GB2265513A - Radar systems - Google Patents

Abstract

Polarised radar is used for vehicle detection and recognition. The transmitter includes aerial means (10) adapted for outputting and receiving left-hand circularly polarised electromagnetic radiation, and right-hand circularly polarised electromagnetic radiation, switch means (12) for continuously switching transmissions between left-hand and right-hand circularly polarised radiations, and transmitter frequency varying means 26 for varying the frequency of the transmitted radiation. The receiver employs the same aerial means (10), and includes first and second channels for receiving the left and right handed polarised radiation, respectively, mixers (18, 30) in each of the channels for combining the signals therein with signals from the local oscillator (22) to product IF signals, receiver frequency varying means (26) for varying the frequency of the local oscillator, multiplier means (36) for multiplying the IF signals together to produce an output signal, and resonance detection means for determining resonance in the output signal. The frequency of the transmitted radiation may be varied randomly. and/or chirped. <IMAGE>

Description

RADAR SYSTEMS The invention concerns improvements in or relating to radar systems, and provides a radar system receiver and transmitter.

Radar systems exploit the electromagnstic radiation reflecting properties of bodies in order to detect their presence and are used in for example military applications and air/sea rescue.

A problem encountered in such applications is reduced effectiveness due to clutter from surrounding man-made objects and natural background.

An object of the present invention is the provision of a radar system with reduced corruption due to clutter and which offers a degree of selectivity of size of object to be detected.

According to a first aspect of the invention there is provided a radar transmitter including aerial means adapted for outputting left-hand circularly polarised electromagnetic radiation and right-hand circularly polarised electromagnetic radiation, switch means for continuously switching trarsmission between left-hand and right-hand circularly polarised radiations, and transmitter frequency varying means for varying the frequency of the transmitted radiation.

The transmitter frequency varying means may include means for varying the frequency in discrete steps and may be combined with means for continuously varying the frequency within each step. The transmitter frequency varying means may additionally include a transmitter code generator for controlling the frequency variation in discrete steps according to an output code which may employ pseudo-random variation.

According to a second aspect of the invention there is provided a receiver for receiving radiation from the transmitter in accordance with the invention, wherein the receiver includes first and second channels for receiving left-hand circularly polarised radiation and right-hand circularly polarised radiation respectively, a mixer in each of the channels for combining the signals th in with signals from a local oscillator to produce IF (Intermediate Frequency) signals, receiver frequency varying means for varying the frequency of the local oscillator, means for multiplying the IF signals together to produce an output signal, and resonance detection means for determining resonance in the output signal.

The receiver frequency varying means may be arranged to vary the frequency of the local oscillator in steps and may be arranged to do so under the control of a receiver code generator which outputs a code for determining the magnitude of each step. The receiver code generator may have the ability to output a pseudo-random code.

The receiver preferably includes a sample and hold stage for detecting the maximum of an output from the mixer at each variation in frequency. The receiver preferably also includes a store for storing the maximum values held by the sample and hold stage. The receiver may also include an analysis stage for analysing the contents of the store. The analysis stage may additionally include a Fourier transform device which may be connected to a comparator stage which compares an output from the Fourier transform device with a knotm signature.

Preferably the transmitter and receiver are combined to provide a transceiver and the aerial means arranged for signal transmission and reception.

The invention will now be described by way of example only with reference to the accompanying drawings consisting of Fig 1 illustrating in block diagram form a radar system according to a first aspect of the invention, and Fig 2 illustrating in block diagram form the radar system of Fig 1 with additional features, common elements in Figs 1 and 2 being numbered identically.

The radar system of Fig 1 is for use in detecting vehicles on land.

The radar system operates by transmitting alternate pulses of right and left-hand circularly polarised electromagnetic radiation and, after each pulse, receiving co-polar and orthogonal (ortho -polarlcircularly polarised signals in separate channels and simultaneously processing these signals in order to identify the required vehicles.

A bi-polarised circular horn aerial, 10, is used both to transmit the pulses of radiation and to receive incoming signals.

The aerial, 10, transmits either right-hand circularly polarised waves or left-hand circularly polarised waves according to the setting of a polarise switch 12.

Electromagnetic radiation at a frequency of around 35 GRz ie:wavelength 8.6 mm is radiated from aerial 10 by the following process:- a 40 kHz pulse repetition frequency (PRF) generator 14 drives a step recovery diode (SRD) impulsor 16 which, in turn drives an IF surface acoustic wave (SAW) pulse generator 18.

