GB2305044A

GB2305044A - Radar receiver

Info

Publication number: GB2305044A
Application number: GB8412990A
Authority: GB
Inventors: Stuart John Ingram
Original assignee: Racal Radar & Displays Ltd
Current assignee: Racal Radar & Displays Ltd
Priority date: 1983-06-20
Filing date: 1984-05-21
Publication date: 1997-03-26
Anticipated expiration: 2004-05-21
Also published as: GB8412990D0; GB2305044B

Abstract

A radar arrangement is described which is capable of recognising different types of target (e.g. classes of helicopters) which each produce a characteristic frequency-time variation in the reflected radar signal. The received radar signal is time-compressed and multiple replicas are produced each of which, after frequency conversion in a mixer (29), is mixed with a particular one of a set of predetermined frequency sweep signals produced by a controllable chirp generator (34). The predetermined frequency sweeps produced by the chirp generator each have a form such that, when mixed with a processed reflected signal from a particular one of the target types to be recognised, the result is the production of a composite frequency sweep which is the mirror image of the frequency/time characteristic of a signal processing unit (35) to which the output of the mixer is fed. When a match is made the output from the signal processing unit (38) is a pulse.

Description

Tne present invention relates to radar receivers particularly for high frequency radar systems using wavelengths in the millimetre range. In such radar systems a radar pulse reflected from a target may show some structure depending on the type of target, for example a helicopter.

The structure introduced into the reflected signal may result in spreading of the pulse and a variation in the frequency with time within the received signal.

Because of this spreading the magnitude of the received signal at each wavelength will be reduced.

The type of frequency time structure of the received signal can also typically contain information about the target.

The technical problem which this invention is intended to solve is the provision of a receiver for a high frequency radar system that has an improved gain.

Moreover the invention is intended to solve the technical problem of detecting the type of frequency time structure in the received signal.

The present invention accordingly provides a radar receiver for a high frequency radar system comprising means for receiving a radar signal reflected from a target which received signal has a frequency variation in time which is dependent on the target, wherein the receiver further comprises means for applying a set of modifications to the received signal, means for delaying each modified signal as a fixed function of frequency, and means for identifying the particular modification of the set (if any) which produces the largest substantially pulse-form output above a predetermined threshold from said delay means.

When the frequency time characteristic of the delay means is a time inverted replica of the frequency time variation of the modified signal the whole received signal energy is compressed into a pulse thereby substantially increasing the gain. Identification of the appropriate signal modification to produce this result enables the original frequency structure to be assessed.

The invention further provides a radar receiver for a high frequency radar system comprising means for receiving a radar signal reflected from a target which received signal has a frequency variation in time which is dependent on the target, wherein the receiver further comprises means for modifying the received signal in a number of different ways to provide a set of resulting signals, frequency dependent delay means for receiving each of the resulting signals and and producing an output signal, whereby when the frequency variation in time of a resulting signal is substantially a time inverted replica of the frequency time characteristic of said delay means the output signal duration tends to zero, and means for determining which modified signal produces an output signal from said delay means which exceeds a predetermined magnitude.

The invention also provides a method of processing a received radar signal which has a frequency variation in time comprising the steps of modifying the received signal and selecting the type of modification (if any) which provides a substantially pulse output after a predetermined frequency variable delay to establish information regarding the frequency variation of the received signal.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a circuit diagram of a radar receiver; Figures 2, 3 and 4 show frequency/time characteristics of radar signals to be recognised by the receiver; and Figures 5 and 6 show frequency/time characteristics of signals occurring in the receiver.

The circuit arrangement now to be described with reference to Figure 1 is for recognising radar signals containing frequency time structures of a predetermined class which in one particular example consists of a signal sweeping through a range of frequencies. The frequency sweep may be in either direction and may or may not be linear. Such a signal may be produced as a result of radar reflection from the moving blades of a helicopter, the transmitted radar signal having, for example, a fixed wavelength of the order of 4 mm. Bowever, it is to be understood that the circuit arrangement to be described is by means restricted to the recognition of such signals reflected by helicopter blades.

