GB1605249A

GB1605249A - Radar receivers

Info

Publication number: GB1605249A
Application number: GB3112572A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1971-11-29
Filing date: 1972-07-03
Publication date: 1986-03-05
Also published as: DE2159106C1; IT1123650B

Description

(71) We, SIEMENSAKTIENGESELLSCHAFT, a German Company of Berlin and Munich, Germany, 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: The present invention relates to radar receivers of the type provided with means to suppress transient interference signals, successive amplitude values of signals received from a target or any other source when operating being compared to provide difference signals which may be analysed to identify transient interference signals, prior to display.

In one known receiver, a control signal is obtained characteristic of a maximum or peak in the difference signals by detecting any change of polarity of these signals.

In another known receiver, means are provided for suppressing long-term echo signals, automatic control of a threshold level being effected by splitting the radar video signals into amplitude stages and directing the signal components in each amplitude stage to a selector circuit which only passes signals having durations corresponding to one or two range elements. It will be readily apparent that this arrangement is unsuitable for application to the controlled suppression of transient interference signals, i.e. disturbances occurring within a specific range sector, because such transient interference components would satisfy the analysis and be assumed to represent useful information.

In another known arrangement for reducing non-periodic interference in radar signals, difference signals are formed from an input signal pattern made up of periodic individual signals to provide new signals having the same signal pattern but opposite polarity, and these are fed with a certain delay to a rectifier system which only passes those signals whose polarity has a specific relationship to the input signals.

The simultaneous application of signals of opposite polarity, which has to be carried out several times in order to eradicate interference, means that any effective signals are relatively severely impaired, so that during the occurrence of interference signals, information about the effective signals is drastically reduced. For detection of the occurrence of a non-synchronous individual interference pulse, a mere change of sign is not an adequate criterion to adopt in order to eliminate interference, because of the varied nature of the possibilities which arise in the many and varied modes of operation. For example, when an antenna lobe of a radar sweeps over an object moving transversely with respect to the sweep of the main beam, the echo signals exhibit initially rising and then decaying amplitudes. A maximum or peak is thus produced when the target passes the main radiation direction, and a change in sign occurs, which in the known arrangement is considered as an interference criterion. Similar problems arise if the object detected is moving radially towards a radar device whose antenna is carrying out a swinging or rotating motion, as is the case for example with omnidirectional radar systems.

One object of the present invention is to provide an improved radar receiver of the type described in the opening paragraph, which is capable of positively and reliably detecting individual interference pulse sources as such, without significantly impairing the probability of detection of any moving targets, although maintaining the cost and complexity of the circuitry within reasonable limits.

The invention consists in a radar receiver in which a comparator circuit is provided which operates to suppress transient interferences, successive amplitude values of echo signals from any target and of any other received signals being compared with one another in a first subtraction circuit and the difference signals thus formed being employed to effect the suppression of any transient interference signals prior to analysis or display, the magnitude of the difference signal between any successive amplitude values being determined and compared with a predetermined reference value, and means being provided to operate if the reference value is exceeded to ensure that any said difference signal or other signal exceeding said reference value is excluded from analysis.

As the difference signal formed between the amplitude values of successive echo signals is additionally related to a predetermined reference value the specific parameters of any particular radar system, e.g. its sensitivity, the particular form of the antenna radiation pattern (lobe), the speed of'rotation of the antenna in the case of a surveillance radar system, and any other similar distinguishing characteristic of a receiver in a system can be included, so that it is possible to ensure that substantially no moving target signals are inadvertently attenuated.

