mitte
dfreque
ncy
receive
dfre
quen
cy
frequency
Fig 3.3 The principle of FM - CW radar
2. FM-CW, frequency modulated continuous wave radar
t = 2 x Rc
t =
t
2 x R
c
[Eq. 3.2]
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37
3. Types of radar
FM - CW wave forms transmitted frequency
received frequency
Fig 3.6 Saw tooth wave
Most commonly used
on most FM - CW
process radar level
transmitters
Fig 3.5 Triangular wave
Used on FM - CW
radar transmitters
frequency
4.4GHz
4.2GHz
10 GHz
9 GHz
frequency
time
time
time
frequency
Fig 3.4 Sine waveCommonly used on air-
craft radio altimeters
between 4.2 and
4.4 GHz
In practice, the FM - CW signal hasto be cyclic between two different fre-
quencies. Radio altimeters modulatebetween 4.2 GHz and 4.4 GHz. Radarlevel transmitters typically modulatebetween about 9 GHz and 10 GHz or
24 GHz and 26 GHz.The cyclic modulation of FM - CW
radar transmitter takes different forms.These are sinusoidal, saw tooth ortriangular wave forms.
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38
If we look at a triangular waveform we can see that there is an inter-
ruption in the output of the differencefrequency , fd. In practice, the receivedsignal is heterodyned with part of thetransmitted frequency to produce thedifference frequency which has a posi-
tive value independent of whether themodulation is increasing or decreasing.
The diagram below makes theassumption that the target distance isnot changing. If the target is moving,there will be a doppler shift in the dif-ference frequency.
frequency
time
time
difference
frequency
fd
Fig 3.7 & 3.8 The change in direction between the ramping up and down of the frequency
creates a short break in the measured value of the difference frequency.
This has to be filtered out. The transmitted frequency is represented by the
red line and the received frequency is represented by the dark blue line.
The difference frequency is shown in light blue on the bottom graph
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Pulse radar is and has been usedwidely for distance measurement since
the very beginnings of radar technolo-gy. The basic form of pulse radar is apure time of flight measurement. Shortpulses, typically of millisecond ornansecond duration, are transmittedand the transit time to and from the tar-get is measured.
The pulses of a pulse radar are notdiscrete monopulses with a single peak
of electromagnetic energy, but are infact a short wave packet. The number
of waves and length of the pulsedepends upon the pulse duration andthe carrier frequency that is used.
These regularly repeating pulseshave a relatively long time delaybetween them to allow the return echoto be received before the next pulse istransmitted.
39
3. Types of radar
The inter pulse period (the timebetween successive pulses) t is theinverse of the pulse repetition
frequency fr or PRF. The pulse durationor pulse width, , is a fraction of theinter pulse period.
The inter pulse period t effectivelydefines the maximum range of theradar.ExampleThe pulse repetition frequency(PRF) is defined as
If the pulse period t is 500 microsec-onds, then the pulse repetition frequen-cy is two thousand pulses per second.
In 500 microseconds, the radar pulseswill travel 150 kilometres. Consideringthe return journey of an echo reflectedoff a target, this gives a maximum the-oretical range of 75 kilometres.
If the time taken for the returnjourney is T, and c is the speed of light,then the distance to the target is
3. Pulse radar
Fig 3.9 Basic pulse radar
t
3rd
pulseTransmitted pulses
2nd
pulse 1st
pulse
1t Tx c2
fr = R =[Eq. 3.3]
a. Basic pulse radar
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The pulses transmitted by a standardpulse radar can be considered as a very
short burst of continuous wave radar.There is a single frequency with nomodulation on the signal for the dura-tion of the pulse.
If the frequency of the waves of thetransmitted pulse is ft and the target ismoving towards the radar with velocityv, then, as with the CW radar alreadydescribed, the frequency of the return
pulse will be ft + fdp , where fdp is thedoppler beat frequency. Similarly, thereceived frequency will be ft - fdp if thetarget is moving away from the radar.
Therefore, a pulse doppler radar canbe used to measure speed, distance anddirection.
The ability of the pulse dopplerradar to measure speed allows the sys-tem to ignore stationary targets. This is
also commonly called moving targetindication or MTI radar.
In general, an MTI radar has accu-rate range measurement but imprecisespeed measurement, whereas a pulsedoppler radar has accurate speed mea-surement and imprecise distance mea-surement.
The velocity of the target in thedirection of the radar is calculated in
equation 3.4:
This is the same calculation as for
CW radar. The distance to the target iscalculated by the transit time of thepulse, equation 3.3.
As well as being used to monitorcivil and military aircraft movements,
pulse doppler radar is used in weatherforecasting. A doppler shift is measuredwithin storm clouds which can be dis-tinguished from general ground clut-ter. It is also used to measure theextreme wind velocities within a torna-do or twister.
40
b. Pulse doppler radar
c
R
2
2
=
=
=x fdp c x fdp
2 x ft
T x c
[Eq. 3.4]
[Eq. 3.3]
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3. Types of radar
Fig3.10
Pulsedopplerradarprovidestargetspeed,distanceanddirection
ft+fdp
ft
R
Pulse
dopplerrad
ar
41
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42
With pulse radar, a shorter pulseduration enables better target resolution
and therefore higher accuracy.However, a shorter pulse needs a sig-nificantly higher peak power if therange performance has to be main-tained. If there is a limit to the maxi-mum power available, a short pulsewill inevitably result in a reducedrange.
With limited peak power, a longer
pulse duration,
, will provide more
radiated energy and therefore range but(with a standard pulse radar) at the
expense of resolution and accuracy.Pulse compression within a Chirp
radar is a method of achieving theaccuracy benefits of a short pulse radartogether with the power benefits ofusing a longer pulse. Essentially, Chirpradar is a cross between a pulse radarand an FM - CW radar.
Fig 3.11 Chirp radar wave form. Chirp is a cross between pulse and FM - CW radar
c. Pulse compression and Chirp radar
f1
f2
t1 t2
time
time
frequency
amplitude
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43
3. Types of radar
Each pulse of a Chirp radar has lin-ear frequency modulation and a con-
stant amplitude.The echo pulse is processed through
a filter that compresses the echo bycreating a time lag that is inversely
proportional to the frequency.Therefore, the low frequency that
arrives first is slowed down the mostand the subsequent higher frequenciescatch up producing a sharper echo sig-nal and improved echo resolution.
Another method of echo compres-sion uses binary phase modulationwhere the transmitted signal is special-ly encoded with segments of the pulseeither in phase or 180 out of phase.The return echoes are decoded by a fil-
ter that produces a higher amplitudeand compressed signal.The name Chirp radar comes from
the short rapid change in frequency ofthe pulse which is analogous to thechirping of a bird song.
The above methods of radar detec-tion are used widely in long range dis-tance or speed measurement. In thenext chapter we look at which of thesemethods can be applied to the uniqueproblems involved in measuring liquid
or solid levels within process vesselsand silos.
Pulse compression of chirp radar echo signal
Long frequency modulated echo pulse
Time
lag
Frequency
Filter
Compressed
signalFig 3.12 Pulse compression of chirp radar echo signal
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Part IIRadar level measurement
Radar antennas
Radar level installations