GB1579477A - System for simultaneous listening and jamming - Google Patents

Description

(54) A SYSTEM FOR SIMULTANEOUS LISTENING AND JAMMING (71) We, THoMsoN-CSF, a French Body Corporate, of 173 Boulevard Haussman, 75008, Paris, France, 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 a system for listening and jamming simultaneously.

The system includes in particular a transmission antenna which is connected to circuits for producing and transmitting jamming signals in a predetermined frequency band and a reception antenna which is coupled to a receiver for listening for signals emitted by other systems.

Direct coupling between the transmitting and receiving antennas and unwanted closerange reflections result in the jamming signals being transmitted to the listening receiver. An object of the present invention is to improve the effective decoupling between the jamming circuits and the reception circuits in order to allow the jammer and the listening receiver to operate simultaneously.

Accordingly the present invention consists in a system for simultaneous jamming and listening including means for generating jamming signals to be radiated by a first antenna, and means for receiving signals which, in operation, are picked up by a second antenna, said jamming signals being repeated in time in operation with a predetermined period T and said means for receiving signals including means for subtracting the signals picked up at time t from the signals picked up at time tT thereby reducing interference coupling between said two antennas.

By subtracting the received signals with a Itime lag of T, the delay and subtracting means enable the jamming signals, which recur in identical form with a period T, to be eliminated. However, any reception signals which there may be are preserved and divided into two by the delay means.

By adding second means for subtracting and applying a delay of T, it is possible to eliminate in addition the first order drifts of the modulation of the jamming signal.

The received signals are increased three fold.

The decoupling which is achieved in accordance with the present invention enables the sensitivity of the receiver associated with the jammer to be improved.

Tn order that the present invention may be more readily understood, an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: FIG. 1, a block diagram of a listening system according to the invention; FIG. 2, a modification of the decoupling arrangement, and FIG. 3, an improved jamming and listening system.

FIG. 1 shows a jamming and listening system according to the invention.

The system is intended to emit jamming signals over a wide frequency band (one of 200 MlIz for example about a centre frequency Fo) whilst at the same time allowing signals which may be emitted by other transmitting stations (telecommunications stations or radars for example) in the same frequency band or in a wider band, containing all or part of the jamming band, to be listened for.

The jamming signals are emitted, for example, by using a transmitter 1 which produces a sinusoidal wave of frequency Fo. This wave is applied to a modulator 2 which, by known techniques, converts it into jamming signals covering a band of width 2AF centred on Fo. These signals are then amplified by an amplifier 3 and radiated by an antenna 4.

Alongside the jammer, a receiver an tenna 5 picks up any signals which may be emitted by known or unknown transmit- ting stations and this antenna is coupled to a receiver 9 via filtering circuits 7 and 8.

By reason of the fact that the transmitting antenna 4 and receiving antenna 5 are necessarily not very far apart, and by reason of the fact that a proportion of the energy radiated by antenna 4 is reflected from nearby objects, a part, even though very small, of the energy of the jamming signals is picked up by the receiving an tenna. There is thus a non-physical link between the jammer and the listening system which may result in the latter suffer ing a considerable loss in sensitivity. One object of the invention is to reduce the effect of this link, that is to say of the undesirable coupling between the jammer and the receiver.

Thus in the embodiment being described the coupling is reduced by means of filter ing circuits 7 and 8 which are specially adapted to the characteristics of the modu lator 2 which produces the jamming signals.

Each of the filtering circuits includes means 'to subtract from the signal received previously, with a predetermined timelag T.

The first filtering circuit 7 includes a delay line 11 and a subtractor circuit 12 whose input terminals are connected to the input and output of the delay line 11. The received signals are applied to the input of the delay line. At the output of the subtractor the signals received at a time t are subtracted from the signals received at a time t-T, T being the delay applied by the line 11. The useful signals are thus divided into two, that is to say are emitted twice with a time-lag T. The jamming signals on the other hand are eliminated since they are repeated in identical form with a period of T.

A second filter 8 identical to filter 7 con tains a delay line 13 and a subtractor 14 which are connected together as in filter 7.

By this second filtering, the first order de rivatives of the modulation of the jamming signals are also cancelled out.

The filtering circuits may of course be produced by analogue or digital techniques, in which case coding and decoding circuits are associated with them. Means for balancing gain are provided in the inputs to the subtractors to achieve complete can cellation of the jamming signals. These means are not shown in order to overload the Figures.

