Method of operating a detection radar according to two sub-modes: Doppler and associated detection radar

Abstract

Method of operating a two-submode Doppler detection radar and associated detection radar. The present invention relates to a method of operating a radar comprising a signal transmission / reception step (110) comprising the following substeps: + generation (111) of two consecutive pulses associated with a first and a second Doppler submode; + transmission (112) of the pulses in different frequency bands; + reception (113) in a common time window of the pulse echoes; + preprocessing (114) of the received echoes in relation to the first Doppler submode to determine a clutter zone; + preprocessing (115) of the received echoes in relation to the second Doppler submode outside the clutter zone; the process further comprising the following step: - implementation (120) of an extraction processing from a preprocessing result of the echoes received in relation to the second Doppler sub-mode.Figure for the abbreviation: Figure 2.

Description

Title of the invention: Method for operating a detection radar according to two sub-modes: Doppler and associated detection radar

[0001] The present invention relates to a method of operating a detection radar following at least two Doppler sub-modes.

[0002] The present invention also relates to a detection radar implementing such a method.

[0003] The technical field of the invention is that of radar systems embedded for example on board aircraft, boats, submarines or satellites, implementing target detection / identification.

[0004] The general problem solved by the invention is the management of the presence of soil and sea clutter which disrupts the operation of detectors classically used in radar, based on an analysis of power maps.

[0005] As is known per se, radar modes for detecting aerial and moving ground targets employ Doppler waveforms. Phase coherence between pulses makes it possible to exploit the Doppler effect related to the relative movement of targets with respect to the radar. This conventional technique allows targets to be separated according to their radial velocity. It has considerably increased the discriminating power of radars against stationary clutter (generally from the ground) or slow-moving clutter (generally from the sea).

[0006] Doppler processing is a coherent processing (exploiting phase information) allowing to increase the coherent gain with respect to a Gaussian white noise and for a given target its signal-to-noise ratio (SNR).

[0007] Doppler processing allows for the separation of spread echoes, such as ground and sea clutter, along the frequency axis. The resolution cell of the Doppler processing, also called a cell, is inversely proportional to the integration time. The average equivalent radar area (also called RCSA) of a surface clutter over a range-velocity cell is then proportional to the Doppler resolution cell.

[0008] As a general rule, the repetition frequencies Fr are chosen according to the speed ranges to be processed by the mode, due to the fact that the ambiguous speed is a function of the wavelength A and the Fr:

[0009] amb = XFr / 2.

[0010] Target detection is then performed on two-dimensional power maps (distance, speed). A contrast between the cell under test and the ambient noise is performed and then a thresholding is carried out to decide if a detection is present on the cell under test.

[0011] To combat clutter zones in the context of aerial and ground target detection, one solution is to disable the detection function across the entire velocity (or Doppler frequency) domain of the ground clutter, that is, in the velocity (or Doppler frequency) domain where the ground clutter is located. This avoids the detection of the main echoes. This technique is commonly called "Doppler notch": a minimum detection velocity is imposed, ensuring that this velocity is greater than the trace of the ground (or even sea) clutter.

[0012] One drawback of this solution is that it does not allow any detection in the velocity domain corresponding to soil clutter, causing a notch in the axis of detected velocities. It also does not allow for the management of secondary clutter zones. The main reason is that the notch width is not adaptive.

[0013] To eliminate secondary clutter echoes, antenna processing can be performed, for example by using an ancillary or spread spectrum channel to suppress any detection originating from the secondary lobes. This technique therefore consists of pulling all primary detections to the power card level and then filtering them a posteriori via antenna processing. This technique has two limitations: the use of several receiving channels on the one hand, and on the other hand, areas near clutter remain desensitized depending on the topology of the chosen detector.

[0014] Another known solution consists of a technique for a priori projecting clutter zones onto power maps based on carrier information and an antenna beam model. The zones thus defined on the power maps are filtered at the detector level to avoid any detection in these areas. This technique is an advanced and adaptive version of the Doppler notch and is particularly well-suited to secondary clutter zones.

[0015] A similar technique can be used to project clutter zones into the unambiguous domain (distance, velocity). Any detection after extraction falling within these zones can be eliminated (or conditionally subjected to a suppression SER).

[0016] Image processing on power maps (Da, Va) can also be used to identify clutter areas. While effective, these processes have two drawbacks: they can be computationally intensive and they can include targets near the clutter (primary or secondary) within the clutter areas to be eliminated.

