GB2078048A - Radar apparatus - Google Patents

Abstract

Radar apparatus has a non- resonant linear array antenna 1 that is supplied with signals at two different frequencies f1,f2 such that two beams are propagated at different angles alpha 1, alpha 2. As the antenna 1 is rotated about its axis 11 one beam leads the other and receives echo signals in respect of the same location prior to the other beam. The echo signals in respect of the two beams are supplied to a comparator 30, the signals from the leading beam being delayed so as to be supplied to the comparator simultaneously with the signals from the other beam. The comparator 30 passes, to a display 3, only those signals that are within predetermined limits of one another, rejecting those signals that change substantially between the leading and the other beam, and which are thereby in response to transient phenomena. <IMAGE>

Description

SPECIFICATION Radar apparatus This invention relates to radar apparatus and to methods of operating radar apparatus.

The invention is more particularly, but not exclusively, concerned with radar apparatus for marine use.

One problem that occurs, especially with marine radar apparatus, is that of discriminating between radar reflections from objects such as ships, buoys or land masses, and unwanted radar reflections from waves on the water surface. In general, objects of interest to the radar operator are stationary or move along continuous paths, whereas waves are short-lived, transient features that are continually changing in size and shape. Radar reflections from objects of interest will, therefore, in general, be of the same amplitude on successive scans, but radar reflections from such transient features as waves will have different amplitudes on successive scans. This fact has been appreciated by others, and radar apparatus has been proposed utilising this to discriminate between wanted and unwanted reflections.This previously proposed radar apparatus employs a rapidly rotating radar antenna responsive to both the wanted and unwanted reflections. The echo signals received during one scan are stored by the apparatus and compared with signals received during the next scan. Those signals which are of approximately the same amplitude for both scans are assumed to originate from objects such as ships, or land masses and are passed for processing or display on the radar screen. Signals from one scan which are not approximately matched in amplitude by signals from the next scan are assumed to be reflections from transient phenomena, such as waves, and are thereby rejected. The disadvantage of such a system is that the radar atenna must be rotated at very high speeds so that reflections from, for example, rapidly moving ships at close range will not be rejected.Such rapidly rotating antennas require excessively large radomes or the use of special drive motors and bearings, that are more liable to mulfunction.

It is an object of the present invention to provide radar apparatus and methods of operating radar apparatus that enables discrimination between wanted and unwanted radar reflections without the need for a rapidly rotating antenna.

According to one aspect of the present invention there is provided radar apparatus including means for propagating at least two beams of radar energy and scanning said beams over a target area, said beams being at an angular displacement with respect to one another so that one beam leads the other during said scan; means for receiving reflected energy arising from said beams of radar energy and for deriving first and second amplitude signals representative of the amplitude of energy reflected from within said target area respectively from said leading beam and said other beam; comparing means for receiving said first and second amplitude signals; means for supplying to said comparing means substantially simultaneously said first and second amplitude signals in respect of the same location in said target area; said comparing means supplying signals to display or other utilising means only when said first and second amplitude signals received by said comparing means are within predetermined limits of one another, such that reflected energy from transient objects which change between incidence of said leading and said other beam can thereby be identified and rejected.

In this way, echo signals can be produced twice upon each scan from within the target area with a short time separation between them. This avoids the need for a rapidly rotating antenna.

The leading and other beams may be of a different frequency from one another. The apparatus may have an antenna that is provided by a non-resonant linear array of the kind that is arranged to propagate radar energy at an angle dependent on the frequency of energy supplied to the array.

According to another aspect of the present invention there is provided a method of operating radar apparatus comprising the steps of propagating two beams of radar energy; scanning said beams over a target area, said beams being at an angular displacement with respect to one another so that one beam leads the other during said scan; receiving reflected energy arising from said beams of radar energy; deriving first and second amplitude signals representative respectively of the amplitude of reflected energy from said leading beam and said other beam; delaying said first amplitude signals with respect to said second amplitude signa Is such that said first and second amplitude signals in respect of the same location in said target area are substantially simultaneous; comparing said first and second amplitude signals after said delay; and supplying signals to display or other utilising means only when said first and second amplitude signals are within predetermined limits of one another, such that reflected energy from a transient object which changes between incidence on it of said leading and said other beam can thereby be identified and rejected.

