George W. Stimson introduction to Airborne Radar (Se)

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CHAPTER 25 The Crucial Choice of PRF

lapsed (telescoped) to the point that mainlobe clutter occu-

pies most of the doppler passband (Fig. 13). Consequently,

when the clutter is rejected the return from most of the tar-

get region will be rejected. Also, since target echoes of

widely different true doppler frequencies are indistinguish-

ably intermixed, not only is it impossible to resolve doppler

ambiguities, but the radar is susceptible to interference

from ground moving targets, GMTs.

Because of the severity of the mainlobe clutter problem,

the use of low PRFs for air-to-air operation in fighters,

which employ short wavelengths and comparatively small

antennas, is today restricted largely to situations where

mainlobe clutter can be avoided:

• When flying over water, which (because of its more

nearly mirrorlike surface) has a relatively low

backscattering coefficient at moderate to low grazing

angles

• When looking up in search of targets at higher alti-

tudes

• When the mainlobe strikes the ground beyond the

maximum range of interest (Fig. 14), and clutter from

beyond the first range zone is rejected through other

means than doppler resolution.

For ground mapping, low PRFs are ideal. Because main-

lobe ground return is then the only return of interest, its

overwhelming strength is an asset, not a liability. Moreover,

the unambiguous observation of range which low PRFs

provide is essential.

What about synthetic array ground mapping? For it (Fig.

15), the unambiguous observation of doppler frequencies,

too, is essential. Happily, the PRF can generally be made

high enough to prevent the repetitions of the mainlobe

clutter spectrum from overlapping, while providing an ade-

quately long maximum unambiguous range.

High PRF Operation. The problem of mainlobe clutter

can be solved by operating at high PRFs. The width of the

mainlobe clutter spectrum is generally only a small fraction

of the width of the band of true target doppler frequencies,

so that at high PRFs mainlobe clutter does not appreciably

encroach on the region of the spectrum in which targets are

expected to appear. Moreover, since all significant doppler

ambiguities are eliminated at high PRFs, mainlobe clutter

can be rejected on the basis of doppler frequency without at

the same time rejecting echoes from targets. Only if a target

is flying nearly at right angles to the line of sight from the

radar—a condition which occurs rarely and is usually

maintained for only a short time—will its echoes have the

same doppler frequency as the clutter and so be rejected.

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14. Use of low PRFs for air-to-air operations in fighter-type aircraft

is limited to situations where mainlobe clutter is not a prob-

lem—e.g., over water or where mainlobe does not strike

ground within ranges of interest.

15. For ground mapping, unambiguous range provided by low

PRFs is essential. However, for SAR mapping, PRF must also

be high enough that repetitions of mainlobe clutter do not

overlap.

13. IF PRF is made low enough to provide reasonably long unam-

biguous ranges, most of target return will be rejected along

with mainlobe clutter, MLC. Also, ground moving targets,

GMTs, cannot be directly discerned from airborne targets.

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PART VI Air-to-Air Operation

16. High PRFs provide clutter-free region in which to detect high

closing-rate targets.

17. But, return from low closing-rate targets must compete with

sidelobe return, much of it from short range.

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Operation at high PRFs has other important advantages.

First, between the band of central-line sidelobe clutter fre-

quencies and the first repetition of this band, a region

opens up in which there is absolutely no clutter (Fig. 16).

It is here that the doppler frequencies of approaching tar-

gets lie—those for which long detection ranges normally

are desired. Second, closing rates can be measured directly

by sensing doppler frequencies. Third, for a given peak

power, average transmitted power can be maximized sim-

ply by increasing the PRF until a duty factor of 50 percent

is reached. High duty factors can also be obtained at low

PRFs, but this requires increasing the pulse width and

employing large amounts of pulse compression to provide

the degree of range resolution which is essential in low PRF

operation.

The principal limitation of high PRF operation is that

detection performance may be degraded by sidelobe clutter

when operating against low closing-rate tail-aspect targets.

