DESCRIPTION
Title of the invention: Proximity imaging radar system with multichannel
antenna
[0001] The invention relates to the field of radar systems, notably the radar systems that are intended to be embedded in a mobile carrier, for example a rotary wing aircraft such as a helicopter.
[0002]The invention relates more specifically to the proximity and anticollision radar systems that make it possible to ensure the onboard navigational safety of a rotary wing aircraft.
[0003] In the field of helicopter navigation safety, one general problem relates to the accidents linked to collisions in take-off and landing phases or when moving in proximity to obstacles such as cliffs and buildings. The risk of collision is all the greater when the environmental conditions are degraded and the visibility of the pilot is reduced (mist, gust cloud, night).
[0004] In this field, there is a need for radar equipment that makes it possible to provide the crew with a panoramic proximity view, regardless of the optical visibility conditions, in order to be warned sufficiently early of a danger of collision.
[0005] This equipment must be compatible with the demanding constraints concerning its installation on a helicopter, which includes considerations of weight, of consumption, of bulk, of positioning and of cost.
[0006] Furthermore, the function performed by the radar equipment must have a very significant coverage angle with a refresh time that is short enough to make it possible to best anticipate the possible collision hazards, in particular during manoeuvres in direct proximity to obstacles.
[0007] Moreover, some helicopter fleets may already be equipped with monitoring radar, and may have difficulty in embedding an additional radar payload. Likewise, some helicopters are equipped with weather radar, with similar payload constraints.
[0008] One general problem to be resolved is therefore how to design a proximity radar system which can be based on a monitoring radar or weather radar architecture in order to re-use a part of this existing equipment and limit the bulk onboard the carrier.

[0009] Various solutions from the prior art have been considered to address the problem of proximity obstacle detection for mobile carriers.
[0010] One first known solution relies on the use of satellite radio navigation systems, for example GPS systems, to locate the carrier, associated with a digital terrain map which locates the potential obstacles. This solution offers the advantage of using only a GPS receiver which is also generally available. Nevertheless, the major drawbacks of this solution are the excessively rough resolution and the excessively high refresh time of the GPS system. That can however be improved by data from the navigation unit. Another disadvantage lies in the weak robustness of the GPS system which may be effected by interference or unavailable in some zones. Moreover, this system relies on terrain models which are not necessarily up to date, or which may lack accuracy. In particular, vehicles or temporary installations are not referenced therein, which poses a problem of reliability of the system with respect to these unlisted obstacles.
[0011] A second known solution consists in implementing laser sensors to produce an optical mapping of the surroundings of the helicopter. This solution presents the drawback of not operating in reduced visibility conditions (for example mist), or even towards the sun.
[0012] Another major family of solutions is based on radar sensors, which make it possible to obtain a mapping even when optical visibility is low.
[0013] The systems based on mechanical scanning radars do not make it possible to observe the trade-off between short refresh time and extended scanning range.
[0014] In the solutions based on solid-state radars, there are the installations that implement electronic scanning radars and solutions that rely on the use of a large number of solid-state sensors.
[0015] The electronic scanning radars produce a mapping by scanning the peripheral space of the helicopter. The anticollision application targeted requires a very significant angular coverage. The scanning figure produced therefore has a long refresh time, or else a short illumination time in each direction of the space. This trade-off is unfavourable for the application which requires both a short refresh time and an illumination time that is as long as possible to benefit from an accurate measurement of the Doppler speed. In fact, the measurement of the Doppler speed

makes it possible to obtain better mapping of the environment and better identify and anticipate the potential dangers.
[0016] The second solution, based on the installation of a large number of solid-state sensors, makes it possible to obtain permanent coverage of the space. Nevertheless, obtaining a fine angular resolution means using a large number of sensors. The plurality of the sensors impacts the price and the ease of installation on the carrier. An alternative is to use computing-based beam-forming antennas. These antennas have multiple distinct reception channels. It is then possible to transmit and receive in a large portion of the space. The digitization of each of the individual reception channels makes it possible to form beams simultaneously, a posteriori, by digital processing. The trade-off between the size of the coverage range and the angular resolution of the system is reflected by the number of distinct channels to be digitized and to be processed, which impacts the complexity and the cost of the solution first and foremost.
[0017] A variant of this solution consists in using several orthogonally coded transmission channels and several independently digitized reception channels (technique known as MIMO, for Multiple Input, Multiple Output). It is then possible to extract the signals originating from each transmitter at each receiver, so as to reconstruct a number of virtual channels corresponding to the product of the number of transmission channels by the number of reception channels. The complexity of these systems and the throughput of the data to be transmitted and to be processed increases with the number of reception channels.
[0018]The invention proposes a proximity radar system for helicopters, or more generally for rotary wing aircraft, based on an antenna with multichannel 2D electronic scanning in transmission and/or in reception, of which the individual antennas can be phase-modulated.
[0019] Advantageously, this radar system can be the same one which handles monitoring functions and/or the weather radar function, the current architectures of which widely implement active antenna radars composed of individually active modules coupled to individual antennas.

