GB2249890A - An Iff receiving device for use with laser interrogation units - Google Patents

Abstract

An IFF laser interrogation receiver achieves a constant false alarm rate with a regulating loop Fig. 3. The theoretical value UW, and the actual value UX derived by a retriggerable monostable W from the output of threshold K, of the false alarm rate are compared (DI) and used to control a control element from whose output a constant false alarm rate is obtained. The threshold circuit K having a controllable threshold value or a regulating amplifier ("video-amplifier") having a controllable amplification factor, can be used as the control element. <IMAGE>

Description

AN IFF RECEIVING DEVICE FOR USE WITH LASER INTERROGATION UNITS: The invention relates to an IFF receiving device having laser sensors for use with laser interrogation units, the useful signals of which sensors are obtained from the output of the receiving device via a threshold circuit, and a device being provided to maintain a constant false alarm rate (CFAR).

For IFF (identification friend-or-foe) using laser pulses in the near infra-red range for interrogation, receivers are used which contain one or more photo-diodes which supply for further processing an output comprising video pulses superimposed on noise.

The propagation of the laser signals is strongly influenced by atmospheric and weather conditions. Moreover, the noise level at the receiver input is dependent upon the background brightness, and therefore highly variable receiving conditions must be- expected, and it is not advisable to design the receiver with dimensions dependent upon the maximum occurring noise, but to match its sensitivity to the input noise as influenced by the currently prevailing environmental conditions, such as for example temperature and lighting, which are themselves dependent upon the time.

The video signals can be detected by the use of a signal threshold whose value must be reached in order that a useful signal may be detected.

To maintain a constant false alarm rate in the presence of such noise, it is already known to change the level of the detection threshold in dependence upon the long-term mean value of the noise power. Such adjustment of the threshold value has the disadvantage that if a comparator is used as a threshold device, the off-set voltage drift of the comparator disadvantageously influences the desired false alarm probability.

One object of the present invention is to provide an improved arrangement of the type described in the introduction, in which the constancy of the false alarm rate is enhanced.

In accordance with the present invention there is provided an IFF receiving device for use with laser interrogation, in which the useful signals of laser sensors are fed to the output of the receiving device via a threshold circuit, a device being provided to maintain a constant false alarm rate, those output signals which exceed the threshold (false alarms) being quantized in a retriggerable monostable flip-flop and integrated in an integration circuit to form an actual value of a false alarm rate, and the difference value, formed by comparison with a predetermined theoretical value being amplified and used to control a control element in a regulating circuit by way of regulation deviation, in such manner that a false alarm rate which is regulated to a constant value is obtained from the output of the threshold circuit.

In a receiving device designed in this way, during normal interrogation operation, for the majority of the operating time the laser sensor does not receive useful signals but supplies output signals resulting from received noise. On the other hand, the short signals used in laser interrogation require quantization following the threshold circuit and in the case of a low false alarm rate the quantized signals must be integrated in order to obtain clearly defined results.

The regulating circuit which serves to maintain a constant false alarm rate can be designed in such manner that a threshold device having a controllable threshold value is used as a control element. The level of the threshold value is dependent upon the magnitude of the regulation deviation, which is determined by comparing the theoretical value with the actual value.

If the threshold value is fixed, it is possible to use a regulating amplifier having a controllable amplification factor to serve as a control element.

To improve upon the one-sided dependence of the actual value formation upon the noise, the influence of the useful signals is advantageously taken into consideration by the use of a second threshold device having its own threshold value.

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 receiver for laser signals with a CFAR-circuit; Figure 2 is a fundamental circuit diagram of one regulating circuit for maintaining a constant false alarm rate, using a threshold value device as a correcting element; Figure 3 is a fundamental circuit diagram of a regulating circuit for maintaining a constant false alarm rate, using a regulating amplifier; Figure 4 shows circuit details of the regulating circuit illustrated in Figure 2; Figure 5 is a circuitry detail showing a modified regulating circuit for a constant false alarm rate using a controllable regulating amplifier as a control element with two threshold devices having different threshold values; and Figure 6 illustrates a threshold value device with a S/N-dependent threshold value regulation unit.

