GB2085252A

GB2085252A - Radar

Info

Publication number: GB2085252A
Application number: GB8122718A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1980-09-27
Filing date: 1981-07-23
Publication date: 1982-04-21
Also published as: GB2088667A; GB2085251A; GB2085252B; GB2088667B; GB2085251B

Abstract

A transmitter 2 generates a train of pulses arranged in groups of two, each group including a long pulse and a short pulse of different frequencies. These long and short pulses are separated in a receiver 3 by a range gate 12 which passes the pulses through the upper receiving channel when reflected from long ranges and the lower receiving channel when received from short ranges. The upper channel includes a filter 19 which passes only the long pulses (which are frequency swept). These pulses are compressed at 20. The lower channel includes a filter 15 which passes only the short pulses which, in this particular embodiment, are not frequency swept. The transmitter 2 reverses the order of the pulses in adjacent groups so that there are adjacent frequency swept pairs of pulses in the pulse train. <IMAGE>

Description

1 GB 2 085 252 A 1

SPECIFICATION

Radar sonar and similar systems This invention relates to a radar system which is defined for the purpose of this specification as apparatus for deriving information about the position or movement of a feature in a medium surrounding the apparatus by transmitting energy, through the medium, to a feature in said medium and observing the energy travelling to the said apparatus from the aforesaid feature. Whilst the invention is particularly applicable to apparatus which transmits and receives microwave radiation it is also applicable to acoustic or optical systems. Also, whilst the invention is particularly applicable to radar systems which sense the direction and time of arrival of the energy it is applicable to systems which sense just the direction or just the time of arrival. It is also applicable to equipment which observes a Doppler shift between the transmitted and received signals. The term "radar" thus includes range finders, direction finders, and speed measuring equipment.

The invention is particularly concerned with a type of radar which transmits groups of pulses, each group containing a long pulse, which is used for long range detections, and a short pulse, which is used for short range detections. One known radar of this type is described in our U.K. Patent Specification 1424026 and employs a transmitter which modulates the transmitted long pulses to that they are swept in frequency. This allows the receiver to compress the long pulses on reception.

Hitherto each long, frequency swept pulse, in such systems has immediately been preceded orfollowed by a short unswept pulse.

This invention provides radar apparatus comprising a transmitter fortransmitting a train of pulses including some of a first type and some of a second type, the transmitter including means for modulating some of the pulses so as to allow pulse compression on reception, characterised in that the transmitter is constructed and arranged to apply said modulation to adjacent pulses of said pulse train.

The invention also provides radar apparatus comprising a transmitter adapted to transmit groups of pulses each group containing pulses of different types and to modulate the last pulse of one group and the first pulse of the next group in such a way that these pulses can be pulse compressed on reception.

The aforementioned modulation, which allows pulse compression on reception, is preferably a continuous frequency sweep but alternative possibilities are feasible. For example a discontinuous frequency sweep (sometimes called a "staircase"); or phase changes; or even amplitude modulation can be used to achieve the same effect.

The aforementioned first and last pulses may be modulated (e.g. frequency swept) in the same way or in different ways. Modulation in different ways (e.g. by frequency sweeping in different directions or at different rates) facilitates separation of the pulses in the receiver.

The transmitter is preferably adapted to transmit, in each alternate group of pulses, a long modulated pulse followed by a short pulse; and in each other group of pulses a short pulse, followed by a long modulated pulse.

Certain advantages can be obtained if each group of pulses comprises a long, frequency swept, pulse preceded and followed by short pulses, also frequency swept. In this case it is preferable for the short pulses to be swept in opposite directions or with different rates of sweep to facilitate separation thereof in the receiver.

In order to increase the pulse repetition frequency of the short pulses, additional short pulses may be inserted between the groups.

The individual pulses of each group may be separated by a time interval; though this is not necessary if they have sufficiently different characteristics (e.g. frequency) to allow separation in the receiver. If there is a time interval between pulses of a group, this is preferably but not necessarily less than the time intervals between groups of pulses.

According to another aspect of the invention there is provided a transmitter adapted to transmit during groups of periods each group containing periods of first and second differenttypes of transmission and to modulate or code the transmission during the last period of one group and the frist period of the next group in such a way that the signal transmitted during these first and last periods can be pulse compressed on reception and a receiver adapted to distinguish between received signals of the said different types and between signals received after reflection from features in first and second range zones, the receiver, when in operation, using received signals of the first type to give an indication of the positions of pulses in the first range zone and received signals of the second type to give an indication of the positions of features in the second range zone, the receiver including a pulse compressor for compressing received signals which are modulated or coded as aforesaid.