The circuit arrangement to be described may be precedd by conventional radar receiving circuitry shown diagrammatically by the block 10 which receives and processes the signal received by the radar antenna B so as to produce a signal having the form shown in Figure 2. In Figure 2, waveform A shows one form of signal which may be produced by the reflection of a fixed frequency transmitted signal by a moving helicopter blade of a stationary (that is, hovering) helicopter. As there shown, the signal comprises a frequency sweep over a frequency band of (in this example) about 15 kHz to 110 kHz.The length of time from to to tl may be of the order of 0.5 to 4 milliseconds, the length of this time period being affected by such factors as the position, attitude and movement of the helicopter target and the shape and size of the rotor assembly. The frequency sweep is caused by the doppler shift resulting from the movement of the helicopter blade.

If the helicopter is not hovering but moving towards the radar antenna, there will be a resulting doppler shift imposed on the signal A, causing the signal to have the form shown at B, for example, where the signal has been shifted so that its frequency range extends from 50 kHz to lbO kHz, say. If the helicopter is moving in the opposite direction, that is, away from the radar antenna, there will clearly be a doppler shift in the opposite direction but such target movement will normally be of less practical interest.

Figure 2 shows signals A and B as being substantially linear. However, they may not necessarily have this shape but have the shapes shown at A and B in Figure 3, that is, be in the form of non-linear frequency sweeps. Figure 4 shows further versions of the signals A and B where they are non-linear over a part of the frequency range and substantially linear over the remaining part.

In each case, however, the signals are processed and frequency-shifted to a suitable band 8o as to be received on a line 12 by the circuit arrangement now to be more specifically described with reference to Figure 1.

The signals arriving on line 12 are converted into digital tom by means of an analogue-to-digital converter 14 which is sampled at a predetermined rate (for example 375 kHz) by means of sampling signals on a line 16, and the digital samples are fed into and stored in a buffer 18 at an input clock rate controlled by a clock input on one of a pair of lines 24. When the buffer 18 contains the samples representing the input during a suitable length of time, say 2ms, then these samples are passed from buffer 18 under the control of a clock signal received the other of the pair of lines 24, to a digital-toanalogue converter 22 at a clock rate of, say, 15 MHz.

During this reading-out process samples continue to be stored in buffer 18 without interruption. In this way, the incoming signal on line 12 has been compressed in time by a factor of 40. This enables the time-compressed version of each input. signal on line 12 to be fed out on line 26 a number of times in succession from buffer 18, before the next incoming signal on line 12 has been digitised and fed into the buffer 18. In practice this number of times will be typically less than forty. Each of these timecompressed replicas of the incoming signal will of course have a frequency sweep corresponding to that of the incoming signal. The high frequency components of the time-compressed replica introduced by the sampling process are removed by a low-pass filter 28.

The time-compressed signal is then translated to a suitable intermediate carrier frequency by mixing it in a mixer 29 with the output of an oscillator 30 and one sideband, only, is selected by filter 31. The resultant single sideband modulated signal is applied to a mixer 32.

In the mixer 32, the signal is mixed with a linearly frequency modulated or chirp signal received on a line 33 from a signal or chirp generator 34. The signal generator 34 can be controlled by means of control signals on a line 36 so that the sweep signal on line 33 has any one of a number of predetermined frequency sweeps, that is, any one of a number of frequency sweeps of predetermined rate of change (which need not be constant) between predetermined values, as will be described in more detail below. The timing of the frequency sweeps relative to the received signal may also be varied.

The resultant output of the mixer 32 is fed into a processing unit 38 which is a frequency dependent delay means and may be a linearly dispersive delay line (of any suitable type, e.g. of a surface acoustic wave type) having a fixed and known frequency/time characteristic to be described in more detail below.

The output of the processing unit 38 is fed into a detector unit shown diagrammatically as a rectifying diode 40 and thence into a threshold comparator 42 which produces an output on a line 44 when the output of detector 40 exceeds a predetermined threshold.

