RADAR INTERROGATOR APPARATUS

Abstract: The invention regards a perfectioning in reply radar systems, in particular SSR (Secondary Surveillance Radar) or IFF identification Friend or Foe). The purpose of this invention is to reduce undesired replies due to reflections from fixed obstacles or from structural peculiarities in the main radar system (i.e. radome) and other similar effects. The suppression of undesired replies is attained by varying azimuthally the emission power of the radio-frequence im­ pulses according to a memorized law.

Description: The present invention regards a perfectioning in reply type radar systems, in particular the SSR (Secondary Surveillance Radar) or IFF (Identification Friend or Foe) types. These radar systems consist of a transceiver on land (interrog a tor) having a system of rotating antenna and a transceiver on board (transponder) which basically utilizes a multi-directional antenna. The interrogator positioned on land typically emits three impulses (identified later as P1,P2,P3) in order, for example at a frequency of 1030 MHz, with for example peak power levels included between 0,5 and 4 KW. The time interval between the impulses P1 and P3 irradiated by means of a directional antenna, is related to the particular "mode" of interrogation. The impulse P2 which presents a fixed delay with respect to the impulse P1, is irradiated from a so called "control" beam of the pseudo-multi-directional type, which has the function of avoiding interrogations from the lateral lobes of the directional beam (SLS - Side Lobe Suppression). From this it follows that, at the input of the transponder the ratio between the amplitudes of the impulses P1 and P2 turn out to depend on the azimuth of the air-craft referred to the axis of the directional antenna beam. Basically the following two cases exist: - for the azimuths included in a section of the directional beam, so called "effective beam" the ratio P1/P2 is nominally greater than 9 dB. - for the impulses arriving from the lateral lobes of the-dire c tional beam, the ratio -P1/P2 is nominally less than ⊘ dB. In case "a" the transponder has to reply, in case "b" nominally there should be no replies what has been described is subjected to international normatives (ICAO, STANAG). In practice several legal mal-functions are revealed at the system and at the interactions between the system and the ambient In particular: a) Reflecting surfaces such as airport structures, aircraft on land, buildings, ships ect. are causes of partial deviation of the directional beam for which interrogations of aircraft can be about which are outside the effective interrogation beam. b) The surrounding ambient, in particular the radome structure often used to protect the antenna systems, tends to notice a bly deform in an unpredictable fashion the two antenna beams. c) A weakness of the SSR/IFF radars consists in the lack of a n gular discrimination which creates problems especially in the "terminal" covers where an elevated air-traffic density can be manifested. In an "on site" optimization procedure use is generally made of reducing the transmission power. This is realized by means of fixed power attenuators, placed in cascade at the transmitter. In other words: the general power of the transmitter is reduced by a fixed amount and in a discriminative way with respect to the specific directions from which the inconvenience was coming. As regards the reflections, optional units exist, which operate on the transmission impulses in such a way as to force blindness of the transponder illuminated by the lateral lobes (IISLS: Improved Interrogator Side Lobe Suppression). A prior system in which the transmission power is reduced by means of an attenuating member is disclosed in US-A-4,099,180. Moreover, this reference discloses several theoretical possibilities for suppressing or reducing undesired reflecting pulses caused by fixed obstacles i.e. reflectors. As one theoretical alternative for obtaining a suppression of pulses, which constitute false replies, use of a technique called interrogator transmitted power programming is proposed however, this alternative is said to be unuseful, since attenuators capable of handling peak power on the order of 2 to 3 KW, which would be required for this function, are still in the stage of development. The problem underlying the present invention is to provide an interrogator transponder radar apparatus in accordance with the pre-characterizing clause of claim 1 which overcomes the drawback of the prior art. This problem is solved by the features stated in claim 1. Specifically, the apparatus of the present invention overcomes several of the disadvantages and inconveniences of the prior art apparatus, which may be summarized as follows: little efficiency; the forced blindness of the transponder tends to diminish the reply probability of the said transponder, to another interrogating sensor; notable hardware impact which in particular is felt either with an appreciable r.f. loss (at least 3 dB) or with the necessity