GB1583240A

GB1583240A - Doppler pulse radar receivers

Info

Publication number: GB1583240A
Application number: GB5138872A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1972-02-29
Filing date: 1972-11-07
Publication date: 1981-01-21
Also published as: IT1037044B; NL162212B; BE791787A; DE2209571C1; FR2435045A1; NL162212C

Description

(54) IMPROVEMENTS IN OR RELATING TO DOPPLER PULSE RADAR RECEIVERS (71) We, SIEMENS AKTIENGESELL SCHAFT, 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 Doppler pulse radar receivers of the type provided with range channels containing moving echo filters, said channels being successively connected to the input section of the receiver and their outputs being connected through a line to a common display of analyser unit, and means being provided to suppress jamming signals.

The German Patent Specification No.

1,124,103 describes a pulse radar system in which jamming signals are suppressed by anti-jamming means which convert the video signals into signals which are at least approximately differentiated. After conversion, the signals are then supplied to two parallel channels one of which simply reverses the polarity and rectifies the signals, whilst the second of which additionally delays the signals and then rectifies them, a coincidence circuit being connected at the output of the two channels, so that, because the delay in the second of the two channels is shorter than the duration of a transmitted pulse, jamming pulses of short duration become suppressed. However, the complexity and cost of this kind of circuit is extremely high, in particular because of the delay devices which are required.

The British Patent Specification No.

960,836 describes a radar receiver in which a wide-band intermediate frequency amplifier is followed by a narrow-band intermediate frequency amplifier. The output of this narrow-band amplifier is fed to a detector operating as a demodulator. Only a small component of any wide-band jamming signals of low amplitude pass through the narrow-band amplifier, and the jamming signals are therefore suppressed to a certain extent. However, because very strong pulsetype jamming signals may lead to blockage of the narrow-band amplifier, a special monitoring circuit is provided, which is connected to an omni-directional antenna, and as soon as a signal whose amplitude exceeds a predetermined threshold level appears in this circuit, a blocking circuit located between the wide-band and the narrow-band amplifiers is rendered operative.The additional monitoring circuit, including the omni-directional antenna that is required, involves a considerable additional cost, and it is also a drawback that the anti-jamming function is operative in the intermediate frequency stages, so that the components thereof have to be specially designed.

Special problems arise in the case of radar systems using range channels. Antijamming circuits which are arranged before the inputs of the range channels lead, amongst other things, (and in particular with rapid changes in receiver sensitivity) to the production of interference signals which can be mistakenly assessed as signals from moving targets. Also, prior to the input of the range channels it is difficult to make a distinction between jamming signals and moving or fixed echo signals.

One object of the present invention is to provide a radar receiver design by which it is possible to effect reilable distinction between jamming signals and moving or fixed echo signals, and moreover to make it possible for the receiver sensitivity to be modified very quickly, without producing new disturbances.

The invention consists in a Doppler pulse radar receiver in which a plurality n of separate range channels containing moving echo filters are provided, said channels being successively connected to the input section of the receiver, their outputs being connected via a common line to a common display or analyser unit, and in which an anti-jamming circuit is arranged between the output of said range channels and the input of said display or analyser unit.

At the output of the range channels, the anti-jamming function is simplified by virtue of the fact that at any instant a full range of jamming signals plus the echo signals from fixed targets is no longer present.

Furthermore, any rapid changes in receiver sensitivity that may be effected can no longer generate signals of the type which would be passed by moving echo filters and falsely indicated as echoes from moving targets if anti-jamming procedures were adopted before the range channels. A further advantage resides in the fact that, with range channels operating in the intermediate frequency stages, the signals appearing at the anti-jamming circuit are video signals, and are therefore particularly simple to exploit for analysis.

Advantageously, the individual range channels each incorporate an amplifier having a gain characteristic which is nonlinear, in particular logarithmic, relative to the input amplitudes, thus yielding a dynamic compression function. This has the advantage that the correcting elements used to alter the receiver sensitivity need not have a very wide dynamic range. The requirements imposed upon the non-linear gain charactenstic are relatively minor, and all that has to be ensured is that the system is unambiguous, i.e. that each input level corresponds with a specific output level. Gain characteristics of this kind can be obtained particularly easily.

In a preferred embodiment, the removal of interference is effected in a simple manner by determining the overall energy of the output signals from all, or from a group of said range channels, and the threshold level required for the display or analysis of any received signals is automatically located a certain amount above the mean value of said overall energy at any instant. Thus, the threshold level automatically adapts itself to the values determined by the particular jamming components.

