EP0097073A1 - Method and device for the reduction of jamming signal power received by the side lobes of a radar antenna - Google Patents

Abstract

The method for reducing the power of the interference signals received by the side lobes of a radar antenna belonging to the method known as the secondary lobe opposition (OLS) uses auxiliary directional radiation patterns, with a zero. in the direction of the maximum radiation of the main antenna, a phase center close to that of the diagram of the main antenna and a minimum gain in the areas where the level of the side lobes of the main antenna is low enough to avoid detecting jammers in these areas.

Application to multibeam antennas, or to an aplanatic lens or to network antennas with candlestick supply.

Description

The present invention relates to the protection of a radar system against jamming. It relates more particularly to a method and devices for reducing the power of the interference signals received by the side lobes of a radar antenna.

These interference signals are generally active, natural or artificial, continuous or cut-off interference signals, sometimes transmitted by several independent jammers and which combine with the internal noise of the associated receivers. As a rule, these interference signals are received by the secondary lobes of the radar antenna with a level such that they considerably reduce the signal to noise ratio and completely disrupt the operation of the radar.

In order to reduce the interference thus produced on the useful signals, techniques known under the name of secondary lobe opposition or OLS ("Side Lobe Cancelation" in English terminology) have been developed. This countermeasure technique is described schematically in an article by MA. JOHNSON and DC STONER "ECCM From the radar designer's view point" published in the review "Microwave Journal" of March 1978, pages 59 and 60 (electronic countermeasures against jamming from the point of view of the radar designer). According to this technique, an effort is made to adapt the characteristics of the radiation of a reception antenna to the surrounding medium, so as to maximize the ratio of the useful signal to the total of the parasitic signals. This adaptation is obtained by using auxiliary antennas, with their reception channels, whose diagrams are combined with that of the main antenna considered so as to obtain a global diagram having zeros, or at least minima, in the directions of external jammers while avoiding excessive amplification of the internal noises of the receivers associated with the auxiliary antennas.

FIG. 1 recalls the classic diagram of a multi-jammer OLS system, comprising a certain number of decoration loops.

A conventional OLS system is a "looped" system mainly comprising a main antenna 1 and auxiliary antennas 2,3 each associated with a reception channel 200, 300. Each of these reception channels comprises a loop constituted by an amplifier 4, 40 , an integrator 5, 50, a correlator 6, 60 and a control mixer 7, 70. In such an OLS system of the prior art, each signal b, b 'received by an auxiliary antenna is cut off in a circuit 8 at signal b received by the main antenna, after each auxiliary signal (b, b ') has been multiplied by a weighting coefficient (W, W') subject to the correlation existing between the auxiliary signal and the signal used. The signal processed takes the form b -bW-b'W. We show that the total noise is then minimum. The adaptation to the environment mentioned above is thus carried out.

Non-looped systems are also known, in which the optimal weighting coefficients W are determined by a calculation which amounts to inverting the covariance matrix of the signals received by the main antenna and the auxiliary signals.

However, whatever the algorithm used, it can be shown that the choice of auxiliary antennas is not indifferent to the speed of convergence of the process, the final improvement factor, the signal to interference ratio, the bandwidth of the system and the vulnerability of the system to additional interferers.