The result is a 50 ns pulse of IF signals at a frequency, f, of at least 300 MHz. The IF signals are then up-mixed in a mixer 20 with signals from an agile stable local oscillator (STALL) 22 and passed through a filter 23 to reject the undesired mix frequency before being amplified by amplifier 24 to produce a 10 W pulse at 35 GHz.

The STALO 22 has a degree of frequency agility enabling it to step up or down in frequency. The stepping is under the control of a ladder stepper 26 which allows the STALO to jump frequency in 64 steps of 8 MEz ie:total of 512 MEz (1.46%) in 1.6 ms.

The polarise switch 12 is switched alternately between positions so that left-hand and right-hand circularly polarised pulses of 35 GIIz radiation are launched alternately from aerial 10.

In the intervals between transmission of pulses, the aerial 10 serves to receive incoming radiation. In order to do this co-polar signals (ie signals of the opposite, polarisation to those output in the previous pulse) and orthopolar signals (ie signals of the same polarisation as those output in the previous pulse) are extracted from separate ports on aerial 10. The co-polar signals are then passed through a co-polar down-mixer 28 and the ortho-polar signals are passed through an ortho-polar down-mixer, 30 both of which are connected to the STALL 22.The mixers 28 and 30 therefore output an IF signal of not less than 300 MEz. Where the incoming waves are reflected waves and the aerial 10 is moving relative to the source of reflection, the IF output from mixers 28 and 30 is Doppler-shifted by an amount, D, in frequency relative to freqency, f, output from the SAW generator 18. Doppler shift does not affect the operation of the radar system as the detection process is Doppler invariant.

In order to detect vehicles, the radar system of Fig 1 relies on the fact that the phase centre for reflection of lefthand circularly polarised radiation differs from the phase centre for reflection of right-hand circularly polarised radiation and also that the two phase centres fluctuate as the number of wavelengths propagated along a prime direction of the vehicle changes. When one additional wavelength is created by frequency agility, there is an excursion of phase change between left- and right-hand circularly polarised waves which may be translated into a cyclically varying detector signal.

This may be done by effectively multiplying together co-polar and ortho-polar radiation emanating from the area of the vehicle and detecting a resonance as the wavelength of the radiation changes. By performing a Fourier analysis on the multiplied signals, detection according to the nature of the vehicle is possible.

A further important factor is that naturally occurring objects are much less depolarising than are man-made objects.

Clutter from, for example, rain, is significantly reduced in the ortho-polar channel compared with the co-polar channel.

A larger signal/clutter ratio in the ortho-polar channel allows improved detection of reflections from man-made objects.

The signals output from co-polar downmixer 28 and orthopolar down-mixer 30 are then amplified in IF amplifiers 32 and 34 respectively. It is important that the phase dispersion in IF amplifiers 32 and 34 is identical and this may be ensured by having a single IF amplifier with co- and ortho-polar signals in segregated bandwidths.

It is important that the IF amplifiers have amplitude limiting means to significantly reduce the effect the pulse by pulse amplitude reflection from the target, and thereby optimise the phase sensitive detection.

After amplification in amplifiers 32 and 34 the orthoand co-polar channel signals are multiplied together in a multiplier 36 after first passing the orthogonal polar IF signal through a w/2 phase shifter 38. The purpose of the phase shifter is to reference phase detection to the centre point of the cyclic variation of phase change so that both positive and negative phase changes are detected with respect to the zero points and with respect to earth, thereby eliminating unwanted pulse amplitude and optimising the signal which is a relatively small fluctuation on the tops of the pulses. This improves efficiency of analogue to digital (A/D) conversion in subsequent parts of the system.

An output signal from multiplier 36 is then passed to a boxcar sample and hold 40 which holds the maximum value of the output from the multiplier. This value is converted to a digital signal in an A/D converter 42 and then passed to a store 44.

The radar system of Fig 1 is arranged so that the store 44 has 64 storage sites which store signals over a period of 1.6 ms. Thus the signals stored in a completely full store represent a full excursion of steps in frequency due to the ladder stepper 26. When store 44 is full a second store 46 becomes operative so that the signals resulting from a subsequent 64 steps in frequency are stored therein. When store 46 is full, store 44 again becomes operative.

A fast fourier transform (PET) section 48 connected to stores 44 and 46 so that as each store becomes full, a Fourier transform of its contents is performed. The result is output to a comparator stage 50 which is also connected to a memory 52.

The PET output which is characterised in frequency and amplitude components, is thus compared with known vehicle signatures which are stored in memory 52 in order to decide if a particular vehicle has been detected.

