Method of operating a radar according to a synthetic aperture imaging mode and a second operating mode using alternating polarization and associated radar

Abstract

Method of operating a radar according to a synthetic aperture imaging mode and a second operating mode using alternating polarizations and associated radar. The present invention relates to a method of operating a radar according to a first operating mode and at least one second operating mode, the first operating mode being a synthetic aperture radar imaging mode and each second operating mode being different from the first operating mode, the method comprising the implementation of several recurrences of a signal transmission / reception step, each Nth recurrence of said step comprising the following sub-steps: - generation of a sequence of consecutive pulses, each pulse of the sequence being associated with the first operating mode or at least one second operating mode, - transmission of the pulses in different frequency bands,The pulses associated with the first operating mode and at least one second operating mode are emitted using different polarizations, and the pulse echoes are received within a common time window. Figure for the abbreviation: Figure 4.

Description

Title of the invention: Method of operating a radar according to a synthetic aperture imaging mode and a second operating mode using alternating polarizations and associated radar

[0001] The present invention relates to a method for operating a detection and imaging radar. The present invention also relates to a detection and imaging radar implementing such a method.

[0002] The technical field of the invention is that of the management of the time budget for detection, imaging and identification by radar systems.

[0003] Traditionally, a radar system can be used in a "single-task" manner, meaning that it operates in a single Doppler or non-Doppler mode throughout the mission. This is the case, for example, with a maritime (or "MMTI," from "Maritime Moving Target Indicator") or land (or "GMTI," from "Ground Moving Target Indicator") Doppler surveillance mode, which is adapted to a given altitude and target type. This adaptation includes, for example, the use of a fixed space scanning logic, waveforms, and processing. In other words, in such a case, the frame does not vary over time, as long as the operator does not change the mission or mode. The time budget is then associated solely with this task and with technical tasks for radar self-calibration.

[0004] For many years, radar operators have sought to broaden the range of applications for radar detection systems and have requested that they become "multi-tasking." For example, for a single radar system, it is advantageous to simultaneously have a maritime tactical situation (MMTI), an air situation (known as "AIR"), and possibly weather condition feedback. The radar system must then define the time budget to be allocated to each of the tasks to be performed.

[0005] Obviously, the more time a task is allocated, the more effective it will be, for example in terms of detection and / or discrimination capacity. Managing and optimizing the time budget therefore appears crucial for new radar systems.

[0006] Traditionally, radar systems employ "short-time" (at the processing block level) or "long-time" (at the scan level) interleaving strategies to perform their various tasks. A time budget is allocated to each of these tasks based on a performance trade-off for each function taken individually (refresh time, detection range, etc.).

[0007] Radar block interleaving is then a technique which temporally orders tasks that are not simultaneous.

[0008] To achieve simultaneous tasks, a known technique consists of decomposing the radar antenna system into several sub-arrays and allocating a task to each sub-array to perform what is called a colored transmission. This operation is found mainly in MIMO (Multiple Input Multiple Output) radar systems.

[0009] The simultaneous emission of several orthogonal waveforms is thus achieved to color the space, that is to say, to associate a {sub-array, waveform] pair with a {azimuth-elevation} direction. Colored emission makes it possible either to obtain a complete view of the environment by considerably reducing or improving the refresh time of a task, or to perform several tasks simultaneously.

[0010] This decomposition of the antenna space into sub-arrays and colored emission are not necessarily available or desirable for every radar architecture. Indeed, such a type of emission can degrade the performance of a radar system, particularly in terms of range.

[0011] The present invention aims to solve this problem and thus provide a solution for implementing a multitasking radar system while using a refresh rate equivalent to that of a single-tasking system. This makes it possible to adapt the radar system to multitasking combinations while maintaining system performance.