According to another aspect of the invention there is provided a transmitter for producing a transmis- sion including some periods when there is a transmission of a first type and other periods when there is a transmission of a second type, the transmitter including means for coding or modulating the signals during some of the periods so as to allow pulse compression on reception, and a receiver adapted to distinguish between received signals of the respective different types and between signals received after reflection from different range zones, the receiver, when in operation, using signals of the first type to give an indication of the positions of features in a first range zone and the signals of the second type to give an indication of the positions of features in a second range zone; the receiver including a pulse compressor for compressing those received signals which were coded or modulated as aforesaid and the transmitter being constructed and arranged so as to apply said coding or modulation to adjacent signal periods.

One way in which the invention may be performed will now be described byway of example with 2 GB 2 085 252 A 2 reference to the accompanying drawings in which:

Figure 1 is a schematic block diagram of a radar constructed in accordance with the invention; and Figure 2 shows, at A to J, waveforms appearing at points denoted by respective reference letters on Figure 1.

Referring firstto Figure 1, the radar comprises a timer 1, a transmitter 2, a receiver 3, a T/R cell 4 and an antenna 5.

The transmitter receives timing signals Afrom the timer 1 and these timing signals are passed to a monostable circuit 6, which produce pulses B as shown on Figure 2. The pulses B are passed to the drive circuit of a microwave pulse generator 7 which is caused to generate a microwave frequency output 80 during the period of each pulse B. Pulses B are also fed to a selector 8 which passes alternate pulses directly to a monostable circuit 9 whose period is slightly less than that of the monostable 6. The other alternate pulses are fed indirectly to the monostable circuit 9 via a delay line 10. Thus the waveform at point C is as shown on Figure 2 where alternate pulses are slightly delayed with respect to the timing pulses A.

The pulses D are passed to a ramp geneator 11, which produce pulses E whose timing is similar to that of pulses D, but show amplitude increases with time. The pulses E are passed to a control input of the microwave generator 7 and serve to increase its frequency by an amount depending on the voltage at 95 the control input. Thus the outputfrequency of the generator 7 is, during the periods of pulses E, swept through a range of frequencies slightly above the basic frequency of the microwave generator 7, i.e., the frequency at which it operates when no control pulse is present at its control input. The output of the microwave generator is thus as shown at F on Figure 2. It should be noted that waveforms F to J on Figure 2 are plots of frequency against time in contrast to waveforms A to E which are plots of amplitude 105 againsttime.

The microwave generator 7 is a voltage controlled oscillator followed by an amplifier. The voltage controlled oscillator can be a PIN diode.

Referring to waveform F it will be seen thatthis consists of spaced pairs of groups of pulses, each pair comprising a relatively long frequency swept pulse for a period defined by a monostable 9, immediately adjacent, without any time spacing, a relatively short, unswept pulse whose period or length is defined by the difference in periods of monostables 6 and 9. Furthermore it will be noted that in this particular embodiment of the invention the order of the long and short pulses is reversed in alternate pairs of pulses which means that the frequency or mean frequency of consecutive pulses does not change between pulse pairs. The pulses generated by the microwave generator 7 are passed through the T/R cell 4 and are transmitted as electromagnetic waves into the surrounding medium by an antenna 5.

Reflections of the trahnsmitted pulses are received by the same antenna 5 and passed through the TIR cell 4to the receiver 3. Where they pass through a filter 3A and a mixer 313 where they are mixed with a frequency from a local oscillator 3C. Waveform G on Figure 2 shows signals received from two targets at different ranges. These are separated by a range gate 12 into channels 13 and 14for short range and long range returns respectively. The short range channel 13 incorporates a filter 15 which filters out the frequency swept pulses to leave the short unswept pulses as shown at H. These are detected by the circuit 16 and then passed through a moving target indicator circuit 17, which can operate according to any known principle, for example, using additional range gates and Dopper filters or using a delay line canceller. The effect of the circuit 17 is to remove all pulses derived from stationary targets which are assumed not to be of interest. The circuit 17 receives a reference timing signal from the timer 1. The output of the moving target indicator circuit 17 is passed to a video display system 18, which also receives timing signals from the timer 1, to control the timing of successive scans.

The channel 14 has a filter 19 which removes the short unswept pulses leaving only the swept pulses as shown at 1 on Figure 2. These swept pulses are subjected to pulse compression in a circuit 20, the output of which is shown at J. The output J is subjected to a treatment similar to that of pulses H, by a detector 21, a moving target indicator circuit 22 and a video display 23.

