GB2318237A

GB2318237A - Pulse doppler radar device with elimination of jammer by neutralization

Info

Publication number: GB2318237A
Application number: GB8802255A
Authority: GB
Inventors: Daniel Paillard
Original assignee: Dassault Electronique SA
Current assignee: Thales SA
Priority date: 1987-02-06
Filing date: 1988-02-02
Publication date: 1998-04-15
Anticipated expiration: 2008-02-02
Also published as: GB8802255D0; FR2737578A1; GB2318237B

Abstract

A pulsed Doppler radar comprises a principal antenna (10) and a receiving system (11 to 15), providing distance gates (p) each subdivided by fast Fourier transformation (16) into Dopper filters (n). An anciliary antenna (20) has a second receiving system identical to the first. A module (5) establishes the average relative gain of the two antennas over selected ones of the range/Doppler gates. The signal emanating from the ancillary receiving system, weighted by this gain in a multiplier (6), is then subtracted in a comparator (172) from the signal of the principal processing system. The jammer may thus be eliminated.

Description

2318237 PULSED DOPPLER RADAR DEVICE, WITH JAMMER ELIMINATION BY

NEUTRALIZATION The invention relates to pulsed Doppler radars.

In order to permit such a radar to retain its sensitivity in the presence of adverse jamming, it is necessary to eliminate the signal detected by the radar in the direction of the jammer, such elimination being referred to as the "creation of zeros". More generally, this is a technique relating to an adaptive antenna.

There are now available aerial systems with electronic scanning, these being formed of a plurality of radiating elements whose phase can be controlled individually with a high degree of accuracy. This permits the creation of the zeros in the radiation pattern of the antenna itself. The interfering signal from a noise jammer is then no longer detected at all by the entire receiving system.

Although this solution is particularly convenient, it suffers from two disadvantages.

The first is that it is necessary to provide, in the time frame of the radar, service time intervals during which the phaseshifter commands are readjusted.

It will be noted, in passing, that in the case of a Doppler radar this readjustment presents great problems with regard to the electron control system, whose beam is not mechanically stabilized.

2 The second disadvantage stems from the fact that the creation of zeros in the radiation pattern can be envisaged only in relation to sources exhibiting limited spatial dimensions, that is to say in relation to sources of which noise which can be considered as point-localized. Now, the aircraft carrying the radar is most frequently itself also equipped with a jammer. The signal emitted by its local jammer cannot be eliminated by the techniques of creation of zeros at the location of the radar antenna, since the entrance paths of the signal of the jammer into the receiving system are poorly defined, that is to say numerous, complex and capable of varying with time.

The present invention proposes a different solution based on a neutralization of the received signal in the receiving system, on the basis of information collected at one or more auxiliary aerial systems which will likewise be referred to as ancillary aerial systems since only service aerial systems may be involved.

From this starting point, one of the objects of the invention is to permit the elimination of a jammer even if it originates from a source which is not well localized angularly.

A further object of the invention is to permit a jammer to be eliminated while avoiding the constraints of the previously known techniques. It will be appreciated that the invention is applicable both in the case of a 3 mechanically scanning antenna and in the case of an electronically scanning antenna.

According to the invention, we provide a pulsed Doppler radar device, comprising emission means; a principal antenna which is coupled to a principal receiving system with automatic gain control at the head, and with distance/Doppler digital processing means; a second antenna coupled to a second receiving system identical to the principal receiving system; means which react to the presence of a jammer by determining the gain of the principal antenna relative to the second antenna, for a selected subsystem of the distance ports and of the Doppler filters of the two receiving systems; and neutralizing means capable of subtracting, for each distance port and each Doppler filter, from the output of the principal receiving channel the output of the second receiving system weighted by said relative gain.

A person skilled in the art is aware that the principal antenna is aimed in a direction different from that of the jammer. According to the invention, the second antenna will have in the direction of the jammer a gain greater than that of the first antenna, that is to say at least outside the direction of aiming of the principal antenna. Advantageously, the second antenna is substantially omnidirectional.