Advantageously, for detection of the presence of jamming signals a monitoring circuit is provided, which responds whenever at least a predetermined number m of said n! range channels is occupied by output signals, said monitoring circuit then acting to signal an interference condition and initiate operation of the anti-jamming circuit. Normally, only a very small number of airborne targets is picked up by a radar, e.g. a surveillance radar, so that the existence of a jamming condition can be distinguished quite simply from this normal operating condition by the fact that output signals appear in many range channels, and advantageously a simple counting operation will suffice to monitor the presence of jamming.By way of jamming transmitters, virtually all kinds of pulse and continuous wave transmitters mut be considered and the set criterion should be one responsive to jamming of all kinds. The last mentioned would only fail to operate satisfactorily if the jamming occurred at very large time intervals compared with the radar repetition period, and the check is preferably repeated every receiving period T = 1 /fop where fp is the pulse repetition frequency.

The invention will now be described with reference to the drawings, in which: Figure 1 is a block schematic circuit diagram of one exemplary embodiment of a radar receiver constructed in accordance with the invention; Figure 2 is a block schematic circuit diagram of a modified form of the anti-jamming circuit; and Figure 3 is a graph illustrating the behaviour of threshold values as a function of time.

In the exemplary embodiment shown in Figure 1, an antenna 1 of a surveillance radar is connected to a transmit/receive switch 3 that is controlled by a pulse generator 2 to selectively connect a radar transmitter 4 or a demodulator 5, to which is also connected a superheterodyne oscillator 6. The received signals, demodulated in cophasal fashion are selectively fed to range channels K1 to Kn which are each successively connected for a specific period of time to the input section of the radar receiver by the action of a switching device SE.

The range channels, in a manner known per se, contain Doppler filters (moving echo filters), followed by rectifiers and possibly amplifiers, low-pass integrating devices and threshold circuits. The outputs from these channels therefore comprise any remaining echo signals due to moving targets, as well as any jamming components which might simulate a moving target, and the separate outputs are successively scanned by means of a switching device SA which connects them to a common video line VL. This common video line VL is connected to an anti-jamming circuit SU, the output of which is connected to an analyser or display unit 6.

The anti-jamming circuit SU shown in Figure 1 contains a monitoring device 8 connected to the video line, which device is in this case followed by an amplifier 9 to produce a correct output level of a control signal applied to a correcting element 10 which is in series with the video line VL.

The correcting element 10 may be a variable impedance preferably a resistor, or a variable-gain amplifier or threshold stage. The correcting element 10 is followed by a threshold stage 11, likewise connected in series with the video line - VL, and the threshold of this stage 11 determines what signal amplitudes will be displayed or analysed. As soon. as a jamming condition or interference is recognised as such, because of the appearance at the monitor ing device 8 before the correcting element 10 of a signal form, signal amplitude or other significant criterion, the monitoring device 8 operates through the agency of the correcting element 10 to reduce the receiver sensitivity until the threshold of the stage 11 ceases to be exceeded by disturbances.

The control operations required for this to happen in the correcting element 10 can be carried out at any arbitrary rate so that short-period disturbances can be deliberately blocked out, and after the disturbances have ended the receiver sensitivity can be restored virtually straightaway to its original high level.

In Figure 2, an anti-jamming circuit SU of modified form is shown, in which there are two threshold stages, 11 and 1 la. The threshold stage 11 is connected in series with the line VL and has a variable threshold, so that it serves to replace the correcting element 10 of Figure 1, whilst adjustment of its threshold value alters the sensitivity and thus determines what signal amplitude will be detected and processed by the analyser or display unit 6. Between the threshold stage 11 and the display or analyser unit 6, a buffer amplifier 14 is provided in this exemplary embodiment. To vary the threshold level of the stage 11, a control circuit 12 is used, which raises the threshold of the stage 11 when jamming ocurs and lowers it when there is no jamming.The information indicating whether there is any jamming interference or not is obtained from a counter circuit 13 operating as a monitoring circuit, this circuit being fed via the second threshold stage ila in a branch path to monitor the output signals from all the range channels and count the number of range channels in which such output signals actually occur. For example, if a total of forty range channels is provided, then the counter circuit can be adjusted so that it activates the anti-jamming circuit, if it finds on scanning all the outputs of the range channels that of the n = 40 channels there are m carrying a signal, where my is a predetermined value, in this case conveniently lying in the range between 5 and 10.Generally, the ratio m/n will lie in a range from 0.1 to 0.5 for evalution, and m is fixed at a corresponding intermediate value. It is advantageous to arranged for the value m to be variable, because, under the differing conditions of operation of the radar receiver, differing densities of occupation of the range channels by genuine moving echo signals may be expected. In regions where the signal population level is low, the value of m can be chosen to be relatively low, whilst in regions where there is a high moving echo density, the value of m should be correspondingly higher. The numerical value at which the anti-jamming circuit SU responds is advantageously contained in a store which may be construc tibnally integrated with the counter circuit 13 and is not independently illustrated.It is also possible to employ the limiting value of the counter as the numerical value of m.

