GB2318006A - A radar apparatus - Google Patents

Abstract

A radar apparatus (Figure 5) includes an oscillator (102) arranged to generate pulses of microwave radiation which are transmitted at T x . Some of the radiation generated by oscillator (102) is tapped off before transmission and applied as a local oscillator (LO) signal to a mixer (104). The mixer also receives a corresponding return pulse and produces a video pulse having a pulse length related to the delay between transmission and reception, i.e. the range of a target. The video pulse is compared in different channels (1', 2') of the apparatus with respective reference signals R1,R2 each of a form such that the integral thereof with time vanishes over the interval of occurrence of each generated pulse. This alleviates adverse effects of interference signals which may be derived from an external source and or by direct transmission between the transmitter T x and the receiver R x . The transmitted and generated pulses have the same pulse length, and one of the reference signals has an amplitude in the negative polarity sense significantly greater than in the positive polarity sense. In another example (Figure 1) each transmitted pulse has a pulse length of only half that of the corresponding generated pulse and both reference signals are bipolar square waves.

Description

A RADAR APPARATUS This invention relates to a radar apparatus.

According to the invention there is provided a radar apparatus comprising generator means for generating pulses of microwave radiation, transmitter means for transmitting microwave radiation generated during a part, at least, of each said generated pulse, means capable of comparing a pulse, returned from a target, with the corresponding generated pulse to produce a further pulse during the common interval of occurrence of the returned and generated pulses, first and second comparing means for comparing the further pulse with respective first and second reference signals to generate respective first and second comparison signals, each said reference signal being of a form such that the integral thereof with time vanishes over the interval of occurrence of each generated pulse, and means for utilising the comparison signals to generate an output signal indicative of the range of the target.

In an embodiment of the invention the transmitter means is arranged to transmit each said generated pulse in its entirety and one of said reference signals comprises a bipolar signal having an amplitude in one polarity sense substantially greater than that in the other polarity sense.

In another embodiment of the invention the transmitter means transmits microwave radiation during half the interval of occurrence of each generated pulse, and the first and second reference signals comprise bipolar square wave signals of periods respectively one half and one quarter the interval of each generated pulse.

In order that the invention may be more readily understood and carried into effect two specific embodiments thereof are now described, by way of example only, by reference to the accompanying drawings of which: Figure 1 shows a block schematic diagram of one embodiment of the radar system, Figures 2a - i show a series of pulse diagrams useful in understanding operation of the system of Figure 1, Figure 3 shows the variation with target range of output signals developed in different parts of the system of Figure 1, Figure 4 shows the modulation circuit used in the system of Figure 1, Figure 5 shows a block schematic diagram of another embodiment of the radar system, Figures 6 a - h show a series of pulse diagrams useful in understanding operation of the system of Figure 5, and Figure 7 shows the variation with target range of output signals developed in different parts of the system of Figure 5.

Referring firstly to Figure 1, a continuous wave (CW) oscillator 1 is connected to a modulator 2 and generates pulses of microwave radiation, illustrated in Figure 2a, having a pulse length D and a pulse repetition period T.

The output from the oscillator is connected to a p-i-n switch 3 which allows half only of each generated pulse (in this case the trailing half) to pass to a transmitter aerial 4.

Transmitted pulses, therefore, shown in Figure 2b, also have a pulse repetition period T, but have a pulse length of only D/2.

A portion of the output from the C.W. oscillator is fed via a coupler 5 to a mixing circuit 6 and serves both as a local oscillator signal LO (shown in Figure 2c) having the periodity of the oscillator output signal, and as a gating signal. The mixing circuit 6- is also connected to a receiving aerial 4' and so operates as a range gate which generates video pulses (shown in Figure 2g)during the common interval of occurrence of each local oscillator pulse LO and a corresponding returned pulse, received at 4'. Pulses returned from a target are illustrated, by way of example only, in Figure 2d, their phases being shifted in relation to the corresponding transmitted pulses by an amount related to the range of the target. It will be appreciated, therefore, that a pulse received from a target at zero range, for instance, overlaps completely that portion of the L.0.

signal which is coincident with the corresponding transmitted pulse, and so the resulting video pulse, generated at the output of mixing circuit 6, has a width D/2. If, on the other hand, the target has a finite range overlap is no longer complete and the video pulse has a width reduced by an amount (d' in Figure 2g) related to range.

Video pulses generated in the manner described hereinDCfo-re are fed to respective video correlator circuits 7 and 8 in separate channels 1 and 2. Each correlator circuit also receives a respective reference signal R1, R2 which is derived from modulator 2, and in this example the reference signals have the form illustrated in Figures 2e and 2f respectively.