The subsystem is separate from the subsystem comprising the distance ports/Doppler filters and containing the altitude return and the principal ground echo, this being so for the purpose of not interfering with the estimations of the noise of the jammer.

Likewise, there will be available at the output N independent samples, which are the outputs of the P Doppler filters, and which are not contaminated by the clutter of each of the Q distance ports of the sequence of the radar.

The automatic gain control situated at the head of the receiving systems permits the latter to have a suitable operating regime, in spite of the high dynamic level of the signal of a noise jammer.

The invention, may also be provided with a third receiving system, also identical to the principal receiving system, and coupled to a further antenna which is advantageously one of the other channels of the principal antenna, preferably its difference channel. In this case as well, means react to the presence of another jammer by determining the gain of the principal antenna and of the second antenna relative to this further antenna, for a selected subsystem of the distance ports and Doppler filters of the receiving systems. Finally, the device comprises second neutralizing means, capable of subtracting, for each distance port and each Doppler filter, from the outputs of the' principal receiving channel - 5 and of the second receiving channel respectively the output of the third receiving system weighted by said relative gains.

Further features and advantages of the invention will become evident from the following detailed description and the accompanying drawings, in which:

Figure 1 is a basic diagram, in part detail, of a radar device implementing the invention, having a single jammer; and Figure 2 is the basic diagram of a variant of the invention with two jammers.

The accompanying drawings comprise, essentially, elements having definite characteristics. Consequently, they possibly serve not only to provide an improved understanding of the detailed description given hereinbelow, but also to contribute to the definition of the invention, as appropriate.

In Figure 1, the reference 1 designates the emission means of a pulsed Doppler radar. As is known to a person skilled in the art, these emission means are coupled to the principal antenna 10 of the radar which is assumed to be carried by an aircraft. The emission devices are traditional, and have been omitted for the sake of simplification.

The local reception signals are likewise incorporated in the emission circuits 1.

The principal antenna 10, in its summation channel, is connected to a mixer 11, which receives a first local signal, and is followed by an amplifier 12 having an automatic gain control. The output of the amplifier 12 is connected to a second mixer 13 receiving a second local signal from the unit 1. In a traditional manner, the output of the mixer 13 is broken down into two complex channels, defined by two mixers 141 and 142 receiving a third local signal from the unit 1. In order to ensure that the phase is taken into account, the second mixer 142 receives the local signal through a quadrature phase shifter designated 143. The outputs of the two mixers 141 and 142 are applied through respective amplifiers 144 and 145 to an encoder 15 which converts the signals into digital form.

After the emission time frame of the radar has been taken into account, the output of the encoder 15 can be broken down into distance ports of which there may for example be thirty-six, the current port being designed leport p".

The signals relating to each of the distance ports are applied to Fast Fourier Transformation (FFT) stages designed 16-1 to 16-36 respectively for the. thirty-six distance ports.

By the operation of these circuits 16, each distance port is broken down into a predetermined number of 0 7 Doppler filters, for example 128 Doppler filters. The current Doppler filter is illustrated as "filter n".

The second receiving system according to the invention proceeds from an antenna 20 which is omnidirectional, that is to say that it has an angular coverage of approximately 18C.

Its output is applied to a first mixer 21 followed by an automatic gain control amplifier 22, and then a second mixer 23. These components are identical to those of the first receiving system, and these reference numerals are simply increased by 10. Downstream of the output of the mixer 23 the configuration passes to complex mode by means of two mixers 241 and 242, the second fed by the local third signal through a quadrature phase shifter 243. Further, amplifiers 244 and 245 pass the two complex channels to the encoder 25 which itself also supplies thirty- six distance ports. By means of FFT circuits 26 the distance ports are, in their turn, broken down into 128 Doppler filters. As before the filter of rank n in the distance port of rank p is exemplified.

Under the control of a central processing unit (not shown), a selected subsystem of the distance ports and Doppler filters is applied to a circuit 5 whose function is to estimate the relative gain of the two antennas in the direction of the jammer.

