GB2335103A

GB2335103A - Cancelling second trace clutter in pulse doppler radar

Info

Publication number: GB2335103A
Application number: GB8024118A
Authority: GB
Inventors: Peter Lambert Hartley; Clive David Summerfield; Andrew William Phillips; Robert James Mort; Charles Freeman Robinson
Original assignee: Decca Ltd
Current assignee: Decca Ltd
Priority date: 1980-07-23
Filing date: 1980-07-23
Publication date: 1999-09-08
Anticipated expiration: 2000-07-23
Also published as: GB8024118D0; GB2335103B

Abstract

A pulse doppler radar has p.r.f. jitter with successive pulse repetition intervals determined by a p.r.i.counter 28 in accordance with a predetermined pseudo-random sequence. At intervals less than the beam dwell time of the radar, the p.r.i. counter 28 produces an extra long p.r.i. During the extra long p.r.i., the received doppler radar signal values for range cells corresponding to record trace retums forthe normal p.r.i.'s are stored in a memory 25. The stored values for respective corresponding second trace range cells are read out during subsequent ordinary p.r.i.'s under the control of a range counter 40, and subtracted from the received signal values for the range cells, so as to cancel second trace clutter.

Description

2335103 PULSE DOPPLER RADAR APPARATUS AND SECOND TRACE CLUTTER CANCELLING

METHOD THEREIN The present invention is concerned with pulse doppler radar apparatus and particularly with an arrangement.for and a method of cancelling second trace clutter signals in such apparatus.

Because of the relatively high pulse repetition frequency (p.r.f.) of pulse doppler radar apparatus it is common for radar echoes from reflecting objects at ranges greater than the maximum unambiguous range of the apparatus to be received during the interval following the subsequent radar transmitter pulse. Such return signals are known as "second trace" signals. Third and higher or-ler trace signals can also arise in same apparatus in- subsequent Pulse repetition intervals.

Since such second trace signals are at greater than the normal maximum operational (unambiguots) range of the radar apparatus, they are usually produced by relatively large reflecting surfaces. Thus, the majority of second trace signals are "clutter" signals from unwanted stationary 1 or very slow moving distant objects, such as mountains or hills. In pulse doppler apparatus with constant p.r.f., such second trace clutter signals will hppear at a constant corresponding range in the subsequent pulse repetition interval (p.r.i.), and also with zero doppler. As a result, the usual doppler cancellation circuitry of a pulse doppler radar apparatus would cancel also second trace clutter signals.

llowever, it is sometimes desirable to provide pulse doppler radar apparatus with so-called p.r.f. agility, i.e. a p.r.f. which is varied in a predetermined fashion. It will be understood by those practised in the art of doppler radar that a high p.r.f. is desirable to provide a maximum range of unambiguous received doppler frequencies.

The received doppler frequency velocity of the target and the greater is the first so-called is dependent upon the radial higher the radar p.r.f. the "blind speed", i. e. a speed providing a doppler frequency equal to or close to the p.r.f. or its harmonies. Furthermore, it will be appreciated that with fixed p.r.f.,the maximum doppler frequency which can be resolved unambiguously is equal to half the p.r.f.

However, increasing the p.r.f. of the radar apparatus reduces the maximum unambiguous range at which target returns are received in the interval before the following radar pulse.

It is known to mitigate the conflicting requirements of increasing the unambiguous detection range and the maximum resolvable doppler frequency by producing so- called p.r.f. stagger or p.r.f. jitter so that the radar apparatus has effectively two or more p.r.f.'s which are time shared. With such p.r.f. agility, pulse doppler radar apparatus can have a usefully large unambiguous range and yet still resolve unambiguously doppler frequencies corresponding to target velocities up to a maximum likely to be encountered.

However, where the p.r.f. varies at each pulse (p.r.f. jitter), such variations cause the second trace clutter signals from a single distant clutter target to appear in different range cells on successive p.r.i.1s. AS a result, on analysing the doppler content of the signals received by the apparatus, these second trace clutter signals appear to have a doppler frequency and are ther.efore not cancelled by the usual clutter cancellation techniques. Where "block stagger" is employed, i.e. the p.r. f. remains constant for a block of successive pulses before changing, thp.above second trace clutter problem also arises on the first p.r.i. after the change. The present invention sets out to alleviate this problem.