The invention will now be described with reference to the drawings, in which: Figure 1 is a graph which illustrates an amplitude-time diagram pertaining to a typical train of received signals; Figure 2 is a graph which illustrates the resultant signal after pattern difference signal formation has been performed between each two successive amplitude values; Figure 3 is a graph which illustrates an amplitude-time diagram after correction has been effected in a receiver constructed in accordance with the invention; Figure 4 is a block schematic circuit diagram of one exemplary embodiment of a radar receiver constructed in accordance with the invention; Figure 5 is a block schematic circuit diagram that illustrates details of an interference suppressor circuit used in the embodiment shown in Figure 4; Figure 6 is a graph showing the amplitudetime diagram of received signals after passing through a first fixed target echo filter; Figure 7 is a graph which illustrates an amplitude-time diagram of the received signals after passing through a second fixed target echo filter; Figure 8 is a block schematic circuit diagram of an alternative preferred interference suppressor circuit for use in an embodiment of the invention; Figure 9 is a graph which illustrates an amplitude-time diagram of a train of received signals that has steep rise and decay flanks; and Figure 10 is a graph which illustrates the behaviour of the received signals shown in Figure 9, after passage through a fixed target echo filter.

In the graph shown in Figure 1, the amplitude A of demodulated echo signals has been plotted as a function of time, for a sequence of discrete amplitude values A0to An, these echo signals pertaining to a fixed target or interference sources and having been extracted as individual samples of the video signal forming the envelope of the amplitude values.

The picking off of scanning samples in a pulse radar is conveniently carried out at the radar pulse repetition frequency, for each range sector. Apart from an unusually high amplitude value signal A3, attributable to an asynchronous interference pulse, the envelope curve of the amplitude values illustrated has a substantially bell-shaped profile as a function of time. For example, envelope curves of this kind can occur in a radar system due to the fact that the directional lobe of the antenna radiation pattern has swept over a fixed or radially moving target, or that a tangentially moving target has passed through the fixed directional lobe of an antenna. Knowing the antenna characteristic and the size of any anticipated targets, it is possible to determine the basic nature of such an envelope curve in respect of any given radar receiver, and it is then possible to derive from this characteristic curve data on how large the maximum possible difference between successive amplitude values can be allowed to be, bearing in mind the time sequence of the scanned samples taken from the envelope curve, i.e. the scanning frequency for the system. In the present example the maximum value of the difference between successive amplitude values in the absence of interference signals is substantially equal to that between the amplitude values A4 and A5, and this has been marked as AA.

Figure 2 shows the difference signals formed between successive amplitude value signals of the train in accordance with Figure 1, with the ordinate scale A' enlarged. The difference amplitude A'l is given by the difference signal formed by Al-A0 in Figure 1, the value A'2 being that given by the difference signal formed by A2-Al in Figure 1, and soon.

Because of the presence of an assumed interference pulse A3, there are two unusually large difference signal values A'3, from A3-A2, and A'4 from A4-A3, the latter having a negative sign. The value A'3 considerably exceeds the difference value AA which is the value corresponding to that of an assumed maximum sized fixed target in the steepest part of the antenna characteristic, and therefore this value can be assumed to be formed by an interference signal and excluded from further analysis. In order to ensure that any transient noise signals of small amplitude, and any small instabilities do not immediately trigger interference suppression, the reference value VA at which the suppressor circuit to be provided will respond is made significantly greater than AA.

A simplified block schematic circuit diagram of an exemplary embodiment of a pulse radar receiver constructed in accordance with the invention is shown in Figure 4, in a radar installation that comprises an antenna 1, followed by a transmit/receive switch 2, to which is connected a transmitter 3.The transmit/receive switch 2 is operated by a pulse train from a source 2a. In the receiver section, a mixer 4 is provided, to which the output, of a local oscillator 5 is supplied, to form an i.f.

signal in which echo signals from individual targets may be separately processed in suitable circuits, and for example range channels or specific range gates can be used for individual target tracking. One such individual branch, assigned to a specific range, is shown in Figure 4, being provided with an input switch 6 and an output switch 7. The echo signals pertaining exclusively to a target located within this specific range are fed to this range channel, and there is no risk of mixing with echo signals from other targets at other ranges from the receiver. Echo signals from non-synchronous interference pulse sources may be produced in accordance with a statistical distribution, to coincide with pulses in the most varied range sectors. In the embodiment illustrated, which represents the receiver of a Doppler pulse radar system, there is a fixed target echo filter 8, and possibly a second such filter 9 provided, and an interference suppressor circuit 10 is arranged after the first fixed target echo filter, and if two such filters are provided, it is preferably arranged between the two. The processed video signal is supplied to a rectifier arrangement 11 which is followed by a storage and low-pass filter circuit 12, whose output feeds any signals which exceed a predetermined threshold through a threshold detector 13 to be applied across the periodically operated range output switch 7 to an analyser unit, e.g. a display screen 14.

