Radar Book Chapter3

7/29/2019 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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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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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.

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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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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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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

Documents

Radar Book Chapter3