The receiver circuit illustrated in Figure 1 consists of a pluality of laser sensors, in this case six are represented, each having their own regulating circuit, S1 to S6, whose respective outputs are connected to a common signal processing circuit SV via a logic OR-gate LO. The actual sensors are formed by photo-diodes, Phl to Ph6, and each of the photo-diodes has its output connected via a controlled regulating amplifier, V1 to V6 respectively, to separate threshold devices, SE1 to SE6 respectively, which form part of the regulating circuit. Any voltage peak which exceeds a predetermined, set threshold value is regarded as a useful signal. The digitallised output signals of the sensor elements are combined by the logic OR-gate LO.

Each individual photo-diode can only receive signals from a limited spatial angle. To obtain a desired uniform sensitivity of the receiver in all directions at least six laser sensors S1 to S6 must be uniformly distributed in the laser receiver. All the laser sensors of the laser receiver are of identical design. From the output of the logic OR-gate the useful signals are fed for quantization and demodulation, to a synchronisation and detection stage in the common signal processing circuit SV, whose output passes the data on to a crypto-analyser for further processing.

The function of the regulating circuit in each of the laser sensors is to relate the threshold value to the noise level in such a manner as to set a constant false alarm rate. For this purpose either a controllable regulating amplifier or a comparator having a controllable threshold value can be used as a control element in the regulating circuit.

The circuitry and mode of operation of the regulating circuit in the laser sensors,S1 to S6, to ensure for the production of a constant false alarm rate will now be described with reference to the exemplary circuit shown in Figure 2. In this regulating circuit the input is fed via an adder, a comparator K having a controllable threshold value is used as a control element. Only those signals whose peaks exceed the threshold value are passed to the output FAR of the comparator. These output signals are fed back via a converter circuit W arranged in a feedback arm of a regulating loop containing a regulator. The converter circuit consists of a retriggerable monostable flip-flop which serves to quantize the generally very short pulse-like output signals. In dependence upon the desired false alarm rate, then, integration is carried out for a greater or ]esser number of quantized signals in order to obtain an accurate actual value ux which is compared with a predetermined theoretical value uw.

The difference between the theoretical and actual values (regulation deviation uw-ux) is formed in a difference integrator DI, whereupon it is amplified and used to control the threshold value in the comparator K. Where the threshold value tends towards increase, the number of times when the threshold is exceeded (false alarms) is reduced and thus a reduction in the actual value ux is achieved.

Finally, via the feedback of the actual value to the control element (comparator), a constant false alarm rate is automatically achieved at the output of the regulating loop.

Alternatively, in accordance with an exemplary embodiment illustrated in Figure 3, the regulating loop can be designed in such manner that in place of a controllable threshold value, a controllable amplification of a regulating amplifier is used to carry out the regulation. The regulating amplifier then has the function of the control element whereas the comparator operates with a fixed threshold value. The mode of operation and circuitry construction of the two regulating circuits is identical in the upper parts of the circuit.

Since a comparator is used as signal threshold in both exemplary embodiments, the setting of the false alarm probability is in each case dependent upon the offset voltage drift of the comparator. In order to avoid this disadvantageous influence the offset voltage of the comparator may be fed into the regulating circuit as a disturbance variable via an adder prior to the control element, for example, and in this way is directly compensated. Other disturbance variables which occur elsewhere can be included into this disturbance variable compensation.

A regulating circuit corresponding to that illustrated in Figure 2 constructed from integrated circuit elements is shown in Figure 4. The formation and amplification of the regulation deviation (uw-ux), the theoretical value uw, and the actual value ux of the false alarm rate FAR are separately conducted to the inputs of an integrator OP and at the same time the false alarms are integrated for a predetermined period of time. The amplified regulation deviation UR is fed via a voltage divider R1, R2 to one input of a comparator Kp, whose second input is supplied with the noise signal N and any video signal which is to be detected. An arbitrary disturbance variable z can be superimposed upon that component u of the regulation deviation UR which is tapped from the voltage divider.The signals FAR which are obtained from the output of the comparator Kp, and which possess a constant false alarm rate are also fed for digital further processing to the feedback loop of the regulating circuit. Quantization of the pulse duration of the very short, pulse-like signals is carried out in a retriggerable monostable flip-flop having a reset input J and an output Q, which supplies the actual value ux for comparison with the theoretical value uw.

