GB2322260A

GB2322260A - Reducing reflections of electromagnetic signals

Info

Publication number: GB2322260A
Application number: GB8410947A
Authority: GB
Inventors: Kenneth Alfred James Warren
Original assignee: General Electric Company PLC
Current assignee: General Electric Company PLC
Priority date: 1984-04-28
Filing date: 1984-04-28
Publication date: 1998-08-19
Anticipated expiration: 2004-04-28
Also published as: GB8410947D0; GB2322260B

Abstract

To prevent detection by an enemy radar, a target is provided with a monopulse receiver 1, 2, 3, 4, 5, 6, 7, 8 producing an output representing the direction 0 of the enemy radar. A sample of the received signal is amplified and re-transmitted after being subjected to phase and amplitude adjustment at 14 and 15 in such a way that the re-transmitted signal cancels a reflection of the received signal from the target. Store 12 contains the parameters necessary for each reception direction to effect the appropriate phase and amplitude control.

Description

APPARATUS FOR REDUCING REFLECTIONS OF ELECTROMAGNETIC SIGNALS This invention relates to an apparatus for reducing reflections of electromagnetic signals from a target. It was developed for use on board a craft, e.g. an aircraft, boat or spacecraft to reduce the chances of detection by radar systems but it would also be applicable to other potential targets e.g. land-based targets.

The reduction of radar echoes has been attempted in the past with some degree of success by the use of a lossy material applied to the target's surface and serving to absorb incident energy. Another known technique is to modify the geometry of the target so as to deflect as much of the incident energy as possible into directions other than the direction from which it arrives. Whilst both of these techniques do reduce radar returns they are of limited application since the properties of the lossy material and the required changes in geqmetry may well not coincide with other requirements for the surface of the target.

This invention provides apparatus for reducing reflections of electromagnetic signals from a target comprising: a store defining, for each of a plurality of defined directions, characteristics of a cancellation signal required to cancel a reflection, from the target, of a signal received from that direction; means for detecting the direction of a received signal, and for obtaining, from the store, the said characteristics appropriate to that direction; and transmission means responsive to a received signal to transmit a cancellation signal having the characteristics obtained from the store.

The aforementioned characteristics are preferably phase and attenuation (or amplification) values. In a preferred form of the invention this phase shift and attenuation (or amplification) is applied to a received signal which is then re-transmitted. In this way it can be ensured that the frequency and amplitude of the re-transmitted signal is determined by the frequency and amplitude of the received signal thereby ensuring the maximum degree of cancellation.

The store preferably has a storage area or location associated with each of the said directions, each storage area containing values defining the aforementioned characteristics. Another possible technique would be for the store to define a mathematical relationship between the said values and the said directions. This however is less preferable since the relationship would be very complex except for targets having a very simple shape.

The apparatus of the invention is considered to be primarily applicable to the cancellation of radar signals.

However the cancellation of other signals, e.g. in the optical or infra-red regions of the electromagnetic spectrum could also obviously be of benefit e.g. to prevent detection by active detecting systems operating at those wave-lengths.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying drawing which shows a radar echo canceller constructed in accordance with the invention.

Referring to the drawing, the illustrated radar echo canceller can be one of a number mounted on an aircraft, not shown, and includes an amplitude monopulse receiver built along conventional techniques and including: two antenna elements 1 and 2, a hybrid junction 3; a T/R device 4; a local oscillator 5, mixers 6 and 7; and a phase sensitive detector 8 whose output represents the angular error 0. In this system this angular error represents the direction, relative to a boresight 9, of receipt of a signal from an enemy radar 10.

The error signal from the detector 8 is converted into digital form at 11 and is used to read out from a store 12 two values and a stored in a location corresponding to the angle 6.

A coupler 13 passes a sample of the received signal, via a variable phase shifter 14, a variable attenuator 15, and an amplifier 16, back to the T/Rdevice 4; from whence it is re-transmitted via the hybrid junction 3 and antenna elements 1 and 2.

The values and a are chosen before being originally entered in the store so that the re-transmitted signal has, in front of the antenna elements, the same amplitude but opposite phase to the received signal after the latter has been reflected from the aircraft. The values are preferably determined empirically during experiments in which radiation is directed at the aircraft (or other target). During the experimentsthe phase shifter and attenuator are adjusted until the radar return is a minimum, indicating that the maximum degree of cancellation has been achieved.

The illustrated embodiment is of course only one example of how the invention may be implemented. In an alternative construction the phase and amplitude adjustment can be effected at IF in which case the output of the local oscillator 5 will be mixed with the input to phase shifter 14 and with the output of attenuator 15. In another construction the store 12 could contain an equation or a set of equations defining the required phase and attenuation values as functions of the error angle 8.

In yet another modification the amplitude-comparison monopulse receiver could be replaced by a phase-comparison monopulse receiver or indeed any other direction-finding apparatus.

As an additional sophistication, means could be included for transmitting the cancellation signal primarily towards the enemy radar 10 rather than along the boresight 9 as is the case with the illustrated system.

This could be done by using a separate transmitting antenna steered into the appropriate direction. A further possibility is to include a device for measuring the instantaneous frequency of the incident energy and for controlling the local oscillator 5 accordingly. In such a system the store 12 would need to contain sets of details of the target polar diagram for each frequency, the appropriate set being selected by the output of the frequency measuring device.

Finally it is pointed out that one canceller could well be insufficient for a complex target such as an aircraft, even over a limited aspect angle. A number of similar cancellers, each situated at a gross reflection point, has been found to give a particularly good degree of cancellation in such situations.

Claims

1. Apparatus for reducing reflections of electromagnetic signals from a target comprising: a store defining, for each of a plurality of defined directions, characteristics of a cancellation signal required to cancel a reflection, from the target, of a signal received from that direction; means for detecting the direction of a received signal, and for obtaining, from the store, the said characteristics appropriate to that direction; and transmission means responsive to a received signal to transmit a cancellation signal having the characteristics obtained from the store.

2. Apparatus according to claim 1 in which the said characteristics include phase-shift and attenuation or amplification values.

3. Apparatus according to claim 2 in which the transmission means includes means for applying a phase shift and attenuation to a received signal and means for re-transmitting the received signal after the said phase shift and attenuation have been applied.

4. Apparatus according to any preceding claim in which the store has a storage area associated with each direction, containing values defining the said characteristics.

5. Apparatus according to claim 1, substantially as described with reference to the drawings, and substantially as illustrated therein.