Both reference signals, in this example, comprise constant amplitude bipolar square waves. The signal R2 has a phase and periodicity such that a negative-to-positive transition E (occurring between a "mark" (M) and a "space" (S) of each cycle) occurs within the duration of each transmitted pulse. In this example, a transition E occurs in the middle of each transmitted pulse, the period of R2 being D/2. The signal R7, on the other hand, has no such transition and has a period D, equal to the pulse widths of the L.O. and generated pulses, shown in Figures 2c and 2a respectively. As will be explained in greater detail hereinafter, it is an important aspect of this invention that the integral with time of each reference signal, Rl and R2, should vanish over the duration of each generated pulse and in the present example this condition is satisfied by ensuring that the same number of negative and positive half cycles occur within those periods.

By correlating the reference signals Rl and R2 with video pulses (Figure 2g) generated at the output of the mixing circuit 6, respective output pulses, shown in Figures 2h and 2i, are produced. As will be described in greater detail hereinafter the integrals under these output pulses are related to, and air be used to provide output signals indicative of target range.

Output pulses generated by each video correlator circuit (7, 8) are passed to respective processing circuits 20, 30, each comprising in series arrangement a Doppler amplifier 21, 31; an envelope detector circuit 22, 32; and an integrator circuit 23, 33.

The purpose of the Doppler amplifier is as follows. In practice, a pulse transmitted by aerial 4 reaches the receiver aerial 4' not only via a target, but also directly. Pulses returned from the target are, however, generally subject to Doppler modulation due to the relative movement of the radar and target, and the Doppler amplifier provided in each channel rejects substantially all frequency components, derived from the output of the mixing circuit 6, which fall outside the expected range of Doppler frequencies.

In this manner, therefore, the radar is sensitive only to signals returned from genuine targets and tends to reject unwanted signals derived directly from the transmitter which would otherwise give rise to interference (i.e. jamming).

Integration of the output pulses generated by the video correlator circuits 7,8 is inherent in the operation of the Doppler amplifiers 21, 31, and the envelope detector circuits 22, 32 and the integrator circuits 23, 33 generate respective continuous output signals DS1, DS2 representing the level of output signal produced by the Doppler amplifiers.

Variation with range (proportional to pulse period T)-or the continuous wave output signals DS1, DS2 is shown in Figure 3. A sharp transition occurs in the variation of DS2, at a range Roc/2 corresponding to a time interval T/8, and this is caused by the transition E in the corresponding reference signal R2 whenever a video pulse generated by the mixing circuit 6 (Figure 2g) has a width which overlaps precisely one half cycle only of the reference signal. In contrast, reference signal R1 has no such transition E and the corresponding continuous output signal, DS1, in channel 1 varies linearly between a maximum value at zero range (when overlap of R1 with a video pulse is complete) and zero value at a maximum "cut-off" range, Rc, corresponding to a period T/4 (when there is no overlap).

Typically the pulse repetition period T may be of the order of 200 nS corresponding to a "cut-off" range, Rc, of about 8 metres.

To obtain a unique estimate of target range the continuous output signals DS1, DS2 are applied to a ratio circuit 40 comprising respective logarithmic amplifier circuits 42, 43 whose outputs are subtracted by a subtracting circuit 41 to generate an output signal 0/P of a magnitude proportional to the logarithm of the ratio of DS1 and DS2. The variation of O/P with range is represented by the dashed line curve of Figure 3.

As described hereinbefore, the use of Doppler amplifiers, 21, 31 tends to protect the radar from interference signals caused by direct transmission of pulses between the transmitter and receiver aerials 4 and 4'. Another form of interference, however, could be derived from a source of high energy (CW) microwave radiation applied externally of the radar, and in these circumstances the Doppler amplifiers alone may not provide adequate protection against jamming.

In the present invention, however, this problem is alleviated by suitably choosing the form of the reference signals Rl and R2 so that, as described hereinbefore, their respective integrals with time, over the duration of each generated pulse vanish. In this example, the reference signal R1 is arranged to be in phase with each L.O. pulse (and so each generated pulse) and has a period T/2, namely half that of the L.O. pulse. Reference signal R2 on the other hand has a period T/4. If high power microwave radiation is applied to the radar from an external source for at least the duration of each L.O.

pulse then the output from the mixing circuit 6 will be a coincident pulse of duration D. The periodicities of R1 and R2, in this example, are such, however, that when correlated with such a pulse, of duration D, complete cancellation will occur thereby inhibiting the jamming effect of the interference signal. In a practical example, the effect of an interference signal comprising 2.5s s pulses would be reduced by about 20 dB when detected by a radar of the present invention operating with transmitted pulses of 50 ns duration. If the rise and fall times of the externally applied interference signals happen to be of the order of 50 ns then the reduction in their jamming effect would be even greater.