1 According to the present invention pulse doppler radar apparatus has means for varying the normal radar pulse repetition inverval (p.r.i.) between a \maximum and a minimum p.r.i. in a predetermined fashion; means for temporarily interrupting the production of radar pulses to produce, at predetermined intervals less than the beam dwell time of the apparatus, an extra long p.r.i. greater than said maximum p.r.i.; means for range gating received doppler radar signals into predetermined successive range cells; means for storing, during each said extra long p.r.i., the received signal values for range ce lls at ranges greater than the maximum unambiguous range for said minimum p.r.i.; means for reading from storage, for range cells after the second radar pulse following said extra long p.r.i., the stored values for the respective corresponding second trace range cells; and means for subtracting said respective stored second trace values from the actual received signal values for the range cells.

with this apparatus, during each of the extra long p.r.i,Is received signal values for range cells outside the normal maximum unambiguous range are stored and during subsequent normal p.r.i. 1s, stored valueq are used to provide nominal second trace return values for subtraction from the actual received values in the normal p.r.i.'s. If the way in which the normal p.r.i.'s are varied is known# then it is a straightfonvard matter for each normal p.r.i. to compute for each range cell of the p.r.i. the corres- 01-1.

-5onding second trace range cell which might produce in the range cell a false received signal value.

At least one extra long p.r.i. is produced per beam dwell time so that the second trace.ange cell values stored for each extra long p.r.i. are valid for cancelling second trace returns in the immediately following normal p.r.i.'s until the next extra long p.r.i.

Commonly, said means for varying is arranged to vary the p.r.i. on successive radar pulses in a predetermined series. Further, the p.r.i. series may be a cyclic series. in which case said interrupting means may interrupt the radar pulses to produce a whole number of said extra long p. r.i.'s each cycle. Conveniently, said means for varying and said interrupting means are together constituted by p.r.i. timer means arranged to set the intervals between successive radar pulses in accordance with the predetermined cyclic p.r.i. series and the periodic extra long p.r.i.1s..,, Accordingly, the extra long p.r.i.'s can be produced simply by modifying the p.r.i. jitter series to produce a whole number of extra long p.r.i.Is in each cycle.

The Cycle time of said cyclic series may be less than the beam dwell time in which case the radar pulses are interrupted to produce one said extra long p.r.i. each cycle.

Very preferably, said extra long p.r.i. has a duration no less than the expected longest total duration of any successive pair of normal p.r.i.1s. In'this way, as will become apparent, the extra long p.r.i. enables sufficient 1 distant range cells to be stored to provide second trace clutter cancellation over the full normal unambiguous range. Said extra long p.r. i. may have a duration equal to twice said maximum p.r.i.

In a further embodiment, said extra long p.r.i.may have a duration no less than the expected longest total duration of any group of three successive normal p.r.i.1s. Then, sufficient distant range cells can be stored during the extra long p.r.i. to provide not only second but also third trace clutter cancellation. Conveniently, said. extra long p.r.i. then has a duration equal to three times said maximum p.r.i. in this arrangement, said reading means may be arranged to read the stored values for the respective corresponding second and third trace range cells and said subtracting means may subtract said second and third trace values from the actual received signal values.

In another aspect of the present invention, a method of cancelling second trace clutter signals in a pulse doppler radar apparatus with range gating and agile p.r.f. between a maximum and a minimum p.r.i., comprises the steps of temporarily interrupting the production of radar pulses to produce, at predetermined intervals of les than the beam dwell time of the apparatus, an extra long p.r.i. greater than said maximum p.r.i.; storing, during each said extra long p.r.i., the received doppler signal vAlues for range cells at ranges greater than the maximum unambiguous range for said minimum p.r.i.; reading from storage, for range cells after the second radar pulse following said extra long p.r.i., the stored values for the respective corresponding second trace range cells; and then subtracting said respective stored second trace values from the actual received signal values for the range cells.

An example of the present invention will now be described with reference to the accompanying drawings in which:

FIGURE 1 is a block schematic diagram illustrating an embodiment of the present invention,and FIGURE 2 is a graphical representation of successive radar p.r.i.'s illustrating how second trace clutter can- cellation is achieved.

