GB2318011A - Simulator for radar returns - Google Patents

Abstract

A pulse radar simulator (1) comprises an aircraft (2) carrying a passive transponder incorporating a control unit (8), a delay line unit (6) and a dipole antenna (4). The delay line unit (6) comprises an acoustic delay line (18), a switch element (14) comprising a field-effect transistor for enabling or disabling the acoustic delay line (18), and an interface element (16) comprising a photodiode which is receptive to optical signals for controlling the switch element (14). The delay line unit (6) associated with the dipole antenna (4) imposes time delay between the incidence of investigative radiation on the dipole antenna (4) and the emission of corresponding reflected radiation from the dipole antenna (4). Multiple variable delays allow fuller simulation of targets. The transponder is controlled optically.

Description

ACTIVE SIMULATOR This invention relates to simulators for simulating real objects or environments.

Simulators may be ground-based or, alternatively, airborne. In the case of airborne simulators, sometimes referred to as "active chaff", small unmanned aircraft comprising passive reflectors are often employed to simulate large aircraft. Airborne object location and assessment are often achieved using radar techniques which may utilise electromagnetic radiation in the microwave part of the electromagnetic radiation spectrum, namely in the order of several GHz frequency. For this reason, microwave reflecting properties of a simulator may be very important to its simulation function. In order that a simulator should provide a more effective simulation, it is desirable that a microwave radar-beam reflection from the simulator should appear to a radar system interrogating the simulator as similar as possible to a microwave radar-beam reflection from a corresponding real object.

It is an object of the invention to provide an alternative form of simulator.

The present invention provides a simulator comprising radiation reflecting means incorporating signal-delaying means operative to impose time delay between receipt of incident radiation by the reflecting means and emission of response radiation therefrom.

The invention provides the advantage that time-delayed reflections from a simulator give the impression of a more distant object to an interrogating radar system, thereby simulating a large remote object to that system.

Signal time delay in the simulator may be provided by means of acoustic-wave delay lines. Moreover, a dipole antenna may be included in such a simulator for receiving and emitting microwave radiation. Furthermore, a switch element or switching matrix may also be included in order for the signal time delay to be enabled or disabled as required.

In order that the invention might be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, in which Figure 1 illustrates a simulator into which a microwave delay-line reflector is mounted, Figure 2 is a schematic diagram of a delay-line reflector, Figure 3 illustrates a circuit incorporated into a delay-line reflector of Figures 1 and 2, Figure 4 is a schematic diagram of a delay-line reflector comprising a multiplicity of delay lines, and Figure 5 is a schematic diagram of a delay-line reflector comprising a multiplicity of individually-selectable delay lines.

Figure 6 is a schematic diagram of a delay-line reflector comprising a multiplicity of antennae and a multiplicity of selectable delay lines.

Referring to Figure 1, a simulator, indicated generally by 1, comprises an air-frame 2 having wings 3, an antenna 4 connected to a delay-line unit 6 and a control unit 8. A control link 10 in the form of an optical fibre connects the control unit 8 to the delay line unit 6. The antenna 4 is a microwave dipole located in the wings 3 of the simulator 1.

Referring now also to Figure 2, the antenna 4 and its associated delay-line unit are indicated generally by 12. The delay-line unit comprises a switch element 14, an interface element 16, an acoustic-wave delay line 18 and a termination connection to a signal ground 20. The antenna 4 is connected to a terminal 11 on the switch element 14. The switch element 14 has a terminal 12 which is connected to a terminal 13 on the acoustic-wave delay line 18. The interface element 16 has a terminal G2 which is connected to a terminal G1 on the switch element 14. A terminal G3 on the interface element 16 is connected to the control unit 8 through the control link 10.

The acoustic-wave delay line 18 has a terminal 14 which is connected to the signal ground 20.