Figure 5 shows details of the fixed target echo filter 8 and the interference suppressor circuit 10 of the embodiment illustrated in Figure 4. The fixed target echo filter 8 in this case operates digitally, and input terminal 15 (Figure 4) is connected to the input of an analogue-digital converter 18, which provides quantised and digitalised echo signal samples to the filter X, which comprises a subtraction stage 19 and a delay device 20 whose delay is set at T = I/fp, where fp is the pulse repetition frequency of the radar system. The delayed and undelayed received signal samples are applied with mutually opposed polarities to the subtraction stage 19, as is indicated by plus and minus signs at the two inputs of the subtraction stage 19, so that fixed target echoes are largely suppressed by this circuit whilst echoes from moving targets experience very little attenuation because of their inherent phase modulation. Thus, at a terminal 16 at the output of the filter 8, when the radar sweeps over a target, a digital signal representing the kind of pattern shown in Figure 2 appears in the event of the analogue-digital converter receiving a signal pattern of the kind shown in Figure 1. The fixed target echo filter 8 thus has a dual function, serving on the one hand to suppress or attenuate fixed target echo signals and on the other hand to form the difference signals between successive amplitude values of echo and other signals required for interference suppression. The interference suppressor comprises a circuit 21 in which the values of the difference signals are formed in accordance with Figure 2, and the amplitude values thus obtained are then compared in a comparator circuit 22 with a predetermined reference value which is produced by a reference store 23. If the magnitude of the difference signal between two successive amplitude values exceeds the reference value contained in the store 23, which is the value VA shown in Figure 2, then it is to be assumed that an interference pulse is present, and further transmission of received signals should be inhibited. To obtain this object, the comparator circuit is connected via a line 24a to a monitoring circuit 24. On receipt of a signal via the line 24a the monitoring circuit 24 causes the preceding amplitude value, that was fed via a line 25a into a storage device 25, to be transferred into a correction register 26. Thus, in thus case the amplitude value A'2 will advantageously be employed in place of the difference signals produced by interference pulses, until the interference terminates, i.e. until the difference signal between successive amplitude values drops below the value VA shown in Figure 2.

Therefore, instead of the amplitude values A'3 and A'4 shown in Figure 2, an amplitude pattern of the kind shown in Figure 3 occurs, i.e. at an output terminal 17 of the suppressor 10 the signal A ' I is followed by three signals of the value A'2, and only then is there a change because the interference has ceased, so that the value represented by A'5 is passed, followed by the normal train of signals. This injection of a preceding, undisturbed amplitude value signal in place of any disturbed amplitude value signal results in less impairment of the signal pattern than would be experienced if a simple stop-out or blocking circuit was used, in which the entire signal pattern is reduced to zero during the time affected by an interference pulse. This latter kind of action could possibly lead to the simulation of a moving target which is not really there at all, and thus to an unwanted display upon the screen. In the absence of any signal indicating an interference signal, the particular amplitude value signal received is passed unchanged through the correction register 26 and fed via the output terminal 17 to the next stage, in this case a second fixed target echo filter 9. On the occurrence of any signal indicating interference on the line 24a, the correction register 26 is inhibited to prevent the transmission of incoming amplitude values, and instead the previous, undisturbed, amplitude value is transferred from the store 25 and relayed to the output 17. These replacement amplitude values formed by previous-value-replacement operation are in each case transferred to the store 25 again, and repeatedly relayed by the elements 24 and 26 to the terminal 17 as long as the interference persists.