The operation of the circuit arrangement will now be considered in more detail below with reference to Figure 5. As explained, a replica of each incoming signal emerges from the low pass filter 28 several times in succession. Figure 5A shows one form which this replica can have after being translated in frequency by the mixer 29; that is, Figure 5A shows the output of the filter 31. Each time a replica of the signal on line 36 is changed so that the sweep signal on line 33 with which it is mixed in the mixer 32 is a different one of a set of predetermined signals which are related to the possible frequency time structures of the received signal. Figure 5B shows one such member of the class.The output of mixer 32 will of course be a signal having a composite frequency sweep made up of a combination of the frequency sweeps of the replica of the input signal and of the sweep signal on line 33, as shown for example in Figure SC. Figure 6 shows one form which the characteristic of the processing unit 38 may have.

When the frequency sweep of the composite signal (Fig.SC) from mixer 32 is the "mirror images of the characteristic shown in Figure 6 of the frequency sweep of the signal processing unit 38 (that is, the time-reversed replica), the result will be the production by the unit 38 of a sharp pulse of maximum amplitude. The pulse is produced because the low frequencies at the beginning of the composite output signal from mixer 32 are delayed, by the signal processing unit 38, relative to the higher frequencies at the end of the signal. This pulse is of much greater magnitude than the original received signal and therefore production of a pulse from the received signal increases the gain.

If the frequency/time characteristic (Fig.5C) of the composite output signal of mixer 32 is different from the characteristic shown in Figure 6, the unit 38 will not produce a sharp output pulse but a pulse of longer duration and correspondingly lower amplitude.

Therefore, the threshold comparator 42 ensures that an output signal on line 44 is only produced when the frequency sweep of the composite output signal produced by the mixer 30 is sufficiently close to that shown in Figure 6. In this way, the circuit arrangement can be set up so as to be capable of recognising different members of a predetermined class of radar targets (that is, various types of helicopter in a variety of positions or executing various manoeuvres, in the particular example being considered). Each defined member of the class is related to the frequency/time characteristic of a particular one of the sweep signals which can be produced by the signal generator 34 (relative to the sweep characteristic of the signal processing unit 38).

It will be appreciated that the circuit arrangement can be modified to recognise radar signals having decreasing, instead of increasing, frequency sweeps.

In such cases, the frequency sweeps of the sweep signals on line 33 would, in general, be decreasing frequency sweeps instead of increasing frequency sweeps, and the frequency/time characteristic of the signal processing unit 38 would, in general, be an increasing frequency sweep instead of a decreasing frequency sweep.

The control signals on line 36 may be generated in any suitable way. Advantageously, they are generated by a control unit operating under control of the output clock signal one of the lines 24.

The threshold comparator 42 may be replaced by a circuit which stores the actual magnitude of each pulse produced by the detector 40 so as to enable a determination to be made of the particular sweep signal on line 33 which produces a composite signal having the best match with the characteristic shown in Figure 6.

The chirp generator 34 may take the form of a digital memory containing samples of a plurality of predetermined chirps, which need not produce linear frequency sweeps, the samples relating to any particular chirp being.selected by the signals on line 36 and then preferably output through suitable filtering and thence on to line 33. Alternatively, the same samples of a single chirp may be reproduced at different rates (not necessarily linear rates), again under control, of a signal on line 36 and again output through suitable filtering, to produce different chirp slopes.

The circuit arrangement may also be modified by ommitting or replacing the signal generator 34 by a signal generator producing only a single, predetermined, frequency sweep, and replacing the processing unit 38 with a bank of such circuits, each haviiig a frequency/time characteristic of the same general form as shown in Figure 5A but with different parameter values, these units being connected in parallel to the output of mixer 32. These characteristics will again be such that a number of different members of a particular class of targets giving a swept frequency reflected radar signal could be recognised. Each of the signal processing units would be connected to a detecting circuit and a threshold comparator.Such an arrangement enables the recognition process to be carried out in parallel, rather than serially as the case for the arrangement shown in Figure 1, but it uses considerably more hardware. In such a case, the chirp generator 34 could take the form of a linearly dispersive delay line of appropriate characteristic to which a suitable impulse is periodically applied under the control of line 36.

In a further modification, in which the chirp generator 34 may be omitted or be of a type which produces a single predetermined chirp, the rate at which the sampling signals are produced under control of the output clock signal on one of the lines 24 can be varied as the signal is read out so as to modify the slope of the signal produced on line 26 and thus to alter the characteristic shown in Figure 5A. In this way again, therefore, the output characteristic (Fig.5C) can be modified until it matches the characteristic (Fig.6) of the processing unit 38.