to add a second transmitter. The present invention is now described with reference to the enclosed diagrams which illustrate more clearly the type of problem which we intend to overcome, some aspects of preceding techniques and the preferred solutions according to the present invention. Figures: Figure 1 shows the out-going and return relationship (up down link) of the signals of an SSR system: Figure 2 shows respectively the format of the interrogations and of the replies in an SSR system: Figure 3 shows the amplitude relation in a side lobe suppression (SLS) system and the antenna diagrams. Figure 4 shows the effects of reflections in an SSR system, in the presence of on-land obstacles; Figure 5 shows the distortion effects on behalf of a "radome" which induces false interrogations; Figure 6 shows geometric aspects associated to the phenomenon of undesired reflection interrogations: Figure 7 shows well known implemantations for reducing parasitic effects (SLS) and (IISLS); Figure 8 schematically shows the control of the reflection effects by means of the sectorization of the irradiated power; Figure 9 shows a possible realization for controlling the transmission power in four sectors which make up the emission turn angle; Figure 1 0 shows the block diagram of a transmitter according to the technique in figure 9; Figure 11 shows another possible realization for controlling the transmission power sectoralized in relation to the interrogation a suppression impulses; Figure 12 shows the block diagram of a transmitter according to the technique in figure 11. With reference to the diagrams in figure 1 we repeat what has been previously mentioned, that is, the cooperative aspect of the SSR radar. The on-land system AP emits the interrogation and the on-board transponder positioned on the aircraft V carries out the identification and then in a positive case emits the replica or reply on a different frequency channel. In figure 2 are presented the details of the modulation waveforms associated with the interrogation and the reply. In the schematization of figure 2 related to the interrogation section we note the presence of the impulses P2 whose function will be better illustrated with reference to figure 3. In effect in figure 3 are shown the amplitude relationships between impulses P1,P3 and the control impulse P2 and the antenna diagrams related to the impulses P1,P3 and P2. It is noted that the impulses P1 and P3 are emitted on a directional beam which includes a certain number of secondary lobes in various configurations centred on the emission point, while the impulses P2 are emitted from an antenna having an omni-dire c tional irradiation diagram (practically circular in the on-land projection). In figure 4 the effect of reflections in the presence of fixed structures are shown (figure 4a). In figure 4b it is shown how in the presence of a reflecting surface RS an aircraft V "sees" the irradiation from the radar's antenna R in a different angle from the effective angle. As can be noted from figure 4c, which illustrates a typical PPI presentation, in these circumstances one has a real target RT and a false target FT whose presence is induced by the reflecting surface RS of figure 4b. This is the situation which comes about in the presence of fixed obstacles which are not part of the surveillance radar structure. In figure 5 is shown instead the interrogation effect on behalf of the side lobes induced by distortion caused by the radome, that is the fixed struc t ure which normally encloses the rotating antennas. In figure 5a is seen a radome structure including the radome proper RD which encloses a primary antenna PA and an antenna of the secondary radar SA. In figure 5b is reported in cartesian coordinates the polar diagram of the emitted signal in free space and in the pr e sence of radome, the distortions induced by the radome, and the events of possible interrogations on behalf of the side lobes caused by the distortions introduced by the radome's fixed structure. In figure 6 the geometric aspects associated with the interrogation phenomenon by undesired reflections are shown. In the schematization of this figure, one has to note that if the delays of the reflected path is less than the nominal 35 microseconds, and the direct interrogation impulse P1 has an amplitude less than the predetermined threshold the reflected interrogation does not give rise to a reply. In the left-hand case the situation of a direct i n terrogation from the side lobes and in the right-hand case a reflected i n terrogation from the principal lobe is shown. Whenever the following two conditions are verified: a) the transponder recognizes that the first interrogation comes from lateral lobes and this intercepts the reception; b) the delay of the reflected interrogation is less than the i n terception time interval; then the interrogation from the reflection route will not give r i se to any reply. Assuming that the path difference is such as to satisfy the condition b the capability of intrinsic antireflection