The EFT section 48 is arranged as a bank of 32 filters.

The first filter is arranged to coincide with the vehicle dimension, , which gives a single wavelength change in resolution on a full excursion of frequency steps.

This can be calculated using: X - Th = i 1 2 where B1 = the lowest radiation wavelength and 2 = the highest radiation wavelength available by frequency stepping0 Thus X(X2 - #1) X X1X212 X m 2 y = 1.46% 2 = 29.45 cm where AX = X2 - #1 and X = the approximate wavelength, 8.6 mm.

Thus it can be said that a vehicle of dimension 29.45 cm between scatter centres will give one cycle of resonance in the 1.6 my taken to give a full excursion of frequency steps ie this is equavelent to a faurier frequency of 625 Hz.

The maximum filter frequency which may be detected is given by the Nyquist limit. Since pulses of radiation are emitted at a frequency of 40 kHz, the Nyquist limit gives a maximum filter frequency of 20 kHz. This falls in the 20 kllz = 32nd filter of the bank 625 Hz Since pulses of 100 ns duration are used, the resolution of detection ie the dimension at which the beginning of a pulse reflected from the rear of a reflecting body coincides with the end of the pulse reflected from the front of a reflecting body is 7.5 m. This is the maximum dimension to be detected by the radar system of Fig 1. The 7.5 m dimension is therefore detected in the 7.5 m #25th filter of the bank 29.45 cm Vehicles with prime reflecting directions longer than 7.5 m will not fall in the bank and will not be detected.

Vehicles with prime reflecting dimensions smaller than 7.5 m may be detected by concentrating on the appropriate filter between the first and the twenty fifth.

A general vehicle will have several prime reflecting directions and its full signature will be distributed across the filter bank. By ignoring the lower number filters, larger vehicles may be detected and by ignoring the larger number filters smaller vehicles may be detected.

In order to cover a large area of terrain the aerial 10 may be given an ability to scan.

Where there is a requirement for a degree of security in the radar system, for example if there is likelihood of jamming, the system of Fig 2 may provide this.

The Fig 2 system retains the basic Fig 1 configurator but introduces in addition an IF SAW 1chirp! device 54 which has the effect of continuously varying the IF of the pulses output from the IF SAW generator 18. The result is the transmission of pulses from aerial 10 with continuously varying frequencies which are more difficult to track than pulses of constant frequency. In order to reverse the chirping on the received waves a first SAW compress stage 56 is inserted between IF amplifier 32 and multiplier 36 and a second SAW compress stage 58 between IF amplifier 34 and multiplier 36.

In addition to the provision for chirping a further security device is introduced in the form of a pseudo-random code (PRC) generator 60 connected to the output of ladder stepper 26. The PRC generator depicted in Fig 2 is a 26-1 PRO. The PRC has the effect of randomising the steps in frequency made by the agile STALO 22 so that the frequency of waves output by aerial 10 are less easy to track than if the step increases were made consecutively.

In order to compensate for the randomisation as the received signals, a PRC assemble stage 62 is introduced between stores 44 and 46 and the PET section 48.

The radar systems of Fig 1 and Fig 2 therefore provide a detection system which is doppler invariant and by use of the ortho-polar channel, gives an improved signal/clutter ratio.

In addition, the system exploits the fact that natural clutter sources, for example rain, land-bound areas of water, terrain consist of random scattering centres which do not produce resonance when detected by combining oppositely polarised radiation pulses. Man-made objects do, however, produce resonance. By using the frequency agility, the ratio of ortho-polar receiver signal to clutter may be improved, for example, by 15 dB. The ortho-polar signal may therefore be used as a relatively clear reference for phase detection of the co-polar signal.

The invention is not confined to the details of the above embodiments. The values of the parameters quoted, for example, may be varied to suit requirements. To illustrate this, 35 GHz was quoted as the transmission frequency, for instance.

This may be varied according to available hardware. Pulses of 50 ns duration were used in the Fig 1 and Fig 2 embodiments.

Shorter or longer pulses may, however, be employed to suit requirements.

The ladder stepper of Figs 1 and 2 used 64 frequency steps.

Either more or fewer could, however, be used.

The radar systems of Figs 1 andy2 were for use in tracking vehicles in surrounding land. The system is equally applicable to detection of bodies at sea and would be particularly useful in overcoming sea clutter.

The system may be incorporated in a missile and the comparator stage 50 connected to its guidance system to enable the missile to be guided onto a detected target.

