GB2348756A

GB2348756A - Radar system

Info

Publication number: GB2348756A
Application number: GB8117408A
Authority: GB
Inventors: Anthony Handley Russel Perrett; Peter Alfred Robert Holder
Original assignee: EMI Ltd
Current assignee: EMI Ltd
Priority date: 1981-06-10
Filing date: 1981-06-10
Publication date: 2000-10-11
Anticipated expiration: 2001-06-10
Also published as: FR2928742A1; GB2348756B; GB8117408D0; DE3222254C1

Abstract

A radar system includes an r.f. oscillator (4) which generates an r.f. pulse (C) having a preset pulse length determined by a timing circuit (2). The pulse is transmitted at T<SB>X</SB>. A portion (C<SP>'</SP>) of the transmitted pulse is fed via a coupler (6) to a mixing circuit (5) which also receives the corresponding radar return (D). The mixing circuit produces a video pulse (E) of length equal to the interval of common occurrence of the portion (C<SP>'</SP>) and the radar return (D). The video pulse is mixed in one channel CH2 with a bipolar video signal (F) having a transition from one polarity to the other occurring within the time interval of the portion (C<SP>'</SP>) of the transmitted pulse but having no transition coincident with the leading or trailing edges of that pulse. The resulting signal G is then passed via a doppler filter (11) and an integrating circuit (13) to a ratio circuit (14). The video pulse E is also fed along another channel CH1 without any mixing with the bipolar signal to the ratio circuit (14). The ratio circuit output signal related to target range, is thereby made less subject to errors in timing of the pulse edges and the bipolar transitions.

Description

RADAR SYSTEM The present invention relates to a radar system.

The background to the invention will now be described with reference to Figure 1 which is a block diagram of a previously proposed radar system, and Figure 2 which comprises idealised waveform diagrams explaining the operation of the system of Figure 1.

The previously proposed radar system, an example of which is shown in Figure 1, is described in more detail in application No. 8021849. In the system there are transmitted RF pulses C of pulse length 2R/c where c = speed of light, and R is the desired range interval in which a target is to be detected. The pulses have a duty factor of for example 1 in 4. A portion of the energy C'of each transmitted pulse is fed via a coupler 6 and without significant delay to a mixer 5 which also receives the corresponding received pulse D. The output of the mixer is thus a video pulse E of length equal to the common interval of occurrence of the transmitted and received pulses at the mixer.

The video pulse E is fed by a video amplifier 8 to two channels CH1 and CH2. In the first channel CH1 it is correlated in a first video mixer 9'with a first bipolar video signal of frequency f1/2 which is a"square"pulse, having the same pulse length and time of occurrence as the transmitted pulse C. In the second channel the video pulse is correlated in a second video mixer 9 with a cycle of second bipolar video signal f1, which is also a"square"pulse, having a cycle period which is half the pulse length of the transmitted signal.

In each channel, the baseband component from each video mixer is removed by filtering in a Doppler filter 10 or 11, and detected and integrated (12 or 13) and the ratio of the filtered output amplitudes is taken by a circuit 14 as a measure of the range to a target.

Referring to Figure 2 it is apparent that at zero range, the leading and trailing edges of the first video signal f1/2 ideally coincide with the corresponding edges of the video pulse E (0), and the output S1 (0) of the first mixer 9'is a pulse of the same length as the first video pulse E (0). As range increases, e. g. at half range, the leading edges do not coincide but the output S1 (E) is still the same length as the video pulse E ().

At zero range the leading and trailing edges of the cycle of the second video signal f1 coincide with the corresponding edges of the video pulse E (0), whilst the second video signal f1 has a polarity transition T in the centre of the video pulse.

As range increases, e. g. at half range, the leading edge of the video pulse E no longer coincides with the corresponding edge of the second video signal f1, and the video pulse E (}) no longer has the polarity transition of the second video signal f1 at its centre. However the trailing edges of the video pulse and the second video signal still coincide in the ideal case.