The reference signals R1 and R2, illustrated by the noIlo line curves of Figures 2e and 2f respectively, comprise continuous square waves. However, since those portions of the reference signals prevailing during the time intervals, INT, for example, (i.e. between generated pulses) play no role in generating the output signals indicative of range, the form of the reference signals in these regions is unimportant. They may, for example, vary smoothly as illustrated by the dashed line curves in Figures 2e and 2f.

As described, hereinbefore, the reference signals R1 and R2 having periods T/2 and T/4 respectively, are derived from the modulator 2 used to switch the CW oscillator 1. The output from the modulator and the reference signals R1 and R2 may be generated using a common circuit of the kind illustrated in Figure 4, comprising a clock 50 which generates pulses at the frequency of the reference signal R2, and two "divide-by-two" circuits 51, 52. The reference signal R2 (of period T/4) is derived directly from the output of clock 50, the reference signal R1 (of period T/2) is derived from the output of the first divide circuit 51 and the modulation signal applied to the CW generator is derived from the output of the second divider circuit 52. By deriving all these signals from a common circuit in this way their synchronism is ensured.

It will be appreciated that although the above-described example relates to reference signals R1 and R2 of a particular form, signals having other periodicities satisfying the cancellation criterion could be used. Moreover, the transif=ion E of signal R2 could occur at a time other than centrally of the transmitted pulse, thereby resulting in a range law of a form different to that shown in Figure 3.

In the above-described example the log ratio output signal O/P, derived from the continuous output signals DS1, DS2 provides a measure of target range of up to half the "cut-off" range R . It will be noted, however, that the sensitivity in c channel 1 to objects at very close range is considerably greater than that in channel 2, the sensitivity in the latter channel vanishing at zero range.

In the presence of heavy rainfall, for example, this difference in sensitivity in the two channels may result in a distortion of the log ratio, since most of the total rain clutter is derived from nearby raindrops, the clutter being weighted by a factor proportional to (range) . The magnitude of this distortion will depend on both the target range and the rate of rainfall, but in the case of a radar operating at a range of 300 metres when the rate of rainfall exceeds 5 - 10 mm/hr, for example, performance could be significantly affected.

In another example of the present invention, illustrated by reference to Figures 5 to 7 the problems introduced by rain clutter are substantially alleviated. Referring to Figure 5, a solid state switch or modulator 100 is controlled by a timing circuit 101 to modulate periodically the output of a CW microwave generator 102 which is connected directly to CL transmitter Tx. A portion of the output signal from the CW generator 102 is fed via a coupler 103 to a mixing circuit 104 and so acts as a local oscillator L.O. generating pulses which are coincident with both the transmitted and generated pulses.

The transmitted and local oscillator pulses are illustrated in Figures 6a and 6b respectively and in this example have a pulse width t. The mixing circuit is also connected to a receiver Rx responsive to pulses returned from a target and in this regard acts as a range gate to generate video pulses, shown in Figure 6f, during the common interval of occurrence of each local oscillator pulse and the corresponding received pulse. Pulses returned from a target are illustrated, by way of example only, in Figure 6c, their phase being shifted in relation to that of the transmitted pulses by an amount related to target range.

The video pulses generated in the illustrated example, at the output of the mixing circuit 104 has a width smaller than that of the corresponding local oscillator pulse by an amount (t' in figure 6f) related to target range.

The video pulses are amplified in a video amplifier circuit 105 and then correlated in separate channels of the system (Channels 1' and 2') with respective reference signals R1', R2' of a form shown in Figures 6e and 6d respectively. Correlator circuits, 106, 107, described hereinafter, are used to carry out these correlations.

The reference signal R1' comprises a series of bipolar pulses each having an overall width t and a negative-to- iX e transition E' occurring at a mark space ratio of 1 to 4.

Moreover, the amplitude of each such pulse in the positive portion (P) thereof is only one quarter that in the negative portion (N) so that, as in the previous example, the integral with time of the pulse over the duration of each transmitted (and so generated) pulse is zero. The reference signal R2' has a form identical with that of reference signal R2 in Figure 2f, namely a bipolar square wave having a negative-to-positive transition occurring at the centre of each transmitted pulse.

Signals generated in channels 1' and 2' by correlating video pulses (Figure 6f) with reference signals R1', R2' are illustrated in Figures 6h and 6g respectively. These signals are then passed to respective processing circuits 20', 30' identical with circuits 20, 30 of Figure 1, to generate respective continuous output signals DS1', DS2' which vary with target range as illustrated in Figure 7. As before the log ratio of DS1' and DS2' is formed in a ratio circuit 40' of the kind illustrated at 40 in Figure 1.