• - Etant peu directive, parfois presque omnidirectionnelle, une antenne auxiliaire peut englober dans son diagramme plusieurs brouilleurs, entrainant une diminution de l'efficacité et de la rapidité de convergence des boucles de pondération.
• - Le gain d'une telle antenne auxiliaire est faible ; par suite, le coefficient de pondération à attribuer au signal qu'elle délivre est relativement élevé, ce qui risque de ramener dans la voie principale une fraction d'autant plus importante du bruit thermique du récepteur associé et de diminuer le facteur d'amélioration final du rapport signal sur brouilleur. On rappelle que le facteur d'amélioration est le rapport du rapport signal sur bruit en l'absence au rapport en présence du procédé de réduction de la puissance du bruit utilisé, c'est-à-dire en fait le quotient du rapport signal sur bruit avant et après l'opération de réduction de la puissance de bruit envisagée.
• - Le diagramme auxiliaire est large et recueille donc des échos parasites, appelés aussi clutter, ce qui diminue l'efficacité du système.
• - Le centre de phase d'une antenne auxiliaire est généralement éloigné de celui de l'antenne principale et le coefficient de pondération W associé est très sensible à la fréquence. Dans le ,cas d'un radar à fréquence aléatoire par exemple, ce coefficient devra varier très rapidement, empêchant ainsi le système d'être à très large bande.
• - Le diagramme global, résultant de la combinaison du diagramme de l'antenne principale et des diagrammes des antennes auxiliaires qui sont peu directifs, présente des lobes latéraux perturbés par le fait que les lobes des antennes auxiliaires captent des brouilleurs qui ne gênaient pas l'antenne principale seule.
• - On peut montrer qu'il existe des combinaisons entre directions de brouilleurs et antennes auxiliaires non directives qui ne convergent vers aucune solution. L'ensemble des sources auxiliaires quasi-ponctuelles avec leurs coefficients de pondération présente un diagramme à caractère angulairement périodique, alors que les lobes latéraux de l'antenne principale ne le sont pas. Comme le système OLS revient à opposer les uns aux autres, cette opposition, si elle est réalisée dans une direction, a peu de chance de l'être dans les directions séparées par une ou plusieurs périodes angulaires.

Generally, the auxiliary antennas associated with known OLS systems are poorly directed and often placed around the periphery of the main antenna. Such a configuration has certain drawbacks:

• - Being not very directive, sometimes almost omnidirectional, a auxiliary antenna can include in its diagram several jammers, resulting in a decrease in the efficiency and speed of convergence of the weighting loops.
• - The gain of such an auxiliary antenna is low; as a result, the weighting coefficient to be assigned to the signal it delivers is relatively high, which risks bringing back into the main channel an even greater fraction of the thermal noise of the associated receiver and reducing the final improvement factor signal to jammer report. It is recalled that the improvement factor is the ratio of the signal-to-noise ratio in the absence to the ratio in the presence of the noise reduction method used, that is to say in fact the quotient of the signal-to-noise ratio. noise before and after the planned noise reduction operation.
• - The auxiliary diagram is wide and therefore collects parasitic echoes, also called clutter, which reduces the efficiency of the system.
• - The phase center of an auxiliary antenna is generally far from that of the main antenna and the associated weighting coefficient W is very sensitive to frequency. In the case of a random frequency radar for example, this coefficient will have to vary very quickly, thus preventing the system from being very broadband.
• - The global diagram, resulting from the combination of the diagram of the main antenna and the diagrams of the auxiliary antennas which are not very directive, presents side lobes disturbed by the fact that the lobes of the auxiliary antennas pick up jammers which did not interfere with the main antenna only.
• - We can show that there are combinations between directions of jammers and non-directive auxiliary antennas which do not converge towards any solution. The set of quasi-point auxiliary sources with their weighting coefficients presents an angularly periodic diagram, while the side lobes of the main antenna are not. Like the system OLS amounts to opposing each other, this opposition, if it is carried out in one direction, is unlikely to be in the directions separated by one or more angular periods.

The object of the invention is both a method and a device for reducing the power of the interference signals received by the side lobes of a radar antenna which overcomes the drawbacks mentioned above.

According to the invention, the method of reducing the power of interference signals capable of being received by the side lobes of a radar antenna is characterized in that the auxiliary diagrams are directional, have a zero in the direction of the radiation maximum of the main diagram, have a phase center close to that of the main diagram and have a minimum gain in the areas where the level of the side lobes of the main diagram is sufficiently low to avoid picking up the jammers in these areas.

According to one embodiment of the invention, there are also limiters in different correlation loops, increasing the speed of convergence of said loops. The role of the limiter is to reduce the spread of the spectrum of the eigenvalues of the covariance matrix. If b. are the signals of the various auxiliary channels (i = 1.2 ...), the covariance matrix is the one whose general term is the correlation coefficient between the signals of the auxiliary channels marked i and K or Rik = mean value of ( bi bk ^* ). Under these conditions, the speed of convergence of the so-called optimization correlation loops is increased.