[0017] The known solutions are therefore not sufficiently effective in terms of detecting cluttered areas or require a significant computational load. Moreover, the aforementioned techniques, without prior knowledge of the clutter velocity of sea, do not allow for effective management of the Doppler notch (adaptive or not) in the presence of sea clutter.

[0018] The present invention aims to solve this prior art problem and to propose means for efficiently determining clutter zones without excessive computational burden.

[0019] In particular, the invention makes it possible to recognize clutter areas, upstream of detection techniques, in order to help the latter to solve the detection of targets on power maps filtered from the main clutter and the secondary clutter.

[0020] Moreover, one of the advantages of the invention is that it works regardless of the configuration and type of surface clutter (sea speed, mix between land and sea clutter, etc.).

[0021] To this end, the invention relates to a method of operating a detection radar, a method of operating a target detection radar in Doppler mode, the method comprising the implementation of several recurrences of a signal emission / reception step, each nth recurrence of said step comprising the following sub-steps:

[0022] + generation of two consecutive pulses associated with a first sub-mode Doppler and a second Doppler sub-mode, said Doppler sub-modes defining different energy balances;

[0023] + emission of pulses in different frequency bands according to different emission directions;

[0024] + reception in a common time window of pulse echoes;

[0025] + preprocessing of received echoes in relation to the first Doppler submode for identify at least one clutter zone;

[0026] + preprocessing of the received echoes in relation to the second Doppler sub-mode in outside of the or each area of ​​clutter determined in relation to the first Doppler sub-mode;

[0027] the process further comprising the following step:

[0028] - implementation of an extraction process from a pretreatment result echoes received in relation to the second Doppler sub-mode.

[0029] According to other advantageous aspects of the invention, the method comprises one or more of the following features, taken individually or in all technically possible combinations:

[0030] - the second Doppler sub-mode defines a greater energy balance than the first Doppler sub-mode;

[0031] - the pulse associated with the second Doppler submode has a larger width that the impulse associated with the first Doppler sub-mode;

[0032] - the detection radar is mounted on an aircraft;

[0033] - the direction of emission of the pulse associated with the first Doppler sub-mode corresponds to the land / sea direction;

[0034] - the preprocessing step of the received echoes in relation to the first sub-mode Doppler includes the determination of at least one primary clutter zone and / or at least one secondary clutter zone;

[0035] - each of the preprocessing substeps of the received echoes includes the determination of a power map of the echoes, defining a distance axis and a velocity axis;

[0036] - the area or each clutter zone is determined by a thresholding of the map of power determined in relation to the first Doppler sub-mode;

[0037] - the or each clutter zone determined in relation to the first sub-mode Doppler is transferred in the form of a mask onto the power map determined in relation to the second Doppler sub-mode;

[0038] - the mask is determined by a constant noise value;

[0039] - the echo preprocessing step in relation to the second Doppler sub-mode defines a first detection path and a second detection path, the first detection path being implemented from an unmasked power board and the second detection path being implemented from a masked power board;

[0040] said preprocessing step further comprising a fusion of detections from the two detection channels;

[0041] - during the emission substep, the corresponding pulses are emitted in using different slopes of chirps used to emit them or by adding a random phase;

[0042] - during the reception sub-step, echoes associated with different pulses are distinguished by determining the slopes of the corresponding chirps or the corresponding phase;

[0043] - during the emission substep, the corresponding pulses are emitted in using different polarizations;

[0044] - during the reception sub-step, echoes associated with different pulses are distinguished by determining their polarizations;

[0045] - a polarization is emitted for each pulse or a set of polarizations forming a signature is emitted for each pulse.

[0046] The invention also relates to a target detection radar comprising technical means configured to implement the method as defined above.

[0047] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0048] - [Fig. 1] [Fig. 1] is a schematic view of a detection radar according to the invention,

[0049] - [Fig.2] [Fig.2] is a flowchart of a method for operating the radar of the [Fig. 1], and

[0050] - [Fig.3] [Fig.4] [Fig.5] Figures 3 to 5 are different views illustrating the implementation work of the process of [Fig.2].

[0051] Figure 1 illustrates a detection radar 10 according to the invention. This radar 10 is intended, for example, to be mounted on a mobile platform moving in the air and / or on a land surface and / or on a sea surface. Advantageously, the radar 10 is intended to be mounted on a platform moving in the air, such as an aircraft. Alternatively, the radar 10 is fixed in place.