If any individual interference pulses undetected by the previous measures are still too large, and would lead to an unwanted display upon the screen, then by the introduction of additional measures the sensitivity of the detection and suppression of individual interference pulses can be increased.

However, the difficulty then presented is that a small echo signal from a moving target that is only just strong enough to be detected may be inadvertently attenuated.

Figure 6 shows a graph illustrating the conditions affecting such a situation, typical output signals A' of the fixed target echo filter 8 shown in Figure 5 being illustrated. The train of signals F' represents a fixed target, whilst a train of signals B' represents a moving target and signals S' represent an individual interference pulse. If the moving target signals B' are to remain unattenuated, then the threshold governing the response of the interference suppressor circuit 10, i.e. the reference voltage in the reference value store 23 of Figure 5 must be made sufficiently high to ensure that it is not quite exceeded by the moving target signals B', and a corresponding line VAl represents such a reference voltage.

However, a reference voltage VA1 of this magnitude in the reference value store 23, can mean that a small individual interference pulse source such as that indicated by S' is not suppressed, and will therefore be displayed on the display screen 14.

Figure 7 graphically illustrates the output voltages of the signals shown in Figure 6 when these signals have been passed through the further fixed target echo filter 9, as will be explained with reference to Figure 8. The fixed target echo signal F' that has already been attenuated by the first fixed target echo filter 8, is now additionally attenuated, to appear at the ouput of the second fixed target echo filter 9 with the form indicated by the curve F . The moving target signal B' of Figure 6 experiences no attenuation in the second fixed target echo filter 9, whilst the double interference pulse signal S' of Figure 6 produces a triple interference pulse S" with a particularly high negative amplitude component. A reference voltage VA2 can now be used, and this can be made substantially less than the reference voltage VA 1 of Figure 6, so that even the small interference pulses S" lead to this reference voltage being exceeded and are correspondingly suppressed. However, the signals B produced by the moving target also exceed the reference voltage VA2, so that special measures must be taken if this moving target echo signal is to be attenuated as little as possible. Through the measures described hereinafter it is also ensured that even with the superimposition of a fixed target echo signal on a relatively high amplitude moving target echo signal, the latter is not inadvertently affected and prevented from being displayed, although any pure fixed target echo signals F lead to a response by the correcting circuit because the double fixed target echo suppression ensures that they are very heavily attenuated, and are therefore below the reference value VA2.

The block schematic circuit diagram shown in Figure 8 comprises an input terminal 15a, which is intended for connection to the point marked 15 in the block circuit diagram of Figure 4. An output terminal 17a is provided for connection to the terminal 17 of Figure 4. In this case an analogue-digital converter 18a is followed by a fixed target echo filter 8a containing a subtraction stage 19a whose second input is connected to a delay device 20a which produces a delay in the signals of T = 1/fp, where fp equals the pulse repetition frequency, as before. This arrangement operates to ensure that at the ouput the differences between successive amplitude values or scanned samples of the echo or interference signals appear, and are fed to a switch 30, via which a further arrangement is switched in which contains a second fixed target echo filter 8b, comprising a subtraction stage 19b and a delay device 20b whose delay is also equal to T. This fixed target echo filter 8b is connected in parallel with a line 34 linking the switch 30 to the output terminal 17a, and is accordingly inoperative during signal transmission when a switch 33 at the output of the delay device 20b is open. Moreover, at the output of the subtraction stage 19b there are provided a threshold circuit 31 followed by a timer 32, e.g. a counter. In normal operation the received signals pass through the analoguedigital converter 18a, the fixed target echo filter 8a, the switch 30 and onwards to the circuit 8b.

