GB2517659A

GB2517659A - Radar for helicopters

Info

Publication number: GB2517659A
Application number: GB9408634.5A
Authority: GB
Inventors: Bernard Dortomb; Jean-Claude Charpentier; Jean-Claude Langot
Original assignee: Dassault Electronique SA
Current assignee: Thales SA
Priority date: 1993-06-23
Filing date: 1994-04-27
Publication date: 2015-03-04
Anticipated expiration: 2014-04-27
Also published as: ITTO940344A1; GB9408634D0; GB2517659B; ITTO940344A0

Abstract

On the rotor mast of the helicopter, there is mounted the 5 antenna casing BA of a pulsed Doppler radar operating in the S band, with a form factor of the order of 1/10, and a high repetition frequency of the order of 15 to 20 Khz. The antenna BA 100 is fixed to the rotor. Post processing of the radar return signals after reception is effected, for each distance gate, on groups of samples which overlap according to an overlap factor n so that for samples spread over R radar repetitions a new group is available after R/n repetitions, as shown distance gate samples are Doppler filtered by the first Fourier transform and post-integrated in temporal mode to detect air frame echoes and in frequency mode to recognize signatures of sporadic blade flashes from neighboring helicopters.

Description

RADAR FOR HELICOPTERS

The invention concerns radar surveillance undertaken on board helicopters, in particular with the object of detecting air targets and determining their position.

While studying the installation of a surveillance radar on board a helicopter, the Applicant has explcired the idea of fixing the radar antenna in a fixed manner at the centre of the main rotor of the carrier helicopter. This has the advantage that the radar is not disturbed by the blades of the rotor, since the movements are synchronous. At the end of the studies undertaken on a helicopter fitted with a mock-up of the antenna casing, it has become apparent that the presence of the casing did not impair the stability of the aircraft in flight.

Such a solution is nevertheless unusual as far as the radar operators are concerned, sinc they prfer to control the angular scanning speed of. the ±Sar, all the more so, since the speed of the main rotor of a helicopter, typically approximately 300 r.p.m. is considerably higher than the usual radar scanning speeds which are, at most, of the order of 50 to 60 r.p.m.

Other technical difficulties appear, in particular as regards the electric power supply for the antenna casing, the exchanges of data between the antenna casing and the part of the radar situated in the cabin, and cooling of the antenna casing.

Thus substantial technical problems remain to be resolved.

Moreover, a particular feature of helicopters is that in response to a radar signal, they return not only an "air-frame" echo like fixed wing aircraft and other moving units in general, but also echoes of a different kind, called "blade flashes". The processing of these blade flash echoes is worthwhile on many counts, but scanning them is difficult and so is their processing itself.

Even more difficult problems arise when one wishes to detect from a helicopter (carrier) the blade flashes of neighbouring helicopters (targets). Indeed, th' main rotor speeds which are generally of the same order of magnitude for all heli-copters, are moreover virtually identical for craft of the same type. As a result, if the radar of the carrier receives blade flashes from a target craft of the same type as its own, the radar of the caErj helicopter can, depending on the relative angular phase dithplacement between its owit main rotor and that of its neighbour Qtroboscopic effect): -see all the blade flashes (synchronism) -see none of the blade flashes (antisynchronism), or -in the present case, see a variable proportion of the blade flashes.

The object of the present invention is to provide solutions for these problems; as regards the stroboscopic effect, an optimized processing is proposed so as to use the small number of available data regarding blades.

According to a first aspect of the invention, the antenna casing of the radar is mounted on the rotor mast, preferably on the shaft, of the helicopter. The radar is of the pulsed Doppler type, its transmitter /receiver preferably operating in the S band (for example, with a font factor of the order of 1/10 and a high pulse repetition frequency, of the order of 15 to 20 Khz) Advantageously, the antenna casing is subjected to an angular sweep that is synchronous with the rotation of the main rotor of the carrier helicopter, and fixed so that the main antenna lobe is situated between two blades of the rotor of the carrier helicopter.

The post-processing of the radar return signals after their reception is effected in a special manner: on the basis of radar samples spread over R repetitions, a fresh group of samples is established ever R/n repetitions, where n is termed the "overlap factor"; this makes it possible to increase the number of data that can be used for the process-ing, in particular for targets with a short presence period.

Preferably, the post-processing includes: -a first post-processing with a temporal integration in each distance gate, for identifying conventional fluctuating targets, such as the fuselages of aircraft, and -a second post-processing with a frequency integration for identifying targets with a brief presence period, such as rotor blades of helicopters.

According to another aspect of the invention, the radar is split into a top part incorporated into the antenna casing (or aerial casing -this is the casing wherein the antenna is incorporated), and a bottom part mounted in the cabin or fuselage of the helicopter.

The connection between these top and* bottom parts may be ensured through an axial element, fixed or rotating, which passes through the rotor shaft, whilst a rotating collector ensures the link between the fixed part connected to the helicopter and the rotating part to which the antenna casing is fixed.

An angle encoder is mounted between the fixed part and the rotating part for the acquisition of the instantaneous position of the antenna casing.

As a variant or in addition, the communication between the two parts includes a digital radio or infrared link, without wires.

Moreover, the power supply for the antenna casing may be ob-tained at least partly by means of an alternator.

According to yet another aspect of the invention the antenna casing, which has a form of revolution coaxial with the axis of the main rotor, includes an antenna covered by a radotne forming a lateral part of the casing, with a sealed space between the antenna and its radome, and means cooperating with louvres arranged in the outer wall of the antenna casing so as to ensure circulation of cooling air in the rest of the antenna casing, taking into account its rotational movement.

