GB2080654A - Improvements in or relating to radar systems - Google Patents

Abstract

A radar system, e.g. for distinguishing a target from clutter, comprises a transmitter TX arranged to transmit a signal comprising two orthogonally polarised waves and a receiver system RX for receiving from targets the signal after reflection, the receiver system comprising means 18 responsive to the received signal for providing, for each of the waves, a resultant signal derived in dependence upon the phase difference between the copolar and cross polar constituents of the received signal, and means 21 for comparing the resultant signals and for providing, in dependence upon the results of the comparison, an output signal indicative of the character of a target. <IMAGE>

Description

SPECIFICATION Improvements in or relating to radar systems This invention relates to radar system and more particularly to a radar system having means for descriminating between different characters of target such as the discrimination of wanted (coherent) radar signal reflections from clutter echoes (diffuse reflections).

When a monochromatic electromagnetic wave is incident upon a target, scattering occurs and a back-scattered field is produced which has three parameters available for measurement. These are echo amplitude, a change of polarisation of the echo and a reduction in phase coherency of the echo. The latter two parameters do not seem to have been fully exploited in radar technology to date.

This invention seeks to provide a radar system which employs the above mentioned parametres to distinguish different characters of target.

According to the invention there is provided a radar system comprising a transmitter arranged to transmit a signal comprising two orthogonally polarised waves and a receiver system for receiving from targets the signal after reflection, the receiver system comprising means responsive to the received signal for providing, for each of the waves, a resultant signal derived in dependence upon the phase difference between the copolar and cross polar constituents of the received signal, means for comparing the resultant signals and for providing, in dependance upon the results of the comparison, an output signal indicative of the character of a target.

The means for comparing the resultant signals may comprise a subtractor and means for determining when the subtracted phase difference of the two resultant signals exceeds a predetermined value. The predetermined value may be one which distinguishes valid signals from clutter echoes.

In accordance with one aspect of the invention the transmitter may be arranged to transmit the two waves alternately. The receiver system may comprise two phase matched receivers arranged to receive the copolar or cross polar constituents of the received signal respectively and to provide output signals which are phase compared to provide the resultant signal. The receivers may be fed from an orthogonal mode transducer which transducer is fed from an unpolarised receiving aerial. The receiver may comprise means for delaying or storing the resultant signals of one wave to facilitate comparison with a subsequent resultant signal derived from the other wave. Sample and storage means may be provided for each wave for repetatively sampling and storing the phase difference signal of that wave for comparison.

In accordance with an alternative aspect of the invention the transmitter may be arranged to transmit the two waves simultaneously.

One way of enabling separation of the two waves at the receiver is for each wave to be transmitted at a different frequency. The two signal frequencies may be fed to a common non polarised aerial via an orthogonal mode transducer to provide a multiplexed signal. The receiver system in this case comprises a demultiplexing system for separating the received signal into separate copolar and cross polar components corresponding to each of the transmitted frequencies which components are detected by respective phase receivers prior to phase comparison.

Another way of enabling separation of the two waves at the receiver is for each wave to be transmitted at the same frequency but to comprise a different modulation coding in which case the receiver system is provided with discrimination means responsive to the modulation codes for separating the two waves. The modulation coding may comprise a pair of pseudo random bi-phase codes, the modulation on one polarisation being chosen to be orthogonal to that on the other polarisation.

The transmitted signal may be pulsed or continuous carrier.

In a refinement of the invention means may be provided for detecting when the ratio of copolar to cross polar exceeds a predetermined value when the cross polar level is below a predetermined value to provide a signal indicative that the received signal is to be considered to be a valid signal.

In order that the invention may be more easily understood the basic theory behind the invention will now be related and then some embodimenis of the invention will be described, by way of example only, with reference to the drawings, in which: Figure 1 is a schematic illustration of vector directions with respect to local surface elements, Figure 2 is a scatter diagram showing the distribution of values of difference between the relative phases of the copolar and cross polar constituents, received after transmission of two orthogonal waves, plotted against transmitter (scatterometer) depression angle, Figure 3 is a schematic block diagram of a radar system constructed in accordance with the invention, Figure 4 is a more detailed schematic block diagram of the receiver of the system of Figure 3, Figure 5 is a diagram and table showing the phase relationship between the relative phase of the two copolar components, resulting from two orthogonal transmitted waves, for distinguishing the presence of a wanted signal from a signal formed by clutter, Figure 6 is a block schematic diagram of the transmitter part of an alternative radar system constructed in accordance with the invention, and Figure 7 is a block schematic diagram of the receiver part of the system of Figure 6.