It will readily be seen from Figure 2F that, because of the reversal of the order of the long and short pulses in successive pulse-pairs, the period between successive short pulses and successive long pulses can have one of two values. There are thus two pulse repetition frequencies. This is known as p.r.f. stag- gering and is introduced deliberately in known radar systems to prevent blind spots in a manner known perse, e.g., as explained in the 1962 edition of Skolnik's Book 1ntroduction to radar systems" on page 131. The timing signals supplied to the video display and the MT] circuits need to take this staggerd p.r.f. into consideraton.

Another advantage accrues from the use of the illustrated radar when there is a large source of clutter at a slightly closer or further range than a small close-range target. The return from such a source of clutter may saturate the common parts 3A, 313 and 3C of the receiver during periods when some of the short return pulses are being received from the target. These short pulse returns are thus lost in the viewer. However, since some of the long, swept. pulses precede, and other follow the associated short pulse, it is assured that not all of the short pulse returns are lost.

It will be appreciated that the described embodiment of the invention is only one of many different forms thatthe invention can take. The invention is, for example, applicable to sonar systems. Also, whereas in the illustrated system the different types of pulse differ in frequency and in length, it would be possible in an alternative system for them to differ only in frequency or only in length or to have some other distinguishing characteristic. Another possible modification would be for each group of pulses to incorporate three or more pulses some or all of which are of different types. Another modification 3 GB 2 085 252 A 3 would be for the transmitter to produce groups of pulses similar to those shown in Figure 2F but with the frequency sweep in opposite directions in adja cent groups. Another modification would be to use a single video system insted of the two systems shown at 18 and 23 in Figure 1 so that targets of all ranges are displayed on one screen. In such an arrangement, it would be possible for the range gate 12 to be positioned at the ends of the channels 13 and 14 instead of at the beginning. In some radar applications, it is not necessary to have a video display and the video system shown at 17 and 23 could be replaced by some form of digital proces sing system.

Claims

1. Radar apparatus as herein defined comprising a transmitter for transmitting a train of pulses including some of a first type and some of a second type, the transmitter including means for modulat ing some of the pulses so as to allow pulse compression on reception, characterised in that the transmitter is constructed and arranged to apply said modulation to adjacent pulses of said pulse train.

2. Radar apparatus accoding to claim land including a receiver adapted to distinguish between the different types of pulse and between pulses received after reflection from features in different range zones, the receiver, when in operation, using the first type of pulse to given an indication of the positions of features in a first range zone and the second type of pulse to give an indication of the positions of features in a second range zone; the receiver including a pulse compressor for compress- 100 ing the modulated pulses.

3. Radar apparatus according to claim 1 or2 wherein the pulses are arranged in groups and wherein the transmitter is adapted to apply said modulation to the last pulse of one group and to the first pulse of the next group.

4. Radar apparatus according to claim 1, 2 or 3 wherein the said modulation is a frequency sweep.

5. Radar apparatus according to claim 4 wherein the frequency sweep is in the same direction for each of said adjacent pulses.

6. Radar apparatus according to claim 4 wherein the frequency sweep is different for each of said adjacent pulses.

7. Radar apparatus according to claim 3 or any one of claims 4,5 or 6 when dependent on claim 3 wherein the transmitter is adapted to transmit in each alternate group of pulses, a modulated pulse of a first type, followed by a pulse of a second type and, in each other group of pulses, a pulse of the second type followed by a modulated pulse of the first type.

8. Radar apparatus according to any preceding claim adapted to operate at microwave wavelengths.

9. Radar apparatus according to claim 3 or any other preceding claim, when dependent on claim 3, wherein the pulses of each group are separated by a time space.

10. Radar apparatus according to claim 3 or any one of claims 4to 8, when dependent on claim 3, wherein the pulses of each group are not separated 130 by a time space.

11. Radar apparatus according to any preceding claim wherein the pulses of differenttypes have different lengths.

12. Radar apparatus according to claim 11 having means for distinguishing between the pulses of different types, means for distinguishing between pulses received from different range zones, and means for using the long pulses when received from long ranges and the short pulses when received from short ranges.

13. Radar apparatus according to any preceding claim wherein the train of pulses includes long and short pulses and includes a series of four consecu- tive and successive pulses the first and fourth of which are short, the second and third being modulated to allow pulse compression on reception.

14. Radar apparatus according to any preceding claim wherein the train of pulses includes long and short pulses and includes a series of four consecutive and successive pulses the first and fourth of which are long and the second and third of which are short.

15. Radar apparatus according to any preceding claim wherein the train of pulses includes long and short pulses and includes a series of four consecutive and successive pulses the second and third of which are short and the first and fourth of which are modulated to allow pulse compression on reception.