The reference value set in the threshold circuit 31 associated with the filter 8b is so chosen that it is reliably assured that this value will not be reached by the smallest just detectable moving target signal when the latter has superimposed upon it the maximum possible remaining fixed target echo signal residue, which represents the most unfavourable case. Consequently, these signals are transmitted via the line 34 to the terminal 17a without impairment, so that the smallest moving target signal is not affected and there is no reduction in the probability of its detection. If the reference value of the threshold circuit 31 is exceeded, however, either by an individual interference pulse or iby a moving target signal which is correspondingly higher and lower than the smallest signal to be detected, then the counter 32 is started by the output of the circuit 31 and advances in the rhythm of the pulse repetition frequency. For a predetermined period determined by the chosen first steps of the count, the switch 30 is held open and switch 33 is closed, by outputs from the counter 32 indicated by lines 35 and 36. This inhibits direct transmission of any received signals for a specific time, and signals which have arrived previously and have been stored in the delay device 20b, which may be a rotary store, are fed in at a specific rate to form substitute signals that are fed to the terminal 17a to effect previous-value-substitution, until the counter 32 has reached a specific, predetermined first count. On reaching this count, the switch is closed again, and the switch 33 opened, so that previous-value-substitution is no longer effected, but direct transmission is resumed whilst the counter 32 runs on to a preset second count, returns to zero and stops.

Whilst the counter 32 is running, the threshold circuit 31 is rendered inoperative, as is indicated by a line 37 to the threshold circuit 31. This ensures that moving targets whose echoes exceed the preset value of the threshold circuit 31 are not inadvertently attenuated.

Only after the elapse of the dead time during which the circuit 31 was inoperative can any exceeding of the threshold lead to a renewed starting of the counter and renewed previousvalve-substitution.

Very small amplitude moving target echo signals which can still be detected are thus not attenuated, because they do not exceed the reference value of the threshold circuit 31.

Higher amplitude moving target echo signals (e.g. B" in Figure 7) may trigger the counter to initiate previous-value-substitution, if they exceed the reference value VA2. However, this previous-value-substitution is effective only for a short time in relation to the duration of the moving target echo signal, the effective time being set by the first count to be reached by the counter 32, so that the circuit only briefly attenuates the corresponding moving target signal. This brief attenuation does not produce any suppression of the indication of the moving target. The second count set on the counter 32 will conveniently he made higher than the maximum possible anticipated duration of exceeding of the reference value VA2 (Figure 7) by any moving target echo signal.

When using an antenna system having a highly directional characteristic, in particular when using phased-array antennas, and when using sensitive receivers with correspondingly short starting transient times, it may happen that a non-uniform change in direction of the antenna results in a fixed target producing an echo signal with a virtually rectangular envelope, of the type indicated in Figure 9, with a pulse train of the discrete amplitude values Al to An. The initial steep rise, and possibly steep decay, at the end of this kind of echo signal leads to very high amplitude individual pulses at the start and end of the echo signal train if a fixed target echo filterS, of the kind shown in Figure 5 is used for fixed target echo suppression for example. The resultant pulse shapes A' have been shown in Figure 10.The first pulse A'1 is produced by the rapid rise from virtually zero to the value Al of the signals shown in Figure 9, whilst the value -'n is produced by the steep decay from the signal An shown in Figure 9 to zero. The amplitude values A'1 and A'n could falsely simulate moving targets.

If the reference value in the fixed-value store 23 shown in Figure 5 is correspondingly low, as indicated for example by the value VA3 in Figure 10, then previous-value-substitution reduces the individual pulses A'1 and -A'n to zero, so that they are suppressed as far as any further analysis is concerned. Therefore it is not possible for any fixed target echo signal which has occurred to cause a moving target to be falsely indicated.

When using a correcting circuit of the kind shown in Figure 8, it should be ensured that previous-value-substitution does not terminate prematurely, since this could partially nullify the desired effect. The first count to be achieved by the counter 32 should therefore be made higher than that of any anticipated duration of the train of a fixed target echo signal as shown in Figure 9, and therefore greater than the time of dwell of a radar antenna within a specific directional increment in a given system.