Depolarisation takes place whenever a scatterer is not symmetrical about either the E or Hvector of the incident field; and, in the present context, phase diffusion is increased whenever non-mirroring (non-specular) neighbouring scatterer surface elements are contributing to the back-scatter. There are, of course, phase differences due to returns from non-neighbouring specular surface elements but this type of phase difference is not relevant in the present context. From hence on, phase coherent and phase diffuse field components, as described, will be referred to simply as coherent and diffuse components.

In practice all surface elements of a non-planar non-perfectly conducting scatterer will produce both coherent and diffuse components. However, it has been shown (in Simpson, S.H.W. "Remote sensing of terrain with an X-band scatterometer having an adjustable complex polarisation response", PhD Thesis University of Sheffield, May 1979. (Reference 1)) that it is those surface elements orientated at angles of incidence between normal and the Brewster (or quasi-Brewster) angle that produce most of the coherent back-scatter, and those between the Brewster angle and grazing incidence that produce most of the diffuse back-scatter. The coherent and diffuse components will be precisely defined by means of mathematical expressions in the next section.

It is the difference between the Brewster angle for highly conducting scatterers (targets) and scatterers of relatively low conductivity (clutter), i.e. it is close to grazing incidents for targets but not for clutter, that results in different levels of coherency in the back-scattered signal thus facilitating discrimination. Assuming always, that there is sufficient scatterer/incident field asymmetry to generate measurable cross-polar (linear incident polarisations being assumed).

It is convenient to consider the total back-scattered field to be composed of three components, a coherent co-polar component ES,,, a diffuse co-polar component Esdiff and a diffuse cross-polar component ECdiff (there is no coherent cross-polarised field). These components have been shown in reference 1 and in (Beckmann, P., "The Depolarisation of EM Waves". Golem Press 1968. (Reference 2)) to be proportional to the following three integrals.

2# EsGO # ## cos # (R±R-)e-j2(k.r)# dS (3) 2# Esdiff # - ## cos # [cos2#(R++R-)]e-j2(k.r)# dS (4) 2# Ecdiff # - ## cos # [sin2#(R++R-)]e-j2(k.r)# dS (5) R+ and R- are the perpendicular and parallel Fresnel reflection coefficients respectively, see Figure 1.

These integrals have been derived by employing the Kinchoff approximation to the electromagnetic boundary conditions on the scatterer surface, and so they are only approximate when the scatterer has sharp points and edges. However, in the present context it is the relative phase of the three components that is important, and experiments detailed in reference 1 have shown that, in spite of their approximate nature, inegrals (3) to (5) have proved to be very useful for explaining the relative phase of co-polar field components as a function of incident polarisation as measured on several terrain samples at 9.7GHz.

For the purpose suggested in this application the following points are relevant.

a) Consideration of Lorentz location reciprocity suggested that ECdjff for one transmitted field polarisation will be equal to ECdjfffor a transmitted field having an orthogonal linear polarisation.

b) The coherent component ES,,, does not contain any polarisation dependent factor, and so will be reflected with the same phase irrespective of the polarisation of the incident field.

c) The diffuse components Esdjff and ECdjff contain polarisation dependent factors Cos2tp and Sin2w4 respectively, which produce phase reversals when 7t is replaced by ap + tithe orthogonal polarisation).

d) Ignoring the spatial phase change factor, which is identical for all three field components, the absolute phase determining factor for both diffuse components is identical (R++R-), so that they will always be in-phase.

e) The absolute phase determining factor of the coherent components (R± R-), is different from that of the diffuse components (R++R-), so that the phase difference between coherent and diffuse components can be any value between + 180 .

Typical radar targets which have relatively high conductivity, such as aircraft, give rise to predominantly coherent co-polar back-scatter, to the extent that EsCo > > Esdiff and integral (4) can be neglected. Provided sufficient cross-polar is still generated then the phase difference between the co-polar and cross-polar components can be measured.