Preferably, this "rest of the antenna casing" has a double-walled structure permitting the circulation of the outer cooling air. Moreover, an internal closed circuit air circulation may take place under mechanical actuation, and provision is made for a heat exchange surface between the said closed internal air circuit and the air admitted from the outside.

Other characteristics and advantages of the invention will emerge on examining the detailed description given below and the attached drawings wherein: -Figure 1 schematically illustrates a helicopter; -Figure 2 schematically illustrates the centre of the main rotor of the helicopter provided with a device in accordance with the invention; -Figure 3 is a sectional view showing the mechanical installation of the device in accordance with the in'vention, on a hub of the main helicopter rotor; -Figures 3A and 3B are two sectional views showing the connection between the top part of the radar system and the cabin of the helicopter; -Figures 4A and 43 are two very general diagrams of the installation of various elements of the device in accordance with the invention, on board a helicopter; -Figure 5 is a general diagram of an arrangement of the device in accordance with the invention, with a separation between the aerial casing and the elements installed in the fuselage or cabin of the helicopter;

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-Figures 6A and GB are two more detailed diagrams of the antenna part of the device in accordance with the invention, respectively with and without electronic scanning unit in the angle of elevation mode; -Figure 8 is a more detailed diagram of the ultrahigh frequency receiver part of the device in accordance with the invention, -Figure 9 is a more detailed diagram of the medium frequency receiver part of the device in accordance with the invention, -Figures 10 and bA are two variously detailed diagrams of the video-reception and processing parts and those for the link with the cabin unit of the device in accordance with the invention; -Figure 11 is a diagram affording a better understanding of the digital filtering by a Fourier transform; -Figure 12 is a table of the sample sequences obtained by a Fourier transform with overlap; -Figures 13A and 133 are respectively a temporal diagram and a frequency diagram affording a better understanding of the response of a helicopter blade to a radar signal; -Figure 14 is a schematic sectional view of the antenna casing of the device in accordance with the invention; -Figure 15 is a partly sectional perspective view of the device in accordance with the invention; and -Figures 16A and l6B are diagrams of the cooling circuit of the casing.

The attached drawings are, in essence, of a definitive nature. They therefore form an integral part of the description. They may thus not only serve to render the latter more readily understood, but also to contribute to the definition of the invention if required.

Figure 1 shows schematically a helicopter with a fuselage HC, equipped with a tail rotor HR and a main rotor RP comprising a mast/hub assembly MC, to which there are fixed for example four blades HP1 to HP4, and which enters into a base ER.

This hub KR is surmounted by the aerial part or antenna casing BA of the device in accordance with the invention.

More precisely (Figure 2) the casing BA is mounted on the rotor hub.

In Figure 3, which is a schematic sectional view, the fixed part and rotating parts have been distinguished by a differ-ent representation. There are seen again the blade mounting stubs PP1 and PP3 which are mounted on the rotor hub MR fixed to the rotor shaft. In Figure 3A, the rotor shaft MC is joined by a ball bearing BB to a fixed hollow tube FC which comes down to pass through the main gear box BTP that is associated with the engine HE and with the tail rotor transmission shaft RST. A mechanical interface MI allows the antenna casing BA to be mounted with the interposition of a labyrinth seal SM which in particular prevents water from rising.

At its centre, the casing BA includes a cylindrical cutout of the same geometry as a tube FC so that it is possible to pass other equipment through this cutout as, for example, the con-nection required for an optical viewer Os of the mast.

In the embodiment illustrated (Figure 3), the casing BA rotates round a shaft segment FS which passes through its central cutout. Provision is made for a flexible coupling FL between the tube FC and the shaft segment FS.

Between this fixed shaft FS and the casing BA, there is mounted an angle or bearing encoder CC: the angle delivered by this encoder directly represents the antenna bearing. It will be noted that the angle data are available on the movable rotating part constituted by the casing BA.

Moreover, an electric cable EC]. comes from the fuselage of the helicopter, which contains the electric power supply -10 -lines and the signal lines. At a connector ECC, the cable EC]. is connected to another electric cable EC2 which is itself connected to a rotating collector RC that ensures the exchange of signals and the electric power supply between the fixed shaft FS and the rotating antenna casing BA. Figure 3A shows the emergence of the cable EC1 at the bottom of the gearbox BTP.

As a variant, or by way of addition, an alternator (not shown) may be provided for the electric power supply, and will then be mounted above or below the angle encoder GC.

In Figure 3A, the hollow tube FC is fixed and joined to the fuselage of the helicopter.

The hollow tube FC may also be replaced by a rotating semirigid cable REC (Figure 38) connected by a connector BA-C to the antenna casing BA. In this case, the rotating collector RC placed under the gearbox BTP takes over the cable Ed. The angular encoder GC is disposed even lower and its data are sent back at RAC towards the rotating collector. The disposition of the elements Rd and GC may be inverted.

The general installation of the device on board a helicopter will now be considered.

-11 -In the fuselage, the connections to the antenna casing arrive in an interface unit lu. The interface unit IU transmits its power supply to the antenna casing BA and exchanges signals therewith.

In the case of a helicopter without a bus system (Figure 4A) the unit Ill controls the visualization and radar control ele-ntents RCV through a graphics processor GP. At PS, it also receives suitable electric power supplies and the parameters relating to the helicopter, in particular its flight charac-teristics, and other information available on board.

In the case of a helicopter (Figure 48) equipped with a bus RB (which may be of the type l553B, or also of the Applic-ant's "Digibus" type), the unit IU is connected to this bus by exchange lines HBC. The bus HB is itself connected to an on-board computer BC which, though a graphics processor GP, controls visualization and iuultifunction control elements RCV operating in particular for the visualizations and commands for the radar. The interface unit lu also receives suitable electric power supplies at PS.