Referring to FIGURE 1, received radar signals at receiver intermediate frequency (i.f.) are supplied on a line 10 from the receiver or. any prior signal processing equipment.. A reference intermediate frequency signal is also supplied on a line 11. In accordance with normal practice, the received radar signals are supplied on line 10 to a pair of multipliers 12 and 13. The reference i.f. on line 11 is split into two signals separated in phase by -7r /2 and one phase is supplied to the multiplier 12 and the other phase to multiplier 13. r'Llhus, the multipliers 1 r 1 x 1 12 and 13 produce respective signals on lines 14 and 15 which represent the real and imaginary parts of a complex number or vector representative of the"instantaneous amplitude and phase of the received radar signals relative to the reference i.f. The signals on lines 14 and 15 can be considered the received doppler radar signals.

Again, in accordance with normal practice, the received dop-.)ler radar signals on lines 14 and 15 are range gated in gates 16 and 17 controlled by pulses from a range clock generator 18 on a line 19. The range clock pulses produced by the generator 18 are synchronised with the radar pulses which are fed to the generator 18 on a line 20. The range gated doppler radar signals from the range gates 16 and 17 are converted from analogue to digital form by respective analogue to digital converters 21 and 22. The analogue to digital converters 21 and 22 in the present example produce parallel twelve bit words on data buses 23 and 24 representative of the range gated doppler video signals.

Ihe twelve bit words on buses 23 and 24 are s=plied to a second trace memory 25. The successive twelve bit words presente d to the memory 25 are written into the memory under the control of a write strobe on a line 26 from a write strobe generator 27. The write strobe generator 27 is synchronised with the range clock pulses from the range clock generator 18 and also receives an output from a 1 p.r.i. counter 28.

The radar apparatus of this example is arranged to have so-called p.r.f. jitter, that is t(: say, successive p.r.i.Is are varied. The variations in p.r.i. take place in accordance with a predetermined pseudo-random sequence which is determined by the p.r.i. counter 28. Radar pulses received on a line 29 clock the p.r,i. counter 28 on to the next p.r.i. of the series and the p. r.i. counter 28 defines the duration of the following p.r.i. by setting up an exclusive p.r.i. address code on an output bus 30. The address code is fed to a trigger sequencer 31 which responds to the address code by producing a radar trigger on a line 32. The trigger sequencer 31 generates the trigger after a delay determined by the p.r.i. address code so that the subsequent radar pulse produced in response to the trigger is generated at the correct time in accordance with the series.

In the present example of this invention, the P.r.i. counter 28 is arranged to generate at predetermined inter- vals the address code of an extra long p.r.i. The extra long p.r. i. is at least twice the duration of the longest normal p.r.i. of the sequence. The p.r.i,. counter 28 signals the write strobe generator 27 on a line 33 whenever the address code of the extra long p.r.i. is generated.

The write strobe generator 27 responds to the signal on the - 1 1 line 33 by producing write strobes on its output line 26 to write into the second trace memory 25 the doppler signal values appearing on the buses 23",and 24 during the extra long p.r.i. The write strobe generator 27 may be arranged to generate the write strobe only after a predetermined delay from the beginning of the extra long p.r.i. so as to write into the second trace memory 25 only the doppler radar signal values for gated range cells at ranges outside the maximum totally unambiguous range determined by the normal p.r.i. series. In fact, to ensure the that sufficient distant range cells are written into second trace memory 25 it is necessary to start storing in the memory 25 the signal values for range cells outside the maximum unambiguous range for the shortest p. r.i. of the normal p.r.i. series. The write strobe on line 26 is produced until the end of the extra long p.r.i. so that all the distant range cell values are written into the memory 25.