Referring now to Figure 3, the switch element 14 incorporates a field-effect transistor 22 which has a drain electrode 24, a grounded source electrode 26 and a gate electrode 28. The interface element 16 incorporates a silicon photodiode 30 which is connected to the optical-fibre control link 10, and a ballast impedance 32.

Referring now to Figure 4, a delay-line reflector comprising a multiplicity of delay lines is indicated generally by 34. Three delay lines 18a,18b,18c have terminals 13a,13b,13C respectively which are connected in parallel to the terminal 12 of the switch element 14 in a more complex arrangement as compared to Figure 2.

Referring to Figure 5, an alternative arrangement of delay-line reflector in which the delay lines 18a,18b,18c have associated switch elements 14a,14b,14c with respective inputs 1,a,11b,11c connected in parallel to the antenna 4 is indicated generally by 36.

Referring now to Figure 6, a schematic diagram of a delay-line reflector incorporating a multiplicity of antennae and a multiplicity of selectable delay lines is indicated by 38.

The delay line reflector 38 comprises three antennas 4a, 4b, 4c, a signal switching matrix 40, four delay lines 18a, 18b, 18c, 18d, a signal ground 20 and an interface element 16. The antennae 4a, 4b, 4c are connected to terminals 11a, 11b,11C of the matrix 40 respectively. The matrix has four connection terminals 12a' 12b 12c 12d which are connected to terminals l3a, 13b, 13e and 13d on the delay lines 18a, 18b, 18c and 18d respectively. The delay lines 18a, 18b, 18c, 18d have terminals 14as 14b 14cw 14d respectively which are connected to the signal ground 20.

The mode of operation of the simulator 1 will now be described with reference to Figures 1 and 2. In the case of a simulator comprising a passive reflector but lacking a delay line, microwave radiation from an interrogating radar system incident upon the simulator is reflected from the passive reflector without delay and subsequently received by the radar system; the amplitude of the reflected microwave radiation received at the radar system together with the time interval between emitting the microwave radiation from the radar system and receiving the corresponding reflected microwave radiation at the radar system give an indication of the size of the passive reflector and its location in space. If, however, the microwave radiation incident on the simulator is delayed before being reflected, the delay will make the simulator simulate a more distant object to the radar system. Moreover, if the microwave radiation impinging on the simulator is delayed before being reflected and is of comparable intensity to a directly reflected undelayed signal, the radar system will perceive the delayed reflection to correspond to a more distant target having a large microwave reflection cross-section, that is a large remote object. The simulator 1 provides a delayed reflection of incident microwave radiation when the control unit 8 enables the delay line 6 for a delaying signal resulting from radiation received at the antenna 4. Microwave radiation is both received and subsequently transmitted from the antenna 4.

Referring to Figure 2, the acoustic-wave delay line 18 provides a propagation delay for microwave signals injected at the terminal 13 to reach the terminal 14. The terminal 14 is connected to the signal ground 20 so that a microwave signal which has propagated along the acoustic-wave delay line 18 to the terminal 14 will be reflected back into the acoustic-wave delay line 18 in a reverse direction and will be received again at the terminal 13. The acoustic-wave delay line 18, with a connection to the signal ground 20, is coupled via the switch element 14 to the microwave dipole antenna 4. Microwave radiation from an interrogating radar system (not shown) and incident on the dipole antenna 4 passes as a microwave signal through the switch element 14, from the terminal Ii to the terminal 12, to the terminal 13 of the acousticwave delay line 18. The signal propagates from the terminal 13 to terminal 14, from whence it is reflected back along the delay line 18 to the terminal 13, and then passes through the switch element 14 from the terminal 12 to terminal 11. It is then reradiated from the dipole antenna 4 to provide a delayed microwave reflection to be received by the interrogating radar system.