In greater detail, (Figure 5), the antenna casing BA includes an antenna 100, connected to a duplexer or circulator 170, itself connected to the transmitter 30 receiving the fre-quency to be transmitted from a source 200.

-12 -In the receiving mode, two channels, sum and difference, (in a variant, one sum channel and two difference channels) are respectively applied to high frequency stages 41 and 42, fol-lowed by medium frequency and video stages 44 and 45, whereupon the filtering operations 46 and 47 are proceeded with so as, finally, to effect the post-integrations 50 and 60.

After the post-integration, the sum and difference data (bearing and/or angle of elevation) are applied to an ex-traction unit 70 which receives the data from the bearing encoder GC. The echoes recognized after extraction in a top control unit 80 (which nay, or may not, incorporate the tracking and pursuit functions) form part of the data exchanged through the rotating collector RC. In the opposite direction, the collector RC also transmits power supplies to a stage PS (the detail of the connections is not given) and the commands from the radar system to the top control unit 80.

In the case of Figure 4A, the rotating collector RC operates on the fuselage side of the helicopter with the interface unit 91 or lu to control the visualization unit 92 or RCV and to receive instructions from control elements 93 (advantage-ously incorporated in the visualization unit RCV). The operation is similar in the case of Figure 4B.

-13 -Figures 6A and GB detail the antenna functions in two variants.

The antenna 100 is advantageously a mains antenna with slots operating in horizontal polarization and fed, for example, by wave guide transitions with a coaxial cable.

Corrections of the antenna lobe may be effected so as to optimize the radiation diagram, by adjusting the number, size and position of the slots.

The antenna is separated into four quadrants 110 to 140.

In Figure 6A, phase shifters are associated with each quadrant permitting electronic scanning in the angle of elevation mode, that is to say, 111 to 113 for the quadrant 110, 121 to 123 for the quadrant 120, and so on. The phase shifters of each quadrant are followed by a summation in the angle of elevation mode on the vertical edges of the antenna: blocks Dli, 1112 on the left and D13, Dl4 on the right.

The outputs of the blocks Dli and 012 are applied to a magic Tee 161, a difference output of which is applied to an input of a 3 decibel coupler 163 and a sum output to another magic Tee 165. Similarly, the outputs of the phase shifter blocks 013 and 014 are applied to a magic Tee 162 whereof one difference output is applied to the coupler 163 and a sum -14 -output to the magic Tee 165.

The sum output of the magic Tee 165 is then applied to a circulator 170, which on the one hand receives the signal to be transmitted from the transmitter 30, whilst on the other hand, it feeds the part 41 (the sum channel) of the receiver 40.

The other output of the magic Tee 165 is a difference in bearing. Next to it, the output of the 3 decibel coupler 163 is a difference in the angle of elevation.

In the variant without electronic scanning (Figure GB), the outputs of the elements (hA to liD; 12A to 120; 13A to 13D; 14A to 140) of the antenna array fan the object of a simple summation for each quadrant, which may be effected on the horizontal edges of the antenna, (as illustrated). Subse- quently, the second output of the magic Tee 166 is a differ-ence in the bearing, and the output of the 3 decibel coupler 164 is a difference in the angle of elevation.

An electronic switch 180 makes it possible to choose one of the differences in the angle of elevation and the bearing for application to the part 42 (difference channel) of the receiver 40. In a variant, these two differences are processed in parallel.

-15 -Figures 7A and 7B illustrate the transmitter in greater detail.

Provision is made separately for a frequency source 200 which is for example, capable of providing eight different fre-quencies. One of these frequencies will be designated Ft.

It. is applied to a coupler 210 which, on the one hand, provides the signal of the local ultrahigh frequency local oscillator which will be described below for mixing at the frequency of the source 200, and on the other hand, includes a test output so as to know the frequency transmitted.

After this unit 210, there cones a modulation unit 310 in-cluding a head insulator 312, the modulator 315 proper and an output insulator 317. The power available at the output of the modulator may be greater than 30 milliwatts, being typically of the order of 50 milliwatts.

From a frequency source which may be the source 200 (the con-nection is not shown), the modulator 315 receives the chopping signal giving the repetition frequency and the width of the transmission pulse.

The output of the unit 310 is applied to a preamplifier unit 320 which includes two input stages 322 and 324 followed by an output insulator 328 for supplying an output power of the order of 5 watts.

-16 -There then comes a driver stage 340 and the output stage 380.

In the case of electronic scanning in the angle of elevation mode (Figure 7A), the driver stage 340 includes a 30 watt head amplifier 341 and two amplifiers 348 and 349 mounted in opposition, each of which has a nominal output of 55 watts.

The power stage 380 is constituted by two pairs of amplifier stages, 381 and 382 on the one hand, and 383 and 384 on the other hand, each having a nominal output of 80 watts, which makes it possible to have available an output power of 250 watts, the amplification being effected in class C. In the absence of electronic scanning in the angle of elevation mode (Figure 7B), the driver stage 340 is reduced to two amplifiers 341 and 345 and the power stage 380 is reduced to two amplifiers 386 and 387 in class C for an output power of at least 120 watts.

The various components of the receiver will now be described, starting with Figure 8, which concerns the ultrahigh fre-quency part, that is to say, the high frequency of the receiver.

In this Figure 8, there will first be found the duplexer 170 receiving the sum channel and, moreover, connecting the transmitter 300 to the antenna.

-17 -By way of symmetry, provision is made in the difference channel or channels for another duplexer 180 that is not connected on its transmission terminal.

The remaining part of the assembly is symmetrical and in the drawing reference numerals have been used that are separated by ten units., which makes it possible to describe only the sum channel. This is transposed to the difference channel (or to the two difference channels if applicable).