In order to obtain a desired range law, i. e. variation with range of the ratio of the outputs of the first and second mixers, it is important in the previous proposal to ensure the edges of the pulses and the polarity transition occur at the appropriate times. Errors in timing produce errors in the range law.

It is an object of the present invention to provide a radar system in which problems associated with the relative timings of pulse edges are reduced.

According to the present invention, there is provided a radar apparatus comprising: means for transmitting an RF pulse having a pulse length of 2R/c where R is the range interval for detecting a target and c is the speed of light, means for receiving the corresponding return pulse, means responsive to the return pulse and to a further RF pulse having a pulse length substantially equal to that of the transmitted pulse and having a predetermined timing relative thereto to produce a video pulse of length equal to the interval of common occurrence of the further and return pulses, means responsive to the video pulse and to a bipolar video signal having at least one transition from one polarity to the other or vice versa occurring within the time interval of the further pulse but no such transitions substantially coincident with the leading and trailing edges of the further pulse to produce a signal representing the amplitude of the whole video pulse weighted in accordance with the bipolar video signal, and means for forming a signal representing the ratio of the said signal and a further signal representing the unweighted amplitude of the video pulse and derived from the video pulse without the use of any further signal.

For a better understanding of the invention, reference will now be made, by way of example, to Figures 3 to 8 of the accompanying drawings, in which: Figure 3 is a schematic block diagram of a radar system in accordance with the invention, Figure 4 comprises graphs having idealised range laws of the system of Figure 3, Figure 5 is a schematic block diagram of the timing circuit of Figure 1, Figure 6 comprises idealised waveform diagrams explaining the operation of the system of Figure 3, Figure 7 is a schematic block diagram of a modification of the system of Figure 1, and Figures 8A and B are waveform diagrams illustrating the effect of timing and the compensation thereof on the operation of the system of Figure 7.

Referring to Figures 3 and 6 a clock 1 of a timing circuit 2 generates clock pulses of frequency f1/2 of period from which a logie circuit 2'drives a pulse train of duty factor 1: 4 of pulses having length"/2. These pulses are fed to an oscillator driver circuit 3 which produces pulses B of the length L/2 and duty factor 1: 4 suitable for driving an RF oscillator 4 which produces corresponding RF pulses C for transmission by an antenna TX. The length of each pulse C is chosen to be tp = 2 R/c where R is the range interval for detecting a target and c is the speed of light.

The corresponding return pulse D from a target is received by receiver antenna RX and fed to an RF mixer 5. The mixer 5 also receives a portion of the energy of the transmitted pulse via an RF coupler 6, as an RF local oscillator pulse C'which is delayed by a delay 7, and is of smaller amplitude, relative to pulse C.

If the range to the target is zero, pulses C'and D will be coincident in time at the mixer 5 as shown in Figure 6 at C', D (0). (Although the delay 7 is shown in Figure 1 between the coupler 6 and mixer 5, the delay is insignificant being provided only to compensate for the aerial cable delay from point P to point Q). Thus at zero range the output of the mixer 5 is a video pulse H which is fed to a video amplifier 8 to produce a ideo pulse E.

The length of the video pulse E is equal to the common interval of occurrence of the pulses C'and D at the mixer, as shown for zero range at E (0) in Figure 6. At half full range R, the pulse D will be delayed relative to pulse C'by half the length of pulse C'as shown at D () in Figure 6. The video pulse is then as shown at E () in Figure 6.

The video pulse E is fed to two parallel channels CH1 and CH2. Channel CH2 comprises a video mixer 9 which receives the video pulse E and a bipolar video signal F. In this example the video mixer 9 comprises a phase splitting amplifier 91 and a diode switch 92. In accordance with this example of the invention, video signal F has a polarity transition T (see Figure 6, F) timed to be in the centre of the RF local oscillator pulse C', but has no polarity transitions substantially coincident with the leading and trailing edges of pulse C', the opposite polarity transition T occurring midway between successive pulses C'.

