GB2174868A

GB2174868A - Simultaneous listening out and jamming

Info

Publication number: GB2174868A
Application number: GB08028244A
Authority: GB
Inventors: Roland Carre; Benoit Rabouille; Jean Michel Coiffard
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1979-09-07
Filing date: 1980-09-04
Publication date: 1986-11-12
Also published as: GB2174868B; FR2582409A1; DE3033243C1; IT1148721B; IT8024445A0; FR2582409B1

Abstract

Radar jamming transmitter 1 and listening antenna 2 operate simultaneously, and to avoid direct interference 20 from 1 to 2 the received signal is demodulated at 5, in correlation with a sample from 3 of the transmitted jamming signal suitably delayed at 4. The result is filtered at 9 and further correlated at 10 with a filtered (11) form of the delayed signal from 4. <IMAGE>

Description

SPECIFICATION A device for simultaneous listening out and jamming The present invention covers a device for simultaneous listening out and jamming.

Modern combat aircraft are often fitted with jamming devices which make it possible to transmit waves with sufficiently high power to blind radars likely to detect their presence. To arrive at this result, the jamming device contains a transmitter and a receiver. The purpose of the receiver is to listen out to radars, analyze the frequency spectra of the waves transmitted by these radars and pilot the transmitter of the jammer device so that the wave transmitted corresponds to the characteristics of the radar it is desired to jam.

As these jamming devices are of small size so that they can be fitted in aircraft, the transmission and reception antennas are very close to one another and may sometimes be a single antenna. This proximity and the external elements of the aircraft which support the jammer device cause interference reflections that produce coupling between the transmission and reception antennas. As these jammer devices are only of the maximum effectiveness if their transmission is continuous, it is necessary to have decoupling between the transmission and reception so that the radar waves can be analyzed and the corresponding jamming signals transmitted simultaneously.

To correct this nuisance of coupling between the transmission and reception antennas prior practice uses digital filters of the MTI type fitted between the reception antenna and the device analyzing the signal received by this reception antenna. The noise generated by the jammer device being periodic in accordance with a given code, the digital filters in the reception channel are effective provided that the delay in one of the channels of these digital filters corresponds exactly to the repetition period of the noise transmitted by the jammer device. This condition of repetition of the noise transmitted by the jammer device is a very strict one.

In the case in which coupling between the transmission and reception antennas deforms the signal in such a way that the repetition is reduced, the effectiveness of the digital filters is very much attenuated. Also, apart from the nuisance of strict repetion, these digital filters are very difficult to adjust.

The present invention seeks to correct these disadvantages by the use of the correlation between the coupling signal received by the reception antenna and the signal transmitted by the transmission antenna.

In accordance with a main characteristic of the invention, the device for simultaneous listening out and jamming contains a transmitter assembly, a reception antenna, which receives a useful signal Ru(t) and an interference signal R(t) from coupling between the transmission and reception antennas, and a demodulation circuit, which receives the signal R(t)+Ru(t) and a signal E(t-t) corresponding to the signal transmitted by the transmission assembly delayed by a prearranged time by a delay circuit, this demodulation circuit delivering to the first rejection filter, whose prearranged bandwidth Af is centred on frequency F0 of the transmission assembly, the signal E(t-l)[R*(t)+Ru*(t)], in which R*(t) and Ru*(t) are the respective conjugate quantities of the functions R(t) and Ru(t).

Other advantages and characteristics of the invention will oppear in the description which follows of two variants of manufacture shown by means of the figures which represent: in Figure 1, the envelope of the frequency spectra of the coupling interference signal and the useful signal after demodulation ut before filtering; in Figure 2, the envelope of the frequency spectra of the coupling interference signal and the useful signal after demodulation and filtering; in Figure 3, a block diagram of the device in accordance with the invention; in Figure 4, a variant of the device in accordance with the invention in Figure 3.

In the rest of the description, E(t) is the signal transmitted by the jammer device, R(t) the coupling signal between the transmission and reception antennas and Ru(t) the useful signal coming from the radar that is to be analyzed.

Experimental measurements with jammer devices and their antennas in particular have shown that the shift in time of the coupling signal R(t) between the transmission and reception antennas with respect to the transmitted signal E(t) is small and, in any case, less than the coherence time of the waves transmitted by the jammer device being considered. These findings show that it is possible to obtain rejection of the coupling signal by correlation with the transmitted signal. The procedure for processing the signal by this correlation method is as follows: In the first stage a fraction of the transmitted signal E(t) is sampled and delayed a time z which corresponds to the mean delay of the transfer function for the coupling between the transmission and reception antennas.The signal RE(t) given by RE(t) = E(t-t) [R*(t)+Ru*(t)j in which the sign * shows that it is a question of the complex conjugate quantity is then calculated by demodulation.