The acoustic-wave delay line 18 is fabricated from a piezo-electric material which may be either quartz, lithium niobate, lithium titanate or yttrium aluminium garnet in which an acoustic wave may propagate. Alternatively, the acoustic-wave delay line 18 may be fabricated from silicon onto which a zinc oxide piezo-electric transducing layer has been deposited. Acoustic-wave delay lines customarily exploit a substrate in which an acoustic wave propagates as a tightly bounded surface wave; such an acoustic surface wave is often referred to as a Rayleigh wave" or "surface-acousticwave" (SAW). Such delay lines propagating a SAW, referred as SAW delay lines, are able to operate efficiently with microwave signals whose frequency approaches 4 GHz; propagation attenuation through a SAW delay line may exceed 70 dB at frequencies above 4 GHz. At signal frequencies below 4 GHz, signal propagation delays of several microseconds may be achieved using a SAW delay line; such delays of several microseconds correspond to the propagation period required for free-space microwave radiation to traverse a distance of several kilometres.

In order to operate an acoustic-wave delay line efficiently at frequencies above 4 GHz, for example at X-band microwave frequencies between 9.2 GHz and 10.3 GHz employed in advanced modern radar systems, bulk acoustic wave propagation in a delay-line substrate may be utilised; such bulk acoustic wave propagation occurs with a low attenuation of approximately 10 dB at these X-band frequencies if the delay-line substrate is thinned to tens of micrometres in a direction perpendicular to the direction of propagation of an acoustic bulk wave along the thinned delay-line substrate. In this manner, delay lines providing delays of typically 200 nanoseconds are practicable which are able to offer a wide bandwidth in excess of 4 GHz together with approximately 10 dB attenuation. Such bulk acoustic delay lines may be used for the acoustic-wave delay line 18 to facilitate delayed reflection of microwave radiation incident upon the simulator at X-band frequencies which are employed in advanced modern radar systems.

Referring now to Figure 3, the switch element 14 allows the transmission of microwave signals between the terminals Ii and 12 to be controlled so as to enable or disable the delayed-reflection characteristic of the simulator 1. The switch element 14 comprises a wide-bandwidth field-effect transistor (FET) 22, such as a gallium arsenide FET. In order that the reflector 4 and the delay-line unit 6 should not require a power supply to operate, the interface element 16 comprises a photodiode 30 so that an optical signal, conveyed by means of the fibre-optic link 10, generates a bias voltage at the terminal G2 which connects via the terminal G1 to the gate electrode 28 of the FET 22. The bias voltage applied to the gate electrode 28 creates a highimpedance path between the drain electrode 24 and the source electrode 26 so that a microwave signal does not propagate through the FET 22 but is diverted to the delay line 18. In an absence of an optical signal and hence absence of a bias voltage, the ballast impedance 32 ensures that a low-impedance path is created between the drain electrode 24 and the source electrode 26 so that a microwave signal applied to the drain electrode 24 passes to the signal ground 20, is reflected at the signal ground, then passes from the source electrode 26 to the drain electrode 24 and is finally re-emitted without delay as radiation from the antenna 4 rather than passing to the delay line 18.

Referring now to Figure 4, the delay-line unit 6 comprise three delay lines 18a,18b,18c configured in parallel. These three delay lines provide propagation delays which are of approximately of similar duration but arranged to be slightly different from one another. These differences in propagation delay are necessary to simulate reflections from a large structure whose various parts reflect incident interrogating radiation at slightly different times on account of these various parts being at slightly different distances from an interrogating radar system. SAW delay lines are small light-weight inexpensive compact devices which are suitable for use in such a parallel configuration in simulators; these SAW delay lines are particularly suitable for simulators which are used as expendable targets. More than three delay lines 18 may be used in the simulator 1 in order to obtain a more complex, and hence more convincing, reflection from the simulator 1.

Referring now to Figure 5, the delay-line unit 6 comprises three switch elements 14a,14b,14c connected to three delay lines 18a,18b,18c where the interface element 16 controls each of the switch elements independently. The interface element 16 in this case is a modified form of that shown in Figures 3 and 4. The advantage of the configuration shown in Figure 5 is that the control unit 6 may periodically enable and disable one or more of the delay lines and thereby change the degree of complexity presented to an interrogating radar system.