At the high PRFs typically employed in fighter radars, the

return from virtually all ranges is collapsed (telescoped) into

a range interval little wider than that occupied by a target’s

echoes. Consequently, the sidelobe clutter can only be

rejected by resolving the return in doppler frequency.

Much of the sidelobe clutter falling within the same

resolvable frequency increment as a target’s echoes will have

been reflected from very much shorter ranges and so will be

quite strong (Fig. 17).

When flying at moderate to low altitudes over terrain that

has a high backscattering coefficient, unless the target has a

large radar cross section or is at short range, its echoes may

be lost in the clutter, and there will be no way of extracting

them from it.

Also, if little or no range discrimination is provided, zero

closing-rate targets will be rejected along with the altitude

return.

Another disadvantage of very high PRFs is that they make

pulse delay ranging more difficult. As the PRF is increased,

range ambiguities become more severe. To resolve them, the

radar must switch among more PRFs.

Ultimately, a point is reached where range must be mea-

sured by more complex, less accurate techniques, such as

frequency modulation ranging. In any event, because of

losses incurred in resolving ambiguities, range can be mea-

sured only at the expense of a reduction in maximum detec-

tion range.

Nevertheless, when mainlobe clutter is a problem and

long detection ranges are desired against approaching tar-

gets, the advantages of high PRFs far outweigh any of these

disadvantages.

HIGH PRFs

ADVANTAGES LIMITATIONS

1. Good nose-aspect 1. Detection range

capability—high-clos- against low-closing-

ing-rate targets appear rate targets may be

in clutter-free region of degraded by sidelobe

spectrum. clutter.

2. High average power 2. Precludes use of sim-

can be provided by in- ple, accurate pulse de-

creasing PRF. (Only lay ranging.

moderate amounts of

pulse compression, if

any, are needed to

maximize average

power.)

3. Mainlobe clutter can be 3. Zero-closing rate tar-

rejected without also gets may be rejected

rejecting taget echoes. with altitude return

and transmitter

spillover.

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CHAPTER 25 The Crucial Choice of PRF

Medium PRF Operation. Medium PRFs were conceived

as a solution to the problems of detecting tail-aspect targets

in the presence of both mainlobe and strong sidelobe clut-

ter, thereby providing good all-aspect coverage. If the maxi-

mum required operating range is not exceptionally long,

the PRF can be set high enough to provide adequate separa-

tion between the periodic repetitions of the mainlobe clut-

ter spectrum without incurring particularly severe range

ambiguities.

Mainlobe clutter can then be isolated from the bulk of

the target return on the basis of its doppler frequency. And

individual targets can be isolated from the bulk of the side-

lobe clutter through a combination of range and doppler

resolution.

Also, ground moving targets—being close to the fre-

quency of the mainlobe clutter—can be rejected along with

it, without rejecting an unacceptable additional amount of

possible target return (Fig. 18).

Range ambiguities are more easily resolved than at high

PRFs so that pulse delay ranging is possible. While doppler

frequencies are ambiguous, these ambiguities are also mod-

erate enough to be resolved.

On the negative side, because of the range and doppler

ambiguities, both nose- and tail-aspect targets may have to

compete with close-in sidelobe clutter (Fig. 19). This prob-

lem, of course, can be avoided by switching among several

different PRFs.

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18. With medium PRFs, repetitions of mainlobe clutter are widely

enough separated so that it and return from ground moving

targets can be rejected without rejecting undue amount of

target return.

19. While targets may still have to compete with close-in sidelobe

clutter, the clutter can be avoided by switching among several

different PRFs.

But the resulting reduction in the integration time for

each PRF limits the maximum detection range.

Nevertheless, where extremely long detection range is

not required—as when operating at moderate to low alti-

tudes, in lookdown situations, or in tail chases—adequate

detection range can generally be achieved.

Another consequence of the range and doppler ambigui-

ties encountered at medium PRFs is that sidelobe return

from ground targets of large radar cross section can be a

serious problem. Special measures must generally be taken

to eliminate this return lest it be confused with return from

airborne targets.