[0012] To this end, a method for operating a target detection radar is described, according to a first operating mode and at least one second operating mode, the first operating mode being a synthetic aperture radar imaging mode and each second operating mode being different from the first operating mode, the method comprising the implementation of several recurrences of a signal transmission / reception step, each Nth recurrence of said step comprising the following sub-steps:

[0013] - generation of a sequence of consecutive pulses, each pulse of the sequence being associated with the first operating mode or audit at least a second operating mode,

[0014] - emission of pulses in different frequency bands, and

[0015] - reception within a common time window of pulse echoes,

[0016] during the substep of the emission of each Nth recurrence, at least one of the pulses, called the phase-shifted pulse, being emitted with a random phase associated with the number N.

[0017] 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:

[0018] - each pulse is emitted with a random phase associated with the band corresponding frequency.

[0019] - the method further comprises a preliminary step of selecting a number M corresponding to an ambiguity rank to be dealt with in a beam of signals emitted / received by the radar, the number M varying between 0 and a maximum number of ambiguity ranks in the beam and in which, during the sub-step of the reception of each Nth recurrence, the phase shift of the received echoes being compensated for the frequency band of the phase-shifted pulse, by the random phase associated with the number NM.

[0020] - the reception substep includes compensation for the phase shift of the echoes received in each frequency band, by the random phase associated with the NM number and that frequency band.

[0021] - the pulses of each recurrence share the same frequency support and are emitted with a frequency gap greater than each of the said frequency bands, the frequency gap is chosen to be able to distinguish the different frequency bands at the reception of the echoes.

[0022] - during the generation step, a pulse of the sequence is associated with the first operating mode, each other pulse in the sequence being associated with a second respective operating mode.

[0023] - the sequence has a repetition frequency between 1 kiloHertz and 5 kilohertz.

[0024] - the first mode of operation or the audit at least a second mode of their operation corresponds to the same given direction.

[0025] - the first mode of operation or the audit at least a second mode of their operation corresponds to a respective direction.

[0026] A method for operating a radar is also described, consisting of a first operating mode and at least one second operating mode, the first operating mode being a synthetic aperture radar imaging mode and each second operating mode being different from the first operating mode, the method comprising the implementation of several recurrences of a signal transmission / reception step, each Nth recurrence of said step comprising the following sub-steps:

[0027] - generation of a sequence of consecutive pulses, each pulse of the sequence being associated with the first operating mode or audit at least a second operating mode,

[0028] - emission of pulses in different frequency bands, the pulses associated with the first operating mode and audit at least a second operating mode being emitted using different slopes of chirps used to emit them, and

[0029] - reception in a common time window of the echoes of the pulses.

[0030] According to other advantageous aspects of the invention, the method comprises a or several of the following characteristics, taken individually or in all technically possible combinations:

[0031] - during the reception substep, echoes associated with the first mode of operation or audit at least a second mode of operation are distinguished by determining the slopes of the corresponding chirps.

[0032] - during the generation step, a pulse of the sequence is associated with the first operating mode, each other pulse in the sequence being associated with a second respective operating mode.

[0033] - the sequence has a repetition frequency between 1 kiloHertz and 5 kilohertz.

[0034] - the second operating mode is a Doppler mode.

[0035] - the first mode of operation or the audit at least a second mode of their operation corresponds to the same given direction.

[0036] - the first mode of operation or the audit at least a second mode of their operation corresponds to a respective direction.

[0037] The description also describes a method of operating a radar according to a first operating mode and at least a second operating mode, the first operating mode being a synthetic aperture radar imaging mode and each second operating mode being different from the first operating mode, the method comprising the implementation of several recurrences of a signal transmission / reception step, each Nth recurrence of said step comprising the following sub-steps:

[0038] - generation of a sequence of consecutive pulses, each pulse of the sequence being associated with the first operating mode or audit at least a second operating mode,

[0039] - emission of pulses in different frequency bands, the pulses associated with the first operating mode and audit at least one second operating mode being emitted using different polarizations, and

[0040] - reception in a common time window of pulse echoes.