[0052] The radar 10 allows the detection of targets according to a Doppler mode, for example of the MMTI (Maritime Moving Target Indicator), GMTI (Ground Moving Target Indicator) or AMTI (Aerial Moving Targets) type.

[0053] With reference to [Fig.1], the radar 10 comprises an array of elementary antennas 21 enabling the emission of signals in the form of pulses and the reception of signals corresponding to echoes of these pulses.

[0054] The radar 10 further includes a transmission unit 22 for generating the pulses to be emitted by the antenna array 21 and a reception unit 23 for processing the echoes received by the antenna array 21 in order to deduce the presence of a target and possibly a speed and distance to that target.

[0055] Each of the units 22, 23 is implemented, for example, as a programmable circuit of the FPGA (Field Programmable Gate Array) type and / or of the ASIC (Application-Specific Integrated Circuit) type. In addition or alternatively, each of these units 22, 23 is implemented at least partially as software executable by a processor and stored in memory.

[0056] The operating method of radar 10 will now be explained with reference to [Fig.2] showing a flowchart of its steps.

[0057] This method is considered to be implemented during an electronic and / or mechanical scan (e.g., in azimuth and / or elevation) of the space around the radar 10, for example, by implementing conventional beamforming in transmission and reception. In particular, this method includes the iterative implementation at least of step 110 described below for each radar pointing position 10. Each iteration of this step is called recurrence.

[0058] It is further considered that the pointing positions follow one another according to a predetermined direction of rotation and each defines an angular opening dependent on the pointing angle known to those skilled in the art. The set of pointing positions defines an available visibility cone of the radar 10. In other words, by "visibility cone" is meant the set of different pointing positions during a scan performed by the radar 10. Without loss of generality, the sequence of these pointing positions can also be random to achieve a complete scan of the visibility cone.

[0059] Each nth recurrence of step 110 includes an emission / reception of signals in the corresponding pointing position.

[0060] In particular, during this step, the transmitting unit 22 of the radar 10 emits signals having a particular waveform and the receiving unit 23 receives echoes of these signals.

[0061] The signals emitted / received during this step are of the so-called communalized wave type.

[0062] Each communalized wave comprises at least two consecutive pulses associated with different energy budgets. Thus, each pulse defines a Doppler submode, namely a first Doppler submode and a second Doppler submode, with an associated spatial domain as will be explained later. The different Doppler submodes employ the same set of waveforms and, in particular, the same repetition period and the same emitted bandwidth. Moreover, advantageously, the pulses are emitted in different directions so that the associated spatial domains extend in different directions.

[0063] Figure 3 illustrates an example of a Doppler mode, for example of the AIR type, decomposed into a first sub-mode SI defining a spatial domain DI and a second sub-mode S2 defining a spatial domain D2. The sub-mode S2 has a higher energy budget than the sub-mode SL. Furthermore, according to the first sub-mode SI, the corresponding pulses are emitted towards the ground / sea. Thus, the first spatial domain DI extends in a direction oriented towards the ground / sea. According to the second sub-mode S2, the corresponding pulses are emitted in the direction of movement of the carrier. Thus, the second spatial domain D2 extends in this direction substantially parallel to the ground / sea. The two domains D1 and D2 define overlapping zones, corresponding in particular to the secondary lobes of the antennas during the emission of the corresponding pulses.In particular, according to one embodiment, the side lobes of the SI submode also point in the main direction of the S2 submode, and vice versa.

[0064] Generally, each spatial domain can be defined by an emission direction and, in some cases, by a distance to the radar 10. The emission direction can, for example, be defined by a pair of angular values. These angular values ​​correspond, for example, to the emission elevation (or site) and azimuth, denoted respectively by El and Az.

[0065] In the example in [Fig. 3], the domain D2 can be defined, for example, by a distance Dis2 to radar 10 and an elevation angle of 0°. This domain corresponds to the volume where an aerial target is to be detected during the current pointing operation. In the same example, the domain DI corresponds to the volume for analyzing surface clutter (land and / or sea). The ambiguous distance domain processed by the two submodes is given by the characteristics of the common waveform, i.e.

[0066] Dmin = c * Te / 2 and Dmax = c * (Tr-Te / 2)

[0067] where:

[0068] Te is the duration of an emission window;

[0069] Tr is the duration of a reception window;

[0070] c is the speed of sound.