[0020] The principle of the invention relies on the phase modulation of one or more individual antennas in transmission, and the phase modulation of one or more individual antennas in reception, within each digitized reception channel.
[0021] One advantage of the invention is that it makes it possible to adapt the number of active radiating elements as a function of the speed of movement of the carrier.
[0022] The subject of the invention is an imaging radar system comprising an antenna system comprising a plurality of sectors each composed of a plurality of activatable radiating elements, the radar system comprising at least one transmission channel and at least one reception channel capable of each controlling a sector, each transmission channel being configured to control a sector by means of a signal burst having a predetermined recurrence frequency and of which the phase at the origin of each recurrence follows a predefined phase coding law, each reception channel being configured to apply said phase coding law to the active radiating elements of a sector, the phase coding laws applied to the different active radiating elements of a set composed of a sector controlled by a transmission channel and of a sector controlled by a reception channel, being mutually orthogonal, the system further comprising a computer comprising a virtual channel identification module configured to filter in reception each virtual channel corresponding to the combination of a radiating element of a sector controlled by a transmission channel and of a radiating element of a sector controlled by a reception channel.
[0023] According to a particular aspect of the invention, each phase coding law is defined by a different Doppler frequency for each radiating element, each virtual channel being defined by a Doppler frequency equal to the sum of the Doppler frequency associated with a radiating element in transmission and of the Doppler frequency associated with a radiating element in reception, the values of the Doppler frequencies of the radiating elements being chosen such that all of the Doppler frequencies of the virtual channels are evenly distributed in a frequency band [-Fr/2; Fr/2] with Fr the recurrence frequency.
[0024] According to a particular aspect of the invention, the radar system is intended to be embedded on a mobile carrier having a given speed of movement and the computer further comprises an activation module for the radiating elements and a spectral congestion estimation module that are configured to determine a maximum

number of active radiating elements per sector that observes a constraint of spectral congestion of the virtual channels as a function of the speed of movement of the carrier and of the recurrence frequency of the signals so as to avoid spectral folding.
[0025] According to a particular aspect of the invention, the number of active radiating elements M per sector controlled by a transmission channel and the number of active radiating elements N per sector controlled by a reception channel observe the following constraint: M.N < (AFr/2Vp) in which A is the wavelength of the signals, Fr is the recurrence frequency and Vp is the speed of movement of the carrier.
[0026] According to a particular aspect of the invention, the phase coding law of the radiating elements follows a modulation of "Doppler Division Multiple Access" DDMA type.
[0027] According to a particular aspect of the invention, each reception channel comprises a combiner for summing the signals received by the active radiating elements of a same sector.
[0028]According to a particular aspect of the invention, each transmission channel comprises a modulator configured to frequency-modulate the radar signals on each recurrence.
[0029]According to a particular aspect of the invention, each transmission channel comprises a radar pulse generator configured to generate a burst of radar pulses at the predetermined recurrence frequency.
[0030] According to a particular aspect of the invention, each active radiating element is composed of a plurality of active radiating sub-elements, each transmission channel and each reception channel being configured to apply a predefined beam steering control to the active radiating sub-elements of a same active radiating element.
[0031] According to a particular aspect of the invention, the computer is configured to perform, for each reception channel, a distance filtering processing on the signals on each recurrence and a Doppler filtering processing from recurrence to recurrence.
[0032] Other features and advantages of the present invention will become more apparent on reading the following description in relation to the following attached drawings.