The output G of video mixer 9 is thus the video signal pulse E, parts of which are positively and negatively weighted in accordance with the timing of the transition T of the signal F relative to the pulse E. As shown at G (0) in Figure 6, at zero range, the output G comprises equal negative and positive portions, as the transition T is in the centre of the video pulse E. As range increases, the negative portion decreases, until at half range the output G comprises only a positive portion as the video pulse E comprises only a pulse occurring after the transition T as shown at E (}) and G (;) in Figure 6.

In channel 1, in accordance with the invention, there is no mixer (c. f. the previous proposal discussed hereinbefore), thus avoiding errors due to inexact pulse edge timings.

Both channels CH1 and CH2 comprise a Doppler filter and amplifier 10 or 11, and a detector and integrator circuit 12 or 13. These are provided for practical reasons. In practice, the output of the RF mixer 5 comprises a spillover component due to direct transmission of a signal from TX to RX. A real target return pulse D and the desired video pulse E of length equal to the common interval of occurrence of the real target return D and the local oscillator pulse C'is separable from the spillover component by the Doppler modulation of the target return. Thus in practice, the waveforms D, H, E and G of Figure 6 would be Doppler modulated.

The Doppler filters 10,11 pass the Doppler components, which are detected in. circuits 12 and 13 and integrated to provide a D. C. signal. The output channel of CH1 is thus a signal representing the unweighted amplitude of the whole video pulse E, whilst the output of channel CH2 is a signal representing the amplitude of the whole video pulse E weighted according to its timing relative to the bipolar video signal F. The two signals are fed to a circuit 14 which forms the logarithm of the ratio of the two signals.

Figure 4 shows the variation of the output voltages of the channels CH1, CH2 and circuit 14 with range. They show that the outputs of Channels CH1 and CH2 go zero simultaneously. In practice that is undesirable, so channel CH2 comprises a circuit 15 which ensures that its output does not fall below a predetermined value at near-maximum range to avoid indeterminancy in the output of circuit 14 at near-maximum range.

The circuits for producing signals A, B and F are shown in more detail in Figure 5. Signal A is derived from the clock by a frequency divider circuit 17 which divides by 2. Signal B is derived from A and from the clock by an AND gate 18, whilst signal F is derived by delaying A by 4. Z/2 using a delay line 16.

Figure 7 is a schematic block diagram of a modification of the system of Figures 1 to 3. In it, items equivalent to those of Figure 1 are indicated by the same reference numerals as are used in Figure 1. In addition to those items, which will not be further described, the system of Figure 7 as shown comprises RF filters 19 and RF isolators 20. Furthermore, the system lacks the delay 7 between coupler 6 and mixer 5 as the aerial cable delays are inherently compensated for as will become clear. Figure 7 also shows an example of the log ratio circuit 14 which comprises logarithmic amplifiers 141, 142 and a subtractor 143.

Referring to Figure 7, signals in the system are susceptable to delays in the video circuits and to delays in the microwave circuits.

The timing circuit 2 comprises in this example a pulse repetition frequency generator 201 which produces the signal A (Figure 5A or Figure 2) and a pulse generator 202 which responds to the leading edge of signal A to produce the signal B (Figure 5A or Figure 2) defining the RF transmitted pulse length tp, and which is fed to the RF oscillator 4. The total video circuit delay from the output of generator 201 to the output of oscillator 4 is tv.

The RF pulse of oscillator 4 is fed to antenna TX via the coupler 6 isolator 20 and filter 19 and is subject to a delay tm of which the coupler 6 contributes nothing to the delay.

The transmitted pulse is then subject to a delay t (R) dependent on range to a target between the antenna TX and the receiver antenna RX.

The pulse received at antenna RX is then subject to a microwave delay tm2 through filter 19 and isolator 20, to the input of RF mixer 5.

The video output of mixer 5 is then subject to a delay tv2, through video amplifier 8 to the channels CH1 and CH2.

The bipolar video signal A produced by timing circuit 2 is subject to a delay tv3 in the video delay line 16.

Any other delays are included in the delays tm, tm2 v1 ~ 2 an tv3.