The output of the duplexer 170 is applied to a band filter 411 followed by a low pass filter 412, then by a limiter 413 which precedes a low noise amplifier 414 before an insulator 14 whose function is in particular to match the impedance.

Thereupon, a blocking or blanking stage 416 inhibits re-ception during the transmission. There follows an amplifier stage 417 (in fact an attenuator) whose gain (the attenu- ation) is variable in voltage, then a filter 418 for reject-ing the image frequency of the mixer situated down the line.

On its radio frequency input, this mixer 420 receives the output of the filter 418 via an insulator 419. On its local oscillator input, it receives the signal produced by a local oscillator source 250 through a 3 decibel coupler 220 for supplying the sum and difference channel(s). The output of the mixer 420 is applied to the sum channel of the medium -18 -frequency circuits designated 44.

The same applies to the difference channel(s) where the output of the mixer 430 is applied to the difference channel of the medium frequency part designated 45.

In Figure 9 there has been represented the source 250 based for example on a quartz oscillator 252, which provides the value of the medium frequency Fi. For this purpose, the output of the oscillator 252 is applied to a filter 254 followed by a 3 decibel coupler 256, one output of which passes to a mixer 280 for constituting local oscillators that are necessary in the ultrahigh frequency mode for the trans-mission and the reception respectively.

The other output is applied to an amplifier 256 followed by a second 3 decibel coupler 258 for supplying the sum channel 44 and difference(s) channels 45 in the medium frequency mode.

In the units 44 and 45, there is a reference difference of ten units between the reference numerals for the elements that correspond to each other. Only the sum channel will therefore be described.

After the mixer 420, the stage 44 includes two medium frequency amplifiers 441 and 443, between which is interposed -19 -a medium frequency filter 442 is interposed. There then follows a splitter divider 444 with two outputs which feeds two mixers 447 and 448 respectively.

From the coupler 258 already referred to, there passes a second local signal applied to a phase shifter 449 whose phase output is applied to a mixer 447, while its quadrature output is applied to a mixer 448.

In this way, there is thus obtained a phase output and a quadrature output for the sum channel medium frequency stage 44, and similarly for the bearing mode difference channel medium frequency stage 45, (possibly also for a difference channel in the angle of elevation mode, not shown).

Still with the same conventions, the video stages 46 and 48 will now be described.

In the phase and quadrature channels, there are respectively provided two video amplifiers 46 and 47, which are followed by low pass video filters 463 and 473, and then by analog converters 465 and 475 operating on 12 bits (at least).

The two converters provide in digital form the phase compo-nent and the quadrature component of the received radar signal for the sum channel.

-20 -The same applies on the side of the difference channel(s).

Control lines 467 and 489 make it possible to control the operating band of the video filters of the stages 46 and 48 respectively.

As from flow on, and up to what is termed "extraction", the sum channel and difference channel will be described with a difference of 100 units between their reference numerals.

The sum channel forms the object of a fast Fourier transform indicated at 510, followed by a post-integration 520, followed in turn by two different processing operations, the one 521 for the data received concerning the usual radar targets which are here termed "airframes" and the other 532 for the specific processing of helicopter blade flashes.

The same applies to the difference channel(s).

The sun and difference data coming from the stages 531 and 632. for the airframe are applied to an airframe extractor 710, while the sum and difference data coming from the stages 532 and 632 for the blade channel are applied to a blade extractor 720.

The whole set is controlled by a radar processing sequencer 81 which exchanges data with a top control unit 80 ("top" -21 -because it is situated in the casing at the top of the rotor shaft of the helicopter).

After the extractions, a unit 750 effects a ref ineinent as regards the distance, bearing and possibly the angle of elev-ation of the positions of the detected targets.

Apart from this, the top control unit 80 also receives the data regarding the bearing coding GC which it transmits to the radar processing sequencer 81.

When the antenna is equipped with phase shifters for elec-tronic scanning in the angle of elevation mode, the top control unit 80 also governs a unit 85 for actuating these phase shifters. This unit 85 also receives temporal data from the sequencer 81, so that the actuation of the phase shifters can be effected outside the active stages of the antenna.

As has been mentioned above, the top control unit 80 conununi-cates through the rotating collector RC with the bottom control unit 910, the computer unit 920, and the interface 930 to the bus HB.

Figure iDA corresponds to Figure 10 but with two difference channels and in a functional representation that is more hardware-orientated.

-22 -After the encoding of the coding of the sum channel (46), the angle of elevation difference channel (48S), the bearing dif-ference channel (480), one will here find again the fast Fourier transform units or FFT, 510, 6lOS and 610G.

S

At each FFT output, three matrices are available (one for each channel), each having 16 distance lines and 64 velocity columns. These matrices are dated according to the antenna position. They are transmitted to coordinate transformers 515, 6lSS and G1SG respectively.

A unit 68 receives the results from the FFT to detect therein any possible jamiuers in the zone.

As regards the sum channel, the moduli of the signals received in the 64 filters of each distance gate are memorized and are post-integrated in a temporal mode (air- frame echoes) and a frequency mode (blade echoes). Refer-ence will be made to this point below.

The summation of the energies contained in the filters situ-ated outside the velocity range attributed to the targets also provides a noise level reference for the detection.

For each sample coming from the FF2', this detection lies in verifying the inequality condition S > K.B, where S is the energy of the signal given by its modulus, B is the reference -23 -level of the thermal noise, and K a factor adjusted according to the desired probability of a false alarm. The detection is effected separately for the airframe echoes and the blade echoes.

As regards the sum channel, the moduli of the signals coming from the FFT are memorized. They are only post-integrated in the distance filters and gates where the sum channel has detected an echo.

The radar is a Doppler-type pulsed radar with its trans-mission/reception preferably effected in the S band. In one embodiment, the f on factor is of the order of 1/10th, and the repetition frequency is high, of the order of 15 to 20 Khz, which corresponds to a pulse period of the order of 5 microseconds.