[0071] Moreover, in practice, Dis2 > Disl > Dmax > Dmin.

[0072] Step 110 of signal transmission / reception comprises several sub-steps which will be explained in detail below.

[0073] During substep 111, the emission unit 22 of the radar 10 generates two consecutive pulses associated with different Doppler submodes, i.e., with different energy balances.

[0074] Thus, during substep 111, the transmitting unit 22 generates a first pulse I1 linked to the first Doppler submode (the SI submode in the example of [Fig. 3]) and a second pulse I2 linked to the second submode (the S2 submode in the example of [Fig. 3]). These two pulses are illustrated in [Fig. 4].

[0075] The pulses h and I2 are associated with the same repetition frequency, different emission directions, and different emission frequencies. The emission direction is defined, for example, by a pair of angular values ​​as defined previously.

[0076] The pulses L and I2 are generated within an emission window Te in which each pulse has a width Li and is separated from the other pulse and from one of the boundaries of the emission window Te by a time gap TGAP. This time gap TGAP is chosen to be as small as possible depending on the transmitter's capabilities. In what follows, the subscript i=l denotes any value relating to the first pulse and i=2 denotes any value relating to the second pulse.

[0077] The energy budget of each pulse is defined by its width Li. Thus, to have different energy budgets, the width Li of one of the pulses must be strictly greater than the width Li of the other. In a particular mode, the width Li of one of the pulses is at least twice as large as that of the other. Thus, according to an example, 5% of the emission window Te can be allocated to one of the submodes and 95% of the emission window Te can be allocated to the other submode. This represents a difference in energy budget of approximately 13 dB. For example, in the case of a ratio of l% / 99%, the difference in energy budget is 20 dB.

[0078] In the example in [Fig. 4], the width L2 of the pulse associated with the second submode (i.e., pulse I2) is strictly greater than the width Li of the pulse associated with the first submode (i.e., pulse IJ-). Thus, in this example, the second submode has a greater energy balance than the first submode. Conversely, the first pulse F can have a greater width than the second pulse I2.

[0079] In the frequency domain, the pulses share the same frequency support Brec, with a frequency gap FGAP between the corresponding carriers Fi greater than the frequency bands Bi of these pulses. The frequency gap FGAP is chosen to be sufficient to distinguish echoes of these pulses at the receiver. In what follows, a frequency band is defined by a center frequency and a bandwidth. Advantageously, in what follows, all frequency bands have the same bandwidth. Furthermore, the frequency gap FGAP is measured between a pair of corresponding center frequencies and is greater than the bandwidth of each frequency band.

[0080] The frequency bands Bi and B2 of the first pulse F and the second pulse I2, respectively, are advantageously chosen to be the same for each iteration of step 110. Thus, the same center frequency Fei and the same center frequency Fe2 are chosen for the first and second pulses, respectively, in each iteration of step 110, ensuring phase coherence between the iterations to allow for Doppler processing. Furthermore, advantageously and alternatively, the shortest pulse can be generated without linear frequency modulation (LFM) so that its time width is compatible with the desired distance resolution, without pulse compression (since it is not LFM), i.e., Li = 1 / B2.

[0081] During substep 112, the transmitting unit 22 emits the pulses generated during the previous substep in the corresponding frequency bands.

[0082] During substep 113, the receiving unit 23 receives echoes corresponding to the pulses emitted within a common reception time window. The duration Tr The duration of this common reception window is equal to the total duration of the recurrence TR (i.e., the observation time of the corresponding pointing for a recurrence, as defined previously) minus the duration of the transmission window Te. During reception, the echoes corresponding to the different pulses are distinguished by the different frequency bands, using for example bandpass filters around the center frequencies.

[0083] During substep 114, the receiving unit 23 performs preprocessing of the received echoes in relation to the first Doppler submode.

[0084] This preprocessing includes the implementation of at least some techniques selected from a pulse compression technique, a clutter rejection technique, and a Doppler processing technique. These techniques make it possible, in particular, to establish a power map of the echoes as a function of velocity and distance to the radar 10. In other words, such a power map is defined by a distance axis and a velocity axis. It should be noted that implementing the clutter rejection technique prevents the receivers from being overwhelmed by nearby clutter. Even after rejection, the clutter remains sufficiently powerful relative to the thermal noise floor level.