MEDIUM PRFs

ADVANTAGES LIMITATIONS

1. Good all-aspect capa- 1. Detection range

bility—copes satisfac- against both low and

torily with both main- high closing-rate tar-

lobe and sidelobe gets can be limited by

clutter. sidelobe clutter.

2. Ground-moving targets 2. Must resolve both

readily eliminated. range and doppler

ambiguities.

3. Pulse delay ranging 3. Special measures

possible. needed to reject side-

lobe return from

strong ground targets.

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PART VI Air-to-Air Operation

334

Summary

Pulse repetition frequencies used by airborne radars

range from a few hundred hertz to several hundred kilo-

hertz at X-band. Generally, only at extremely low PRFs is

range unambiguous—and then only if all return from

beyond the first range zone is excluded or negligible.

Conversely, only at considerably higher PRFs are doppler

frequencies largely unambiguous. Thus, the choice of PRF

is generally a compromise.

Three categories of PRF have been established: low, high,

and medium. Which category a particular PRF falls in

depends on the operational situation.

A low PRF is one for which the maximum required oper-

ating range falls within the first range zone. Simple pulse

delay ranging can be used, and sidelobe clutter can be

almost entirely removed through range resolution. But, in

fighter radars, doppler ambiguities are generally so severe

that mainlobe clutter cannot be rejected without rejecting

much possible target return, and GMTs may be a problem.

A high PRF is one for which doppler frequencies of all

significant targets are unambiguous. Mainlobe clutter can

be rejected without rejecting target return, and a clutter-free

region is provided in which approaching targets appear.

Also, high average power can be obtained by increasing the

PRF. While this mode is excellent for nose-aspect targets,

because of range ambiguities sidelobe clutter may severely

limit performance against tail-aspect targets. Range rates

can be measured directly, but pulse delay ranging may be

difficult or impractical because of severe range ambiguities.

A medium PRF is one for which both range and doppler

frequency are ambiguous. But if the value of the PRF is

judiciously selected, the ambiguities are comparatively easy

to resolve. Consequently, good all-aspect performance can

be provided despite both mainlobe and sidelobe clutter, as

well as GMTs. Maximum detection range, however, is limit-

ed by close-in sidelobe clutter, and sidelobe return from

large-RCS objects on the ground may be a problem.

CATEGORIES OF PRF

PRF RANGE DOPPLER

HIGH Ambiguous Unambiguous

MED Ambiguous Ambiguous

LOW Unambiguous Ambiguous

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Low PRF Operation

1. A low PRF is one for which the first range zone extends at least

to the maximum range the radar is designed to handle.

ow PRF is, by definition, one for which the first

range zone—the zone from which first-time-

around echoes are received—extends at least to

the maximum range the radar is designed to

handle (Fig. 1). In the absence of return from beyond this

zone, the observed ranges are unambiguous. (The first

range zone, you’ll recall, extends out to the so-called maxi-

mum unambiguous range, R

, which in nautical miles

roughly equals 80 divided by the PRF in kilohertz.)

Typically, low PRFs range from around 250 hertz (R

320 nmi) to 4000 hertz (R

= 20 nmi). Unfortunately, at

such PRFs unless the wavelength is relatively long and/or

the closing rate relatively low, the observed doppler fre-

quencies are highly ambiguous.

Low PRFs are essential for most air-to-ground uses. And

they are superior to both medium and high PRFs for cer-

tain air-to-air applications, e.g. early warning. But for use in

fighter aircraft, where target return generally must compete

with mainlobe clutter, low PRFs have serious limitations

In this chapter, we will take a closer look at low PRF

operation. We will see how target echoes may be separated

from ground clutter and how the signal processing may be

performed; then, take stock of advantages and limitations

and see how the limitations may be alleviated.

Differentiating Between Targets and Clutter

To see what must be done to separate target echoes from

ground clutter, let us look at the range and doppler profiles

that would be observed by a low PRF radar for a fighter

aircraft in a representative flight situation.