[0033] [Fig. 1] represents a functional diagram of a radar system according to a first embodiment of the invention,
[0034] [Fig. 2] represents an example of wave form used by the radar system of [Fig. 1],
[0035] [Fig. 3] represents a diagram identifying transmission-reception virtual channels in the Doppler space, according to a particular aspect of the invention,
[0036] [Fig. 4] represents a diagram illustrating the spectral congestion of the virtual channels identified in [Fig. 3] as a function of the speed of movement of the carrier,
[0037] [Fig. 5] represents a functional diagram of a radar system according to a second embodiment of the invention.
[0038] [Fig. 1] represents a diagram of a radar system 100 according to a first embodiment of the invention.
[0039] [Fig. 1] presents an architecture of an exemplary multichannel radar system comprising one transmission channel and four reception channels. Without departing from the context of the invention, the number of transmission channels can be greater than one channel and the number of reception channels can be any number strictly greater than 1.
[0040] The radar system 100 comprises an antenna system ANT, an analogue-digital conversion system SYSCAN, a digital computer CAL and a display device AFF.
[0041] In the example of [Fig. 1], the antenna system ANT is composed of several sectors each comprising the same number of antenna elements. For this particular example, the antenna system ANT comprises one transmission sector TX and four reception sectors RX1, RX2, RX3, RX4. Each sector comprises an identical number of radiating elements (9 elements in the example of [Fig. 1]). Each radiating element can be individually activated or deactivated. The invention allows operation with a reduced number of radiating elements activated per sector. In the example of [Fig. 1], two radiating elements ET1-TX, ET2-TX are activated in transmission and two radiating elements ET1-RX1, ET2-RX1 are activated for each reception sector.
[0042] Each radiating element is phase-modulated independently by means of an activation command CMD generated by a coder COD.

[0043]The transmission sector TX is controlled by a transmission channel and the four reception sectors RX1, RX2, RX3, RX4 are each controlled by a dedicated reception channel.
[0044] The antenna system ANT further comprises a summer S1, S2, S3, S4 for each reception sector. A combiner is configured to sum the signals received by the active antenna elements of a same reception sector RX.
[0045]The transmission channel comprises a digital radar signal generator (forming part of the computer CALC but not represented), a digital-analogue convertor CNA and a carrier modulator MOD.
[0046] In a particular embodiment of the invention, the radar signals are generated in the form of a signal burst having a predefined recurrence frequency Fr. On each recurrence, the signal is modulated by a frequency ramp. The wave form used to modulate the signal burst is, for example, of frequency-modulated continuous wave type or FMCW. One advantage in this use of type of wave form is that it makes it possible to obtain a short-distance coverage with no blind zones. Without departing from the context of the invention, other wave forms can be envisaged, as will be explained hereinbelow.
[0047]The phases of the active radiating elements of the emission sector TX are coded with an orthogonal coding law. In other words, the different active radiating elements of a same reception sector have different phases at the origin and follow an orthogonal law of evolution with respect to the phases of the other active radiating elements of the same sector.
[0048] For example, the phase coding law follows a modulation of DDMA (Doppler Division Multiple Access) type, that is to say a linear evolution of the phase at the origin with a different gradient for each radiating element of a same transmission sector. The reference [1] describes the principle of DDMA modulation. The evolution of these phases 4)1,k and 4)2,k correspond to Doppler frequencies denoted Fd_tx1 and Fd_tx2 hereinbelow.
[0049] [Fig. 2] represents, on two time-frequency diagrams, the evolution of the frequency of the radar signal over time for the first active antenna element ET1_TX (top figure) and for the second active antenna element ET2_TX (bottom figure).

[0050] On each recurrence (time period of duration Tr equal to the inverse of the recurrence frequency Fr), the frequency evolves linearly (because of the FMCW frequency modulation) according to a frequency ramp. The phase at the origin of each recurrence is different and evolves according to an orthogonal coding law.
[0051] Thus, the phases at the origin of the active radiating element ETi_TX are denoted 4)1,k, with k varying from 1 to N, for each recurrence of index k and the phases at the origin of the active radiating element ET2_TX are denoted 4)2,k, with k varying from 1 to N, for each recurrence of index k.
[0052] Without departing from the scope of the invention, the frequency-modulated signals can be replaced by constant frequency or frequency-modulated pulse signals via a pulse compression technique.
[0053] The duration of a pulse can be equal to the duration of a recurrence Tr or to a proportion of this duration (for example 10% or 20%). In this latter case, a percentage of the duration of the recurrence is used to transmit radar signals and the remainder of the recurrence is used to receive the echoes of these signals. In this case, the same antenna elements can be used in transmission and in reception (since the transmission and the reception of the signals are temporally segregated).
[0054] In reception, the radar signals are received on the active radiating elements of each reception sector. The phases of these radiating elements are coded via the same coding law as for the sector in transmission.
[0055] In other words, the coding laws of the radiating elements of the sectors in reception are orthogonal to one another and to those of the radiating elements of the sectors in transmission. The Doppler frequencies associated with the coding laws of the phases of the antenna elements ET1_RX1 and ET2_RX1 of a sector in reception RX1 are denoted Fd_rx1 and Fd_rx2.
[0056] The signals received on a same reception sector RX1 are summed in analogue form by means of the combiner S1 after the individual phase coding thereof.
[0057] For each reception sector, the output signal of the combiner is frequency-demodulated via a demodulator DEMi then digitally converted via an analogue-digital convertor CANi.