[0085] An example of such a map is shown in part A of [Fig.5]. According to this example, the power map defines a main clutter zone ZI corresponding to the main lobe of the antennas, two secondary clutter zones Z2 corresponding to the secondary lobes of the antennas and a target zone Z3 comprising targets.

[0086] Advantageously, during substep 114, the receiving unit 23 further implements a technique for suppressing at least some of the secondary clutter zones Z2. This technique may include, in particular, thresholding.

[0087] To achieve this, the suppression technique consists of first identifying the background of the power map corresponding to the average thermal noise power Pbth. Then, knowing the geometry, the energy budget, and a typical RCS of ground clutter (or reflectivity), the technique can deduce a clutter-to-noise ratio (CRN) which corresponds to the clutter power of a resolution cell divided by Pbth. Clutter must be excluded when the CRN is at least greater than the detection threshold, i.e., class 10 to 15 dB (without post-integration). The technique then applies a threshold to the power map of the first submode, equal, for example, to the threshold of the second submode plus a detection margin plus another margin (3-5 dB or even more) to lock onto cluttered areas. The useful targets will not benefit from the antenna gain and therefore do not stand out in the first submode.This will allow the recovery of secondary clutter areas (those targeted by the antenna of the first sub-mode) but also, in some cases, the main clutter (which will be seen by the secondary lobes of the first analysis sub-mode).

[0088] Each determined secondary clutter zone Z2 can then be replaced by a constant value corresponding for example to an average level of thermal noise, as illustrated in part C of [Fig.5] where the ZM2 zones replace the Z2 secondary clutter zones.

[0089] According to some embodiments, in substep 114, the receiving unit 23 further implements a technique for suppressing the main clutter zone. For this purpose, thresholding can also be applied. In particular, the thresholding can be performed using the average thermal noise level (sufficiently high) to determine the position, velocity, and width of the main clutter zone Zl. Then, as in the previous case, the main clutter zone can be replaced by an average thermal noise level. This is illustrated in part B of [Fig. 5] where the zone ZM1 replaces the main clutter zone Zl.

[0090] Finally, all clutter areas (primary and secondary) can be replaced by an average level of thermal noise as illustrated in part D of [Fig.5].

[0091] During substep 115, the receiving unit 23 performs preprocessing of the received echoes in relation to the second Doppler sub-mode. This substep can be implemented at least partially in parallel with substep 114.

[0092] As in the previous case, this preprocessing includes the implementation of at least some techniques selected from a pulse compression technique, a clutter rejection technique, and a Doppler processing technique. Also as in the previous case, these techniques make it possible, in particular, to establish a power map of the echoes as a function of speed and distance to the radar 10, in relation to the second Doppler submode.

[0093] In addition, during this substep 115, the receiving unit 23 implements an ambient noise measurement technique and a detection technique for example of type TF AC (“constant false alarm rate”).

[0094] Furthermore, advantageously during this substep 115, the receiving unit 23 implements a masking technique for each clutter zone determined during substep 114 on the power card determined in relation to the second Doppler submode.

[0095] This masking technique is advantageously implemented before the detection technique (for example, during the implementation of the clutter rejection technique) and comprises masking, on the power map determined in relation to the second Doppler submode, each clutter zone determined in relation to the first Doppler submode. For this purpose, each clutter zone determined in substep 114 can be represented as a mask defined by a constant noise value. This mask can then be applied to the determined power map. related to the second Doppler sub-mode. This power board is subsequently referred to as the masked power board. The detection technique is then implemented on this masked power board.

[0096] According to another embodiment, the detection technique is implemented via two detection channels. In such a case, a first detection channel can be implemented on an unmasked power board (i.e., a power board as initially determined) and a second detection channel can be implemented on the masked power board. The result of such detection can then comprise a fusion of results from the two channels.

[0097] According to the two examples, the detection technique makes it possible, in particular, to pre-detect targets that must be confirmed during an extraction process that will be explained in detail later. Thus, this technique makes it possible to generate a list of pre-detections, that is to say, a list of pre-detected targets, by analyzing the masked power map or both maps, namely the masked map and the unmasked map.

[0098] In a subsequent step 120, the receiving unit 23 performs an extraction process from the list of pre-detections determined in step 110.

