GB1570279A

GB1570279A - Pulse radar apparatus

Info

Publication number: GB1570279A
Application number: GB4415675A
Authority: GB
Original assignee: Decca Ltd
Current assignee: Decca Ltd
Priority date: 1976-11-15
Filing date: 1976-11-15
Publication date: 1980-06-25

Description

(54) IMPROVEMENTS IN OR RELATING TO PULSE RADAR APPARATUS (71) We, DECCA LIMITED, a British Company, of Decca House, 9 Albert Embankment, London, SE1 7SW, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement.

This invention relates to pulse radar apparatus and is concerned more particularly with improving the detection of targets which give only a weak response.

The invention finds particular application in marine and coastal radars for the detection of small targets. One of the problems in detection of such targets arises from the fading of the return signals at the radar apparatus. Such fading, particularly in conditions of sea clutter, may make the detection of small targets difficult.

The present invention makes use of polarisation diversity. Most targets are of complex physical shape and hence have complex reflection characteristics. It has been found that, if a pulse radar signal is transmitted with one form of polarisation, for example vertical or horizontal, because of the complex reflection characteristics of targets, signals from most targets are received back at the radar not only with a polarisation component corresponding to that which has been transmitted but also with other polarisation components. We have moreover found that signals separately received on receivers arranged for reception of signals of diverse polarisation characteristics have different fading patterns.According to the present invention, in pulse radar apparatus having means for transmitting pulse signals which are linearly polarised, there are provided two senarate receiving means, each with a logarithmic amplifier and signal clipper, arranged to receive linearly polarised signals of polarisations respectively parallel and orthoonal to the transmitted signals to provide separate video signals in two video signal channels, means arranged for optionally switching a fast time constant circuit into at least one of said video signal chan nels, and combining means for additively combining the separate video signals in a single display.

The transmitted signals are linearly polarised, for example horizontally or vertically polarised. The receiving means are arranged to receive orthogonal linearly polarised signals with polarisation respectively parallel and orthogonal to the polarisation of the transmitter signals.

It is the general practice in modern radar apparatus to use directional antennae with a cyclically scanning beam. Typically the present invention might be applied to aradar system having a directional transmitting antenna, for example, a parabolic dish with a horn or other feed or a slotted waveguide aerial. Two antennae would be used for reception; as explained below, these may, to a large extent be physically combined and one of these may be used as the transmitting antenna. In one arrangement, for the reception of the signals; two separate slotted waveguide arrays might be employed; these could be arranged in a common flare or alternatively a parabolic.

dish with a horn feed might be employed.

Alternately, two separate horns may be used or a common horn may be used, e.g. a sqare horn, with a polarisation analyser to provide outputs corresponding to orthogonal linearly polarised signals. The two sets of received signals are fed to separate receiving channels which may employ however a common local oscillator.

If the received video signals in the two receiving channels from any given target were of equal ampltude and non-fading, direct addition of the video signals, e.g. in an AND gate would give a 3dB increase in signal strength. The video noise is uncorrelated and the combined video noise would cause a loss of 1.5 dB thereby giving a 1.5 dB overall gain in the combined signal-tonoise ratio. This improvement is small, However, because of the unequal fading characteristics of the signals on the two receiving channels, in practice much larger improvements than this are possible. Often one signal is giving video output signals from a target when the signals from that target in the other channel have faded. Thus targets may be detected on scans when, if reception were on one channel only, there would be no signal visible and hence the overall detection probability is considerably improved.A further important feature of this technique is that rain clutter returns, because of the substantially uniform nature of the raindrops, have a polarisation which is closely dependent on the polarisation of the transmitted signals. In particular, since linearly polarised signals are transmitted, the rain clutter returns on the cross polarised orthogonal channel are considerably lower than on a receiving channel for receiving signals of the same polarisation as the transmitted signals.

The video signals may be processed be- fore and/or after combining. Bottom-clipping the signals in each of the two channels reduces the noise. In one arrangement, the video signals, after bottom-clipping to reduce noise, are combined in an AND gate.

This gives good noise rejection and some rejection of rain clutter and other clutter.

Using linearly polarised signals, and bearing in mind that the rain clutter returns in the cross polarised receiving channel are considerably lower than in the other or parallel channel, it may be preferred to apply known CFAR (constant false alarm rate) technique, e.g. using a fast time constant circuit following the logarithmic receiver, to the signals in the parellel channel to reduce rain before combining the signals in the two channels in an AND gate, CFAR circuits may be used in one or both channels and may be switched independently in accordance with the radar range. Combination in an OR gate would give good rain reiection and the best passed ble detection probability. However, combination in an AND gate, i.e. additive combination, will give reduced detection probabilitv but even better rejection of rain and chaff.