16. Radar apparatus according to any preceding claim wherein: the train of pulses includes long and short pulses; the first of said adjacent pulses is preceded by a consecutive short pulse; and the last of said adjacent pulses is followed by a consecutive short pulse.

17. Radar apparatus as herein defined comprising a transmitter adapted to transmit groups of pulses each group containing pulses of different types and to modulate the last pulse of one group and the first pulse of the next group in such a way that these pulses can be pulse compressed on reception.

18. Radar apparatus according to claim 17 including a receiver adapted to distinguish between the different types of pulse and between pulses received after reflection from features in different range zones, the receiver, when in operation, using a first type of pulse to give an indication of the positions of features in a first range zone and a second type of pulse to give an indication of the positions of features in a second range zone; the receiver including a pulse compressor for compressing the modulated pulses.

19. Radar apparatus according to claim 17 or 17 wherein the transmitter is adapted to apply said modulation in the form of a frequency sweep.

20. Radar apparatus according to claim 19 wherein the said first and last pulses are of the same length and the transmitter is adapted to frequency sweep the said last and first pulses in the same direction.

21. Radar apparatus according to claim 19 wherein the said first and last pulses are respectively long and short pulses and the transmitter is adapted to frequency sweep them in different ways.

4 GB 2 085 252 A 4

22. Radar apparatus according to any of claims 17 to 21 wherein the transmitter is adapted to transmit, in each alternate group of pulses, a modulated pulse of a first type followed by a pulse of a second type and, in each other group of pulses, a pulse of the second type followed by a modulated pulse of the first type.

23. Radar apparatus according to any of claims 17 to 22 adapted to operate at microwave wavelengths.

24. Radar apparatus according to any of claims 17 to 23 wherein the pulses of each group are separated by a time space.

25. Radar apparatus according to any of claims 17 to 23 wherein the pulses of each group have no time space between them.

26. Radar apparatus according to any of claims 17 to 25 wherein the pulses of different types have different lengths.

27. Radar apparatus according to claim 26 including a receiver having means for distinguishing between the pulses of different types, means for distinguishing between pulses received from different range zones, and means for using the long pulses when received from long ranges and the short pulses when received from short ranges.

28. Radar apparatus according to anyone of claims 17 to 27 wherein the train of pulses includes long and short pulses and includes a series of four consecutive and successive pulses the first and fourth of which are short, the second and third being modulated to allow pulse compression on reception.

29. Radar apparatus according to anyone of claims 17 to 28 wherein the train of pulses includes long and short pulses and includes a series of four consecutive and successive pulses the first and fourth of which are long and the second and third of which are short.

30. Radar apparatus according to anyone of claims 17 to 29 wherein the train of pulses includes long and short pulses and includes a series of four consecutive and successive plates the second and third of which are short and the first and fourth of which are modulated to allow pulse compression in reception.

31. Radar apparatus according to anyone of claims 17 to 30 wherein the train of pulses includes long and short pulses; the first of said adjacent pulses is preceded by a consecutive short pulse; and the last of said adjacent pulses is followed by a consecutive short pulse.

32. Radar apparatus as herein defined comprising: a transmitter adapted to transmit during groups of periods each group containing periods of first and second differenttypes of transmission and to modulate or code the transmission during the last period of one group and the first period of the next group in such a way that the signal transmitted during these first and last periods can be pulse compressed on reception and a receiver adapted to distinguish between received signals of the said different types and between signals received after reflection from features in first and second range zones, the receiver, when in operation, using received signals of the firsttype to give an indication of the positions of features in the first range zone and received signals of the second type to give an indication of the positions of features in the second range zone, the receiver including a pulse compressor for compress- ing received signals which are modulated or coded as aforesaid.

33. Radar apparatus as herein defined comprising a transmitter for producing a transmission including some period when there is a transmission of a first type and other periods when there is a transmission of a second type, the transmitter including means for coding or modulating the signals during some of the periods so as to allow pulse compression on reception, and a receiver adapted to distinguish between received signals of the respective different types and between signals received after reflection from different range zones, the receiver, when in operation, using signals of the first type to give an indication of the positions of features in a first range zone and the signals of the second type to give an indication of the positions of features in a second range zone; the receiver including a pulse compressor for compressing those received signals which are coded or modulated as aforesaid and the transmitter being constructed and arranged so as to apply said coding or modulation to adjacent signal periods.

34. Radar apparatus according to claim land constructed to transmit a train of pulses substantial- ly as described with reference to Figure 2G.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1982. Published byThe Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

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