[0058] In the case of a frequency ramp modulation, each demodulator DEM-i, DEM2, DEM3, DEM4 implements a demodulation technique known as "deramping". This frequency-demodulation step can be performed either in analogue form in the conversion system SYSCAN or digitally in the computer CALC.
[0059] The radar system according to the invention comprises a computer CALC which is configured to perform radar signal digital processing operations.
[0060] For each reception channel, a distance filtering processing DIST is performed on the signals on each recurrence, then a Doppler processing DOP is performed along the burst, from recurrence to recurrence. These processing operations are, for example, described in the reference [2].
[0061]A virtual channel identification processing ID is then applied in order to separate the different virtual channels corresponding to the different radiating elements in transmission combined with the different radiating elements in reception.
[0062] In the example of [Fig. 2], four virtual channels are obtained per reception sector which correspond to four different Doppler frequencies at the output of the Doppler processing DOP.
[0063] [Fig. 3] represents, on a frequency diagram, the four Doppler frequencies Fd_rx1+Fd_tx1, Fd_rx1+Fd_tx2, Fd_rx2+Fd_tx1, Fd_rx2+Fd_tx2 corresponding to the four possible combinations obtained from the two active antenna elements in transmission and the two active antenna elements in reception. Generally, the number of virtual channels for a reception sector is equal to the number of active antenna elements in transmission multiplied by the number of active antenna elements in reception.
[0064] The four Doppler frequencies obtained at the output of the Doppler processing correspond to the different combinations of the modulation Doppler frequencies of the phase coding law of the two active radiating elements in transmission and of the two active radiating elements in reception.
[0065] Advantageously, the modulation Doppler frequencies are chosen so as to be able to distinguish the different combinations in the Doppler space. For example, they are chosen such that the different frequency combinations are evenly distributed in the frequency band [-Fr/2; Fr/2].

[0066] After identification of the virtual channels, a beam-forming processing by the computation FFC is applied to all of the reception channels by combining the different identified virtual channels with phase shifts chosen so as to perform a spatial filtering in chosen directions.
[0067] An information processing and data formatting module TRI is then applied to the beams formed digitally in order to recompose a mapping of the environment of the carrier with angular and distance information. This information is then supplied to a display AFF.
[0068] Moreover, the Doppler processing DOP performed further makes it possible to estimate the speed of movement of the carrier in its environment, and thus detect and warn of a risk of collision with any obstacles.
[0069] The various processing operations performed by the modules DIST, DOP, FFC, TRI are not described in detail because they correspond to processing operations that are well known in the field of proximity radars. The person skilled in the art will be able to refer notably to the document [2] to perform these processing operations.
[0070] The above description has been given under the assumption that the carrier of the radar is immobile, which is not generally the case in an operational context.
[0071] When the speed of the carrier is non-zero, the frequencies illustrated in [Fig. 3] and obtained at the output of the Doppler processing are not pure carriers corresponding to the Doppler frequencies associated with the DDMA coding laws. In fact, these frequencies also carry Doppler frequency information linked to the movement of the carrier in its environment.
[0072] It is considered hereinbelow that the environment of the carrier is fixed and that the carrier is in motion at a speed Vp. In this case, based on the angles covered by the radar panel, the Doppler speeds observed around each spectral line (illustrated in [Fig. 3]) can extend at most from -Vp to +Vp, i.e. a width of 2Vp/X in Doppler frequency, with X the wavelength of the carrier frequency.
[0073] [Fig. 4] illustrates this phenomenon and represents the appearance of the spectrum of the signals at the output of the Doppler processing DOP.

[0074] To be able to correctly identify the different virtual channels, their spectra must be separated in the Doppler space, in other words, there must be no folding of the frequencies onto one another. This condition is observed if the spectra are separated by at least 2Vp/X.
[0075] The recurrence frequency Fr is chosen so as to obtain an unambiguous distance range Da compatible with the proximity situation, typically a few hundreds of metres or a few kilometres.
[0076] The unambiguous speed range Va available is then given by the wavelength used, according to the formula:
[0077]Va = A.c/(4.Da)
[0078] That corresponds to a Doppler frequency Fa = 2.Va/X.
[0079] In other words, the recurrence frequency Fr is chosen to be equal to the frequency Fa = 2.Va/A, so to observe the chosen unambiguous distance range, that is to say the maximum distance at which obstacles are wanted to be detected.
[0080] If the virtual channels are distributed uniformly in the Doppler spectrum, their spacing corresponds to the recurrence frequency Fr divided by the number of virtual channels, that will be denoted M.N (M the number of channels phase-coded on transmission and N the number of channels phase-coded in each reception sector).
[0081] In order to avoid an overlapping of the spectra, the number of virtual channels must therefore observe the following condition:
[0082]Fr/(M.N)>2Vp/X.
[0083] Thus, it can be deduced therefrom that the number of active radiating elements must observe the following condition: M.N < (Fr.X)/2Vp.
[0084] Since the recurrence frequency Fr and the wavelength X of the signals are parameters set as a function of the targeted application, it can be deduced therefrom that the number of active radiating elements is limited by the speed of movement of the carrier. The greater the speed Vp, the lower the number of active radiating elements that must be chosen. In other words, the number of virtual channels that can be operated without folding is inversely proportional to the speed of the carrier.