Various types of jamming may be em ployed on the transmission side. All rely .on known techniques which are reviewed below. Common to them all is a repetition with period T, so that it is possible for them to be eliminated at reception.

A phase-noise jamming signal is obtained by phase-modulating the signal from the transmitter 1 with a noise signal of com plex amnlitude. Filtering in a filter of band width AF and equalising amplitude throughout the band enables this jamming signal to be obtained. The amplitude-noise .signal is repeated in identical form with a period T.

Instead of random values, the phase of the jamming signal may assume only a specific number of predetermined values.

The modulation is then phase-code modulation. The phase of the signal may, for example, assume only two states which are -180" apart. The code may be of the pseudo-aleatory type and may repeat with a period T. The spectral width of the phase-coded signal is equal to the reciprocal of the duration of one code unit.

Instead of phase modulation, the jamming signal may also be obtained by a linear, or non-linear, or triangular, or sawtooth frequency modulation or by frequency modulation and phase coding. The only condition is that the modulation should be repeated as faithfully as possible with a period T.

FIG. 2 shows a modulation of the filtering means on the reception side combined with a switching device.

What is involved is preventing a radar, which would perform the same processing on reception as in FIG. 1, from eliminating the jamming signal.

It consists in phase-coding the transmitted signal at a frequency 1/T using a random code employing phases 0 and 7r for example, or any other code.

Since the radar does not know this code it is no longer able at all times to eliminate the jamming signal.

On the other hand, the listening system associated with the jammer, knowing the code used, makes allowance for it by making an appropriate change in the phase of the signal applied directly to the subtractor.

The filtering circuit 10 shown in FIG. 2 thus includes a subtractor 19, and a delay line 16 which is connected to one input of the subtractor and receives the signals picked up by the antenna 5. The other input of the subtractor is connected to the other end of the delay line via means which enable the phase of the signals to be reversed or not reversed. These means comprise, for example, a change-over switch 18 whose moving contact is connected to the subtractor and whose fixed contacts are connected to the delay line, in the one case directly and in the other via a 1800 phaseshifter 17.

The change of phase in the filter 10 is synchronised with the phase-code applied in the transmission circuits, of which only encoder 1 is shown, which codes the whole of the signal transmitted during the period T at O or at .

A switching device 20, termed the listening switch, which is inserted between the antenna 5 and the filtering means 10 is required to prevent transient signals from occurring at the moment when the phasecode changes state (changes from 0 to 7r or from 7r to O). These transient signals cannot be removed by the filtering circuits.

The switch 20 is controlled by the encoding circuit 21. The time for which the switch 20 remains open is short in comparison with the period T (being 1 ,us for example).

An overall diagram of a jamming and listening circuit is shown in FIG. 3. This system consists of a combination of the members described above.

The transmission section includes an oscillator 30 followed by a phase coding circuit 22 controlled by a code generator 21. Signals, having first been coded, are applied to the modulator 2 and then to the amplifier 3 and finally to the antenna 4.

The modulator 2 performs a phase, frequency or phase and frequency modulation as mentioned above with a repetition period T. The encoder 22 performs phase reversals in accordance with a random code which is preferably non-repetitive within the period T.

On reception the signals picked up by the antenna 5 are transmitted by switch 20 to two demodulators 41 and 42. Demodulator 42 also receives a sinusoidal signal from an oscillator 40. This same signal is applied to demodulator 41 after a 90" phase shift.

The output signals from demodulator 41 are applied to two filters 31 and 33 in succession and then to a modulator 45 which is supplied, via a 90" phase shifter 44, by the same oscillator 40.

The output signals from the demodulator 42 are applied in similar fashion to two successive filters 32 and 34 and then to a modulator 46 which is supplied directly by oscillator 40. The outputs of modulators 45 and 46 are combined together at the input to the receiver 9.

A connection 50, shown as a broken line, represents the oscillator 40 being controlled by the oscillator 30, or vice versa, to ensure that the system is stable.

The switch 20 is controlled by the code generator 21.

The filters 31 and 32 are identical to the filter 10 in FIG. 2. The filters 33 and 34 are identical to the filter 8 in FIG. 1.

The reception signals are thus processed in two identical channels and in quadrature.

In both the channels, analogue/digital and digital/analogue converting circuits may be inserted between the demodulator and the modulator to allow the benefits of digital techniques to be obtained.