The antenna incorporated in the casing BA may for example be a slotted antenna, with its aperture angle in the angle of elevation mode 24° (+/-12°). In the bearing mode, its aperture angle is 12° (main lobe).

At a rotor speed of the order of 300 rpm, the antenna casing ràtates in 2/lOths of a second.

It follows therefrom that the presence time of a target in the main lobe of the aerial is of the order of 6 millisec- -24 -onds, which is much shorter than the values encountered in usual radars.

In spite of that, for the applications to which the invention refers, it is desired that the radar should be able to operate with a non-ambiguous distance range of 10 kilometres, broken down into (at least) 16 distance gates for 64 Doppler gates.

The conventional processing effected in a pulsed Doppler radar lies in dividing the repetition period (outside the transmission time) into a desired number of distance gates, in this case 16, in rearranging the (complex) digital samples in a phase component and a quadrature component received in the same distance gate over its successive repetitions (here 64 successive repetitions) and in effecting a fast Fourier transform therefrom, which gives, for each distance gate, frequency components, that is to say, Doppler components on 64 gates.

In conventional radar processing, this operation is effected with 64 consecutive repetitions. Then one starts again with the following 64 repetitions.

An important characteristic of the present invention is that one proceeds quite differently. Indeed, the invention makes provision for an overlap between the groups of samples which -25 - will be taken for effecting the post-processing or post-integration.

The proposed radar processing includes the operations of: post-integration and detection, extraction, refining and control.

Figure 11 shows four successive repetitions, as well as the rearrangement of the samples of the distance gates PD1, respectively applied to Fourier transform circuits FT]., FT2, FT3 FT16.

On this basis, one obtains for the 16 distance gates, 16 Fourier transform data which each include 64 frequency components which are in practice available sequentially.

From this there can be completed the matrix NDF of Figure 11, which includes 16 lines for the distances and 64 columns for the frequency filters. The current element of the matrix relates to the distance gate of rank i and to the Doppler gate of rank j. It is designated: (Di, Fj).

According to the present invention (Figure 12), the updating of the matrix (Di, Fj) is effected differently from the con-ventional processing. At the level of the extraction of the echoes, 4 x 16 (= 64) consecutive complex temporal samples are considered as the valid matrix at a given moment. But for the following matrix, one suppresses, for example, only the first group of 16 temporal samples which are replaced by a fresh group of 16 temporal samples, the total of the 64 available samples remaining. Because the samples are com-pletely renewed in four analyses, it is said that the overlap factor is 4. This overlap factor can be switched over between various values.

Apart from this, the post-integration for the cells 531 or 631 is conducted for each pair of distance/Doppler gates.

It is "temporal" because the summation relates to matrices obtained at different consecutive moments.

For the blade channels 532 and 632, a simple post-integration technique lies in effecting for each distance gate a summa-tion of all the signals encountered on the different Doppler gates in the same matrix.

There results therefrom a considerable advantage. To under-stand its effect, reference should made to Figures l3A and 13B.

Figure 13A illustrates the temporal curve of a blade flash.

The expert will understand that its shape resembles that of aDirac pulse. It follows therefrom that the band width is large. As a result, after the Fourier transform, at the moment when such a pulse occurs it will result in a wide occupation of the Doppler frequency spectrum. This may be seen in Figure 133 where the X axis indicates the Doppler frequency gates (OF), while the 1 axis gives an information of the level N. After the overlap according to the invention a blade flash, when detected, will manifest itself over several consecutive distance-Doppler matrices, which considerably increases the chances one has of detecting it.

Moreover, as the antenna rotates at a speed which is very close to the nominal speed of rotation of all helicopter rotors, and much closer still to that of helicopters of the same type, it is particularly important to increase the probability of the recognition of a single blade flash, taking into account the fact already indicated that the blade flashes may: -either present themselves either in bursts or not at all (antisynchronisin); -or present themselves in a very variable manner in the course of time, because of the "stroboscopic" effect produced by the joint rotation of two helicopters present.

Instead of the simple technique described above, it is possible to use a recognition of the frequency "signature" of known blade flashes (whereof Figure 133 gives a schematic -28 -illustration).

The extraction (Figure 10) is effected separately for the fuselage echoes (710) and the blade echoes (720).

The refining stage 750 can then effect conventional oper-ations lying in particular in interpolations, to the extent that the signal overlaps several distance gates (which is valid both for the fuselage extractor and the blade extrac-tor), and/or Doppler gates (which is only valid to the fuselage extractor). For refining the bearing and time parameters, one uses the set of data collected during the time the beam has passed over the target.

The sequencer 81 ensures the generation of all the clocks necessary for the operation of the radar. It starts with a reference clock generator obtained by dividing the signal delivered by the oscillator for transposition to an inter-utediate frequency, and produces in particular the set of repetition frequencies intended to remove the distance ambi-guities, as well as to ensure the compatibility with nearby radars.

Thus the top control unit 60 knows the echoes recognized as such. This information is available at a sufficiently slow sequence to be transmissible by wire through the rotating collector RC towards the bottom control unit 910 and the -29 -computer unit 920, as well as the bus interface 930.

The top control unit 80 also controls the phase shifters per-mitting electronic scanning in the angle of elevation mode, if required.

In other words, the top control unit 80 ensures the interface between the extracting function of the processing situated in the aerial block installed on the rotor of the helicopter and the handling and interface unit installed in the cabin. The exchanges are effected by means of two series lines passing through the rotating collector.