During normal p.r.i.Is of the series, the stored signal values.are successively read out of the memory 25 and supplied on buses 34 and 35 to respective digital subtractors 36 and 37. The subtractors 36 and 37 also,receive the actual received doppler signal values directly from the analogue to digital convertors 21 and 22 on the buses 23 and 24. It will be appreciated that the memory 25 contains -11 stored received signal values for each of the range cells outside the maximum totally unambiguous range of the apparatus out to somewhat more than twice this range. Furthermore, the p.r.i. counter 28 is arranged to produce the extra long p.r.i. at intervals less than the beam dwell time of the radar apparatus so that following an extra long p.r.i., the values stored in the memory 25 are representative of potential second trace returns during the immediately following normal length p.r.i.'s. However, because of the variation in p.r.i. during the the normal p.r.i. series, different distant range cells,which have values stored in the memory 25, have second trace correspondence with particular range cells on successive normal p.r.i.'s.

The p.r.i. address generated by the counter 28 is supplied on the bus 30 to an initial address memory unit 38. The initial address memory 38 is pre-programmed with the addresses of the stored value for the distant range cell whichis in second trace correspondence with the first range cell of each successive normal n.r.i. Thus, when a p.r.i. address is generated on the bus 30,.,the initial address memory 38 provides a corresponding initial address on a bus 39 to a range counter 40. This initial address on the bus 39 pre-primes the range counter 40 to start for I-.

the new p.r.i. at the initial address supplied by the memory 38. Thus, for the first range cell of the p.r.i., the address supplied from the range counter 40 to identify a location in the memory 30 on a line 41 is the initial address generated by the memory 38. The contents of this indicated address in the memory 25 is non-destructively read from the memory and supplied on the lines 34 and 35 to subtractors 36 and 37. This contents reDresents the predetermined second trace signal value for the particular range cell. The sub- tractors 36 and 37 subtract this second trace value from the actual received values for the range cell and supply on output buses 42 and 43 modified signal values which have had second trace content cancelled.

Following the first range cell of each normal p.r.i., the range counter 40 is clocked by range clock pulses on a line 44 from the range clock generator 18. Thus, the second trace content of subsequent range cells of each normal p.r.i. is cancelled by subtracting the stored value in the memory 25 for the distant range cell which is in appropriate second trace c'orrespondence. As the range counter 40 is clocked, the address of the appropriate second trace value in the memory 25 is generated on the address bus.41.

Referring to FIGURE 2, the manner in which second trace clutter is cancelled is illustrated graphically. In the figure, each of the parallel vertical lines from the abscissa represent successive p.r.i.'s of the radar apparatus. The ordinate axis 51 is divided into successive range cells. As can be seen', successive p.r.i.'s are of different duration, as indicated by the vertical height of the solid parts of the p.r.i. lines 52, and the horizontal separation of the p.r.i. lines along the abscissa.

A sequence of normal p.r.i.'s in the present example comprises only the four relatively short p.r.i.Is 53 to 56. After every fourth normal p.r.i. there is a extra long p.r.i. 57. The shortest duration normal p.r.i. 55 is divided, in the illustrated simplified example, into a total of eight range cells Ri to R8. Range cells more distant than cell R8 are identified as 2R1 2R2 etc. The most distant range cell corresponding to the extra long p.r.i. 57 is 2R14. The longest normal p.r.i. 56 extends a full three range cells beyond the shortest normal p.r.i. 55. The extra long p.r.i. 57 has a total of twenty- two range cells, which is twice the number of range cells in the longest normal p.r.i. 56. In FIGURE 2, the spacing alongthe abscissa 50 of the p.r.i. lines is proportional to the duration of the previous p. r.i.

Considering firstly the first extra.long p.r.i. at the ordinate 51, suppose there is a radar return from a stationary object, i.e. clutter, which occurs at range cell 2R7, this clutter target is outside the normal un- ambiguous range of the radar apparatus but can produce second trace clutter returns in subsequent normal p.r.i.1s. The example of the present invention''escribed previously operates by storing the received signals during the extra long p.r.i. of each of the range cells 2R1 to 2R14. Thus, if there is a return at 2R7 this will be stored in an appropriate address of the memory 25.

Since all the subsequent normal p.r.i.Is 53 to 56 occur during the beam dwell time of the radar apparatus, it can be expected, for a relatively strongly reflecting clutter target, that radar returns will be received from each of the subsequent radar pulses. However, since the subsequent p.r.i.'s are relatively short, these returns will be received as second trace returns. It can be seen that the first such second trace return arises during the p.r.i. 54. The first trace range cell of the p.r.i. 54 in which the duration exam ple', of p.r.,i. return is pectively.