[0041] According to other advantageous aspects of the invention, the method comprises a or several of the following characteristics, taken individually or in all technically possible combinations:

[0042] - a polarization is emitted for each pulse.

[0043] - a set of polarizations forming a signature is emitted for each pulse.

[0044] - during the reception substep, echoes associated with the first mode of operation or audit at least a second mode of operation are distinguished by determining their polarizations.

[0045] - during the generation step, a pulse of the sequence is associated with the first operating mode, each other pulse in the sequence being associated with a second respective operating mode.

[0046] - the sequence has a repetition frequency between 1 kiloHertz and 5 kilohertz.

[0047] - the second operating mode is a Doppler mode.

[0048] - the first mode of operation or the audit at least a second mode of their operation corresponds to the same given direction.

[0049] - the first mode of operation or the audit at least a second mode of their operation corresponds to a respective direction.

[0050] This description also relates to a radar comprising technical means configured to implement one of the methods as defined above.

[0051] 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:

[0052] - [Fig. 1] [Fig. 1] is a schematic view of a radar according to the invention,

[0053] - [Fig.2] [Fig.2] is a flowchart of a method for operating the radar of the [Fig. 1], and

[0054] - [Fig.3] [Fig.4] [Fig.5] [Fig.6] [Fig.7] Figures 3 to 7 are different views illustrating the implementation of the process of [Fig.2].

[0055] Figure 1 illustrates a 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.

[0056] The radar 10 allows for the detection or characterization of targets according to several operating modes.

[0057] A first mode of operation is the synthetic aperture radar imaging mode.

[0058] Synthetic aperture radar imaging is more often referred to by the acronym SAR, which stands for "Synthetic Aperture Radar".

[0059] SAR imaging is a radar technique that allows imaging of the ground with excellent distance resolution (inversely proportional to the emitted band) and excellent transverse resolution (inversely proportional to the illumination time).

[0060] To obtain images with metric resolution or even lower, the required illumination times exceed ten seconds, depending on the radar carrier's speed. As a general rule, the radar sensor is then entirely dedicated to the ground imaging function.

[0061] The operator can control one or more areas to be imaged in two different ways.

[0062] In SPOTLIGHT operation, the operator selects one or more areas, for example by indicating the coordinates (latitude, longitude) of the central points: the radar scheduler successively commands the waveforms and SAR processing sequentially for each area to be imaged as long as these areas are in the radar's cone of visibility.

[0063] In STRIPMAP operation, the operator declares the central point (distance, azimuth) relative to the radar. As long as the command is active, the radar scheduler commands a succession of SAR illuminations around this central point, so as to have SAR images that scroll according to the speed of the carrier.

[0064] In general, the radar sensor and associated processing are entirely dedicated to the production of SAR images.

[0065] The repetition frequency ranges Fr of a SAR mode are on the order of 1kHz to 5kHz.

[0066] For a multi-panel radar, when the transmission / reception architecture of several panels simultaneously is possible, simultaneous modes can be used: one mode per panel.

[0067] The radar 10 is also capable of operating according to one or more second modes.

[0068] Each second mode of operation is different from the first mode of operation.

[0069] The second operating mode has the same recurrence period as the first operating mode.

[0070] According to a first example, the second mode of operation is a Doppler mode.

[0071] Such a second mode of operation is, for example, chosen from the list consisting of: • an active tracking Doppler mode, and • a GMTI Doppler mode without resolving the distance-speed ambiguity

[0072] More generally, the second mode of operation is any type of Doppler mode that can operate at the repetition frequency of the SAR mode.

[0073] According to a second example, the second operating mode is a non-Doppler mode.

[0074] By way of illustration, the second non-Doppler operating mode is then chosen from the list consisting of: • a weather-to-air mode, • a maritime detection method, and • a Look up type mode without the need to reject soil clutter by Doppler processing.