A real non-spherical object subjected to an interrogating radar beam from a radar system provides a radar reflection which depends upon orientation of the object relative to the radar beam. Moreover, when the object changes orientation, for example when modifying its trajectory, its radar cross-section changes relative to the radar system. This orientation-dependent radar cross-section may be simulated by incorporating several antennae into the reflector 38 of a simulator as illustrated in Figure 6. In Figure 6, the antennae 4a, 4b, 4c exhibit directionality in their polar response characteristics and are orientated in different directions. The matrix 40 incorporates a number of electronic switches which are arranged to connect any of the antennae 4a, 4b, 4c to any of the acoustic-wave delay lines 18a, 18b, 18c,18d.

The matrix 40 connects selected delay lines 18 to selected antennae 4 in a combination specified in a signal sent via the control link 10. The matrix 40 enables a simulator, incorporating the delay line reflector 38, not only to simulate an object having radar cross-section area dependent upon radar interrogation direction but also to simulate an object which is rotating relative to the radar system. Moreover, the matrix 40 may selectively connect the antennae 4 to the delay lines for providing progressively longer or shorter delays in order to simulate an object receding or approaching the interrogating radar respectively even though the simulator itself may not necessarily be changing its location relative to the radar system. The number of antennae 4 and delay lines 18 connecting to the matrix 40 in Figure 6 may be increased, if necessary, for simulating a more complex object.

Altemative embodiments of the invention include submerged simulators, for use with submarines for example, responsive to ultrasonic (sonar) radiation where antennae may be implemented using piezo-electric transponders and the delay-line 18 may either be a SAW delay-line as described earlier or a digital delay line comprising analogue-to-digital converters, digital-to analogue converters and electronic memory devices such as a Random Access Memory (RAM).

Claims (19)

1. A simulator comprising radiation reflecting means incorporating signal delaying means operative to impose time delay between receipt of incident radiation by the reflecting means and emission of response radiation therefrom.

2. A simulator according to Claim 1 wherein the signal delaying means may be enabled or disabled.

3. A simulator according to Claim 1 or 2 including interfacing means for enabling or disabling the signal delaying means in response to control means.

4. A simulator according to Claim 1, 2 or 3 wherein the time delay is variable in response to the control means.

5. A simulator according to Claim 1,2, 3 or 4 installed in an aircraft.

6. A simulator according to Claim 1, 2, 3 or 4 installed in a land-based vehicle.

7. A simulator according to Claim 1, 2, 3 or 4 installed in an aquatic vessel.

8. A simulator according to any preceding claim responsive to microwave radiation.

9. A simulator according to any preceding claim wherein the reflecting means incorporates a microwave antenna.

10. A simulator according to Claim 9 incorporating multiple microwave antennae providing directional response and orientated in a plurality of directions.

11. A simulator according to Claim 9 or 10 incorporating a microwave dipole antenna.

12. A simulator according to any preceding claim wherein the signal delaying means is an acoustic-wave delay line.

13. A simulator according to Claim 12 wherein the acoustic-wave delay line is fabricated from quartz, lithium niobate, lithium titanate or yttrium aluminium garnet.

14. A simulator according to Claim 12 wherein the acoustic-wave delay line is fabricated from silicon having a zinc oxide surface transducing layer.

15. A simulator according to Claim 12, 13 or 14 wherein the acoustic-wave delay line propagates surface acoustic waves.

16. A simulator according to Claim 12, 13 or 14 wherein the acoustic-wave delay line propagates bulk acoustic waves.

17. A simulator according to any preceding claim wherein the interfacing means comprises a photodiode responsive to optical signals applied to the interfacing means.

18. A simulator according to Claim 1 wherein the signal delaying means incorporates a plurality of delays to provide for the simulator to be capable of simulating an extended object.

19. A simulator according to Claim 18 wherein at least one of the delays is switchable between enabled and disabled states.