GB2198905A - Radar jammer apparatus - Google Patents

Abstract

A radar jammer apparatus comprises a CW oscillator 11 the output of which is coupled to a biphase modulator 12 fed with pseudorandom digital noise 13. This provides an output signal whose bandwidth may be controlled by control of the noise characteristics to provide spot or barrage jamming. Thus, jamming may be effected against radars whose characteristics may or may not be known. <IMAGE>

Description

RADAR JAMMER APPARATUS This invention relates to apparatus for generating and transmitting noise signals for radar jamming application.

Conventional jammer systems for use against unfriendly radars usually include the capability to radiate a noise modulated signal at or near the frequency of the victim radar. If the characteristics of the victim radar are well known, and in particular if its frequency is precisely established, then the jammer power can be restricted to the victim bandwidth. This technique is known as spot noise jamming. If, however, the characteristics of the victim radar are not well known, or if it is equipped with a 'frequency hopping' facility as a countermeasure against spot jamming, then, to be effective, the jammer power must be spread over a band of frequencies to ensure that the victim frequency is covered. This is known as barrage jamming. The particular jamming technique employed is thus dependent on the nature of the radar to be jammed.However, conventional jammers do not have the ability to provide both barrage and spot jamming without a considerable duplication of equipment. This is a particular disadvantage for airborne applications where both weight and power requirements are at a premium.

The object of the present invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a radio jammer apparatus adapted to provide both spot and barrage jamming, the apparatus including a continuous wave oscillator, and means for modulating the oscillator with a digital noise signal of controllable code length so as to provide an output signal of controllable bandwidth.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a schematic diagram of the radar jammer apparatus; Fig. 2 shows a digital noise generator for use in the jammer apparatus of Fig. 1; and Fig. 3 illustrates the power spectrum output of the jammer.

Referring to Fig. 1, the jammer apparatus includes a continuous wave (CW) oscillator 11 the output of which is fed to a biphase modulator 12. A suitable biphase modulator is described in "Microwave Diode Control Devices" by R.V. Garrer, At'etch House 1976, ISB 0-89006-022-3. The modulator 12 is supplied with digital noise signals from a pseudo-random noise generator 13. It will be appreciated that purely random digital noise could also be employed.

However the generation of purely random digital noise signals is a somewhat complex operation whilst we have found that pseudo-random noise in the form of pseudo-random numbers can be generated by simple circuits and is equally effective for the present application.

The modulated output from the biphase modulator 13 is fed to an antenna 14. In some applications an amplifier 15 may be used to couple the modulator to the antenna.

Because of the constant amplitude nature of the signal produced by the modulator this amplifier 15 can be a limiting amplifier. Unlike conventional systems it is not necessary to use a relatively costly linear amplifier for this purpose.

Fig. 2 shows one form of digital noise generator.

The noise is provided by a pseudo-random number generator comprising a shift register 21 of n bits providing an output code length L of 2 -1. Selected stages of the register are fed back via a gate array 22 to the register input. Timing of the register is provided by a clock 23 of frequency F. The clock frequency determines the data bit duration Tp which is equal to /F. The code length can readily be altered by the use of one or more further gate arrays 221 which arrays provide feedback from stages only part way along the register. Selection of a particular gate array, and thus selection of a particular code length L, is effected via control 24.

Methods of generating random number sequences are described in !'Spread Spectrum in Communications" R. Skaug, J.F. Hjelmstrad (IEE Telecommunications Series 12) Peter Peregrinus, London, 1985 ISBN 0-863410034-0.

The purpose of providing for control of code length and bit duration will become apparent from the following description with reference to Fig. 3 which illustrates the power spectral density of the output from the modulator 13 of Fig. 1. As can be seen from Fig. 3 the spectrum comprises a series of lines of noise emission bounded by an envelope having the general shape of the function (sin x)/x. In Fig. 3 this function is written as sinc x, S(f) is the spectral power density function, is the carrier frequency and & is the Dirac delta function which is equal to 1 when the argument is zero and is equal to zero for all other values. The main features of the spectrum are as follows:1. The spectrum is centred on the CW oscillator frequency Fc.

2. The first null occurs at Fc + the chipping rate, i.e. at e 1/T . The chipping rate corresponds to the p number of pseudo-random noise symbols per second.

3. The spectral lines occur at multiples of /L times the chipping rate where L is the code length.

4. The power at the centre frequency Fc, i.e. at the singularity, corresponds to the original CW generator power 12 reduced by a factor of 1/L2.

For example, if we choose a chipping rate of 13 50 MHz and a code length of 8191 (i.e. 2 -1) then the first nulls will be at Fc + 50 MHz and the line spacing will be 6.1 KHz. The -3 dB point will be at F Fcfi 22 MHz approximately.

If the chipping rate is reduced by a factor of N, e.g. by inserting a digital divider into the clock path of the PRN generator, then both the total bandwidth of the signal and the spectral line spacing are reduced by the same factor N. Hence a 'spot noise' requirement can be implemented using the same hardware as a 'barrage noise' requirement simply by dividing the chipping rate by a suitable factor.

If the code length is increased e.g. by using a longer shift register within the PRN generator whilst maintaining the chipping rate constant, then the spectral line spacing is decreased but the total bandwidth remains constant.

Conversely, the total bandwidth may be reduced whilst maintaining the same spectral line spacing by reducing both the clock rate and the code length together in proportion.

It may be desirable to ensure that all possible frequencies within the total bandwidth receive at least some jammer energy - in other words to break up the discrete spectral lines as far as possible. This can be done (a) by applying linear frequency modulation, e.g. ramp or sine wave or noise-like, to the PRN generator clock frequency, or (b) by applying pseudo-random frequency modulation to the PRN clock generator. The former method is suitable for analogue techniques, whilst the latter is more suited to digital techniques.

The jammer apparatus described herein is particularly suited to airborne applications but it may of course also be used in marine or ground based applications.

The technique may also be used for the jamming of radio communications signals.

Claims (9)

CLAIMS:-

1. A radio jammer apparatus adapted to provide both spot and barrage jamming, the apparatus including a continuous wave oscillator, and means for modulating the oscillator with a digital noise signal of controllable code length so as to provide an output signal of controllable bandwidth.

2. A radar jammer apparatus adapted to provide both spot and barrage jamming, the apparatus including a continuous wave (CW) oscillator, a biphase modulator coupled to the output of the oscillator, and a digital random noise code generator of controllable code length coupled to the modulator whereby to provide an output jamming signal of bandwidth corresponding to the reciprocal of the digital noise data bit duration and spectral line spacing corresponding to the reciprocal of the product of the noise code length and the data bit duration.

3. A radar jammer as claimed in claim 2, wherein said noise generator is a pseudo-random number generator.

4. A radar jammer as claimed in claim 3, wherein control of the bandwidth is provided by control of the clock rate of the pseudo-random number generator.

5. A radar jammer as claimed in claim 3 or 4, wherein said noise generator comprises a shift register provided with feedback.

6. A radar jammer as claimed in claim 2, 3, 4 or 5 and including means for analogue or digital frequency modulation of the noise generator bit rate.

7. A radar jammer as claimed in any one of claims 1 to 6, wherein the spectral line spacing is controlled by adjustment of the noise code length.

8. A radar jammer apparatus substantially as described herein with reference to and as shown in the accompanying drawings.

9. An aircraft provided with a radar jammer apparatus as claimed in any one of claims 1 to 8.