The numerical value which leads to triggering of the anti-jamming circuit can be made direction-dependent, i.e. in one specific spatial zone swept by the directional antenna of Figure 1, the value of m may be set relatively high, whilst in another zone the value m is made lower. The control of the value of m can conveniently be effected by means of the sweep generator connected to the antenna in such an embodiment.

Whereas the first threshold stage 11 is in series in the video line VL, the second stage 1 la is in a branch line from the video line VL. The output of the second threshold stage 1 la is connected to the counter circuit 13. The threshold value of the stage 11 is so adjusted that with undisturbed operation, the requisite false alarm rate (CFAR) of the receiver, which should be constant if possible, is maintained. Via the threshold stage 1 la, pulses are supplied to the counter circuit 13 which may stem from false alarms, noise peaks, jamming interference, and genuine targets.The threshold level of the threshold stage 1 la will conveniently be lower than that of the threshold stage 11, and it is convenient to arrange for the threshold of the stage 1 la to be lower by such an amount that for each level, the requisite false alarm rate is achieved. If, per receiving period T = 1/fop (fp = pulse repetition frequency), there are more than m occasions of overshooting the threshold of the stage 1 la, then anti-jamming operation is initiated.This produces the following reactions: 1 ile control circuit 12 raises the threshold of the stage 11, preventing jamming signals from reaching the analyser or display unit 6 unmodified. - Moreover, conveniently through the agency of the control circuit 12, the threshold of the stage 1 la is likewise raidsed. It may be convenient to arrange for the two thresholds to rise in a timedependent fashion, advantageously at a uniform - rate. Furthermore, the change in the threshold level or levels can be contrived in such a fashion that each time the value m is exceeded, the threshold level or levels 'is or are raised by a specific value.

In the next receiving period, prior to the commencement of which the count - is conveniently reset to zero, the counting opera7 tion is repeated. If the number of pulses is again in excess of the fixed, value m, then the threshold values of the stages 11 and 1 la is again increased, but if this is not so; then advantageously instead Qf increasing the threshold values of the' two stages 11 and 1 la, a reduction is initiated, this being repeated until the threshold values corresponding to undisturbed operation, are re gained. This ensures that the threshold values follow the magnitude of the jamming interference, and in so doing they are alternately above or below this.

In Figure 3 the threshold values W of the stages 11 (threshold value Will) and 1 Ia (threshold value Wlla) have been plotted as a function of time t. It has been assumed here that a jamming signal ST is being produced which continues for a protracted period of time (e.g. the time taken for the radar receiver to sweep over the target). This results in an increase in the threshold value up to a limiting value which is such that the jamming signal ceases to be displayed.

As the radar sweeps over the jamming source, subsequently a drop in the jamming signal ST occurs. It can clearly be seen how the threshold value Wlla hunts about the jamming voltage because of the fact that the jamming signals do not in reality rise uniformly due to the presence of noise peaks.

The interval between the threshold value W11 of the stage 11 and the threshold value Wlla of the stage 11a must be just sufficient to ensure that at the times when jamming signals ST exceed the threshold value of the stage 1 la, the threshold level of stage 11 is not exceeded, because otherwise the jamming source would be displayed. This interval is of critical importance with regard to the probability of detection of targets whose echo signal power is not substantially stronger than the jamming power. The greater the interval between the two threshold levels, the greater must be the power difference between the target and jamming sources.The interval can be made smaller, the smaller are the ampliutdes of the hunting actions of the threshold levels, and this amplitude depends, amongst other things, upon the speed with which the threshold levels change.

If the display of a jamming source upon the screen or its indication by the analyser device, is suppressed, then conveniently the fact that there is jamming interference present will be indicated in some fashion or other. Furthermore, the strength of the jamming transmitter and in particular its direction, may be of interest. These three pieces of information can be provided in the form of a so-called amplitude-versus-azimuth (AVA) display, in which the screen shows a trace whose interval from the screen centre is a measure of the intensity of the jamming signal from the particular direction.