The signal DS2' varies with range in the same way as DS2 in Figure 4, but the variation of DS1' is significantly different from that of DS1. This difference is due to the transition E' occurring within each pulse of Rl'. When the target range is RT such that the resulting video pulse has a sufficiently reduced width to overlap in its entirety the positive portion only of each pulse of R1', then DS1' attains the maximum value. At ranges less than RT, however, the video pulse also overlaps the negative portion of Rl' and the value of DS1' is correspondingly smaller. In the limit of zero range, overlap is complete, and since the areas under the positive and negative portions of each pulse of R1' are equal the value of DS1' is zero.

In contrast to the range variation of DS1, therefore, DS1' has a reduced sensitivity to targets at close range (RT) RT) and so channel 1 (and therefore the radar system) is relatively insensitive to rain clutter, thereby alleviating the problem discussed hereinbefore. Moreover, as in the previous example of this invention, since the form of both reference signals R1', R2' is chosen so that the integral with time of these signals over the duration of each generated pulse is zero complete cancellation generally will occur when the reference signals are correlated with video pulses derived from externally applied interference signals, thereby providing protection against jamming. Also, since both DS1' and DS2' vanish at zero range additional protection is provided against self-induced signals, such as those passing directly from the transmitter to the receiver.

Circuit 106, used to correlate the video pulses with the reference signal R1', comprises logic gates G1, G2 which are opened in response to timing signals generated at 101. Gate G2 is opened for a time interval t2 and allows a video pulse to pass to a correlator circuit CR2, having a gain of -4 in this example. This corresponds to correlation with the negaB portion of R1'. Gate G2 is then closed and gate G1 is opened for a further time interval tl to allow the video pulse to pass to a different correlator circuit CR1 having a gain of +1, corresponding to the positive portion of R1'. Output signals from the correlator circuits are then summed in an adding circuit AD. Circuit 107 comprises a video correlator circuit which receives a reference signal R2'; generated by timing circuit 101 in the manner described hereinbefore in relation to Figure 1.

In the illustrated example, the transition E' occurs at a time tT = t/5, so that RT = Rc/5. It is possible if desired, however, to set RT at a smaller value as low as RCS50, for example. This is achieved by generating the transition E' at a time tT = t/50by suitably adjusting the time intervals tl, t2. To ensure cancellation in the presence of externally applied interference signals the amplitude of R1' in the negative portion of each pulse needs to be -49 in this case, and so the correlator circuit CR2 will have a gain of - 49 rather than -4.

Although the present invention has been described in relation to particular forms of reference signal suitable for providing protection against both externally and internally derived interference signals, it will be appreciated that the invention is not limited to these particular forms. Reference signals of alternative form suitable for providing such protection could be used.

Claims (7)

What we claim is:

1. A radar apparatus comprising generator means for generating pulses of microwave radiation, transmitter means for transmitting microwave radiation generated during a part, at least, of each said generated pulse, means capable of comparing a pulse, returned from a target, with the corresponding generated pulse to produce a further pulse during the common interval of occurrence of the returned and generated pulses, first and second comparing means for comparing the further pulse with respective first and second reference signals to generate respective first and second comparison signals, each said reference signal being of a form such that the integral thereof with time vanishes over the interval of occurrence of each generated pulse, and means for utilising the comparison signals to generate an output signal indicative of the range of the target.

2. A radar apparatus according to Claim 1 wherein the transmitter is arranged to transmit each said generated pulse in its entirety, and one of said reference signals comprises a bipolar signal having an amplitude in one polarity sense substantially greater than that in the other polarity sense.

3. A radar apparatus according to Claim 2 wherein the means for comparing said further pulse with said one reference signal comprises first gating means for gating said further pulse during a portion only of the interval of occurrence of that pulse, second gating means for gating said further pulse during the remaining portion only of the interval of occurrence of that pulse, means for amplifying the gated signals by respective amounts inversely proportional to the said portions and means for combining the gated signals, so amplified, to generate the corresponding comparison signal.

4. A radar apparatus according to Claim 1 wherein the transmitter is arranged to transmit microwave radiation during half of the interval of occurrence of each generated pulse, and the first and second reference signals comprise bipolar square wave signals of periods respectively one half and one quarter of the interval of occurrence of each generated pulse.

5. A radar apparatus according to Claim 4 including switching means, connected electrically in series between the generator means and the transmitter means and being capable of allowing transmission of microwave radiation during said half only of the interval of occurrence of each generated pulse.

6. A radar apparatus according to any one of Claims 1 to 5 wherein said means for utilising the first and second comparison signals includes respective first and second processing circuits each comprising the series arrangement of a Doppler amplifier circuit, arranged to receive a respective comparison signal, an envelope detection circuit and an integrator circuit, and also including a common ratio circuit capable of generating said output signal, related to the ratio of signals generated by the first and second processing circuits.

7. A radar apparatus substantially as hereinbefore describeS by reference to and as illustrated in the accompanying drawings.