Its main functions are: -the initialization of the radar processing (when ener-gized), -the handling of the "high part" -"low part" link, -configuring the processing according to the commands passed by the operator; -dating the echoes or "contacts" before they are refined, -the calculations for compensating the displacement of the carrier, -switching the repetition frequencies (removing the dis-tance ambiguities), -switching the transmission frequencies in the presence -30 -of jammers, -switching the pulse widths between modes (watch and for example, high resolution, which will be described below), -monitoring the proper functioning of the whole set.

In the bottom unit a handling and control module, constituted by a box of printed circuit cards with the associated 400 lIz supply, comprises the whole set of the processing operations necessary for the operational functioning of the radar: -a bottom control unit 910, -a computer unit 920, -interfaces 930 with the helicopter bus and with the operator.

The bottom control unit 910 has a structure that is identical with the top control unit 80 situated in the antenna block wherewith it is intended to be in dialogue. It is associ- ated with the computer unit 920 which has available a co- processor with a floating decimal point; their main func-tions comprise: -control and activation of the bidirectional series link with the antenna block, -handling of the radar image including fixed markers and the detected echoes, -controlling the display of the radar parameters and the -31 -echo parameters, -taking into account, controlling and sifting the detected echoes, -displaying the detected echoes, -recognition and identification of the echoes, -tracking and pursuit of the echoes.

In a variant,the functions of the units 910 and 920 may be taken up in whole or in part into the top control unit 80.

Of course, the present invention would also apply in the case where the antenna would be mounted on the rotor but with a drive device which would allow it variable mechanical scanning relative to the rotor speed in the bearing mode, which may be advantageous in certain cases for improving the processing of blade flashes still further.

The invention would also apply in the case of an antenna with electronic scanning in the bearing mode that is synchronous or asynchronous with the rotation of the helicopter.

In short, by obtaining an overlap regarding the data avail-able in the course of time, suitably set according to the distance gates, and by effecting a summation or other frequency integration in each distance gate, the identif i-cation of fluctuating targets with a short presence period, and, in particular, of helicopter blade flashes can be -32 - considerably improved -In the preceding discussion, it has been indicated that the power supply of the casing BA passes through the cables EC1 and EC2 at the sane tine as the signal lines.

An advantageous variant lies in placing an alternator between the casing BA (in its cylindrical axial cutout) and the upper fixed shaft FS. This alternator can then provide the supply lines for the casing BA.

Moreover instead of, or in addition to, the communication lines EC, a digital link without wires may be created by using an infrared, radio or other system. In the case of certain angular positions relative to the casing BA and the fuselage of the helicopter, the link without wires exchanges a burst of digital data. The Applicants have observed that the sequence rates required for the exchanges between the rotating aerial casing and the airframe of the helicopter are sufficiently low to permit such a functioning. Of course! provision will be made for a link of this type in both directions.

It has been indicated that eight transmission frequencies are available. But (friendly) helicopters frequently fly in formation. They may meet other friendly helicopters similarly equipped. The disposition of eight different -33 -transmission frequencies is therefore not always sufficient for preventing interference between two helicopters which might use the same transmission frequency. One can, of course, increase the number of the transmission frequencies.

The repetition frequency can also be selected by switching, preferably in correspondence with the chosen transmission frequency, in particular to permit the flight of helicopters in formation.

To increase the possibilities of discrimination and also to make it possible to have a better distance resolution in a nearby zone, the invention makes provision so that the trans-mission period can be switched over between two (or more) different values. The shorter period increases the distance resolution in the proximity of the aircraft.

The position of the aerial casing BA at the head of the rotor mast has required not only feasibility and viability studies in flight, but also the installation of very many functions inside this casing which has, in turn, involved many elec-tronic circuits and therefore the various problems related thereto, in particular as regards heating in operation due to the transmitter and to the other functions.

The solution of this problem will now be described with -34 -reference to Figure 14 in which any numerical or alphabetical reference which has for most of the time already been used in

the present description, will be prefaced by BA.

In Figure 14, the axial cylindrical cutout of the casing BA will be seen which accommodates the rotating collector BA-Re and the angular encoder or bearing encoder BA-Ge.

On the left, the antenna BA100 is mounted on a support BA-Si.

It is covered by a radome BA19O which follows the shape of revolution of the antenna casing. Thus a frontal empty space FV is formed between the two.

Symmetrically with the support BAS1, is another support BA- 52. The electronic circuits are installed between the two supports.

For example:

-the transmission circuit BA3O is found in the bottom portion of Figure 14, as is the high frequency or ultrahigh frequency component of the receiving circuits BA41 and BA42, -the medium frequency and video circuits BA44 and BA45 are found on the right, -the other processing cards described above which are -35 -designated in general BA-PU are found at the top, -the power supply BA-PS is mounted synnuetrically on the right of the structure BA-S2, thus forming a counterweight for the antenna BA100. It starts from the on-board mains of the helicopter (three-phase 115V./400 hertz and d.c. 28 V.).

The direct voltages required are provided by converters which advantageously operate at a hanonic frequency of the radar repetition frequency used (no additional oscillators, which reduces the mass and the volume of the power supply).

In a variant, provision may be made for two separate power supplies, one for the transmission and one for reception, which are accommodated in the respective compartments for these two functions. The symmetrical space of the antenna is then free for another electronic sensor such as a passive alarm detection system.

On the other side of the support BA-S2 and on either side of its centre (occupied in the example by the power supply), provision is made for two fans BA-AF1 and BA-AF2, which will be referred to below. These fans are covered by a closing cap BA-CC.

Finally, the periphery of the casing is provided in its bottom portion with air intake louvres BA-Al. In the top portion, it comprises air outlet louvres BA-AO.

-36 -In practice, the supports BA-Si and BA-S2 form part of a double-walled structure BA-SS (Figures 16A and 168).