• - La figure 2a, un réseau linéaire avec son illumination ;
• - La figure 2b, le diagramme typique du réseau de la figure 2a ;
• - La figure 3, l'échantillonnage du diagramme ;
• - La figure 4a, une antenne multifaisceau de façon schématique;
• - La figure 4b, les diagrammes d'échantillonnage de l'antenne de la figure 4a ;
• - La figure 5, une antenne multifaisceau, variante de celle de la figure 4a ;
• - La figure 6, une antenne réseau alimentée par une lentille ;
• - La figure 7, une source primaire complexe alimentant une antenne multifaisceau ;
• - La figure 8, une antenne multifaisceau avec alimentation en chandelier ;
• - La figure 9, les lois d'illumination correspondant aux diagrammes de l'antenne de la figure 8 ;
• - La figure 10, le diagramme de la voie somme (S) ;
• - La figure 11, le diagramme de la voie différence (D) ;
• - La figure 12, le diagramme de la voie écart (E) ;
• - La figure 13, le diagramme de la voie double différence (D'), et
• - La figure 14, le dispositif suivant l'invention avec des limiteurs.

Other characteristics and advantages of the invention will emerge from the description which follows given with the aid of the figures which represent, in addition to FIG. 1 relating to the prior art:

• - Figure 2a, a linear network with its illumination;
• - Figure 2b, the typical network diagram of Figure 2a;
• - Figure 3, the sampling of the diagram;
• - Figure 4a, a multibeam antenna diagrammatically tick;
• - Figure 4b, the antenna sampling diagrams of Figure 4a;
• - Figure 5, a multibeam antenna, variant of that of Figure 4a;
• - Figure 6, a network antenna powered by a lens;
• - Figure 7, a complex primary source supplying a multibeam antenna;
• - Figure 8, a multibeam antenna with candlestick supply;
• - Figure 9, the illumination laws corresponding to the antenna diagrams of Figure 8;
• - Figure 10, the diagram of the sum channel (S);
• - Figure 11, the diagram of the difference channel (D);
• - Figure 12, the diagram of the deviation path (E);
• - Figure 13, the diagram of the double difference channel (D '), and
• - Figure 14, the device according to the invention with limiters.

In these different figures, the same references relate to the same elements.

In the introductory part to the present application, it has been shown that the drawbacks of the OLS systems which it is desired to remedy arise from the low directivity of the auxiliary antennas which are combined with the main antenna of the radar, in order to obtain a reduction of the power of the interference signals received by the side lobes of the main antenna.

• - Les antennes auxiliaires doivent être très directives. Ainsi, chaque diagramme d'antenne auxiliaire ne capte, en règle générale, qu'un seul brouilleur dans son lobe principal. Un ensemble d'antennes auxiliaires à grande directivité exécute ainsi un préfiltrage spatial. Cette grande directivité entraîne généralement un gain important pour l'antenne auxiliaire : le coefficient de pondération à utiliser est de la sorte faible et le rapport du bruit du récepteur de l'antenne auxiliaire au bruit total est faible, donnant ainsi un bon facteur d'amélioration.
• - Le diagramme auxiliaire présente un zéro dans la direction de rayonnement maximal de l'antenne principale, ou tout au moins un gain minimum dans cette direction ; cela évite de recueillir le clutter ; le diagramme principal n'est pas perturbé et le gain dans les autres zones utiles est renforcé.
• - Le centre de phase des diagrammes auxiliaires est proche de celui du diagramme principal ; cela est favorable à une optimisation à large bande.
• - Les diagrammes auxiliaires présentent de plus, un gain minimum dans les zones où le niveau des lobes latéraux de l'antenne principale seule est suffisamment bas ; cela évite d'y capter des brouilleurs.

According to the invention, the conditions which the diagrams of the auxiliary antennas must meet so that, by their combination with the main diagram, the reduction in power of the interference signals is obtained without undergoing the drawbacks indicated, are therefore the following:

• - The auxiliary antennas must be very directive. Thus, each auxiliary antenna diagram does not, as a rule, capture only one jammer in its main lobe. A set of auxiliary antennas with high directivity thus performs spatial pre-filtering. This high directivity generally results in a significant gain for the auxiliary antenna: the weighting coefficient to be used is thus low and the noise ratio of the receiver of the auxiliary antenna to the total noise is low, thus giving a good factor of improvement.
• - The auxiliary diagram presents a zero in the direction of maximum radiation of the main antenna, or at least a minimum gain in this direction; this avoids collecting the clutter; the main diagram is not disturbed and the gain in the other useful zones is reinforced.
• - The phase center of the auxiliary diagrams is close to that of the main diagram; this is favorable for broadband optimization.
• - The auxiliary diagrams also present a minimum gain in areas where the level of the side lobes of the main antenna alone is sufficiently low; this avoids picking up jammers.