[0075] More generally, the second mode of operation is any type of Doppler mode that can operate at the repetition frequency of the SAR mode.

[0076] According to another mode of operation, the repetition frequency of the SAR mode can vary within the image acquisition in order to make possible the operation of a Doppler mode, GMTI for example, with distance-speed ambiguity resolution.

[0077] Furthermore, in the case of the present application with a common reception, each mode of operation is carried out on the same antenna panel on the same cone of antenna visibility.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] The operating method of radar 10 will now be explained with reference to [Fig.2] showing a flowchart of its steps.

[0082] It is considered that this method is implemented to perform a scan or image of the surroundings of the carrier carrying the radar 10, according for example to a direction of movement of the carrier.

[0083] This process includes in particular the implementation of several recurrences of a step 110 of signal transmission / reception.

[0084] The frequency of repetition of these recurrences is chosen according to repetition frequencies associated with the first operating mode (SAR mode).

[0085] Each Nth recurrence of step 110 includes the implementation of substeps 111 to 113 explained in detail below.

[0086] During substep 111, the transmission unit 22 generates a sequence of at least two consecutive pulses associated with a respective operating mode.

[0087] In particular, during this substep, the transmission unit 22 generates a first pulse Ii associated with the first operating mode and a second pulse I2 associated with the second operating mode.

[0088] Each pulse is associated with a transmission and reception direction defined, for example, by a pair of angular values. These angular values ​​correspond, for example, to the transmission elevation (or site) and azimuth, hereafter denoted respectively by El; and Az;. The transmission of each pulse is oriented according to these angular values ​​by means of conventional beamforming (FFC) performed at the transmission point. In what follows, the subscript i=1 denotes the first operating mode of the radar 10 and i=2 denotes the second operating mode of the radar 10.

[0089] The pulses are generated in an emission window Te in which each pulse has a width Li and is spaced from the other pulse and from one of the boundaries of the emission window Te by a time gap TGAp.

[0090] It can be noted here that without loss of generalities, the pulse widths can be of different widths.

[0091] Advantageously, in the following, it is assumed that Lil = Li2, this corresponding to an easier implementation.

[0092] In the frequency domain, the pulses share the same reception 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 all that follows, a frequency band is defined by a center frequency and a bandwidth.

[0093] Without loss of generality, the frequency bands can be of different widths between the two modes.

[0094] Advantageously, in the following, all frequency bands have the same width.

[0095] The frequency band Bi of the first pulse L, that is to say of the pulse associated with the first detection configuration, is chosen to be the same for each recurrence.

[0096] Advantageously, this choice is independent of the application of radar 10.

[0097] This is schematically illustrated in Figures 3 and 4, which show several consecutive recurrences corresponding to two distinct applications of the radar 10. Thus, the same center frequency Fei is chosen for the first pulse in each recurrence in each application in order to be able to perform a coherent processing on the pulse train associated with pulse II, the frequency Fei and the direction (Azi,Eli).

[0098] The frequency band of the second pulse I2, i.e. of the pulse associated with the second detection configuration, is chosen according to the application of the radar 10.

[0099] In particular, for a first operating variant, the same frequency band, and more specifically the same center frequency for the second pulse I2, is chosen in each kth recurrence. This technique can be viewed as a barrel mechanism, where at each instant TR, a center frequency is chosen in the barrel modulo k. In other words, in such a case, k different center frequencies are chosen alternately for the second pulses I2 in k consecutive recurrences.

[0100] In the example of [Fig.3], when k=2, two frequency bands B2 and B3 (i.e. two center frequencies) are then chosen alternately for each second pulse I2.

[0101] For a second variant, the same frequency band B2 for the second pulse I2 is chosen in each recurrence, as illustrated in [Fig.4].

[0102] During substep 112, the transmitting unit 22 emits the pulses generated during the previous substep in the corresponding frequency bands.