The external air entering at BA-Al penetrates the structure S (through louvres not visible in the Figures but which are Symmetrical with those indicated by arrows on the front right-hand post). It circulates first of all (Figure 1GA) inside the structure BA-SS, at the top (arrows FS) and at the bottom (arrows Fl), to emerge again at BA-AO. It also emerges through the front louvres of the left-hand post (Figure 168) so as to circulate also in the space RV arranged between the cap BA-cc and the outer left-hand wall BA-EG of the casing. This is indicated by the circuit BA-EA arrowed in long dashes (Figure 14), as well as in Figure 1GB. This creates a flow of air surrounding the power supply.

An internal space is also defined, where the internal air (not mixed with the external air) circulates under the action of the two fans BA-AF1 and BA-AF2, as indicated by the circuit BA-IA (Figures 14, l6A and 168). Other fans may be used for forcing the circulation of the external air if the natural circulation due to the rotation of the antenna casing, taking into account the font of the louvres, is not sufficient.

The structure BA-Ss leaves the internal air with the possi-bility of passing through the printed circuit cards such as -37 -BA-PU, which are also seen in Figure 15.

Because of the large exchange surface between the two air circuits, the cooling of the apparatus is excellent.

In a variant, the fans may be dispensed with, the evacuation of the heat then being ensured by conduction between the hot points and the structure; the modules are then provided with thermal drains connected to thermal grooves which serve for the mounting of these modules on the double wall.

It should be noted that in order to simplify the drawing, the casing BA has been given a strictly cylindrical shape in Figure 11, this shape being different from that previously illustrated.

Figure 15 shows, moreover, a coyer BA-CP for closing the cen-tral cutout of the casing BA. In the variant with an optical viewer OS of Figure 3, this cover is, of course withdrawn.

Claims

-38 -CLAIMS1. A helicopter provided with a surveillance radar, wherein the antenna casing of the radar is mounted on the S rotor mast of the helicopter; wherein the radar is of the pulsed Doppler type; and wherein the post-processing of the radar return signals after reception is effected, at least for each distance gate, on groups of samples which overlap according to an overlap factor n, which makes it possible to have available on the basis of the samples spread over R radar repetitions, a new group of samples at all k/n repeti-tions, and hence to increase the number of samples available for targets with a short presence time.
2. A helicopter according to claim 1, wherein the antenna is fixed to the rotor, which provides mechanical scanning that is synchronous with the rotation of the rotor, and is so positioned that the main antenna lobe is situated between two of the rotor blades of the helicopter.
3. A helicopter according to either of claims 1 and 2, wherein the radar transmitter/receiver operates in the S band.
4. A helicopter according to any one of claims 1 to 3, wherein the radar transmitter/receiver operates with a form factor of the order of 1/10, and a high repetition frequency, of the order of 15 to 20 Khz.
5. A helicopter according to any one of the preceding -39 -claims, wherein the transmission frequency can be switched over.
6. A helicopter according to any one of the preceding claims, wherein the repetition frequency can be altered.
7. A helicopter according to claims 5 and 6 together, wherein the transmission frequency and the repetition frequency can be altered in correspondence.
8. A helicopter according to any one of the preceding claims, wherein the post-processing is effected on groups of 64 complex samples with a f on factor that can be altered, and wherein one of the values of the form factor corresponds to 1/4.
9. A helicopter according to any one of the preceding claims, wherein the post-processing includes: -a first post-processing with a temporal integration in each distance/Doppler gate for the identification of conven-tional fluctuating targets such as airframes, and -a second post-processing without any temporal integra-tion for the identification of targets with a short presence time.
10. A helicopter according to claim 9, wherein said target with a short presence tine corresponds to a helicopter rotor blade echo.
li A helicopter according to claim 9 or 10, wherein the second post-processing includes a frequency integration in each distance gate at every instant.12. A helicopter according to claim 11, wherein the second -40 -post-processing includes a search for the spectral signature of a rotor blade echo.13. A helicopter according to any one of the preceding claims, wherein the post-processing is effected in a sum channel, and in at least one difference channel in the bearing mode.14. A helicopter according to claim 13, wherein the post-processing is also effected in a difference channel in the angle of elevation mode.15. A helicopter according to any one of the preceding claims, wherein the post-processing is effected on the basis of at least 16 distance gates for at least 64 Doppler gates.16. A helicopter according to any one of the preceding claims, wherein the radar is split into a top part incorpor-ated in the antenna casing, and a bottom part mounted in the fuselage of the helicopter.17. A helicopter according to claim 16, including a link between the top and bottom parts, ensured through an axial element with a rotating collector. - 18. A helicopter according to claim 17, and further including, an angular encoder mounted between the said axial element and the rotor or the antenna casing for the acquisi-tion of the angular antenna position.19. A helicopter according to any one of claims 16 to 18, wherein the communication between the two parts includes a digital link without wires.20. A helicopter according to any one of claims 16 to 19, -41 -wherein the top part of the radar includes its ultrahigh frequency stage and medium frequency and video stages, as well as the radar processing and post-processing circuits and their power supplies; and wherein the bottom part includes a control unit that is capable of actuating the visualization of the targets and of searching for their identification.21. A helicopter according to claim 20, wherein the power supply for the casing is at least partly effected by means of an alternator.22. A helicopter according to any one of the preceding claims, wherein the antenna casing, which has the shape of a body of revolution round its axis mounted on the rotor, includes: -an antenna covered by a radome fonning a lateral portion of the casing with a sealed space between the antenna and its radome, and -means cooperating with louvres arranged in the outer wall of the said antenna casing, to ensure a circulation of cooling air in the remaining part of the antenna casing, taking into account its rotational movement.23. A helicopter according to claim 22, wherein said remaining part of the antenna casing includes a double-walled structure admitting said cooling air.24. A helicopter according to claim 23, wherein an internal circulation of the cooling fluid is effected in a closed circuit under mechanical actuation; and wherein the said double-walled structure forms a surface for exchanging heat -42 -with said closed circuit.25. A helicopter according to any one of the preceding claims, wherein the antenna is a slotted mains antenna.26. A helicopter according to claim 25, wherein the antenna array is provided with phase shifters permitting electronic scanning in the angle of elevation mode.27. A helicopter according to any one of the preceding claims, wherein the radar is capable of transmitting pulses with a second, shorter, period, to provide a better distance resolution at close range.28. A helicopter according to any one of the preceding claims, the antenna casing accommodates, on the opposite side to the antenna, a further electronic sensor.29 A helicopter according to claim 28, wherein said further electronic sensor is a passive alan detection systeni.30. A helicopter substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.AAmendments to the claims have been filed as follows -CLAIMS1. A helicopter provided with a surveillance radar, wherein the antenna casing of the radar is mounted on the S rotor mast of the helicopter; wherein the radar is of the pulsed Doppler type; and wherein the post-processing of the radar return signals after reception is effected, at least for each distance gate, on groups of samples which overlap according to an overlap factor n, which makes it possible to have available on the basis of the samples spread over R radar repetitions, a new group of samples at all R/n repeti-tions, and hence to increase the number of samples available for targets with a short presence time.2. A helicopter according to claim 1, wherein the antenna is fixed to the rotor, which provides mechanical scanning that is synchronous with the rotation of the rotor, and is so positioned that the main antenna lobe is situated between two of the rotor blades of the helicopter.3. A helicopter according to either of claims.1 and 2, wherein the radar transmitter/receiver operates in the S band.4. A helicopter according to any one of claims 1 to 3, wherein the radar transmitter/receiver operates with a f on factor of the order of 1/10, and a high repetition frequency, of the order of 15 to 20 Khz.5. A helicopter according to any one of the preceding claims, wherein the transmission frequency can be switched over.6. A helicopter according to any one of the preceding claims, wherein the repetition frequency can be altered.7. A helicopter according to claims 5 and 6 together, wherein the transmission frequency and the repetition frequency can be altered in correspondence.8. A helicopter according to any one of the preceding claims, wherein the post-processing is effected on groups of 64 complex samples with a form factor that can be altered, and wherein one of the values of the form factor corresponds to 1/4.9. A helicopter according to any one of the preceding claims, wherein the post-processing includes: -a first post-processing with a temporal integration in each distance/Doppler gate for the identifiction of conven-tional fluctuating targets such as airframes, and -a second post-processing without any temporal integra-tion for the identification of targets with a short presence time.10. A helicopter according to claim 9, wherein said target with a short presence time corresponds to a helicopter rotor blade echo.