Referring now to the antenna 10, it can be seen that at an angle d in relation to its aiming direction this antenna exhibits a gain GS for a noise jammer signal. For the sake of simplification it is assumed that in the same direction d the ancillary antenna 20 has a unit gain.

The conventional processing means of the principal receiving system can identify in known manner the clutter, especially the ground noise, which comprises the altitude return and the principal ground echo. The distance ports and Doppler filters containing this information are not taken into account at the inputs of the module 5.

If necessary, a person skilled in the art will readily find other elements to define the selection of the subsystem of the distance ports and Doppler filters applied to the module 5.

At this point, it will simply be indicated that this module will, on the said subsystem, generate a mean of the ratio between the signal received in each port and each filter, on the one hand via the principal receiving channel and on the other hand via the second receiving channel associated with the ancillary antenna 20.

It will now be assumed that there is present a whitenoise jammer, of amplitude B. The total signal at the input of the principal receiving channel may then be written: SE = SU + GS. B 9 where SU is the useful signal originating from the target situated in the vicinity of the axis of the principal antenna, and GS is the complex. gain (amplitude and phase) of the principal antenna in the direction of the jammer, normalized in relation to the gain of the ancillary antenna.

on the other hand, the following is applicable on the ancillary channel: SE' = SUI + B The function of the module 5 is to determine a mean estimate <K> of the quantity GS.

It is then sufficient to take, for each Doppler filter of each distance port, the signal YA available at the output of the ancillary channel, weighted in a multiplier 6 by the gain <K>, and subtracted in a comparator 172 from the signal YS relating to the same distance port p and to the same Doppler filter n.

The resultant signal R(p,n), the expression for which is illustrated in Figure 1, is then virtually cleared of the jammer noise.

It is recalled that the estimation of <K> is undertaken on the basis of M independent measurements K(p,n) of the relative gain of the principal antenna and of the ancillary antenna.

These independent measurements are obtained in the various distance ports p and the various Doppler - 10 filters n forming part of the selection, that is to say of the selected subsystem as defined above.

It has been observed in practice that a number of independent measurements m of the order of 50 will allow a satisfactory estimate of the coefficient K to be obtained.

Experiments have also shown that, in the situations where the gain of the principal antenna is equal to that of the ancillary antenna, there the principal receiving system will be desensitized by of the order of approximately 3 dB; this is acceptable in practice as the most unfavourable case.

It will be noted, in particular, that the desensitization of the receiver is negligible in the secondary lobes distant from the principal antenna. This is particularly important when it is desired to eliminate jammers which are introduced through such secondary lobes.

A further point needs to be considered. The receiving systems which have been described above involve the action of 36 distance ports, each having 128 Doppler filters. A calculation load of very great magnitude might be expected for the estimation of the relative gain. The experiments carried out by the Applicants have shown that this is not in any sense the case: even with a jammer whose level is greater than the thermal noise, it is not necessary to carry out a large number of measurements to compensate for the action of this jammer. The calculation - 11 load required is thus entirely compatible with the modern means available.

The device which has just been described permits the elimination of the interfering signal emanating from a jammer. The Applicants have also examined what happens when there are two jammers present.

If their levels are very different, the device eliminates the stronger jammer, almost as in the case of the single jammer previously.

more generally, it has been observed that on average the more powerful jammer is reduced to the level of the weaker.

In certain applications it may be desired to be more effective. A brief description will now be given, with reference to Figure 2, of how this can be obtained.

In Figure 2, the symbols bS and bA designate respectively the outputs of the principal channel and of the auxiliary channel of Figure 1, for a given distance port and Doppler filter.

There is again a module 5 which establishes, for the stronger jammer, the relative gains between the two channels. The difference from Figure 1 resides in the fact that this calculation is made after further processing which will receive further consideration below. The signal of the auxiliary channel, weighted on each occasion by the gain in question, is applied to the comparator 172 for subtraction from the signal of the principal channel.

The second jammer is processed by means of a third receiving channel which is identical to the other two and makes use of another source of information (in space). This other source is a third antenna. It is possible to use one of the other channels of the principal antenna, in particular one of its difference channels. The third channel is thus designated channel D.