WHAT WE CLAIM IS: 1. A radar receiver in which a comparator circuit is provided which operates to suppress transient interferences, successive amplitude values of echo signals from any target and of any other received signals being compared with one another in a first subtraction circuit and the difference signals thus formed being employed to effect the suppression of any transient interference signals prior to analysis or display, the magnitude of the difference signal between any successive amplitude values being determined and compared with a predetermined reference value, and means being provided to operate if the reference value is exceeded to ensure that any said difference signal or other signal exceeding said reference value is excluded from analysis.

2. A radar receiver as claimed in Claim 1, in which said reference value has a magnitude greater than the greatest difference signal anticipated, as determined by the directional characteristic of the antenna and the receiver sensitivity.

3. A radar receiver as claimed in any preceding Claim, in which said reference value is a fixed value.

4. A radar receiver as claimed in any preceding Claim, in which individual target echo signals are separated by range gating.

5. A radar receiver as claimed in Claim 4, in which each range sector is assigned a respective interference signal suppressor circuit.

6. A radar receiver as claimed in any preceding Claim, in which storage means are provided for each amplitude value, and in the event of one or more amplitude values being detected as interference signals by said

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. stored in the delay device 20b, which may be a rotary store, are fed in at a specific rate to form substitute signals that are fed to the terminal 17a to effect previous-value-substitution, until the counter 32 has reached a specific, predetermined first count. On reaching this count, the switch is closed again, and the switch 33 opened, so that previous-value-substitution is no longer effected, but direct transmission is resumed whilst the counter 32 runs on to a preset second count, returns to zero and stops. Whilst the counter 32 is running, the threshold circuit 31 is rendered inoperative, as is indicated by a line 37 to the threshold circuit 31. This ensures that moving targets whose echoes exceed the preset value of the threshold circuit 31 are not inadvertently attenuated. Only after the elapse of the dead time during which the circuit 31 was inoperative can any exceeding of the threshold lead to a renewed starting of the counter and renewed previousvalve-substitution. Very small amplitude moving target echo signals which can still be detected are thus not attenuated, because they do not exceed the reference value of the threshold circuit 31. Higher amplitude moving target echo signals (e.g. B" in Figure 7) may trigger the counter to initiate previous-value-substitution, if they exceed the reference value VA2. However, this previous-value-substitution is effective only for a short time in relation to the duration of the moving target echo signal, the effective time being set by the first count to be reached by the counter 32, so that the circuit only briefly attenuates the corresponding moving target signal. This brief attenuation does not produce any suppression of the indication of the moving target. The second count set on the counter 32 will conveniently he made higher than the maximum possible anticipated duration of exceeding of the reference value VA2 (Figure 7) by any moving target echo signal. When using an antenna system having a highly directional characteristic, in particular when using phased-array antennas, and when using sensitive receivers with correspondingly short starting transient times, it may happen that a non-uniform change in direction of the antenna results in a fixed target producing an echo signal with a virtually rectangular envelope, of the type indicated in Figure 9, with a pulse train of the discrete amplitude values Al to An. The initial steep rise, and possibly steep decay, at the end of this kind of echo signal leads to very high amplitude individual pulses at the start and end of the echo signal train if a fixed target echo filterS, of the kind shown in Figure 5 is used for fixed target echo suppression for example. The resultant pulse shapes A' have been shown in Figure 10.The first pulse A'1 is produced by the rapid rise from virtually zero to the value Al of the signals shown in Figure 9, whilst the value -'n is produced by the steep decay from the signal An shown in Figure 9 to zero. The amplitude values A'1 and A'n could falsely simulate moving targets. If the reference value in the fixed-value store 23 shown in Figure 5 is correspondingly low, as indicated for example by the value VA3 in Figure 10, then previous-value-substitution reduces the individual pulses A'1 and -A'n to zero, so that they are suppressed as far as any further analysis is concerned. Therefore it is not possible for any fixed target echo signal which has occurred to cause a moving target to be falsely indicated. When using a correcting circuit of the kind shown in Figure 8, it should be ensured that previous-value-substitution does not terminate prematurely, since this could partially nullify the desired effect. The first count to be achieved by the counter 32 should therefore be made higher than that of any anticipated duration of the train of a fixed target echo signal as shown in Figure 9, and therefore greater than the time of dwell of a radar antenna within a specific directional increment in a given system. WHAT WE CLAIM IS:

1. A radar receiver in which a comparator circuit is provided which operates to suppress transient interferences, successive amplitude values of echo signals from any target and of any other received signals being compared with one another in a first subtraction circuit and the difference signals thus formed being employed to effect the suppression of any transient interference signals prior to analysis or display, the magnitude of the difference signal between any successive amplitude values being determined and compared with a predetermined reference value, and means being provided to operate if the reference value is exceeded to ensure that any said difference signal or other signal exceeding said reference value is excluded from analysis.

comparator circuit the value of the immediately preceding signal or difference signal is used as a substitute signal for further analysis.

7. A radar receiver as claimed in any preceding Claim, in which the amplitude values are extracted in the form of scanned samples from the video signal and processed in digital form.

8. A radar receiver as claimed in any preceding Claim, in which a fixed target echo filter is provided for the suppression of fixed target echo signals, which filter produces at its output the requisite difference signals.

9. A radar receiver as claimed in Claim 8, in which an interference suppressor circuit is provided after said fixed target echo filter.

10. A radar receiver as claimed in Claim 6, or any one of Claims 7 to 9 when dependent upon Claim 6, in which said substitution signals are injected for a predetermined time, after which passage of the received signals is resumed.

11. A radar receiver as claimed in Claim 10, in which after a period during which substitution signals have been employed, a dead time is provided during which no such substitution can take place.

12. A radar receiver as claimed in Claim 11, in which'the duration of said dead time is longer than that time for which any anticipated high amplitude moving target echo signal can exceed said reference value.

13. A radar receiver as claimed in Claim 11 or 12, in which the duration of said dead time is controlled by the pulse repetition frequency of said radar receiver.

14. A radar receiver as claimed in any one of Claims 10 to 13, in which the duration of the use of substitution signals is relatively short compared with the time during which any anticipated high amplitude moving target echo signal may exceed said reference value, so that said echo signal is attenuated only slightly by use of said substitution signals.

15. A radar receiver as claimed in Claim 8, or any one of Claims 9 to 13 when dependent upon Claim 8, in which said first fixed target echo filter is followed by a second such filter connected in parallel with the signal transmission path, a first series switch being connected before the second fixed target echo filter, said first series switch being open during use of substitution signals, in order to block the signal transmission path.

16. A radar receiver as claimed in Claim 15, in which a subtraction circuit is provided in the output of said second fixed target echo filter, followed by a threshold circuit with a threshold value serving as reference value that is so chosen that it is not exceeded by the smallest moving target echo signal to be indicated even when a fixed target echo signal is superimposed upon it.

17. A radar receiver as claimed in Claim 15 or 16, in which the output of a delay device of the second fixed target echo filter is connected via a second switch to the signal transmission path, said second switch being closed during use of substitution signals.

18. A radar receiver as claimed in Claim 16, or Claim 17 when dependent upon Claim 17, in which said threshold circuit defining said reference value is followed by a timer circuit which controls the duration of use of substitution signals.

19. A radar receiver as claimed in Claim 18, in which said timer circuit acts after any use of substitution signals to inhibit the response of the threshold circuit for a predetermined dead time period, during which renewal of use of substitution signals cannot be initiated.

20. A radar receiver as claimed in any preceding Claim, in a system provided with a variable directional antenna system, in which means are provided to suppress interference signals simulating moving targets from being produced by the steep rise and/or decay occurring when the radar antenna is aligned on a fixed target.

21. A radar receiver as claimed in Claim 20, in which the duration of use of substitution signals is at least equal to the time of dwell of the radar antenna in a specific directional sector.

22. A radar receiver substantially as described with reference to Figure 4, Figures 4 and 5, or Figures 4 and 8.