at the system results inefficient since the shape of the side lobes of the directional beam impedes the verification of condition a. In fact, for circa 80% of the azimuth directions related to the side lobes, the amplitude of the impulse P1 so small that it is not recognised by the transponder which therefore does not detect the interrogation from the side lobes. The IISLS technique which cannot be generalized since it is exclusively concerned with receivers which satisfy condition b consists in assuring that the condition a is always verified. The action is that of increasing, artificially the level of the side lobes of the directional beam. In figure 7 are schematically presented the on-land sensors which are either of the conventional type or of the type equiped with IISLS (figures 7a and 7b). In figure 7c are respectively shown the irradiation diagrams related to the two realisations 7a and 7b. Now the solution adopted by the present invention will be d e scribed: a) as regards the "reflections" it has been brought to notice that there is a big advantage in irradiating the minimum power. The transmitted power in fact is generally "redundant". A reduction of the transmitted power, of the order of 3 to 6db, does not dramatically reduce the range of the reflected path. There are therefore large power margins available, and this is schematically represented in the diagrams of figure 8. The disadvantage of this approach is in the reduced performance of the system in terms of radar cover. It is also known that the "reflectors" occupy a relatively limited azimuthal sector (order of magnitude of the antenna beam). The technical knowledge forming the basis of the present invention therefore consist in limiting the transmission power o n ly in the angular sectors necessary. b) As regards the interrogation on behalf of the side lobes, it is proposed to strengthen in an additive way the level of the impulse P2 with respect to the impulses P1 and P3. If the phenomenon presents aspects of azimuthal stazionarity the action can be limited only to the angular sector of interest. The strengthening of the ratio P2/P1 is evidently realizable by means of a reduction in power associated with the impulses P1, P3. It is still evident that the safeguard of the power margins, will be carried out by applying this function in a sectorial method. c) The same technique mentioned in the preceding paragraph (b) can be used for narrowing the effective interrogation beam and the azimuthal sectoralization can evidently also be applied. Practical ways of realizing the technical knowledge of the pr e sent invention will now described. With reference to figure 9, in 100 is indicated a part of the interrogation transceiver. From the block 101 are available azimuth data supplied by the r o tation system of the directional antenna. The data from block 101 are taken to the detecting unit 102 of the active sector, to which are supplied also the data from block 103 which is pre-selection of the extremes of the four sectors in which the antennas special turn angle is divided. At the output of block 102 there is practically a sector address for commanding block 104 which is the pre-selection of the tran s mission power levels for each of the four sectors for which it is intended to act. The output of block 104 is composed of an i n terface with the transmitter indicated in 105 for the command of the transmitter 106 which in turn is connected with a duplexer 107. The structure of the transmitter is illustrated in more detail in figure 10. As can be noted in figure 10 we forsee four power amplification units 108 connected to a summing network 109, whose output, by means of the circulator 110 constitutes the radio-frequency ou t put. The units 108 are driven by amplifier stages 111 operating in class C with controllable power, biased on one side by a radio-frequency power dividing network 112. The units 111 are controllable for example by means of their direct-current supply, with numerous commands 113. Typically the commutation time is of the order of one millisecond for which the impulses constituting a message in transmission present the same level. In figure 11 another realisation is shown, in which the azimuth data are supplied from a block 200, and from a block 201 are su p plied data related to the sector information from a central processor of the radar system. In the block 203 which constitutes part of the transceiver su b division of the antenna's exploration turn angle into 128 sectors is carried out in block 204. At the same time block 205 supplies an updating of the power levels. On the output of block 204 there are sector addresses which are supplied to a first memory 206 of the interrogation and suppression power levels for each sector. At the same time the sector addresses and the updating data of the power levels supplied by block 205 are conducted to another memory 207 up-datable with relation to the p o wer levels. The outputs of the memories 206 and 207 are combined in a selection logic 208 for the command of