[0099] In particular, and in a manner known per se, such an extraction process utilizes information gleaned by the use of several repetition periods (or repetition frequency Fr) played within the same tracking sequence in order to resolve ambiguities regarding distance and speed by correlating the different pre-detections obtained on each Fr. Any other process enabling the identification of targets in a standby mode can be used, for example, and without limitation, a complementary extraction process of the "turn-by-turn" type, to enhance control of the false alarm rate upstream of the tracking algorithm.

[0100] A target is considered to be detected when, at the end of this extraction process (also called unambiguity removal), the receiving unit 23 concludes that such a target is present in the area considered.

[0101] A target is considered definitively detected when, after several iterations of steps 110 and 120, at least K detections are present and correlate within the same area over a horizon of N iterations of these steps. In this notation, the number K represents the number of steps 120 during which the target was considered detected. The coefficient K / N can then be compared with a threshold called the target extraction threshold.

[0102] At the end of this step 120, the receiving unit 23 generates a list of detections, that is to say a list of detected targets.

[0103] Then, depending on different embodiments of the invention, the tracking of at least some of the targets can be carried out.

[0104] It is therefore understood that the process according to the invention has a number of advantages.

[0105] In particular, decomposing the detection mode into at least two sub-modes oriented in different directions makes it possible to mask clutter areas, especially secondary clutter areas, upstream of the detection point. This allows the detector, regardless of the topology chosen, to detect targets as close as possible to these clutter areas, without desensitization (consideration of clutter samples in the ambient noise power) or false alarms from the primary clutter. This makes the process particularly efficient, adaptable to any geometry and combination of surface clutter, and undemanding and predictable in terms of computational capacity.

[0106] In certain embodiments, the operating method as explained above further includes the implementation of at least one technique for separating, during the transmission / reception of the shared waves, the echoes of pulses corresponding to different submodes in order to reconstruct a complete image of the environment. These techniques are, for example, applicable when the same transmission frequency is used for both pulses.

[0107] According to a first technique, during the implementation of the nth recurrence of step 110, and in particular during the emission substep 112, the emission unit 22 selects one of the pulses, for example the first pulse, and adds a random phase 0 to that pulse. Advantageously, the emission unit 22 adds a different random phase (p) to each of the pulses. The pulse or pulses having an added random phase (p) are hereafter referred to as a phase-shifted pulse.

[0108] It should be noted that the choice of the pulse to be phase-shifted can remain the same for each recurrence of this substep 112. In other words, when only one pulse is phase-shifted during this substep, the same pulse is phase-shifted in each recurrence of this step. When both pulses are phase-shifted during this substep, these pulses are also phase-shifted in each recurrence of this substep.

[0109] Then, during the reception substep 113, the receiving unit 23 compensates for the phase shift of the received echoes in the frequency band of the or each phase-shifted pulse, by the corresponding random phase. In other words, the phase shift is carried out by subtracting the value (p in the band corresponding to the index i.

[0110] Thus, during the subsequent processing, only the echoes corresponding to the relevant submode can be processed coherently. The phase shifting of the other echoes cannot be done correctly, so they are considered as white noise. This technique also requires processing the distance ambiguity ranks in parallel and having a sufficient number of recurrences to ensure the necessary isolation.

[0111] Other techniques for obtaining better isolation of echoes corresponding to different submodes upon their reception are also possible. The techniques explained below, in particular, allow for a single Doppler processing step, unlike the previously mentioned random phase technique.

[0112] Thus, according to a second technique, during the implementation of the nth recurrence of step 110, and in particular during the emission substep 112, the emission unit 22 implements different slopes for the chirps used to emit the pulses associated with the different submodes. In other words, during this substep 112, the emission unit 22 emits the pulses using either an ascending or a descending slope depending on the submode associated with each pulse. The same slope is then used for all pulses of this type in all recurrences of step 110.

[0113] For example, for all recurrences, an upward slope is chosen for the pulses associated with a particular submode and a downward slope is chosen for the pulses associated with another particular submode.

[0114] Then, during the reception substep 113, the receiving unit 23 receives echoes having different frequency slopes. This receiving unit 23 therefore determines the received slopes (using, in particular, suitable filters) in order to isolate the echoes corresponding to the different submodes.