It follows that, according to the invention, the main and auxiliary antennas must present diagrams such that the set of antennas constitutes a sort of spatial filter of the environment of the antenna.

According to a first variant of the invention, an antenna structure can be defined, the optimized auxiliary diagrams of which have the characteristics which have been given. Such diagrams are so-called sampling diagrams, produced from a linear network.

,

). Ce réseau rayonne, dans une direction 0 repérée par rapport à la normale N au réseau 9, un diagramme F ( τ) bien représenté, sur la figure 2b, par la transformée de Fourier de f(x), soit

avec :

, λ étant la longueur d'onde.FIG. 2a shows schematically a linear network 9 of length L, identified by an abscissa x and which is the seat of an illumination IL, defined by the scalar complex function f (x) bounded by the domain (

,

). This network radiates, in a direction 0 identified with respect to the normal N to network 9, a diagram F (τ) well represented, in FIG. 2b, by the trans Fourier formed of f (x),

with:

, λ being the wavelength.

This diagram having a spectrum with bounded support, we know from the sampling theorem that it can be represented, as illustrated in Figure 3, as a linear combination of elementary sampling diagrams in the form

Each sampling diagram has the characteristics which are those required according to the invention for an auxiliary diagram.

Note that, if the antenna structure is such that each sampling diagram (in number N) is the subject of a separate entry, as is the case for example of a network excited by a matrix of Buttler or an equivalent matrix, it is possible to control the coefficients a _k so as to cancel the resulting diagram in the N directions of jammers, by carrying out as was recalled the sum of the signals received by the elementary antennas constituting the network, weighted by coefficients subject to the condition of maximization of the signal to total noise ratio.

Elementary diagrams meeting the stated characteristics are thus obtained with a multibeam antenna, the elementary diagrams of which are directional, adjacent diagrams, preferably orthogonal, covering the angular range to be protected against jammers.

Such a multibeam antenna is shown in FIG. 4a very schematically.

We recognize the linear array 9 of elementary antennas fed by a matrix 10, which can be a Buttler or Maxson matrix. The supply channels all include a weighting device 11, assigning to the signal which passes through them a weighting coefficient (Wi) determined in known manner; these channels are connected to a summing device 8, also receiving the main channel and supplying a receiver 12 which delivers the signal cleared of the jammers, or at least a signal in which the effect of the interference is greatly attenuated.

FIG. 4b shows the diagrams of the various elementary antennas 1 to N which play the role of the sampling diagrams defined above.

FIG. 5 schematically represents a multibeam antenna whose elementary diagrams meet the characteristics which have been defined previously and which is advantageously used to reduce the power of the jammers picked up by the antenna.

The network antenna 9 is supplied by a power divider 13 through phase shifters 14, creating the main channel. The auxiliary channels are created from couplers 15, placed in front of the phase shifters 14, which derive part of the incident energy towards a Buttler matrix 10 whose other terminals are connected to weighters 11 connected to an adder 8 receiving the channel main VP. The adder is connected to a receiver 12.

Other network antennas also respond to the problem and one can cite, among other things, a network antenna powered by a lens, preferably aplanatic. In an antenna of this type, shown diagrammatically in FIG. 6, the primary sources 17 illuminating the lens 16 generate, in a domain surrounding the main channel 18, the auxiliary directional diagrams 19 sought. The weighted addition, in phase and in amplitude, of the signals received by the auxiliary diagram 19, receiving a jammer B, with the signals received by the main diagram 18 makes it possible to obtain resulting signals in which the jammer is attenuated.