Consideration of points (a), (b) and (c) previously mentioned suggest that a relative phase difference between copolar and cross polar components of 0 for one incident polarisation will change to + 0 + z for the orthogonal polarisation. This phenomenon may be useful as a signature for identifying conducting scatterers such as aircraft.

Reference 1 suggests that some types of terrain, at X-band frequencies, appear to have relatively low conductivity and few specular regions. A scatterer of this nature should produce relatively diffuse co-polar back-scatter, such that Esdiff > > EsCo and integral (3) can be neglected. Consideration of point (d) previously mentioned suggests that there will be approximately zero phase difference between co-polar and cross-polar diffuse components irrespective of the incident polarisation. This phenomenon may be useful as a signature for identifying some types of clutter, i.e. when there is negligible EsCo but large Esdjff and ECdiff, clutter is signified.

The only known source of experimental evidence of clutter induced diffusion has been acquired at 9.7GHz (Reference 1), so the amount of diffusion induced may be different at other frequencies. However, for 9.7GHz Figure 2 presents a scatter diagram of the relative phase of the co-polar components of the back-scattered field when horizontal and vertical incident fields are separately transmitted (using the reciprocal cross-polar component as a reference), equivalent to QPdjff (to be defined later).

The surfaces represented in Figure 2 are a wide cross-section of terrain types, e.g. Kale crop, potato crop, rough soil, grass of various types etc; some of which were gathered under differeing degrees of surface moisture (an increase of surface moisture content will increase coherency).

In Figure 2 (PPdiff points close to zero degrees represent highly diffuse back-scatter whilst points close to the represent relatively cohereent back-scatter. In general however, both diffuse and coherent back-scatter is present.

One way of exploiting the phenomena discussed is by employing a dual polarised radar transmitting alternate pulses in orthogonal linear polarisations, and receiving the co-polar and cross-polar components of the back-scattered field. Such an arrangement is illustrated in Figure 3. Using the reciprocal cross-polar components as a reference, the phase between the co-polar and cross-polar components can be measured for each pulse pair (a pulse pair consists of two adjacent pulses which are orthogonally polarised). The pair phase difference will then be characteristic of the relative coherency of the signal.

(ppdiff = Oc oe 0o = phase difference between co-polar and cross-polarforthe 1st (odd order) pulse.

0e = phase difference between co-polar and cross-polar for the 2nd (even order) pulse.

Pdiff = the pair phase difference.

If all the points in Figure 2 are regarded as the population of false alarm signals due to clutter from terrain, then a confidence interval of 90" centred about 180 encloses only 8 of the 43 points. A reduction of the confidence interval width would reduce the probability of false alarm still further, but it is expected that a limit will be reached where the reduction of false alarm is outweighed by a reduced probability of target detection. The optimum confidence interval will probably need to be found experimentally.

In Figure 3 a transmitter section of the radar system comprises two pulsed oscillators 10, 11, which are synchronised to produce alternate pulses. The outputs of the oscillators are used to drive respective modes of a high power wide-band orthogonal mode transducer 12 which is coupled via a circularwaveguide 13 to an unpolarised transmitter aerial 14. In this way the transmitter is arranged to transmit signal pulses alternate ones of which are orthogonally polarised.

The receiver section in Figure 3 comprises an unpolarised aerial 15 which is coupled via a circular waveguide 16 to an orthogonal mode transducer 17 which during one pulse routes the copolar component of the received signal to a first input of a subtractor 18 and the cross polar component of the received signal to a second input of the subtractor. During the next pulse, the transducer 17 will route the cross polar component of the received signal to the first input of the subtractor and the copolar component of the signal to the second input. In each case the signals on the first and second inputs are subtracted and fed via a respective output to a store 19 or 20 so that the stores each contain a phase difference value related to alternate pulses. The stored values are subtracted in a subtractor 21 and the subtracted value in representative of the received signal.

Vertical and horizontal polarizations can be employed. However the circuit in Fig u re 3 shows antennas polarised at 450 to the vertical and horizontal planes. This polarisation will, on average, generate the strongest cross-polar signal (because level terrain is always quasi-symmetrical about the vertical plane, so that these polarisations provide maximum asymmetry). However, the cross-polar component is a maximum because the Sin 2factor in integral (5) is maximised, but this is only achieved at the expense of minimising the cos 2factor in integral (4) (Esdiff), this factor is maximised under conditions of maximum symmetry.