The invention is applicable to listening radar stations and to countermeasures systems.

WHAT WE CLAIM IS:- 1. A system for simultaneous jamming and listening including: means for generating jamming signals to be radiated by a first antenna, and means for receiving signals which, in operation, are picked up by a second antenna, said jamming signals being repeated in time in operation with a predetermined period T and said means for receiving signals including means for subtracting the signals picked up at time t from the signals picked up at time t-T thereby reducing interference coupling between said two antennas.

2. A system as claimed in claim 1, wherein said means for subtracting the signals picked up at time a from the signals picked up at time t-T comprise at least one delay circuit and at least one subtractor circuit having two inputs connected to the input and output respectively of said delay circuit.

3. A system as claimed in claim 2, wherein said means for subtracting the signals picked up at time t from the signals picked up at time t T include means for reversing the phase of the signal applied to one of the inputs of said subtractor circuit and means for simultaneously reversing the phase of the jamming signal to be transmitted.

4. A system as claimed in claim 3, further including means for momentarily interrupting the reception signals, said means being operated in synchronisation with each reversal of the phase of the jamming signals.

5. A system for simultaneous jamming and listening substantially as described with reference to the accompanying drawings.

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. or from 7r to O). These transient signals cannot be removed by the filtering circuits. The switch 20 is controlled by the encoding circuit 21. The time for which the switch 20 remains open is short in comparison with the period T (being 1 ,us for example). An overall diagram of a jamming and listening circuit is shown in FIG. 3. This system consists of a combination of the members described above. The transmission section includes an oscillator 30 followed by a phase coding circuit 22 controlled by a code generator 21. Signals, having first been coded, are applied to the modulator 2 and then to the amplifier 3 and finally to the antenna 4. The modulator 2 performs a phase, frequency or phase and frequency modulation as mentioned above with a repetition period T. The encoder 22 performs phase reversals in accordance with a random code which is preferably non-repetitive within the period T. On reception the signals picked up by the antenna 5 are transmitted by switch 20 to two demodulators 41 and 42. Demodulator 42 also receives a sinusoidal signal from an oscillator 40. This same signal is applied to demodulator 41 after a 90" phase shift. The output signals from demodulator 41 are applied to two filters 31 and 33 in succession and then to a modulator 45 which is supplied, via a 90" phase shifter 44, by the same oscillator 40. The output signals from the demodulator 42 are applied in similar fashion to two successive filters 32 and 34 and then to a modulator 46 which is supplied directly by oscillator 40. The outputs of modulators 45 and 46 are combined together at the input to the receiver 9. A connection 50, shown as a broken line, represents the oscillator 40 being controlled by the oscillator 30, or vice versa, to ensure that the system is stable. The switch 20 is controlled by the code generator 21. The filters 31 and 32 are identical to the filter 10 in FIG. 2. The filters 33 and 34 are identical to the filter 8 in FIG. 1. The reception signals are thus processed in two identical channels and in quadrature. In both the channels, analogue/digital and digital/analogue converting circuits may be inserted between the demodulator and the modulator to allow the benefits of digital techniques to be obtained. The invention is applicable to listening radar stations and to countermeasures systems. WHAT WE CLAIM IS:-

1. A system for simultaneous jamming and listening including: means for generating jamming signals to be radiated by a first antenna, and means for receiving signals which, in operation, are picked up by a second antenna, said jamming signals being repeated in time in operation with a predetermined period T and said means for receiving signals including means for subtracting the signals picked up at time t from the signals picked up at time t-T thereby reducing interference coupling between said two antennas.

2. A system as claimed in claim 1, wherein said means for subtracting the signals picked up at time a from the signals picked up at time t-T comprise at least one delay circuit and at least one subtractor circuit having two inputs connected to the input and output respectively of said delay circuit.

3. A system as claimed in claim 2, wherein said means for subtracting the signals picked up at time t from the signals picked up at time t T include means for reversing the phase of the signal applied to one of the inputs of said subtractor circuit and means for simultaneously reversing the phase of the jamming signal to be transmitted.

4. A system as claimed in claim 3, further including means for momentarily interrupting the reception signals, said means being operated in synchronisation with each reversal of the phase of the jamming signals.

5. A system for simultaneous jamming and listening substantially as described with reference to the accompanying drawings.