One can also use, in the same field of antennas, a reflective array antenna supplied by a network of primary sources. In this case, just like elsewhere in the previous one, the primary source can be complex and present a particular layout. FIG. 7 represents such a primary source which allows better use of the antenna in the context of the invention. Indeed the two antenna systems described above are particularly effective against multiple jammers located in directions not too far from that of the main lobe, distance which can be measured in a few widths at 3dB. If these jammers are distributed in a "horizontal" plane around the useful lobe, which is frequent in the case of distant powerful jammers, the sources are distributed as shown in figure 7. The main source SP monopulse giving the main lobe is placed in the center of a system of axes OX, OY and the auxiliary sources Si (i = 1 to 6) are distributed all around, capable of creating diagrams in accordance with the invention but not identical to each other according to the most probable distribution of the jammers.

Other types of array antenna can also be used, in accordance with the invention, to reduce the power of the jammers. These are the network antennas supplied by a candlestick or espalier divider, that is to say a distributor circuit with successive divisions, produced with various technologies such as coaxial waveguides, triplates, printed circuits ... The main channel consists of the main excitation input, or input of the sum channel "S" which produces a symmetrical, equiphase illumination, attenuated on the bell-shaped edges. However, the main channel, as a result of imperfect control, along the network, of the phase and of the amplitude in the frequency band to be covered, is accompanied by diffuse side lobes capable of collecting parasitic signals due to outdoor jammers. To obtain auxiliary diagrams meeting the conditions mentioned at the beginning of the description, the elementary couplers normally existing in the candlestick divider are replaced by directional couplers or of the magic tee type or of the hybrid ring type. All the elementary couplers are not systematically replaced but a certain number of them.

By way of example, FIG. 8 represents, in a very schematic form, the linear network 9 of length L supplied by a candlestick so that one can distinguish there four sub-networks 20, 21, 22, 23 distributed symmetrically and supplied with the same power and equiphase by couplers 25, 26, 27 and 28, for example magic T's. You can then define a certain number of diagrams. The central coupler 25 determines a sum channel S giving the main diagram and a difference channel D giving a difference diagram to constitute an auxiliary diagram within the meaning of the invention.

The couplers 26 and 27 each have a difference channel which are associated by lines of the same length with a coupler 28, magic tee or hybrid ring, which, developing the sum and the difference of the signals it receives, defines two other auxiliary diagrams , corresponding to what has been called, in an earlier publication of the applicant, the gap path (E) and the double difference path (D '). If we represent by a, b, c, and d respectively the amplitudes of the signals created by the networks 20-23, the deviation path E is characterized by a diagram ((ab) + (cd)) and the double difference path From by ((ab) - (cd)).

FIG. 9 represents the illumination laws of the different channels which have been defined from the array antenna of FIG. 8.

• 1. Les diagrammes auxiliaires possèdent un zéro dans l'axe.
• 2. Le diagramme auxiliaire différence D, possède un gain relativement élevé vis-à-vis des lobes latéraux de la voie somme S, même pour les lobes éloignés de l'axe.
• 3. Les centres de phase des illuminations auxiliaires coïncident avec celui de la voie principale S.
• 4. Les diagrammes auxiliaires écart (E) et double différence (D') possèdent des zéros alternés ; ainsi, si un brouilleur tombe dans un zéro d'un diagramme auxiliaire, il est reçu par l'autre diagramme, réalisant une ébauche de préfiltrage spatial.

Figures 10 to 13 represent the diagrams, in decibels as a function of the angle 0 in degrees, of the different main and auxiliary channels (S, D, E and D 'respectively) on which we can observe a certain number of properties which are those defined at the beginning of this description:

• 1. The auxiliary diagrams have a zero in the axis.
• 2. The difference auxiliary diagram D, has a relatively high gain with respect to the lateral lobes of the sum channel S, even for the lobes distant from the axis.
• 3. The phase centers of the auxiliary illuminations coincide with that of the main track S.
• 4. The auxiliary difference (E) and double difference (D ') diagrams have alternating zeros; thus, if a jammer falls into a zero of an auxiliary diagram, it is received by the other diagram, carrying out a draft of spatial pre-filtering.

It was indicated at the beginning of this description that there was a relationship between the spreading of the spectrum of the covariance matrix and the performance of the process considered for reducing the power of the interference signals received by the side lobes of the radar antenna. In fact, if one wants that the system is effective on all the dynamics of the eigenvalues, or on all the dynamics of jamming in the case where the matrix is diagonal, the time of adaptation is proportional to this dynamics.