the interface 209 with the transmitter and then the transmitter 210. In figure 12 is shown a typical diagram of a transmitter in which are utilized PIN diode modulators for controlling the emi s sion power. One must note that the diagrams of figure 12 and figure 10 are substantially changeable, as is evident to a person experienced in this field. One must however notice that in the structure of figure 12, since PIN diode elements are utilized, the commutation times are extremely reduced, of the order of tenths of n a noseconds, as compared with the structure of figure 10. This a l lows a greater resolution in the power variations with the antenna rotation, and also to select in a convenient way the rel a tive amplitudes of the various impulses P1, P2, P3 in ratio with one another. The electronic technique of memorizing and processing data mentioned with reference to figures 9 and 11 are at the fingertips of any expert in the field and consequently it is felt unnecessary to describe them in detail. The technical knowledge according to the present invention presents the following advantages from an operational point of view. The techniques described earlier are "sectorially" inserted, only where needed, in such a way as to leave unaltered the performances in terms of radar coverage, in the "clear" sectors. In the control of the "reflections" with respect to the techniques which are based on the blinding of the transponders from the side lobes of the directional antenna, one tends to diminish the interferences towards the interrogators which realize the adjacent radar coverage. As regards the realization criterion of the transmission unit it is pointed out that with respect to alternative solutions, the solution adopted according to the present invention, permits power variations to be carried out by means of circuits which can be realized by components easily available, are economic, much contained in space taken-up and fully satisfying the systems exigencies mentioned, in terms of both precision and velocity (commutation time ∼ 100ns). This is made possible because the commutators, operating on the radio-frequency input signals of the single amplifier modules, must control power levels which are less, to those nominally emitted from the transmitter, by a factor equal to the product of the number of active elements in parallel and their gain. In practical situations this quantity is between 25 and 50.

Claims: A radar interrogator apparatus for locating an object carrying a transponder, comprising antenna means (SA) including a rotating antenna, transmitter means (106, 210) supplying to said antenna means sequences of pulses at a predetermined radio frequency, receiving means connected to said antenna means, the receiving means being arranged to derive , from transponder reply pulses generated in response to a transmitted pulse sequence, information relating to the location of the object associated with the replying transponder, suppression means for suppressing the generation of transponder reply pulses in response to transmitter pulses travelling to the transponder along an indirect path owing to reflection of the transmitter pulses at obstacles situated at predetermined azimuth directions with regard to the rotating antenna, the suppression means including means (108 - 113) for reducing the power of the pulses transmitted by the antenna means whenever the rotating antenna points in one of said predetermined azimuth directions, characterised in that the power reducing means comprises (i) a plurality of amplifier stages (108), (ii) a plurality of power modulating stages (111) each of which has an output connected to an input of a different one of the amplifier stages, (iii) an RF power dividing network (112) having an input connected to the transmitter means and having a plurality of outputs each of which is connected to the input of a different one of the power modulating stages, (iv) signal combining means (109) arranged to supply to the antenna means the combined output signals from the amplifier stages, (v) control means (101-105; 200-209) for generating modulation control signals for the power modulating stages, said modulation control signals depending on the azimuth position of the rotating antenna in such a way that the power of the combined output signals is reduced by a selected amount for each of said predetermined azimuth directions. Apparatus according to claim 1 wherein the control means are arranged to provide for predetermined amplitude ratios between selected pulses of each pulse sequence. Apparatus according to claim 1 or 2 wherein each power modulating stage consists of a preamplifier (111) whose gain is controlled by the modulation control signals. Apparatus according to claim 1 or 2 wherein each power modulating stage includes a controllable attenuating member consisting of a PIN diode.

Inventors: CHIERICI, ROBERTO, RONCONI, ROBERTO
Applicants: SELENIA INDUSTRIE ELETTRONICHE ASSOCIATE S.P.A.
Application year: 1982
Main classification: G01S13
IPC codes: G01S7, G01S13