The effects of these delays is shown in Figure 8A, in an exaggerated manner.

The transmitted RF pulse C is of length tp but is delayed by tv relative to the leading edge of signal A, whereas ideally it should not be delayed at all.

At zero range i. e. range delay t (R) = t (0), the received RF pulse should be coincident in time with the RF pulse at the output of oscillator 4, but is in fact delayed relative to it by tm + tm + t (0) as shown at D.

The corresponding video signal E at the output of the mixer is further delayed by tv2 relative to the leading edge of RF pulse D instead of being ideally subject to no delay at all.

Accordingly if the bipolar video signal delayed by video delay line 16 to produce signal F were delayed by half the pulse interval of the ideally undelayed RF pulse C, then the output of video mixer 9 in channel 2 would be as shown at G for zero range. Clearly G in Figure 5A indicates other than zero range as may be seen by comparison with G (0) in Figure 2.

In order to compensate for the delays, (i) the RF pulse length is lengthened to (tp + tx) where tx = tm1 + tm2 ; and (ii) the video delay defined by delay line 16 is made tv3 (tm1 + tm2) + (tV1 + tv2) + (tp)/2.

Figure 8B illustrates the result of this modification and shows that signal G consequently correctly represents range for the example of zero range, Signal G also correctly represents range at all other significant values, of course.

Claims

1. A radar apparatus comprising : means for transmitting an RF pulse having a preset pulse length, means for receiving the corresponding return pulse, means responsive to the return pulse and to a further RF pulse having a pulse length substantially equal to that of the transmitted pulse and having a predetermined timing relative thereto to produce a video pulse of length equal to the interval of common occurrence of the further and return pulses, means responsive to the video pulse and to a bipolar video signal having at least one transition from one polarity to the other or vice versa occurring within the time interval of the further pulse but no such transition substantially coincident with the leading and trailing edges of the further pulse to produce a signal representing the amplitude of the whole of the video pulse weighted in accordance with the bipolar signal, and means for forming a signal representing the ratio of the said signal and a further signal representing the unweighted amplitude of the video pulse and derived from the video pulse without the use of any further signal.

2. A radar apparatus according to Claim 1 wherein said means for transmitting an RF pulse comprises timing means for generating a square wave oscillation having said preset pulse length, a dividing circuit responsive to said oscillation to generate therefrom said bipolar video signal having twice said preset pulse length and means for delaying said bipolar video signal by a time interval such that said at least one transition occurs intermediate the leading and trailing edges of the further pulse.

3. A radar apparatus according to Claim 1 or Claim 2 wherein said means responsive to the video pulse and to said bipolar video signal comprises a phase-splitting amplifier circuit, for receiving said video pulse, and a diode switch connected electrically in series with the amplifier circuit to receive the bipolar video signal.

4. A radar apparatus according to any one of Claims 1 to 3 wherein means are provided which couple with said transmission means to generate said further RF pulse.

5. A radar apparatus substantially as hereinbefore described by reference to and as illustrated in Figures 3 to 6 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A radar apparatus comprising: means for transmitting an RF pulse having a preset pulse length, means for receiving the corresponding return pulse, means responsive to the return pulse and to a further RF pulse having a pulse length substantially equal to that of the transmitted pulse and having a predetermined timing relative thereto to produce a video pulse of length equal to the interval of common occurrence of the further and return pulses, means responsive to the video pulse and to a bipolar video signal having at least one transition from one polarity to the other or vice versa occurring within the time interval of the further pulse but no such transition substantially coincident with the leading and trailing edges of the further pulse to produce a first signal representing the amplitude of the whole of the video pulse weighted in accordance with the bipolar signal, and means for forming a second signal representing the ratio of the said first signal and a yet further signal which represents the unweighted amplitude of the video pulse.

3. A radar apparatus according to Claim 1 or Claim 2 wherein said means responsive to the video pulse and to said bipolar video signal comprises a phase-splitting amplifier circuit, for receiving said video pulse, and a diode switch connecte electrically in series with the amplifier circuit to receive the bipolar video signal.