Radar apparatus mounted on a ship, and a method of operating such apparatus, both in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates schematically the radar apparatus; Figures 2 to 4 show a part of the apparatus in greater detail.

With reference to Figure 1, the radar system is mounted on a ship (not shown) and includes an antenna 1, that is supplied with signals at two different frequencies from a transmit/receive unit 2. Received echo signals are processed by the unit 2 and are supplied to a display unit 3 and a control unit or other utilising means 4.

The antenna 1 is of a conventional kind, being a non-resonant linear array mounted for rotation by a motor 10 about an axis 11 at right angles to its length. This form of antenna has the characteristic that the main beam produced in the plane of the normal 12 and the length of the array is displaced from the normal by a small angle. This angle of displacement, or squint angle a, is a frequency dependent.

The transmit/receive unit 2 includes two conventional transceivers 21 and 22 that supply radar signals at frequencies f, and f2 to the antenna 1 along lines 23 and 24 respectively. The lines 23 and 24 may include appropriate filter units 25 and 26 for preventing received signals being supplied to the wrong transceivers 21 and 22; alternatively, the filter units 25 and 26 may be included in the transceivers 21 and 22 themselves.

Echo signals received by the antenna 1 are supplied to respective transceiver units 21 and 22 which produce output signals on lines 27 and 28. Amplitude signals in accordance with the value of received echo signals at a frequency f1 are supplied by the transceiver 21 along line 27 to a comparing unit 30. Signals from the other transceiver unit 22, in accordance with received echo signals at the other frequency f2, are also supplied to the comparing unit 30, along line 28, but in this case a delay is introduced by means of a delay unit 31. Output signals from the comparing unit 30 are supplied via line 40 to the display unit 3 which might be a conventional cathode-ray tube or other display. Signals on line 40 are also supplied to the control unit 4, for navigation or helm-control of the ship.The radar system could include a storage or printer facility for retaining a record of the radar signals.

The operation of the radar system will now be described in greater detail with reference also to Figure 2.

Signals at a frequency f1 supplied to the antenna 1 from the transceiver 21 produce a main beam of propagated energy represented by the line 101. The main beam is produced at a squint angle a1 to the normal 12. Signals at a frequency f2, from the transceiver 22 similarly produce a main beam of propagated energy, represented by the line 102, at a squint angle a2 which is greater than a1. Both main beams are displaced from the normal 12 in the direction of rotation of the antenna 1 so that the beam propagated along line 102 leads the other beam during scanning by the antenna.

In Figure 2, the antenna 1 is at an angle 0 with the axis 13 of the radar-carrying ship, and the leading beam, along line 102, is shown to be incident on a target ship 103. The radar signals reflected from the target ship 103, at a frequency 6, are supplied to the transceiver 22 which in turn supplies amplitude signals in accordance with the vaue of the echo signals to the delay unit 31. At this time no object is in the path of the other beam, along line 101, so no reflected signals are thereby received, at the frequency f1, by the transceiver 21.

After a time t (Figure 3) the antenna 1 has rotated anticlockwise through an angle a2 - a1 so that the antenna is now at an angle 0 + (a2 - al) with the axis 13 of the radar-carrying ship, and so that the beam at the frequency f1 is now incident on the target ship 103.

If the array is rotating at n revolutions each second then t = (a2 - a1)/360n (1) The signal reflected by the target ship 103 at time t and frequency f1 will be of substantially the same amplitude as that initially reflected when the leading beam of frequency f2 was incident on it.

The delay produced by the delay unit 31 is determined in accordance with the speed of rotation of the array 1 and the squint angles a1 and a2. This delay is arranged to be equal to the time difference t (from expression (I)) so that signals in accordance with the two echo signals received from the same location in the target area are passed to the comparing unit 30 simultaneously.

During the time t there will be substantially no change in the radar signature presented by the target ship 103; even if the target ship and the radar-carrying ship are moving away from one another, the change in amplitude of the received echo signals during the time twill be very small. The comparing unit 30 compares the values of its two inputs and produces an output signal on line 40 only when their values are within predetermined limits of one another.