Claims (13)

1. A radar transmitter including aerial means adapted for outputting left-hand circularly polarised electromagnetic radiation and right-hand circularly polarised electromagnetic radiation, switch means for continuously switching transmission between left-hand and right-hand circularly polarised radiations, and transmitter frequency varying means for varying the frequency of the transmitted radiation.

2. A radar transmitter as claimed in claim 1 wherein the transmitter frequency varying means includes means for varying the frequency in discrete steps.

3. A radar transmitter as claimed in claim 2 wherein the means for varying the frequency in discrete steps is combined with means for continuously varying the frequency within each step.

4. A radar transmitter as claimed in claim 3 wherein the transmitter frequency varying means further includes a transmitter code generator for controlling the frequency variation in discrete steps according to an output code.

5. A radar transmitter as claimed in claim 4 wherein the code generator employs a pseudo-random code.

6. A receiver for receiving radiation from a radar transmitter as claimed in any of the preceding claims wherein the receiver includes first and second channels for receiving left-hand circularly polarised radiation and right-hand circularly polarised radiation respectively, a mixer in each of the channels for combining the signals therein with signals from a local oscillator to produce IF (Intermediate Frequency) signals, receiver frequency varying means for varying the frequency of the local oscillator, means for multiplying the IF signals together to produce an output signal, and resonance detection means for determining resonance in the output signal.

7. A receiver as claimed in claim 6 wherein the receiver frequency varying means is arranged to vary the frequency of the local oscillator in steps.

8. A receiver as claimed in claim 7 wherein the receiver frequency varying means is controlled by a receiver code generator which outputs a code for determining the magnitude of each step.

9. A receiver as claimed in claim 8 wherein the code generator employs a pseudo-random code.

10. A receiver as claimed in any one of claims 6 to 9 further including a sample and hold stage for detecting the maximum of an output from the mixer at each variation in frequency.

11. A receiver as claimed in claim 10 further including a store for storing the maximum values held by the sample-andhold stage.

12. A receiver as claimed in claim 11 further including an analysis stage for analysing the contents of the store.

13. A radar system substantially as described herein with reference to the drawings.

13. A receiver as claimed in claim 12 wherein the analysis stage includes a Fourier transform device which is connected to a comparator stage which compares an output from the Fourier transform device with a known signature.

14. A transceiver including a transmitter as claimed in any one of claims 1 to 5 and a receiver as claimed in any one of claims 6 to 13.

15. A transceiver substantially as described herein with reference to the drawings.

CLAIMS 1. A radar system including aerial means adapted for transmiting left-hand circularly polarised electromagnetic radiation and right-hand circularly polarised electromagnetic radiation, switch means for continuously switching transmission between left-hand and right-hand circularly polarised radiations: means for varying the frequency of the transmitted radiation, means for receiving radiation including first and second channels for receiving left-hand circularly polarised radiation and righthand circularly polarised radiation respectively, a mixer in each of the channels for combining the signals therein with signals from a local oscillator to produce IF (Intermediate Frequency) signals, means for varying the frequency of the local oscillator, means for multiplying the IF signals together to produce an output signal, and resonance detection means for determining resonance in the output signal.

2. A radar system as claimed in claim 1 wherein the means for varying the frequency of the transmitted radiation includes means for varying the frequency in discrete steps.

3. A radar system as claimed in claim 2 wherein the means for varying the frequency in discrete steps is combined with means for continuously varying the frequency within each step.

4. A radar system as claimed in claim 3 wherein the means for varying the frequency of the transmitted radiation further includes a code generator for controlling the frequency variation in discrete steps according to an output code.

5. A radar system as claimed in claim 4 wherein the code generator employs a pseudo-random code.

6 A radar system as claimed in claim 1 wherein the means for varying the frequency of the local oscillator is arranged to vary the frequency in steps.

7. A radar system as claimed in claim 6 wherein the means for varying the frequency of the local oscillator is controlled by a code generator which outputs a code for determining the magnitude of each step.

8. A radar system as claimed in claim 7 wherein the code generator employs a pseudo-random code.

9. A radar system as claimed in any preceding claim further including a sample and hold stage for detecting the maximum of an output from the mixer at each variation in frequency.

10. A radar system as claimed in claim 9 further including a store for storing the maximum values held by the sample-and-hold stage.

11. A radar system as claimed in claim 10 further including an analysis stage for analysing the contents of the store.

12. A radar system as claimed in claim 11 wherein the analysis stage includes a Fourier transform device which is connected to a comparator stage which compares an output from the Fourier transform device with a known signature.