[0103] During substep 113, the receiving unit 23 receives echoes corresponding to the pulses emitted within a common reception time window. The duration of this common reception window is equal to the total duration of the recurrence TR minus the duration of the transmission window Te.

[0104] During reception, the echoes corresponding to the different pulses are distinguished by their different frequency bands, using for example bandpass filters.

[0105] A spatial filtering system of the FFC type is applied in the direction of the emitted pulses to separate the echoes from the different emitted pulses. This FFC in reception can be implemented in the same way or differently from that implemented in transmission (different weighting in particular).

[0106] In a subsequent step 120, implemented after the N recurrences of step 110, the receiving unit 23 performs coherent processing of the echoes corresponding to the pulses associated with the first detection configuration and the pulses associated with the second detection configuration. Such coherent processing consists of applying filtering adapted to the waveform of the desired configuration, for example, pulse compression on the short time axis (within a recurrence). and a Doppler processing combining the signals from each recurrence sharing the same emission frequency.

[0107] In a subsequent step 130, implemented only when the radar 10 is operating according to its first variant, the receiving unit 23 further implements non-coherent processing of the outputs of the coherent processing of the pulses associated with the second operating mode. Such non-coherent processing performs the power average of the signals received on each frequency band in the same direction (after the coherent processing).

[0108] As a point of reference, it may be noted here that, for non-Doppler treatment, only the incoherent treatment is implemented.

[0109] In some embodiments, this step is implemented systematically (i.e. independently of the radar application) insofar as k = 1 the average is directly the signal.

[0110] In a subsequent step 140, the receiving unit 23 transmits all the outputs of the coherent processing and possibly of the non-coherent processing, to any interested system allowing for example to implement a detection step, a resolution of ambiguity in distance and / or in speed.

[0111] These outputs can then be used to detect one or more targets according to different operating modes, possibly with speeds and distances associated with these targets.

[0112] In certain embodiments, the operating method as explained above further includes the implementation of at least one additional technique enabling the separation of the echoes of the two modes, or even the rejection of certain echoes which are not necessary or are ambiguous in distance, in order to reconstruct a complete, necessary and sufficient image of the vicinity of the radar according to at least one of the aforementioned operating modes.

[0113] Figure 5 illustrates an example of such a case, allowing the handling of a single range ambiguity rank according to the SAR radar mode. In this example, the radar beam emitted by the radar 10 from the carrier 12 covers several portions of the Earth's surface, the echoes of which overlap due to range ambiguity linked to the choice of the mode's repetition frequency Fr. To avoid processing all the echoes originating from the beam's footprint on the ground, a first technique consists of selecting only one ambiguity rank within the implemented beam, this range zone constituting the entire zone of interest for the mode (SAR or GMTI).

[0114] According to this first separation technique, the operating method of the radar 10 further comprises a preliminary step 105 consisting of selecting a number M corresponding to an ambiguity rank to be addressed in the beam of signals emitted / received by the radar. This number M then varies between 0 and a maximum number of ranks. of ambiguity in the beam. The maximum number depends in particular on the aperture of the radar beam. As illustrated in [Fig. 5], the ambiguity rank M can correspond to the central part of the radar beam.

[0115] In certain embodiments, during this step, several numbers M corresponding to several ambiguity ranks to be addressed are chosen, i.e., when the region of interest of the mode is spread over several ambiguity ranks. In this case, it is subsequently considered that the technique described below is applied in relation to each chosen number M. The processing is carried out, for example, in parallel.

[0116] During the implementation of the Nth iteration of step 110, and in particular during the transmission substep 112, the transmission unit 22 selects one of the pulses associated with one of the radar modes, for example the first pulse, and adds a random phase to that pulse. Advantageously, the transmission unit 22 adds a different random phase to each of the pulses. Each pulse with an added random phase is hereafter referred to as a phase-shifted pulse.

[0117] 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.

[0118] It should also be noted that the value of the random phase for the or each pulse is then stored for all recurrences, i.e. N+M subsequent recurrences of step 110.