Figure 1 shows the frequency spectrum after this demodulation for the useful signal and the interference coupling signal. In this figure, Fo represents the central transmission frequency of the jammer device, AF represents the width of the pass band for this same jammer device and Fu the central frequen cyofthewavetransmitted buy a radar which is to be analyzed. The dotted curve in Figure 1 shows schematically the frequency spectrum envelope of the coupling signal between the transmission and reception antennas of the jammer device after the demodulation operation. In the same way, the continuous line curve shows schematically the frequency spectrum envelope of the useful signal coming from the radar to be analyzed.The width, 2.AF, ofthe residual noise due to coupling between the transmission and reception antennas is due to the demodulation operation which, coming from two signals of pass band AF, gives a noise signal of pass band width 2.AF. In the same way, the useful signal frequency spectrum is widened to a pass band AF F centred round the radar transmission frequency Fu.

In the second stage, the signal RE(t) passes through a rejection filter whose frequency is centred on the jammer device transmission frequency Fo, the width Af corresponding at least to the width of the main peak in the frequency spectrum of the coupling signal between the transmission and reception antennas. At the output of this rejection filter, a signal is obtained which still contains the data of the useful signal coming from the radar to be analyzed and a residual noise due to coupling between the transmission and reception antennas. This residual noise is sufficiently weak for the useful signal to be usable.To use this signal, two solutions are available: - either a second demodulation is carried out which enables the useful signal Ru(t) to be restored but with an error due to the remains of the interfering coupling signal R(t); - or frequency compression of the signal is carried out directly which has the advantage of making the frequency spectrum of the useful signal Ru(t) directly available if E(t) is modulated in frequency in accordance with a law allowing frequency compression and of making the data in the frequen spectrum directly usable.

Figure 3 shows the schematic diagram of the device in accordance with the invention in a first variant which enables the useful signal to be recovered by a second demodulation.

The device contains a transmitter assembly 6 connected to a transmission antenna 1 through coupling means 3 connected to a delay circuit 4. The output of delay circuit 4 is connected to the first demodulator 5 which receives the signals coming from reception antenna 2 and to a second demodulator 10 through a frequency rejection filter 11. The output of first demodulator 5 is also connected to second demodulator circuit 10 through a frequency rejection filter 9. The useful signal is available at the output S of second demodulation circuit 10. Coupling circuit 3 thus enables a fraction of the transmission signal E(t) to be sampled and this is then transmitted to delay circuit 4 which delivers the signal E(t-t), the time T corresponding to the delay in coupling signal 20 between transmission antenna 1 and reception antenna 2.Signal E(t-t) is then applied to first demodulator 5 together with the signal coming from reception antenna 2, this signal being formed of coupling signal R(t) and useful signal Ru(t). Demodulation circuit 5 delivers to frequency rejection filter 9 signal RE(t) previously defined.

Frequency rejection filter 9 cuts out the components of the spectrum centred round transmission signal frequency Fo over a width Af which is sufficient to eliminate the main components of the coupling signal spectrum between transmission antenna 1 and reception antenna 2. In practice, width Af may be of the order of AF/4. Second demodulation circuit 10 then receives signal RE(t) coming from frequency rejection filter 9 on the one hand and signal E(t-l) coming from delay circuit 4 on the other. A frequency rejection filter 11 may be inserted between the output of delay circuit 4 and the input of second demodylation circuit 10. This second frequency rejection filter 11 makes possible an attenuation of the interfering coupling signal residual noise.For demodulation by second demodulation circuit 10 to be as perfect as possible, it is necessary to subject the signal coming from delay circuit 4to the same rejection filtering as that applied to the signal coming from first demodulation circuit SAt the output of second demodulation circuit 10, which is identical to first demodulation circuitS, signal Ru(t) is available but contains the coupling signal residual noise and corresponds to the part not filtered by frequency rejection filter 9.

In certain cases, it may be useful to have the spectral analysis ofthe useful signal coming from the radar to be analyzed directly available at output S. Avariant of the preceding device in the invention corresponding to this case is shown in Figure 4.

Apart from the preceding elements, i.e. transmitter assembly 6, coupling means 3, delay circuit 4, first demodulator circuit 5 and rejection filter 9, the device in accordance with the invention contains an assembly 12 of lines whose delay is variable with the frequency of the signal applied to them. Signal RE(t) is then applied to the input of these delay lines 12 variable with frequency. Signal RE(t) has a frequency spectrum corresponding to that shown in Figure 2.

The delay lines variable with frequency then allow a recompression of the signal which is equivalent to restitution of the useful signal frequency spectrum.

The frequency spectrum thus obtained is slightly wider than the real frequency spectrum of the useful signal because ofthe residual interfering coupling signal remaining at the output of theses delay variable with the frequency. In the case of a frequency modulation in accordance with a periodic law in sawtooth form, a single delay line variable with frequency is required. In the case in which the frequency modulation is in accordance with a periodic law in isosceles triangle form, two delay lines variable with frequency of opposite slopes are required.

In the case of a transmitted signal containing a phase code, demodulation circuits 5 and 10 can also be replaced by decoder circuits which also receive the phase code with a delay to allow for the transfer function which expresses the coupling between the transmission and reception antennas.