[0115] According to a third technique that also provides better isolation of the echoes corresponding to the different submodes during their reception, during the implementation of the nth recurrence of step 110, and in particular during the transmission substep 112, the transmission unit 22 uses different polarizations for the waves used to emit the pulses associated with the different submodes. In other words, during this substep 112, the transmission unit 22 emits the wave carrying each pulse with a polarization chosen according to the submode associated with that pulse. This same polarization is chosen for this type of pulse for all recurrences of step 110.

[0116] For example, two polarizations, namely a vertical polarization and a horizontal polarization, can be chosen for the pulses emitted during substep 112. According to other examples, a 45° or circular polarization can be used.

[0117] Then, during the reception substep 113, the receiving unit 23 receives echoes having different polarizations. This receiving unit 23 therefore determines the polarizations of received echoes (using appropriate filters in particular) in order to isolate the echoes corresponding to the different sub-modes.

[0118] The principle just described can be refined by using several polarizations in the same pulse.

[0119] In such a case, each pulse has a specific polarization signature. Such a signature corresponds to a polarization code.

[0120] This technique thus makes it possible to color the different impulses in space and to obtain an additional rejection of 20 to 30 dB.

[0121] In certain embodiments, the aforementioned techniques are combined to be implemented simultaneously. Furthermore, the extraction process as explained above to resolve ambiguities in distance and velocity and / or along at least one pointing direction can also be used in combination with the second or third technique, as described above.

Claims

Demands

1. A method for operating a target detection radar (10) using a Doppler mode, the method comprising implementing several repetitions of a signal transmission / reception step (110), each nth repetition of said step (110) comprising the following substeps: + generation (111) of two consecutive pulses associated with a first Doppler sub-mode and a second Doppler sub-mode, said Doppler sub-modes defining different energy budgets; + transmission (112) of the pulses in different frequency bands along different transmission directions; + reception (113) in a common time window of the pulse echoes; + preprocessing (114) of the received echoes in relation to the first Doppler sub-mode to determine at least one clutter zone;+ preprocessing (115) of the echoes received in relation to the second Doppler submode outside of the clutter zone or each zone determined in relation to the first Doppler submode; the process further comprising the following step: - implementation (120) of an extraction processing from a preprocessing result of the echoes received in relation to the second Doppler submode.;

2. A method according to claim 1, wherein the second Doppler submode defines a larger energy balance than the first Doppler submode.

3. A method according to claim 2, wherein the pulse associated with the second Doppler submode has a larger width than the pulse associated with the first Doppler submode.

4. A method according to any one of the preceding claims, wherein: - the detection radar is carried in an aircraft; - the emission direction of the pulse associated with the first Doppler sub-mode corresponds to the ground / sea direction.

5. A method according to any one of the preceding claims, wherein the preprocessing step (114) of the echoes received in relation to the first Doppler submode comprises the determination of less a primary clutter area and / or at least a secondary clutter area.

6. A method according to any one of the preceding claims, wherein each of the preprocessing substeps (114, 115) of the received echoes includes the determination of a power map of the echoes, defining a distance axis and a velocity axis.

7. Method according to claim 6, wherein the clutter zone or each clutter zone is determined by a thresholding of the power map determined in relation to the first Doppler sub-mode.

8. A method according to claim 6 or 7, wherein the clutter zone or each zone determined in relation to the first Doppler submode is transferred in the form of a mask onto the power map determined in relation to the second Doppler submode.

9. Method according to claim 8, wherein the mask is determined by a constant noise value.

10. A method according to claim 8 or 9, wherein the preprocessing step (115) of the echoes in relation to the second Doppler submode defines a first detection channel and a second detection channel, the first detection channel being implemented from an unmasked power board and the second detection channel being implemented from a masked power board; said preprocessing step (115) further comprising a fusion of detections from the two detection channels.

11. A method according to any one of the preceding claims, wherein: - during the transmission substep (112), the corresponding pulses are emitted using different slopes of the chirps used to emit them or by adding a random phase; - during the reception substep (113), echoes associated with different pulses are distinguished by determining the slopes of the corresponding chirps or the corresponding phase.

12. A method according to any one of the preceding claims, wherein: - during the emission substep (112), the corresponding pulses are emitted using different polarizations; - during the reception substep (113), echoes associated with different pulses are distinguished by determining their polarizations.

13. A method according to claim 12, wherein a polarization is emitted for each pulse or a set of polarizations forming a signature is emitted for each pulse.

14. Target detection radar (10) comprising technical means (21, 22, 23) configured to implement the method according to any one of the preceding claims.

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