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PART VI Air-to-Air Operation

336

Range Profile. As illustrated in Fig. 2, within the first

range zone the observed ranges are true ranges. The alti-

tude return, sidelobe clutter, and mainlobe clutter are all

clearly identifiable—as is the return from targets A and B,

which are outside the mainlobe clutter. Target C, however,

is completely obscured by mainlobe clutter. Target D,

which is beyond the first range zone, appears falsely at a

much closer range, but we will defer considering it until

later.

From even a cursory inspection of the portion of the pro-

file in which sidelobe clutter alone appears (Fig. 3), one

thing is immediately clear: the echoes from a target will

generally be stronger than the sidelobe clutter received

from the target’s own range.

2. Range profile for low PRF radar in representative operational situation. Out to maximum range of interest, ranges observed by radar directly

correspond to true ranges.

3. A target‘s echoes are generally stronger than sidelobe clut-

ter from target’s own range.

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This is perhaps more clearly illustrated by the range pro-

file observed at the output of the receiver (Fig. 4). In it, the

amplitudes of both the target echoes and the sidelobe clut-

ter are more or less independent of range over the portion

of the profile in which strong sidelobe clutter is received.

This characteristic is due to a feature called sensitivity time

control (STC), which is generally employed when operating

at low PRFs.

Now, if we slice the range profile into increments match-

ing the width of the received pulses

and isolate the energy

CHAPTER 26 Low PRF Operation

337

4. Range profile at output of receiver. Sensitivity time control

(STC) makes amplitude of output independent of range to pre-

vent saturation by close-in return.

1. Compressed width, if pulse

compression is used.

SENSITIVITY TIME CONTROL

At low PRFs, saturation of the receiving system by strong

return from short ranges is commonly avoided without loss of

detection sensitivity at greater ranges through a feature called

sensitivity time control

, STC.

Atter each pulse has been transmitted, the system gain, which

initially is greatly reduced, is increased with time to match the

decrease in amplitude of the radar return with range. Maximum

gain is usually reached well before the end of the interpulse

period.

Thereafter, the increase in sensitivity is continued by

lowering the detection threshold until the noise limit is

reached—i.e., to the point where the threshold is just far

enough above the mean noise level to limit the false-alarm

probability to an acceptable value.

Thus, maximum sensitivity is provided at long ranges,

where it is needed to detect the weak echoes of distant

targets, while the strong return from short ranges is

prevented from saturating the system.

STC may be applied at various points in a system. From

whatever point it is applied, it helps prevent saturation in all

following stages.

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PART VI Air-to-Air Operation

338

contained in each increment, we can differentiate between

target echoes and sidelobe return by noting the differences

in the amplitude from one increment to the next (Fig. 5).

But we are not so fortunate with mainlobe clutter.

Isolating the returns from narrow range increments only

moderates the problem. For the mainlobe clutter from a

target’s own range is generally much stronger than the tar-

get’s echoes. Moreover, even when mainlobe clutter is

received from a range that is some multiple of R

beyond

the target’s range (multiple-time-around return), it will gen-

erally be stronger than the target echoes. To differentiate

between target echoes and simultaneously received main-

lobe clutter, we must look for differences in doppler fre-

quency.

Doppler Profile. Since range is largely unambiguous

when low PRFs are used, the appearance of the doppler

profile varies considerably with the point in the interpulse

period at which the profile is observed. In other words, the

returns from different range increments may have quite dif-

ferent doppler profiles.

The doppler profile for the range increment in which

Target C resides is illustrated in Fig. 6. The most prominent

features of this profile are the periodic repetitions of the

mainlobe clutter spectrum. These occur at intervals equal to

the pulse repetition frequency, f

. Though they may be quite

wide, they are commonly called “lines” or, in view of their

spacing, PRF lines. (The central line is the carrier frequen-

cy; the others are sideband frequencies.)

Between successive PRF lines can be seen the thermal

background noise and Target C. Sidelobe clutter from this

range happens to be so weak that it is below the noise level.

As you may have noticed, had Target C’s doppler frequency

been a little lower, it might have been obscured by the

mainlobe clutter.

Target B is also at a sufficiently long range that the

accompanying sidelobe clutter is below the noise level.