[0119] 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 random phase associated with the number NM. In other words, the phase shift is performed by subtracting the value in the band corresponding to the index i.

[0120] Thus, during the subsequent processing, only the echoes corresponding to ambiguity rank M can be processed coherently. The phase shift of the other echoes cannot be done correctly, so they will behave like white noise.

[0121] This principle is schematically illustrated in Figure 6. According to the example in this figure, the number M is equal to 2 and the maximum number of ambiguity ranks is equal to 3. Thus, during the Nth recurrence of step 110, in order to select only the signals corresponding to the ambiguity rank M=2, the value is used to compensate for the phase shift of the recurrence N in the corresponding frequency band.

[0122] In other words, this random phase compensation and then the coherent processing on the N+M recurrences constitute the filtering adapted to the phase-shifted impulse.

[0123] Other techniques for resolving ambiguities in distance and speed and / or according to at least one pointing direction are also possible, for example by employing several repetition frequencies associated with an extraction process.

[0124] Furthermore, it is also possible to obtain better isolation of echoes corresponding to different radar modes during their reception.

[0125] Thus, according to a second separation 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 of the chirps used to emit the pulses associated with the different detection configurations.

[0126] In other words, during this substep 112, the transmitting unit 22 emits the pulses using either an ascending or a descending slope depending on the configuration associated with each pulse. The same slope is then used for all pulses of this type in all recurrences of step 110.

[0127] For example, for all recurrences, an upward slope is chosen for the impulses associated with the first configuration and a downward slope is chosen for the impulses associated with the second configuration.

[0128] 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 in order to isolate the echoes corresponding to the different detection configurations.

[0129] For this purpose, the receiving unit 23 uses filters (pulse compression) adapted to the corresponding chirp slopes.

[0130] According to a third separation technique allowing also to obtain better isolation of the echoes corresponding to the different radar modes when they are received, during the implementation of the Nth recurrence of step 110 and in particular during the sub-step of emission 112, the emission unit 22 implements different polarizations of the waves used to emit the pulses associated with the different radar modes.

[0131] In other words, during this substep 112, the transmitting unit 22 emits the wave carrying each pulse with a polarization chosen according to the radar mode associated with that pulse. This same polarization is chosen for this type of pulse for all recurrences of step 110.

[0132] For example, two polarizations, namely a vertical polarization and a horizontal polarization, can be chosen for the pulses emitted during substep 112.

[0133] According to other examples, 45° or circular polarization may be used. For example, left-hand circular polarization may be associated with the first configuration and right-hand circular polarization may be associated with the second configuration.

[0134] Then, during the reception substep 113, the receiving unit 23 receives echoes having different polarizations. This receiving unit 23 therefore determines the received slopes in order to isolate the echoes corresponding to the different configurations.

[0135] For this purpose, the receiving unit 23 uses filters adapted to the corresponding polarization slopes.

[0136] The principle just described can be refined by using several polarizations in the same pulse.

[0137] In such a case, each pulse has a specific polarization signature. Such a signature corresponds to a polarization code.

[0138] This technique thus makes it possible to color the different impulses in space and to obtain an additional rejection of 20 to 30 dB.

[0139] In certain embodiments, the aforementioned techniques are combined to be implemented simultaneously. Furthermore, a technique for resolving ambiguities in distance and speed and / or along at least one pointing direction can also be used in combination with the second or third technique, as described above.

[0140] It is therefore understood that the present invention has a number of advantages.

[0141] The method consists of taking advantage of the transmission / reception time of a first SAR imaging operating mode to perform one (or more) other radar tasks (second operating mode) on the same antenna panel.