It will be understood that while the time compression carried out by the analogue-to-digital converter 14, buffer 18 and the digital-to-analogue converter 22 is advantageous, it is not essential.

The processing of the received signals has been described subsequent to the signal compression as being carried out - using analogue components.

However, it will be appreciated that digital systems could be used for these purposes.

Claims

CLAIPS

1. A radar receiver for a high frequency radar system comprising means for receiving a radar signal reflected from a target which received signal has a frequency variation in time which is dependent on the target, means for applying a set of modifications to the received signal, means for delaying the modified signal as a fixed function of frequency, and means for identifying the particular modification of the set (if any) which produces the largest substantially pulse output above a predetermined threshold from said delay means.

2. A receiver according to claim 1, wherein the modifying means comprises means for mixing the received signal with each of a set of selected signals.

3. A receiver according to claim 2, wherein said mixing means comprises means for mixing the received signal with each of the selected signals in parallel and the delay means comprises a corresponding plurality of delay devices for each output of the mixing means.

4. A receiver according to claim 2, further comprising means for producing a time compressed sequence of replicas of each received signal.

5. A receiver according to claim 1, further comprising means for storing a plurality of digital samples representing each received signal, and said modifying means comprises means for reading out said stored samples at selected variable rates.

6. A radar receiver for a high frequency radar system comprising means for receiving a radar signal reflected from a target which received signal has a frequency variation in time to be detected which is dependent on the target, means for modifying the received signal in a number of different ways to provide a set of resulting signals, frequency dependent delay means for receiving each of the resulting signals and producing an output signal, whereby when the frequency variation in time of a resulting signal is substantially a time inverted replica of the frequency time characteristic of said delay means the output signal duration tends to zero, and means for determining which modified signal produces an output signal from said delay means which exceeds a predetermined magnitude.

7. A receiver according to claim 6, further comprising means for time compressing the received signal to produce a plurality of compressed replicas.

8. A receiver according to claim 7, wherein said modifying means comprises means for generating sequentially a number of predetermined signals, and means for mixing each of said signals with a respective one of said replicas.

9. A receiver according to claim 8, wherein each of said predetermined signals is a linear or non-linear frequency sweep having a preselected timing relationship with respect to the replica with which it is mixed.

10. A method of processing a received radar signal which has a frequency variation in time comprising the steps of modifying the received signal and selecting the type of modification (if any) which provides a substantially pulse output after a predetermined frequency variable delay to establish information regarding the frequency variation of the received signal.

11. A radar receiver substantially as herein described with reference to the accompanying drawings.

12. A method of processing a received radar signal substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. A radar receiver comprising means for receiving a radar signal reflected from a target, which received signal has a frequency variation in time which is dependent on the target, means for applying at least one modification to the frequency/time characteristic of the received signal, means for delaying the or each modified signal as a function of frequency, the combined effect of the modifying means and delay means being to produce a set of reproductions of the received signal each of which has been subject to one of a set of modifications in its frequency/time characteristic, and means for identifying the particular modification of the set (if any) which produces either the largest output or an output above a predetermined threshold from said delay means.

3. A receiver according to claim 1 or 2, wherein said delay means comprises a plurality of parallel delay devices each with different frequency/time characteristic.

6. A radar receiver comprising means for receiving a radar signal reflected from a target, which received signal has a frequency/time characteristic to be detected which is dependent on the target, means for modifying the frequency/time characteristic of the received signal in a number of different ways to provide a set of resulting signals, frequency dependent delay means for receiving each of the resulting signals and producing an output signal, whereby when the frequency/time characteristic of a resulting signal is substantially a time inverted replica of the frequency/time characteristic of said delay means the output signal duration tends to zero, and means for determining which resulting signal produces an output signal from said delay means which exceeds a predetermined magnitude.

?O. A method of processing a received radar signal which has an unknown frequency/time characteristic, comprising the steps of modifying the frequency/time characteristic of the received signal in a number of different ways and selecting the way of modification (if any) which provides an output tending to a substantially zero duration after a frequency variable delay to establish information regarding the frequency/time characteristic of the received signal.

11. A radar receiver substantially as herein described with referent to the-accompanying drawings.