Since in this particular flight situation mainlobe clutter is

not received from Target B’s range, this target will appear in

the clear regardless of its doppler frequency. It must com-

pete only with background noise.

At shorter ranges, such as that of Target A, sidelobe clut-

ter is much stronger than noise. Nevertheless, because the

accompanying sidelobe clutter comes from the same range

as the target’s echoes, the target appears above the clutter

(see Fig. 7).

Target D is a second-time-around target and so is not

wanted. Although it happens to be stronger than the

accompanying first-time-around sidelobe clutter, it can be

prevented from reaching the display by PRF jittering.

7. Target A, at short range, must compete with sidelobe clutter.

But since the clutter comes from the target’s own range, the

target echoes are stronger than the clutter.

5. If range profile is sliced into narrow increments, a target’s

echoes can be discerned from sidelobe clutter on basis of

amplitude.

6. A target at a range from which mainlobe clutter is received

can only be detected if it’s doppler frequency is different from

that of the clutter.

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Figure 8 shows the doppler profile at the range of the

altitude return. As was explained in Chap. 22, the altitude

return is generally spread over a band of frequencies whose

width exceeds most low PRFs. Consequently, in the doppler

profile for a range increment from which altitude return is

received, it is indistinguishable from the other sidelobe

clutter.

Because the altitude return is spread over such a broad

band of doppler frequencies, if doppler filtering is employed,

a target may be detected above the altitude return, provided

the target’s echoes are very strong, as they may well be at

very short ranges.

(For the altitude return to be at a short

range, of course, the radar must be at low altitude.)

Figure 9 shows a fairly broad portion of the doppler

spectrum for a range from which mainlobe clutter is

received. To eliminate the clutter, we must reject not only

the band of frequencies in which the central line lies but

bands of equal width at intervals equal to f

throughout

the receiver’s IF passband. (Along with the clutter, some

target echoes may also be rejected.) Only sidelobe clutter

and noise, plus the (unrejected) target echoes will remain.

The target echoes may then be separated from the sidelobe

clutter and noise on the basis of differences in amplitude

and doppler frequency.

CHAPTER 26 Low PRF Operation

339

8. Altitude return is spread over such a broad band of frequencies

it is indistinguishable from the other sidelobe clutter.

9. Doppler profile for range from which mainlobe clutter is received consists of periodic repetitions of mainlobe clutter spectrum, with sidelobe

clutter and target echoes in between.

10. Signal processing functions in a low-PRF radar employing doppler filtering. In applications where mainlobe clutter is avoided, clutter can-

cellers may be eliminated.

PART VI Air-to-Air Operation

340

Signal Processing

One approach to mechanizing the signal processing

functions outlined in the preceding paragraphs is illustrated

by the block diagram of Fig. 10 (bottom of the page).

Basic Mechanization. As illustrated in Fig. 10, the IF

output of the receiver is fed to a synchronous detector

(such as described in Chap. 18), which converts it to I and

Q video signals. The frequency of the reference signal sup-

plied to the detector is such as to place the central line of

mainlobe clutter at zero frequency (dc). The central line is

picked, since its frequency doesn’t change when the PRF is

changed, whereas the frequencies of the other lines do.

(The importance of this will be made clear later on.)

An analog-to-digital converter samples the video signals

at intervals matching the width of the transmitted pulses.

The output of the converter therefore is a stream of num-

bers representing the I and Q components of the returns

from successive range increments. The numbers are sorted

by range increment into separate range bins.

To reduce the amount of mainlobe clutter, the numbers

for each range increment are passed through a separate

clutter canceller. As with the A/D converter, each clutter

canceller has both I and Q channels.

To reduce the mainlobe clutter residue in the output of

the canceller, as well as to minimize the amount of noise

and simultaneously received sidelobe clutter with which a

target must compete, the output of each clutter canceller is

integrated in a bank of doppler filters (as described in

Chaps. 19 and 20). So that the filter bank can be imple-

mented with the fast Fourier transform, the passband of the

bank is made equal to the PRF. Processing of the outputs for

2. Compressed width, if pulse

compression is used.