Ideally both integral (4) & (5) need to be maximised, but since these conditions are in conflict, the optimum polarisations will need to be established experimentally, and those shown in Figure 3 are not meant to be binding. The necessary conditions are that ECdjff needs to be sufficiently large to afford reliable detection, the limits of which will be set by the receiver sensitivity and mechanical precision of the microwave front end, and Esdiff needs to be sufficiently large in comparison to Escoto ensure that 4)Pdiff for the clutter points lies outside the chosen confidence interval.

When a target is illuminated there may well be a very low level of cross-polar (due to the high surface conductivity), so that it may be necessary to have a level trigger on the cross-polar channel. That is, any target generating a ratio of co-polar to cross-polar above a value x, when the cross-polar level is below a minimum value, y should not be subjected to the confidence interval check, and classed as a target.

However, this may turn out to be an unnecessary complexity (not shown in Figure 3).

Another requirement will be precise polarisation alignment of the antennas. This is necessary to ensure that it is true cross-polar that is measured. If the alignment error is (pE, then the condition that must bew satisfied for accuracy is tan 4)2E cross-polar (true) E co-polar (true) In practice it has been found that for accurate measurements it is advantageous if |É| iEI is less than 1".

The receiver is shown in more detail in Figure 4.

The orthogonal mode transducer 17 feeds two phase matched receivers 22 and 23, giving l.F. outputs A < Á and B < ;PB Two automatic level control units 24, 25, feed a phase sensitive detector 18 with constant amplitude signals so that an output proportional to 4)A - XB = 0 is produced. For an "odd" pulse, O= 0o and for an "even" pulse ( = ee Table 1 shows how these outputs relate to the nature of the target(s). The output of the detector 18 is fed via a low pass filter 27 to the inputs of two sample and hold units 19 and 20.

Detectors 30 and 31 and video filters 32 and 33 allow the system to discriminate between "odd' and "even" received pulses. The outputs of the two filters are fed each to a respective input of a differential amplifier 34 which is coupled to a dual level threshold device 35. When the output from filter 32 is sufficient to exceed one threshold value the sample and hold unit 19 is triggered to sample the output of the detector 18 whilst when the output from filter 33 falls below the other threshold value the sample and hold unit 20 is triggered via an inverter 36.In this way the sample and hold units each store a value 0c or 0e which values are subtracted in a subtractor 21 and fed via a threshold window unit 37 to an output which can be used to operate an alarm, indicate on a radar display the presence or absence of valid signals or clutter or to disable the display in the presence of clutter. The filter 27 removes terms in n IF from the phase sensitive detector output.

A suitable transducer is described in the reference Fogel, R.L. "An orthogonal mode transducer", Nat Conv Rec IRE, pt 5, P.53, 1956.

An alternative system is illustrated schematically in Figures 6 & 7 where Figure 6 shows a transmitter and Figure 7 shows a receiver.

In Figure 6 two oscillators 41 and 42 produce slightly different frequencies f1 and f2 respectively and f2 = f1 + Af (where Af is no more than is required to permit separation offal and f2 by filtering). The two oscillators are coupled to respective inputs of an orthogonal mode transducer 43 which feeds a non polarised aerial 44. The two oscillators are preferably synchronously pulsed so that the transmitter radiates a signal pulse containing both frequencies but which are orthogonally polarised.

Figure 7 shows a transmitter for receiving the information as transmitted by the transmitter of Figure 6.

The signal is received by an unpolarised aerial 45 which is coupled to an orthogonal mode transducer 46 which provides two outputs one of which comprises the f1 copolar and f2 cross polar components of the received signal and the other of which contains the fl cross polar and f2 copolar components of the signal.

The outputs are fed to respective demultiplexers 47,48 which separate the f1 copolar, f1 cross polar, f2 cross polar and f2 copolar components of the signal, which components are fed to respective phase matched receivers 49, 50, 51, 52. The f1 components are fed via automatic level control units 53, 54 to a phase sensitive detector 55 whilst the f2 components are fed via automatic level control units 56, 57 to a phase sensitive detector 58.The two phase sensitive detectors produce output signals which are representative of the phase difference between copolar and cross polar components of a respective one of the frequencies f1, f2 and these are routed via low pass filters 60, 61 to a subtractor 62 the output of which is routed via a dual level threshold device 63 operating similarly to the threshold window unit 37 of Figure 4 which provides an output which can be used to operate an alarm, indicate on a radar display the presence or absence of valid signals or clutter or to disable the display in the presence of clutter.