li. A helicopter according to claim 9 or 10, wherein the second post-processing includes a frequency integration in each distance gate at every instant.
12. A helicopter according to claim 11, wherein the second U.S post-processing includes a search for the spectral signature of a rotor blade echo.
13. A helicopter according to any one of the preceding claims, wherein the post-processing is effected in a sum channel, and in at least one difference channel in the bearing mode.
14. A helicopter according to claim 13, wherein the post-processing is also effected in a difference channel in the angle ofelevation mode.
15. A helicopter according to any one of the preceding claims, wherein the post-processing is effected on the basis of at least 16 distance gates for at least 64 Doppler gates.
16. A helicopter according to any one of the preceding claims, wherein the radar is split into a top part incorpor-ated in the antenna casing, and a bottom part mounted in the fuselage of the helicopter.
17. A helicopter according to claim 16, including a link between the top and bottom parts, ensured through an axial element with a rotating collector.
18. A helicopter according to claim 17, and further including, an angular encoder mounted between the said axial element and the rotor or the antenna casing for the acquisi-tion of the angular antenna position.
19. A helicopter according to any one of claims 16 to 18, wherein the communication between the two parts includes a digital link without wires.
20. A helicopter according to any one of claims 16 to 19,-wherein the top part of the radar includes its ultrahigh frequency stage and medium frequency and video stages, as well as the radar processing and post-processing circuits and their power supplies; and wherein the bottom part includes a control unit that is capable of actuating the visualization of the targets and of searching for their identification.
21. A helicopter according to claim 20, wherein the power supply for the casing is at least partly effected by means of an alternator.
22. A helicopter according to any one of the preceding claims, wherein the antenna casing, which has the shape of a body of revolution round its axis mounted on the rotor, includes: -an antenna covered by a radorne forming a lateral portion of the casing with a sealed space between the antenna and its radome, and -means cooperating with louvres arranged in the outer wall of the said antenna casing, to ensure a circulation of cooling air in the remaining part of the antenna casing, taking into account its rotational movement.
23. A helicopter according to claim.22, wherein said remaining part of the antenna casing includes a double-walled structure admitting said cooling air.
24. A helicopter according to claim 23, wherein an internal circulation of the cooling fluid is effected in a closed circuit under mechanical actuation; and wherein the said double-walled structure forms a surface for exchanging heatC 1%.1with said closed circuit.
25. A helicopter according to any one of the preceding claims, wherein the antenna is a slotted mains antenna.
26. A helicopter according to claim 25, wherein the antenna array is provided with phase shifters permitting electronic scanning in the angle of elevation mode.
27. A helicopter according to any one of the preceding claims, wherein the radar is capable of transmitting pulses with a second, shorter, period, to provide a better distance resolution at close range.28.. A helicopter according to any one of the preceding claims, the antenna casing accommodates, on the opposite side to the antenna, a further electronic sensor.29 A helicopter according to claim 28, wherein said further electronic sensor is a passive alarm detection system.30. A helicopter substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.