The way in which the system responds to transient phenomena such as, for example, waves in the target area, is shown in Figures 4 and 5. In Figure 4 the normal 12 of the radar antenna 1 is inclined at an angle 0' to the axis 13 of the radar-carrying ship, and the leading beam, at frequency f2 along line 102, is incident on a wave 104. The radar energy at frequency f2 is reflected from the wave 104 thereby producing an amplitude signal from the transceiver 22 that is supplied to the comparator 30 after a delay oft seconds provided by the delay unit 31. After t seconds has elapsed, the array 1 will have rotated through an angle a2 - a1 so that the other radar beam, at the frequency f1, propagated along line 101 is incident on that part of the target area where the wave 104 was originally located. By this time, however, the wave 104 will have changed its size, it may have reduced or increased in size, or completely subsided. For this reason, the echo return signal supplied to the transceiver 21 will differ from that received by the transceiver 22 t seconds earlier. The comparing unit 30 thereby rejects the amplitude signals from the wave 104 because the difference exceeds the preset limits.

In this way, traces on the radar screen produced by transient phenomena, or "clutter", can be substantially reduced. The reduction in clutter could be of the order of 6dB.

The two radiated beams could lead or lag the normal 12 and the antenna 1 could rotate clockwise or anticlockwise. More than two beams could be used which should result in a greater reduction in clutter.

The propagated rear energy could be of the continuous or pulsed kind and, in this respect, alternate pulses of the two different frequencies could be supplied to the antenna 1.

Frequency diversity exists because there is a difference between the transmitted frequencies. This can additionally be used to decorrelate the clutter and cause a further reduction in the overall clutter level.

It is not essential that the two displaced beams of radar energy be produced by a single non-resonant array using two different frequencies. Alternatively, two arrays, one mounted above the other could be used, the arrays being arranged with a fixed angular orientation between them. Electronically phased arrays could alternatively be used to generate the beams.

Claims (9)

1. Radar apparatus including means for propagating at least two beams of radar energy and scanning said beams over a target area, said beams being at an angular displacement with respect to one another so that one beam leads the other during said scan; means for receiving energy arising from said beams of radar energy and for deriving first and second amplitude signals representative of the amplitude of energy reflected from within said target area respectively from said leading beam and said other beam; comparing means for receiving said first and second amplitude signals; means for supplying to said comparing means substantially simultaneously said first and second amplitude signals in respect of the same location in said target area; said comparing means supplying signals to display or other utilising means only when said first and second amplitude signals received by said comparing means are within predetermined limits of one another, such that reflected energy from transient objects which change between incidence of said leading and said other beam can thereby be identified and rejected.

2. Radar apparatus according to Claim 1, wherein said leading and other beams are of a different frequency from one another.

3. Radar apparatus according to Claim 1 or 2, wherein said two beams of radar energy are propagated from a single antenna.

4. Radar apparatus according to Claim 3, wherein said antenna is provided by a non-resonant linear array of the kind that is arranged to propagate radar energy at an angle dependent on the frequency of energy supplied to the array.

5. Radar apparatus according to any one of the preceding claims, wherein said apparatus includes delay means intermediate said means for receiving reflected energy and said comparing means, said delay means being arranged to delay said first amplitude signals until reception of said second amplitude signals in respect of the same location in said target area.

6. Radar apparatus to any one of the preceding claims, wherein said apparatus includes a cathode-ray tube display.

7. A method of operating radar apparatus comprising the steps of propagating at least two beams of radar energy; scanning said beams over a target area, said beams being at an angular displacement with respect to one another so that one beam leads the other during said scan; receiving reflected energy arising from said beams of radar energy; deriving first and second amplitude signals representative respectively of the amplitude of reflected energy from said leading beam and said other beam; delaying said first amplitude signals with respect to said second amplitude signals such that said first and second amplitude signals in respect of the same location in said target area are substantially simultaneous; comparing said first and second amplitude signals after said delay; and supplying signals to display or other utilising means only when said first and second amplitude signals are within predetermined limits of one another, such that reflected energy from a transient object which changes between incidence on it of said leading and said other beam can thereby be identified and rejected.

8. Radar apparatus substantially as hereinbefore described with reference to the accompanying drawings.

9. A method of operating radar apparatus substantially as hereinbefore described with reference to the accompanying drawings.