A device for simultaneous listening out and jamming has thus been described.

Claims

1. A device for simultaneous listening out and jamming containing a transmitter assembly (6) and a reception antenna (2) which receives a useful signal Ru(t) and an interfering signal R(t) coming from coupling (20) between the transmission (1) and reception (2) antennas, characterized by the fact that it contains a demodulation circuit (5), which receives the signal R(t)+Ru(t) and a signal E(t-t) corresponding to a function of the signal transmitted by transmitter assembly (6) delayed a prearranged time T by a delay circuit (4), demodulation circuit (5) delivering to a rejection filter (9) of prearranged pass band width Af centred on frequency Fo of transmitter assembly (6) the signal E(t-t) [R*(t)+Ru*(t)] in which R*(t) and Ru*(t) represent the respective conjugate quantities of functions R(t) and Ru(t).

2. A device for simultaneous listening out and jamming as in claim 1, characterized by the fact that it contains at least one delay line variable with frequency (12) connected at the output or rejection filter (9).

3. A device for simultaneous listening out and jamming as in claim 1, characterized by the fact that it contains a demodulator (10) connected at the output of frequency rejection filter (9) and delay circuit (4).

4. A device for simultaneous listening out and jamming as in claim 3, characterized by the fact that it contains, between demodulator (10) and the output of delay circuit (4), a second band rejection filter (11), which is identical to first band rejection filter (9).

5. A device for simultaneous listening out and jamming as in claim 1, characterized by the fact that the delay corresponds to the mean delay of the interfering signal R(t) received by reception antenna (2) with respect to the transmitted signal E(t).

6. A device for simultaneous listening out and jamming as in claim 1, characterized by the fact that delay circuit (4) is connected to transmitter assembly (6) by coupling means (3).

7. A device for simultaneous listening out and jamming as in claims 1 and 3, characterized by the fact that, in the presence of a transmitted signal modulated by a phase code, demodulators (5) and (10) are replaced by decoder circuits which receive the delayed phase code.

8. A device for simultaneous listening out and jamming substantially as hereinbefore described with reference to figure 3 or 4 of the accompagnying drawings.

8. A device for simultaneous listening out and jamming, substantially as hereinbefore described with reference to Figure 3 or 4 of the accompanying drawings.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1 to 8 above have been deleted or textually amended.

(b) New or textually amended claims have been filed as follows:

1. A device for simultaneous listening out and jamming comprising transmitting means (6) at the central frequency Fo, connected to a transmission antenna (1), and a reception antenna (2) which receives a useful signal Ru (t) and an interfering signal R (t) produced by a coupling (20) between the transmission (1 ) and reception (2) antennas, wherein it further comprises a demodulation circuit (5), which receives the signal R (t)+Ru (t) from the reception antenna (2) and a signal E (t-l) which is a fraction of the output signal of the transmitting means (6) delayed by a predetermined time X by a delay circuit (4), a rejection filter (9) having a predetermined bandwidth Af centred on the central transmitted frequency Fo and receiving the output signal E(t z)[(R* (t)+ Ru* (t)] in which R* (t) and Ru* (t) represent the respective conjugate quantities of the functions R(t) and Ru (t).

2. A device for simultaneous listening out and jamming as in claim 1, wherein it further comprises at least one delay line (12) variable with frequency which is connected at the output of the rejection filter (9) and restitutes a useful signal (S).

3. A device for simultaneous listening out and jamming as in claim 1, wherein it further comprises a second demodulation circuit (10) receiving on first and second inputs the output signal of the frequency rejection filter (9) and of the delay circuit (4) respectively and delivering a useful signal (S).

4. A device for simultaneous listening out and jamming as in claim 3, wherein it further comprises a second rejection filter (11), connected between the output of the delay circuit (4) and the corresponding second input of the second demodulation circuit 10 and having a bandwidth equal to Af and centred on the central transmitted frequency Fo.

5. A device for simultaneous listening out and jamming as in claim 1, wherein the delay time T corresponds to the mean delay of the interfering signal R(t) received by the reception antenna (2) with respect to the transmitted signal E(t).

6. A device for simultaneous listening out and jamming as in claim 1, wherein the delay circuit (4) is connected to the transmitting means (6) by coupling means (3).

7. A device for simultaneous listening out and jamming comprising transmitting means delivering a signal at a frequency Fo, means for modulating said signal by a phase code, a first antenna (1) for transmitting the modulated signal, a second antenna (2) receiving a useful signal Ru (t) and an interfering signal R (t) produced by a coupling (20) between the first and second antennas, wherein it further comprises a first decoder supplied with the signal Ru (t) + R (t) from the second antenna (2) and with the phase code through a delay circuit, a second decoder receiving on a first and a second input the output signal of the first decoder through a first rejection filter and the delayed phase code from the delay circuit through a second rejection filter respectively and delivering a useful signal (S), the first and second rejection filters having a predetermined bandwidth equal to Af and centred on the frequency Fo.