In order to obtain an accurate actual value for the production of the threshold value, the regulating circuit must integrate a very large number of false alarms. Accordingly the integration time constant must be very high in comparison to the average expected time between two false alarms. In the case of low false alarm rates the regulating circuit consequently becomes very sluggish. This undesirable behaviour can be eliminated by a simple modification of the exemplary embodiment illustrated in Figure 2, as illustrated in Figure 5. Parallel to the comparator A of the regulating circuit illustrated in Figure 2 and Figure 3, a second comparator B is provided, whose threshold value u' differs from the threshold voltage u of the comparator A.As a result of the different threshold voltage the comparator B produces a constant false alarm rate FAR' which differs from the false alarm rate FAR of the comparator A. The new threshold voltage u' can be easily derived from the regulation deviation UR, for example by means of a voltage divider Rll/R12.

A high detection probability at a predetermined signal/ noise ratio is achieved by means of a regulating circuit which also possesses a high false alarm probability. For larger signal/ noise ratios the detection probability very soon tends towards a limit value. Figure 6 represents a circuit which employs two comparators whose threshold values are set in accordance with two different criteria.

The criteria are governed by a small signal/noise ratio in the case of threshold 1 and by a larger signal/noise ratio in the case of threshold 2. If the threshold voltage u2 of the threshold 2 becomes greater than the threshold voltage ul of the threshold 1, then the output is switched over from threshold 1 to threshold 2. In this way the advantage of a regulating circuit for a constant false alarm rate with a high false alarm probability is expolited, and the disadvantage of a constant bit error rate with a high signal/noise ratio is avoided.

Under the simplest circumstances the threshold voltage u2 is produced by a high-speed rectifier G1 followed by a capacitor C acting as a charge store. This has the advantage that it is possible to detect even the second pulse with the most favourable threshold. If sources of interference must be taken into consider ation during the laser interrogation, it is advisable to integrate a plurality of pulses for the threshold regulation.

Claims (3)

Claims:

1. An IFF receiving device for use with laser interrogation, in which the useful signals of laser sensors are fed to the output of the receiving device via a threshold circuit, a device being provided to maintain a constant false alarm rate, those output signals which exceed the threshold (false alarms) being quantized in a retriggerable monostable flip-flop and integrated in an integration circuit to form an actual value of a false alarm rate, and the difference value, formed by comparison with a predetermined theoretical value being amplified and used to control a control element in a regulating circuit by way of regulation deviation, in such manner that a false alarm rate which is regulated to a constant value is obtained from the output of the threshold circuit.

2. A receiving device as claimed in Claim 1, in which the regulating circuit includes an amplifier for constant amplification of the regulation deviation and a comparator having a controllable threshold value as a control element.

3. A receiving device as claimed in Claim 1, in which the regulating circuit includes an amplifier which is controllable by the difference value as a control element, a comparator having a constant threshold value being provided.

3. A receiving device as claimed in Claim 1, in which the regulating circuit includes an amplifier which is controllable by the regulation deviation as a control element, a comparator having a constant threshold value being provided.

4. A receiving device as claimed in Claim 3, in which the regulating circuit has a first comparator with a predetermined or controllable threshold value connected in parallel at the input end to a second comparator having a different threshold value.- 5. A receiving device as claimed in Claim 4, in which the threshold value of the second comparator is derived from the threshold value of the first comparator by means of a voltage divider.

6. A receiving device as claimed in any preceding Claim, in which a threshold regulation which is dependent upon the signal/noise ratio is carried out in such manner that two threshold devices having different threshold values are provided, whose outputs are selectively connected to further processing devices in dependence upon the value ratio of the threshold values, the first threshold value device having the threshold value for a small signal/noise ratio being arranged in the regulating circuit, and the threshold value of the second threshold value device for a high signal/noise ratio being derived from the signal pulses.

7. An IFF receiving device substantially as described with reference to Figure 1 and any one of Figures 2 to 6.

AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS.

1. An IFF receiving device for use with laser interrogation, in which the useful signals of laser sensors are fed to the output of the receiving device via a threshold circuit, a device being provided to maintain a constant false alarm rate, those output signals which exceed the threshold (false alarms) being quantized in a retriggerable monostable flip-flop and integrated in an integration circuit to form an actual value of a false alarm rate, and the difference value, formed by comparison with a predetermined theoretical value being amplified and used to control a control element in a regulating circuit in such manner that a false alarm rate which is regulated to a constant value is obtained from the output of the threshold circuit.

2. A receiving device as claimed in Claim 1, in which the regulating circuit includes an amplifier for constant amplification of the difference value and a comparator having a controllable threshold value as a control element.