[0142] The radar scheduler can test for each of the tracks active or not in the common visibility cone of the SAR image the active tracks compatible for the detection of the Fr of the SAR mode: the tracks are said to be compatible when they fall in a "clear" area of ​​the SAR waveform, i.e. out of distance eclipse, in thermal noise zone and out of ground clutter eclipse (for example, ~+ / -10m / s modulo the ambiguous velocity = XFr / 2).

[0143] The scheduler then periodically plays active pursuit scores at the same time as the impulse-scale SAR.

[0144] The use of a single Tr prevents the lifting of ambiguity, which is not a problem in active pursuit because the unambiguous speed and distance of the tracks are already known.

[0145] Alternatively, it is conceivable to provide the user with an automatic adjustment option on the Fr also in SAR mode in order to be able to perform an ambiguity removal for the associated Doppler mode (GMTI, unnecessary in active tracking).

[0146] This nevertheless requires maintaining phase coherence for the SAR, therefore keeping Fe constant.

[0147] According to one example, to comply with this constraint, the signal feeding the SAR can be resampled to bring it back to a situation where Fr is constant. Since the acquisition windowing in SAR is dynamic and the range of distances to be imaged is predefined, a value of Li and Fr can be chosen to take into account the eclipse phenomenon occurring with distance.

[0148] In each of the embodiments described above, the process therefore makes it possible to benefit from the same information with a gain in speed.

[0149] According to advantageous embodiments, it is possible to reduce the margin taken on the clutter-to-construction noise ratio of the SAR image by cutting the emission time into P sub-pulses: one of the sub-pulses is associated with the SAR imaging mode and the other sub-pulses are associated with one (or more) second mode(s), which may, depending on a particular case, correspond to another SAR image taken on another region.

[0150] It is also possible to implement a frequency step technique.

[0151] The frequency step technique is more often referred to by its English name corresponding to "step-frequency".

[0152] This frequency-stepping technique is a classic SAR technique that allows the recombination of emitted bands to obtain a distance resolution equivalent to a very large synthetic band. An example of its implementation is known from French patent application FR 2766578 B.

[0153] It can also be highlighted with reference to [Fig.7] an operation of the radar 10 in which the first mode of operation or said at least a second mode of operation corresponds to a respective direction.

[0154] In this case, the recurrence period Tr is common on the waveforms, but the pointed directions are uncorrelated.

Claims

Demands

1. A method for operating a radar (10) according to a first operating mode and at least one second operating mode, the first operating mode being a synthetic aperture radar imaging mode and each second operating mode being different from the first operating mode, the method comprising the implementation of several recurrences of a signal transmission / reception step (110), each Nth recurrence of said step (110) comprising the following substeps: - generation of a sequence of consecutive pulses, each pulse of the sequence being associated with the first operating mode or at least one second operating mode, - transmission of the pulses in different frequency bands, the pulses associated with the first operating mode and at least one second operating mode being transmitted using different polarizations,and - reception of pulse echoes within a common time window.

2. A method according to claim 1, wherein a polarization is emitted for each pulse.

3. A method according to claim 1, wherein a set of polarizations forming a signature is emitted for each pulse.

4. A method according to any one of claims 1 to 3, wherein, during the reception substep, echoes associated with the first mode of operation or at least a second mode of operation are distinguished by determining their polarizations.

5. A method according to any one of claims 1 to 4, wherein during the generation step, one pulse of the sequence is associated with the first operating mode, each other pulse of the sequence being associated with a respective second operating mode.

6. A method according to any one of claims 1 to 5, wherein the sequence has a repetition frequency between 1 kiloHertz and 5 kiloHertz.

7. Method according to claim 6, wherein the second mode of operation is a Doppler mode.

8. A method according to any one of claims 1 to 7, wherein the first mode of operation or auditing at least a second mode of operation corresponds to the same given direction.

9. A method according to any one of claims 1 to 7, wherein the first mode of operation or auditing at least a second mode of operation corresponds to a respective direction.

10. Radar (10) comprising technical means (21, 22, 23) configured to implement the method according to any one of claims 1 to 9.

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