The sequence of p.r.i.'s both normal and extra long is predetermined in the radar apparatus and therefore it is a straightforward matter to predetermine for each second trace return arises is dependent on the of the preceding p..r.i. 53. In the illustrated the second trace return occurs in range cell R6 54. In subsequent p.r.i.'s the second trace generated in range cells RS, R7 and R4 res- -is- normal p.r.i. the second trace range cell which is equivalent to the first range cell of the p.r.i. In the illustrated example of FIGURE 2, 2h2 is equivalent to Rl for p.r.i. 54, 2R3 is equivalent to Rl in p.r.i. 55, 2R1 is equivalent to Rl in p.r.i. 56 and 2R4 is equivalent to Rl in p. r.i. 57, which is the next extra long p.r.i. It will be appreciated that second trace clutter cancellation takes place also during the normal unambiguous range portion of each extra long p.r.i.

Accordingly, during each of the p.r.i.'s 53 to 57, the apparatus of FIGURE 1 operates to subtract from the actual returns received during the p.r.i. the equivalent stored second trace values received during the distant part of the preceding extra long p.r.i.

The stored values in the memory 25 are refreshed each extra long p.r.i., so that, for example, the clutter target my now appear at cell 2R6, assuming the clutter pattern may vary.

It can be seen, therefore, that the present invention provides a convenient and effective way of reducing second trace clutter returns in p.r.f. agile pulse doppler radar. The technique need not be restricted to reducing only second trace clutter returns. If the ex.tra long p.r.i. is extended to more distant ranges, third trace returns can also be dealt with in a similar fashion. Then for each range cell of the normal p.r.i.'s, two values are read from the memory representing the equivalent second and third trace values and these are subsequently subtracted from the actual received value. Higheil,.order returns can be cancelled similarly.

i

Claims

CLAIMS:

1. Pulse doppler radar appara"tus having means for varying the normal radar pulse repetition interval (p.r.i.) between a maximum and a minimum p.r.i. in a predetermined fashion; means for temporarily interrupting the production of radar pulses to produce, at predetermined intervals less than the beam dwell time of the apparatus, an extra long p.r.i. greater than said maximum p.r.i.; means for range gating received doppler radar signals into predetermined successive range cells; means for storing, during each said extra long p.r.i., the received signal values for range cells at ranges greater than the maximum unambiguous range for said minimum p.r.i.; means for reading from storage, for range cells after the second radar pulse following said extra long p.r.i., the stored values for the respective corresponding second trace range cells; and means for subtracting said respective stored second trace values from the actual received signal values for the range cells..

2. Radar apparatus as claimed in.claim 1, wherein said means for varying is arranged to vary the p.r.i. on successive radar pulses in a predetermined series.

1 cl - 1.5 -

3. Radar apparatus as claimed in claim 2, wherein the p.r.i. series is a cyclic series and said interrupting means interrupts the radar pulses to pr (>duce a whole number of said extra long p.r.i.'s each cycle.

4. Radar apparatus as claimed in claim 3 wherein said means for varying and said interrupting means are together constituted by p.r.i. timer means arranged to set the intervals between successive radar pulses in accordance with the predetermined cyclic p.r.i. series and the periodic extra long p. r.i.'s.

5. Radar apparatus as claimed in either of claims 3 or 4, wherein the cycle time of said cyclic series is less than the beam dwell time and the radar pulses are interrupted to produce one said extra long p.r.i. each cycle.

6. Radar apparatus as claimed in any preceding claim, wherein said extra long p.r.i. has a duration no less than the expected longest total duration of any successive pair of normal p.r.i.1s.

7. Radar apparatus as claimed in claim 6, wherein said extra long p.r.i. has a duration equal to twice said maximum p.r.i.

(7)

8. Radar apparatus as claimed in any of claims 1 to 5, wherein said extra long p.r.i. has a duration no less than the expected longest total duration of any group of three successive normal p.r.i.1s.

9. Radar apparatus as claimed in claim 8, wherein said extra long p.r.i. has a duration equal to three times said maximum p.r.i.