The manner of processing the received signals may be arranged so that, for the first few miles of radar range, the signals are combined in an AND mode but, for greater ranges, they are combined in an OR mode. This is to give good rain rejection at close range where the signal return may be of high amnlitude but to give better detection posqihility at longer ranges by changing to the OR mode.

Obviously however there are many other possible modes of processing and/or combining the video signals in a range dependent manner, particularly when possibility is taken into account of range-dependent gain control in one or both channels. Digital processing of the video signals may be employed, for example, to give an M out of N integration and scan-to-scan correlation.

The following is a description of two embodiments of the invention, reference being made to the accompanying drawings, in which: Figure 1 illustrates diagrammatically one form of pulse radar apparatus particularly for use in a marine or coastal radar; Figure 2 is a diagram illustrating an alternative form of antenna system for use in the apparatus of Figure 1; and Figure 3 shows a further antenna system.

Referring to Figure 1 there is shown diagrammatically a magnetron 10 with a pulse modulator 11 for producing short duration microwave frequency pulses which, after passing through a coupler 12 providing an output 13 for automatic frequency control of a local oscillator 14, are fed through a TR coupler 15 and a waveguide feed 16 with a twin waveguide rotating joint 17 to a rotating antenna structure 18. This antenna structure, in the apparatus of Figure 1, comprises two slotted waveguide aerials 19 and 20. The pulse signals from the magnetron are radiated from the aerial 19 which is arranged to transmit horizontally polarised signals. The horizontally polarised components of the radar return signals are received on this aerial 19 and vertically polarised components of the return signals are received on aerial 20.These received signals from aerials 19 and 20 pass back separately through the twin waveguide joint. The horizontally polarised components are fed by waveguide 16 to the TR coupler 15 and pass then to a first receving channel 21. The vertically polarised components pass from the rotating joint 17 through a second waveguide 22 to a second receiving channel 23.

In receiving channel 21, the horizontally polarised return signals from the TR coupler 15 are fed to a mixer 25 for mixing the return signals with the output of the local oscillator 14 which is controlled in frequency by an AFC unit 27 making use of the aforementioned signals from coupler 12. These signals at the radiated frequency from coupler 12 are mixed in unit 27 with feedback from the local oscillator via couplers 38 and 28 to give the required control signal for controlling the frequency of the local oscillator 14. From the mixer 25, the intermediate frequency signals are amplified by a logarithmic intermediate frequency amplifier and detector 30 to give a video output and are then fed to a signal clipper and video amplifier 32 to give output video signals on a lead 33.These signals on lead 33 are derived from the horizontally polarised received signals.

The vertically polarised received signals in the receiving channel 23 are fed through a TR crystal protection unit 36 to a signal mixer 37 where they are mixed with output signals from the aforementioned local oscillator 14. The aforementioned coupler 38 is a 3 dB coupler provided for dividing the local oscillator output between the two mixers 25 and 37. From the mixer 37 the vertically polarised return signals are amplified and detected in another logarithmic intermediate frequency amplifier 40 to give a video output to a video amplifier and signal clipper 41 giving an output on a lead 42.

In this particular receiver, selective combining means, indicated diagrammatically at 43 are provided for combining the two video signals on leads 33 and 42 and the combined signals are fed to an output 45 leading to a display unit 46. The video signals may be combined, as has previously been described, in an AND gate. A particularly desirable mode in some case may be to combine the signals from the AND gate with the horizontally polarised signals on lead 33 having been passed through a fast time constant circuit to improve the rain rejection whilst the vertically polarised signals are not passed through such a circuit.

Figure 2 illustrates diagrammatically an alternative form of antenna system comprising a parabolic dish 50 with a square horn 51 leading to a hybrid coupler 52 for separating the horizontally and vertically polarised components. The horn is shown diagrammatically as being supported by stays 53. Such an antenna system may be used with radar apparatus as described with reference to Figure 1 in place of the slotted waveguide aerials 19, 20. The hybrid coupler 52 provides a signal corresponding to the horizontally polarised components in a waveguide 54 and a signal corresponding to the vertically polarised components in a waveguide 55. These waveguides 54, 55 lead to the twin waveguide joint 17 of Figure 1 to provide the received signals in waveguides 16, 22 respectively.

The radiated signals, in the embodiments described above, are linearly polarised.

As shown in Figure 4, two slotted waveguides 70, 71 may be fitted into a common flare 72. The flare 72 is shaped to provide the required vertical coverage pattern. The antenna is provided with an outer weatherproof covering shown diagrammatically by the dashed lines 73. In this particular example, the upper waveguide 70 is arranged to receive vertically polarised signals and the lower waveguide 71 is arranged to receive horizontally polarised signals. One of these waveguides only is used for transmission.