If it is assumed that each interferer is only received by a single auxiliary antenna and that the interference levels received on the auxiliary channels are all equal, the correlation loops are perfectly decoupled and the covariance matrix operates in parallel and identically. However, this is a relatively problem-free situation for a single-boiler OLS system. If the auxiliary diagrams are sufficiently directive and therefore each diagram only picks up one jammer, the other jammers being received by side lobes of the diagram considered, it follows that most often the covariance matrix is predominantly diagonal. The partial decoupling thus obtained for the correlation loops can be exploited to improve the dynamic performance of the system. To do this, according to the invention, a limiter is inserted between each auxiliary antenna and the associated correlation mixer.

FIG. 14 schematically represents the device thus produced. The network antenna 9 determines the main channel VP and the auxiliary channels 200, 300, 400 ... which are all connected to the summing circuit 8. In the correlation loop shown in FIG. 14 is inserted a limiter 29 before the correlator 6, through which the signal b ₁ from the auxiliary antenna passes. This is achieved for each correlation loop.

In the case of auxiliary antennas with weak directivity, even omnidirectional, all the eigenvalues of the matrix are multiplied by the same constant. It results from it that the dynamics of the eigenvalues is unchanged and one gains nothing from the point of view speed of convergence. On the other hand, in the case of directive auxiliary antennas, one gains approximately in a ratio of 2 on the spread of the spectrum expressed in decibels. The result is a marked improvement in the speed of convergence of the system.

A method and devices have thus been described implementing this method, for reducing the power of the interference signals received by the side lobes of a radar antenna.

Claims (10)

1. Method for reducing the power of interference signals capable of being received by the side lobes of a radar antenna, according to which the radiation pattern of this antenna, known as the main pattern, is combined with auxiliary radiation patterns which associated with it in order to obtain a global diagram having minima in the directions of the jammers, the method being characterized in that the auxiliary diagrams (1-N) are directional, have a zero in the direction of the maximum radiation of the main diagram, have a phase center close to that of the main diagram and have a minimum gain in the zones where the level of the side lobes of the main diagram is low enough to avoid picking up the jammers in these zones. 2. Method according to claim 1, characterized in that the signals from the auxiliary diagrams are subjected to a weighting before being added to the signals of the main diagram, the signals of the auxiliary diagrams are furthermore subjected to a limitation of their amplitude during the weighting calculation. 3. Device for reducing the power of interference signals received by the side lobes of a radar antenna, characterized in that it implements the method according to one of the preceding claims. 4. Device according to claim 3, characterized in that the auxiliary diagrams are created by a multibeam antenna whose beams are directional, adjacent, orthogonal and arranged so as to cover the angular range to be protected against jammers, said antenna being consisting of a linear array (9) of elementary antennas supplied by a Buttler or Maxson array (10) and further comprising a weighting (11). 5. Device according to claim 4 implementing the method according to claim 2, characterized in that it comprises limiters (Li), inserted in each auxiliary channel between the weighting device (11) and the radiating element corresponding auxiliary diagram. 6. Device according to claim 3, characterized in that the auxiliary diagrams are created by a network antenna (Fig. 6) supplied by an aplanatic lens (16) illuminated by primary sources (17). 7. Device according to one of claims 3 or 5, characterized in that the auxiliary diagrams are created by a reflective array antenna, supplied by a network of primary sources (Fig 7). 8. Device according to claim 6, characterized in that the network of primary sources (Fig. 7) comprises a main source (SP) of the monopulse type surrounded by auxiliary sources (Si to Si ₆ ) creating separate auxiliary directional diagrams, according to the most likely distribution of jammers. 9. Device according to claim 3, characterized in that the auxiliary diagrams are created by network antennas supplied by a candlestick divider (Fig. 8) in which the couplers (25 to 28) are directional couplers or T-type couplers magic or hybrid ring type. 10. Device according to claim 8, characterized in that the main diagram of the antenna is that created by the sum channel and the auxiliary diagrams are those created respectively by the difference channel (D), the difference channel (E) and the double difference path (D ').

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