10. Radar apparatus as claimed in claim 8 or claim 9, wherein said reading means is arranged to read the stored values for the respective corresponding second and third trace range cells, and said subtracting means subtracts said second and third trace values from the actual received signal values.

11. A method of cancelling second trace clutter signals in a pulse doppler radar apparatus with range gating and agile p.r.f. between a maximum and a minimum p.r.i., comprising the steps of temporarily interrupting the production of radar pulses to produce, at predetermined intervals less than the beam dwell time of the.apparatus, an extra long p.r.i. greater than said maximum p.r. i.; storing, during each said extra long p.r.i., the received doppler signal values for range cells at ranges greater than the maximum unambiguous range for said minimum p.r.i.; reading from storage, for range cells after the second radar pulse following said extra long p.r.i., the stored values for the respective corresponding second trace range cells; and then subtracting said respective stored second trace values from the actual received signal values for the range cells.

12. A method as claimed in claim 11, wherein the normal radar p.r.i. is varied on successive radar pulses in a predetermined cyclic sequence and an extra long p.r.i. is produced a whole number of times each cycle.

13. A method as claimed in claim 12, wherein the cycle time of said cyclic series is less than the beam dwell time and one said extra long p. r.i. is produced each cycle.

14. Pulse doppler radar apparatus substantially as hereinbefore described with reference to and as illus- trated.in the accompanying drawings.

15. A method of cancelling second trace clutter signals in a pulse doppler radar apparatus as claimed in claim 1 and substantially as hereinbefore described.

IMR/REC/BA

15. A method of cancelling second,trace clutter signals in a pulse doppler radar apparatus, substantially as hereinbefore described.

IMR/REC/BA C t Amendments to the claims have been filed as follows 1. Pulse doppler radar apparAus having means for varying the normal radar pulse repetition interval (p.r.i.) between a maximum and a minimum p.r.i. in a predetermined fashion; means for temporarily interrupting the production of radar pulses to produce, at predetermined intervals less than the beam dwell time of the apparatus, an extra long p.r.i. greater than said maximum p.r.i.; means for range gating received doppler radar signals into predetermined successive range cells; means for storing, during each said extra long p.r.i., the received signal values for range cells at ranges greater than the maximum unambiguous range for said minimum p.r.i.; means for reading from storage, for range cells after the second radar pulse following said extra long p. r.i., the stored values for the respective corresponding second trace range cells; and means for subtracting said respective stored second trace values from the actual received signal values for the range cells.

nl 011 3. Radar apparatus as claimed in claim 2, wherein the p.r.i. series is a cyclic series and said interrupting means interrupts the radar pulses to prduce a whole number of said extra long p.r.i.Is each cycle.

6. Radar apparatus as claimed in any preceding claim, wherein said extra long p.r.i. has a duration no less than the expected longest total duration of any successive pair of n ormal p.r.i.1s.

J3 8. Radar apparatus as claimed in any of claims 1 to 5, wherein said extra long p.r.i. has a duration no less than the expected longest total duration of any group of three successive normal p.r.i.'s.

10. Radar apparatus as claimed in claim 8 or claim 9, wherein said reading incans is arranged to read the stored values for the respective corresponding second and third trace range cells, and said subtracting means subtracts said second and third trace values from the actual received signal values.

11. A method of cancelling second trace clutter signals in a pulse doppler radar apparatus with range gating and agile p.r.f. between a maximum and a minimum p.r.i., comprising the steps of temporarily interrupting the production of radar pulses to produce, at predetermined intervals less than the beam dwell time of the.apparatus, an extra long p.r.i. greater than said maximum p.r. i.; storing, during each said extra long p.r.i., the received doppler signal values for range cells at ranges greater than the maximum 2A_ unambiguous range for said minimum p.r.i.; reading from storage, for range cells after the second radar pulse following said extra long p.r.i., the stored values for the respective corresponding second trace range cells; and then subtracting said respective stored second trace values from the actual.received signal values for the range cells.

13. A method as claimed in claim 12, wherein the cycle time of said cyclic series is less than the beam dwell time and one said extra long p. r. i. is produced each cycle.

14. Pulse doppler radar apparatus substantially as hereinbefore described with reference to and as 20 illustrated in the accompanying drawings.