The arrangement of Figures 6 and 7 is advantageous in that there is no need to sample and hold information from alternate pulses for subsequent comparison and this permits doubling of the effective pulse repetition frequency and elimination of problems which might arise due to changes in the scattering system (target and clutter) occurring from pulse to pulse.

Instead of employing two different frequencies, any suitable form of modulation coding for the orthogonal transmitter polarisations could be used, e.g. a pair of pseudo-random bi-phase codes, the modulation code on one polarisation being chosen to be orthogonal to that on the other polarisation.

In their most general form the systems previously described need not be pulsed but for many applications pulsed radar is most suitable.

Claims (21)

1. A radar system comprising a transmitter arranged to transmit a signal comprising two orthogonally polarised waves and a receiver system for receiving from targets the signal after reflection, the receiver system comprising means responsive to the received signal for providing, for each of the waves, a resultant signal derived in dependence upon the phase difference between the copolar and cross polar constituents of the received signal, means for comparing the resultant signals and for providing, in dependence upon the results of the comparison, an output signal indicative of the character of a target.

2. A system as claimed in claim 1, wherein the means for comparing the resultant signals comprises a subtractor and means for determining when the subtracted phase difference of the two resultant signals exceeds a predetermined value.

3. A system as claimed in claim 2, wherein said predetermined value is one which distinguishes valid signals from clutter echoes.

4. A system as claimed in any one of the preceding claims, wherein the transmitter is arranged to transmit the two waves alternately.

5. A system as claimed in any one of the preceding claims, wherein the receiver system comprises two phase matched receivers arranged to receive the copolar or cross polar constituents of the received signal respectively and to provide output signals which are phase compared to provide the resultant signal.

6. A system as claimed in claim 5, wherein the receivers are fed from an orthogonal mode transducer which transducer is fed from an unpolarised receiving aerial.

7. A system as claimed in any one of the preceding claims, wherein the receiver system comprises means for delaying the resultant signal of one wave to facilitate comparison with a subsequent resultant signal derived from the other wave.

8. A system as claimed in any one of claims 1 to 6, wherein the receiver system comprises means for storing the resultant signal of one wave to facilitate comparison with a subsequent resultant signal derived from the other wave.

9. A system as claimed in any one of claims 1 to 6, comprising sample and storage means for each wave for repetatively sampling and storing the phase difference signal of that wave for comparison.

10. A system claimed in any one of claims 1 to 3, wherein the transmitter is arranged to transmit the two waves simultaneously.

11. A system as claimed in claim 10, wherein each wave is transmitted at a different frequency.

12. A system as claimed in claim 11, wherein the two frequencies for transmission are fed to a common non polarised aerial via an orthogonal mode transducer to provide a multiplexed signal.

13. A system as claimed in claim 12, wherein the receiver system comprises a demultiplexing system for separating the received signal into separate copolar and cross polar components corresponding to each of the transmitted frequencies which components are detected by respective phase receivers prior to phase comparison.

14. A system as claimed in claim 10, wherein each wave is transmitted at the same frequency but comprises a different modulation coding and the receiver system comprises discrimination means responsive to the modulation codes for separating the two waves.

15. A system as claimed in claim 14, wherein the modulation coding comprises a pari of pseudo random bi-phase codes, the modulation on one polarisation being chosen to be orthogonal to that on the other polarisation.

16. A system as claimed in any one of the preceding claims, wherein the transmitted signal is of pulses carrier form.

17. A system as claimed in anyone of claims 1 to 5 wherein the transmitted signal is of continuous carrier form.

18. A system as claimed in any one of the preceding claims, wherein including means for detecting when the ratio of copolar to cross polar exceeds a predetermined value when the cross polar level is below a predetermined value to provide a signal indicative that the received signal is to be considered to be a valid signal.

19. A radar system substantially as described herein with reference to and as illustrated in Figure 3 of the drawings.

20. A radar system substantially as described herein with reference to, and as illustrated in, Figures 3 and 4 of the drawings.

21. A radar system substantially as described herein with reference to, and as illustrated in Figures 6 and 7 of the drawings.