The gains of the principal channel S and of this channel D, in relation to the auxiliary channel in the direction of the two jammers, are respectively designated GS and GS1 on the one hand, GD and GD' on the other hand. In each case the auxiliary channel serves as unit gain reference.

Proceeding from a subsystem of the distance ports and Dappler filters, a subsystem which may be the same as for the stronger jammer, the relative gains of the principal channel S and of the auxiliary channel are determined in relation to the difference channel. This is done in two modules 51 and 52, which have the same basic structure as the module 5 mentioned above.

The average gain <K> established by the module 51 serves to weight the output bD of the difference channel, for applying it in subtraction in a comparator 81 on the principal processing system.

The gain <KI> of the module 52 serves also to weight the output bD of the difference channel, applying it - 13 in subtraction in a comparator 182 situated on the path of the signal ba of the auxiliary channel. Further, the outputs of the comparators 181 and 182 serve, on the one hand, to establish the gain <C> of the module 5 for the final subtraction in the comparator 172 of (a) the weighted signal emanating from the auxiliary channel from (b) that of the principal channel already corrected on a first occasion.

In other words, the neutralization process of the invention is applied on a first occasion to eliminate the stronger jammer.

By means of the third receiving channel, which is in this instance the difference channel, the process of the invention is applied on a second occasion to eliminate the weaker jammer.

In this way, it is possible to reduce greatly the interference phenomena created by two jammers, even if they are of different levels.

Another advantage may be drawn from the use of the difference channel as the third channel: the particular form of the radiation pattern of this difference channel appears to authorize the elimination of jammers situated so as to border on the principal lobe of the summation channel.

The invention is, of course, not limited to the described emobidments. It extends to any variant included within the scope of the following claims.

Claims

1. A pulsed Doppler radar device, comprising emission means; a principal antenna which is coupled to a principal receiving system with automatic gain control at the head, and with distance/Doppler digital processing means; a second antenna coupled to a second receiving system identical to the principal receiving system; means which react to the presence of a jammer by determining the gain of the principal antenna relative to the second antenna, for a selected subsystem of the distance ports and of the Doppler filters of the two receiving systems; and neutralizing means capable of subtracting, for each distance port and each Doppler filter, from the output of the principal receiving channel the output of the second receiving system weighted by said relative gain.

2. A device as claimed in claim 1, wherein the second antenna has a gain greater than that of the first antenna at least outside the principal direction of the first antenna.

3. A device as claimed in claim 1 or claim 2, wherein the second antenna is substantially omnidirectional.

4. A device as claimed in any one of claims 1 to 3, wherein the subsystem is selected to eliminate clutter, in particular the altitude return and the principal ground echo.

5. A device as claimed in any one of claims 1 to 4, which comprises a third receiving system identical to the principal receiving system, which is coupled to a further antenna means which react to the presence of another jammer by determining the gains of the principal antenna and of the second antenna relative to this further antenna, for a selected subsystem of the distance ports and of the Doppler filters of the receiving systems; and further neutralizing means capable of subtracting, for each distance port and each Doppler filter, from the outputs of the principal receiving channel and of the second receiving channel respectively, the output of the third receiving system, weighted by the said relative gains.

6. A device as claimed in claim 5, where said further antenna is constituted by the difference channel of the principal antenna.

7. A pulsed Doppler radar device, constructed and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

)G Amendments to the Claims have been filed as follows 1. A pulsed Doppler radar device comprising:

transmission means; a principal antenna fed by said transmission means, and adapted to collect main radar returns; a principal receiving system connected to said main antenna for receiving therefrom said main radar returns, said principal receiving system having main automatic gain control means followed by main time/frequency analysing means operating with a predetermined set of range/frequency gates for distributing said main radar returns into a plurality of respectively corresponding main subreturns; a first auxiliary antenna adapted to collect first auxiliary radar returns, corresponding to transmission by said principal antenna; a first auxiliary receiving system connected to said first auxiliary antenna for receiving therefrom said first auxiliary radar returns, said first auxiliary receiving system having first auxiliary automatic gain control means followed by first auxiliary time/frequency analysing means operating with said predetermined set of range/frequency gates for distributing said first auxiliary radar returns into a plurality of respectively corresponding first auxiliary subreturns; processing means responsive to detection of a jammer signal within said main and first auxiliary returns, tn for determining the average ratio of those of said main subreturns and first auxiliary subreturns corresponding to selected ones of said predetermined set of range/frequency gates; and neutrodyning means adapted to subtract from each main subreturn of said predetermined set of range/frequency gates the corresponding first auxiliary subreturn weighted by said average ratio, whereby said jammer signal is substantially eliminated from the output signal of said neutrodyning means.

2. A device according to claim 1, wherein said first auxiliary antenna has a gain greater than that of said principal antenna at least outside the main direction of said principal antenna.

3. A device according to claim 1 or 2, wherein said first auxiliary antenna is substantially omnidirectional.

4. A device according to any one of claims 1 to 3, wherein said selected ones of said predetermined set of range/frequency gates are different of those of said predetermined set of range/frequency gates which distribute said main and first auxiliary radar returns into corresponding main and first auxiliary subreturns containing information about a predetermined clutter.

5. A device according to claim 4, wherein said predetermined clutter comprises altitude returns and main ground echo. 6. A pulsed Doppler radar device comprising:transmission means; a principal antenna fed by said transmission means, and adapted to collect main radar returns; a principal receiving system connected to said principal antenna for receiving therefrom said main radar returns, said principal receiving system having main automatic gain control means followed by main time/frequency analysing means operating with a predetermined set of range/frequency gates for distributing said main radar returns into a plurality of respectively corresponding main subreturns; a first auxiliary antenna adapted to collect first auxiliary radar returns, corresponding to transmission by said main antenna; a first auxiliary receiving system connected to said first auxiliary antenna for receiving therefrom said first auxiliary radar returns, said first auxiliary receiving system having first auxiliary automatic gain control means followed by first auxiliary time/frequency analysing means operating with said predetermined set of range/frequency gates for distributing said first auxiliary radar returns into a plurality of respectively corresponding first auxiliary subreturns; a second auxiliary antenna adapted to collect ffi second auxiliary radar returns, corresponding to transmission by said principal antenna; a second auxiliary receiving system connected to said second auxiliary antenna for receiving therefrom said second auxiliary radar returns, said second auxiliary receiving system having second auxiliary automatic gain control means followed by second auxiliary time/frequency analysing means operating with said predetermined set of range/frequency gates for distributing said second auxiliary radar returns into a plurality of corresponding respective second auxiliary subreturns; first processing means responsive to detection of a first jammer signal within said main and first auxiliary radar returns, for determining a first average ratio of those of said main subreturns and first auxiliary subreturns corresponding to selected ones of said predetermined set of range/frequency gates; first neutrodyning means adapted to deliver first neutrodyning subreturns by subtracting from each main subreturn of said predetermined set of range/frequency gates the corresponding first auxiliary subreturn weighted by said first average ratio; second processing means responsive to a detection of a second jammer signal within said first auxiliary and second auxiliary radar returns for determining a second average ratio of those of said first auxiliary and second - 20 auxiliary subreturns corresponding to said selected ones of said predetermined set of range/frequency gates; second neutrodyning means adapted to deliver second neutrodyning subreturns by subtracting from each second auxiliary subreturn of said predetermined set of range/frequency gates the corresponding first auxiliary subreturn weighted by said second average ratio; third processing means for determining a third average ratio of those of said first and second neutrodyning subreturns corresponding to said selected ones of said predetermined set of range/frequency gates; and third neutrodyning means adapted to subtract from each said first neutrodyning subreturn of said predetermined set of range/frequency gates, the corresponding second neutrodyning subreturn weighted by said third average ratio, whereby said first and second jammer signals are substantially eliminated from the output of said third neutrodyning means.

7. A device according to claim 6, wherein said first auxiliary antenna is the difference channel of said main antenna.

8. A pulsed Doppler radar device, constructed and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.