The production of such an AVA-display is particularly simple because an electrical quantity proportional to the jamming signal is already available, in the form of the threshold voltage, and this simply has to be converted to a form suitable for display upon the screen.

WHAT WE CLAIM IS:- 1. A Doppler pulse radar receiver in which a plurality n of separate range channels containing moving echo filters are provided, said channels being successively connected to the input section of the receiver, their outputs being connected via a common line to a common display or analyser unit.

and in which an anti-jamming circuit is arranged between the output of said range channels and the input of said display or analyser unit.

2. A Doppler pulse radar receiver as claimed in Claim 1, in which each of said range channels incorporates a respective amplifier having a non-linear gain characteristic relative to the input amplitudes, thus producing dynamic compression.

3. A Doppler pulse radar receiver as claimed in Claim 2, in which each said amplifier has a logarithmic function.

4. A Doppler pulse radar receiver as claimed in any preceding Claim, in which means are provided to determine the total energy of the output signals from all or a group of said range channels at any instant, and said means automatically adjust the threshold level for the display or analysis of received signals, which is set a specific amount above the mean value of said total energy.

5. A Doppler pulse radar receiver as claimed in any preceding Claim, in which a monitoring circuit is provided which responds when at least a predetermined number m of said n range channels is occupied by output signals, to indicate the presence of jamming and initiate operation of the anti-jamming circuit.

6. A Dopper pulse radar receiver as claimed in Claim 5, in which the ratio m/n is between 0.1 and 0.5.

7. A Doppler pulse radar receiver as claimed in Claim 5 or Claim 6, in which said monitoring circuit determines the number of range channels which are occupied by output signals during each respective receiving period.

8. A Doppler pulse radar receiver as claimed in any one of Claims 5 to 7, in which said monitoring circuit incorporates a counter circuit connected to the outputs of said range channels to determine the number of said range channels occupied by output signals, and a storage device is provided to store the permissible number of occupied channels, and when said number is exceeded, said storage device activates said anti-jamming circuit.

9. A Doppler pulse radar receiver as claimed in any one of Claims 5 to 8, in which means are provided to adjust the

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

Claims

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

and 1 la, a reduction is initiated, this being repeated until the threshold values corresponding to undisturbed operation, are re gained. This ensures that the threshold values follow the magnitude of the jamming interference, and in so doing they are alternately above or below this.

In Figure 3 the threshold values W of the stages 11 (threshold value Will) and 1 Ia (threshold value Wlla) have been plotted as a function of time t. It has been assumed here that a jamming signal ST is being produced which continues for a protracted period of time (e.g. the time taken for the radar receiver to sweep over the target). This results in an increase in the threshold value up to a limiting value which is such that the jamming signal ceases to be displayed.

As the radar sweeps over the jamming source, subsequently a drop in the jamming signal ST occurs. It can clearly be seen how the threshold value Wlla hunts about the jamming voltage because of the fact that the jamming signals do not in reality rise uniformly due to the presence of noise peaks.

The interval between the threshold value W11 of the stage 11 and the threshold value Wlla of the stage 11a must be just sufficient to ensure that at the times when jamming signals ST exceed the threshold value of the stage 1 la, the threshold level of stage

11 is not exceeded, because otherwise the jamming source would be displayed. This interval is of critical importance with regard to the probability of detection of targets whose echo signal power is not substantially stronger than the jamming power. The greater the interval between the two threshold levels, the greater must be the power difference between the target and jamming sources.The interval can be made smaller, the smaller are the ampliutdes of the hunting actions of the threshold levels, and this amplitude depends, amongst other things, upon the speed with which the threshold levels change.

If the display of a jamming source upon the screen or its indication by the analyser device, is suppressed, then conveniently the fact that there is jamming interference present will be indicated in some fashion or other. Furthermore, the strength of the jamming transmitter and in particular its direction, may be of interest. These three pieces of information can be provided in the form of a so-called amplitude-versus-azimuth (AVA) display, in which the screen shows a trace whose interval from the screen centre is a measure of the intensity of the jamming signal from the particular direction.

The production of such an AVA-display is particularly simple because an electrical quantity proportional to the jamming signal is already available, in the form of the threshold voltage, and this simply has to be converted to a form suitable for display upon the screen.