WHAT WE CLAIM IS: 1. Pulse radar apparatus having means for transmitting pulse signals which are linearly polarised, wherein there are provided two separate receiving means, each with a logarithmic amplifier and signal clipper, arranged to receive liearly polarised signals of polarisations respectively parallel and orthogonal to that of the transmitted signals to provide separate video signals in two video signal channels, means arranged for optionally switching a fast time constant circuit into at least one of said video signal channels and combining means for additively combining the separate video signals in a single display.

2. Pulse radar apparatus as claimed in claim 1 wherein the receiving means include two separate slotted waveguide arrays for the signals respectively of the two differing polarisations.

3. Pulse radar apparatus as claimed in claim 2 wherein the two waveguide arrays are arranged in a common flare.

4. Pulse radar apparatus as claimed in claim 1 wherein the receiving means include a parabolic dish with two separate receiving horns arranged respectively to receive the signals of the two differing polarisations.

5. Pulse radar apparatus as claimed in claim 1 wherein the receiving means include a parabolic dish with a single horn to receive the signals of the two differing polarisations.

6. Pulse radar apparatus as claimed in claim 5 wherein said single horn is a square horn with a polarisation analyser to provide outputs corresponding to orthogonal linearly polarised signals.

7. Pulse radar apparatus as claimed in any of the preceding claims wherein the two sets of received signals are fed to separate receiving channels which employ a common local oscillator.

8. Pulse radar apparatus as claimed in any of the preceding claims wherein said combining means for additively combining the separate video signals comprises an AND gate.

9. Pulse radar apparatus as claimed in any of claims 1 to 8 wherein said combining means for combining the separate video signals comprises means for bottom-clipping the signals.

10. Pulse radar apparatus as claimed in any of claims 1 to 9 wherein the means for combining the video signals comprises means arranged so that, for the first part of the radar range, the signals are combined in an AND mode but, for greater ranges, they are combined in an OR mode.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. signal clipper and video amplifier 32 to give output video signals on a lead 33. These signals on lead 33 are derived from the horizontally polarised received signals. The vertically polarised received signals in the receiving channel 23 are fed through a TR crystal protection unit 36 to a signal mixer 37 where they are mixed with output signals from the aforementioned local oscillator 14. The aforementioned coupler 38 is a 3 dB coupler provided for dividing the local oscillator output between the two mixers 25 and 37. From the mixer 37 the vertically polarised return signals are amplified and detected in another logarithmic intermediate frequency amplifier 40 to give a video output to a video amplifier and signal clipper 41 giving an output on a lead 42. In this particular receiver, selective combining means, indicated diagrammatically at 43 are provided for combining the two video signals on leads 33 and 42 and the combined signals are fed to an output 45 leading to a display unit 46. The video signals may be combined, as has previously been described, in an AND gate. A particularly desirable mode in some case may be to combine the signals from the AND gate with the horizontally polarised signals on lead 33 having been passed through a fast time constant circuit to improve the rain rejection whilst the vertically polarised signals are not passed through such a circuit. Figure 2 illustrates diagrammatically an alternative form of antenna system comprising a parabolic dish 50 with a square horn 51 leading to a hybrid coupler 52 for separating the horizontally and vertically polarised components. The horn is shown diagrammatically as being supported by stays 53. Such an antenna system may be used with radar apparatus as described with reference to Figure 1 in place of the slotted waveguide aerials 19, 20. The hybrid coupler 52 provides a signal corresponding to the horizontally polarised components in a waveguide 54 and a signal corresponding to the vertically polarised components in a waveguide 55. These waveguides 54, 55 lead to the twin waveguide joint 17 of Figure 1 to provide the received signals in waveguides 16, 22 respectively. The radiated signals, in the embodiments described above, are linearly polarised. As shown in Figure 4, two slotted waveguides 70, 71 may be fitted into a common flare 72. The flare 72 is shaped to provide the required vertical coverage pattern. The antenna is provided with an outer weatherproof covering shown diagrammatically by the dashed lines 73. In this particular example, the upper waveguide 70 is arranged to receive vertically polarised signals and the lower waveguide 71 is arranged to receive horizontally polarised signals. One of these waveguides only is used for transmission. WHAT WE CLAIM IS:

1. Pulse radar apparatus having means for transmitting pulse signals which are linearly polarised, wherein there are provided two separate receiving means, each with a logarithmic amplifier and signal clipper, arranged to receive liearly polarised signals of polarisations respectively parallel and orthogonal to that of the transmitted signals to provide separate video signals in two video signal channels, means arranged for optionally switching a fast time constant circuit into at least one of said video signal channels and combining means for additively combining the separate video signals in a single display.

11. Pulse radar apparatus as claimed in

any of the preceding claims and having a fast time constant circuit in each receiving channel to reduce rain signals.

12. Pulse radar apparatus substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings or to Figure 1 modified as described with reference to Figure 2 or Figure 3 of the accompanying drawings.