WHAT WE CLAIM IS:- 1. A Doppler pulse radar receiver in which a plurality n of separate range channels containing moving echo filters are provided, said channels being successively connected to the input section of the receiver, their outputs being connected via a common line to a common display or analyser unit.

and in which an anti-jamming circuit is arranged between the output of said range channels and the input of said display or analyser unit.
2. A Doppler pulse radar receiver as claimed in Claim 1, in which each of said range channels incorporates a respective amplifier having a non-linear gain characteristic relative to the input amplitudes, thus producing dynamic compression.
3. A Doppler pulse radar receiver as claimed in Claim 2, in which each said amplifier has a logarithmic function.
4. A Doppler pulse radar receiver as claimed in any preceding Claim, in which means are provided to determine the total energy of the output signals from all or a group of said range channels at any instant, and said means automatically adjust the threshold level for the display or analysis of received signals, which is set a specific amount above the mean value of said total energy.
5. A Doppler pulse radar receiver as claimed in any preceding Claim, in which a monitoring circuit is provided which responds when at least a predetermined number m of said n range channels is occupied by output signals, to indicate the presence of jamming and initiate operation of the anti-jamming circuit.
6. A Dopper pulse radar receiver as claimed in Claim 5, in which the ratio m/n is between 0.1 and 0.5.
7. A Doppler pulse radar receiver as claimed in Claim 5 or Claim 6, in which said monitoring circuit determines the number of range channels which are occupied by output signals during each respective receiving period.
8. A Doppler pulse radar receiver as claimed in any one of Claims 5 to 7, in which said monitoring circuit incorporates a counter circuit connected to the outputs of said range channels to determine the number of said range channels occupied by output signals, and a storage device is provided to store the permissible number of occupied channels, and when said number is exceeded, said storage device activates said anti-jamming circuit.
9. A Doppler pulse radar receiver as claimed in any one of Claims 5 to 8, in which means are provided to adjust the

permissible number m of channels occupied at any instant.
10. A Doppler pulse radar receiver as claimed in Claim 9 in which said permissible number m is modified as a function of the angular position of the radar antenna at any instant.
11. A Doppler pulse radar receiver as claimed in any preceding Claim, in which a first threshold stage is provided in the video signal line to said display or analyser unit.
12. A Doppler pulse radar receiver as claimed in Claim 11, in which the threshold value set at said first threshold stage is the lowest value obtained in the case of undistured operation and giving the requisite false alarm rate for the particular radar receiver.
13. A Doppler pulse radar receiver as claimed in Claim 11 or Claim 12, in which a second threshold stage is provided in a branch line from said video line leading to said display or analyser unit, the output of which second threshold stage is connected to the monitoring circuit responsible for activating the anti-jamming circuit.
14. A Doppler pulse radar receiver as claimed in Claim 13, in which said threshold value of said second threshold stage is lower than that of said first threshold stage.
15. A Dopper pulse radar receiver as claimed in Claim 14, in which the threshold value of said second threshold stage is lower than that of said first threshold stage by an amount such that the requisite false alarm rate is achieved for each level.
16. A Doppler pulse radar receiver as claimed in any one of Claims 11 to 14, in which said threshold of said first stage is raised whenever said monitoring circuit responds.
17. A Doppler pulse radar receiver as claimed in any one of Claims 13 to 16, in which said threshold of said second threshold stage is raised whenever m is exceeded.
18. A Doppler pulse radar receiver as claimed in Claim 16 or Claim 17, in which said threshold or thresholds is or are raised by the same amount each time m is exceeded.
19. A Doppler pulse radar receiver as claimed in Claim 17 when dependent upon Claim 16, in which the raising of both thresholds takes place at a constant rate each time m is exceeded.
20. A Doppler pulse radar receiver as claimed in any one of Claims 16 to 18, in which the arrangement is such that if the number of range channels occupied by signals in one period is the same or greater than that during the preceding period the threshold or thresholds is or are raised during the next receiving period, and is or are progressively reduced if the said number is less than that during the preceding period.
21. A Doppler pulse radar receiver as claimed in Claim 13, or any one of Claims 14 to 20 when dependent upon Claim 13, in which the magnitude of the difference between the level of the threshold of said first stage and that of said second stage is such that when the threshold of the second stage is exceeded by output signals, the received signals yield no target display or target analysis.
22. A Doppler pulse radar receiver as claimed in any preceding Claim, in which a display of the received jamming power is provided on the display unit in a form which differs from that employed for the display of echoes from any genuine target.
23. A Doppler pulse radar receiver as claimed in Claim 22, in which said display of the received jamming power is contrived in accordance with the magnitude and direction of the jamming signal.
24. A Doppler pulse radar receiver substantially as described with reference to Figure 1, or as described with reference to Figure 1 when modified by Figure 2.