[0085] The Doppler frequency congestion illustrated in [Fig. 4] is a worst case corresponding to a very wide angular coverage which would generate echoes in directions corresponding to the direction of movement of the carrier, in the direction of movement (echoes converging at Vp), in the opposite direction (echoes diverging at -Vp), and in intermediate directions. Depending on the cases of use (altitude of the carrier, angular coverage), the effective congestion will be a fraction of this spectral congestion.
[0086] The Doppler spread or congestion of the environment linked to the speed of the carrier can be estimated by analysis of the output of the Doppler processing via the module EST ([Fig. 1]).
[0087] Based on the result of this estimation, it is possible to deduce therefrom the maximum number of coded virtual channels possible without having spectral folding. Thus, based on this estimation, it is possible to deduce therefrom the number of active radiating elements per sector. This number is determined by the module EST or the coding module COD and the activation command of the radiating elements (and of their phase) CMD is then generated as a function of this result.
[0088] In other words, the radar architecture described in [Fig. 1] involves a Doppler congestion estimation module EST, which then makes it possible to adjust the number of virtual channels used by modifying the number of individual antennas per sector in transmission and in reception via the command CMD. The coding module COD also makes it possible to calculate the phase laws associated with these individual antennas and to send the necessary phase-shift commands CMD to each individual antenna.
[0089] Thus, when the carrier is at high speed and with a significant Doppler spectrum congestion, a restricted number of virtual channels obtained by phase-coding of the individual transmission and reception antennas will be used. That results in a fairly rough angular resolution, which is not particularly detrimental for potential objects at a great distance.
[0090] Thus, the invention offers the advantage of allowing an adaptation of the number of active radiating elements as a function of the speed of the carrier and more generally of the congestion of the Doppler spectrum so as to avoid spectral folding.

[0091] [Fig. 5] schematically represents a variant embodiment of the radar system 100 of [Fig. 1].
[0092] In this variant, the individual radiating elements are grouped together in groups of elements that have the same number of elements and the same spatial distribution for each sector.
[0093] In the example of [Fig. 5], the active radiating elements of each sector are grouped together by two elements on the vertical axis.
[0094] In this variant, the invention applies identically by replacing the individual active radiating elements of [Fig. 1] with the groups of active elements of [Fig. 5].
[0095] In the example of [Fig. 5], there are two groups G1_TX, G2_TX in the transmission sector, each group being composed of two radiating elements.
[0096] Likewise, in each reception sector there are two groups G1_RX1, G2_RX1 each composed of two radiating elements.
[0097] That makes it possible to improve the range budget in a reduced angular sector, which is advantageous for high speeds of movement.
[0098] Moreover, the individual radiating elements belonging to a same group are assigned a differential phase-shift associated with an electronic beam steering law.
[0099] The elements of a same group are assigned phase-shifts chosen to perform an electronic pointing in a given direction. That makes it possible to combine the electronic scanning with the phase-coding techniques of the individual transmission and reception antennas in order to obtain an adaptive hybrid solution. [Fig. 5] represents a configuration with groupings of two individual antennas subject to the same phase-coding laws, with a constant additional differential phase-shift A4>p between the two antenna elements of each group (in transmission as in reception).
[0100] To obtain an electronic pointing in an angular direction 9 according to the axis of the elements of a group, the differential phase-shift to be applied between these elements is:
[0101] AcpD =^.a.sin0
[0102] With a the distance between the radiating elements of a group, from centre to centre.

[0103] In the case of a greater number of elements in a group, this formula is generalized into a phase gradient applied to the different radiating elements of the group, depending on the desired pointing direction.
[0104] When the carrier is at low speed, the Doppler congestion linked to its speed is reduced and it is possible to use a greater number of virtual channels in order to obtain a better angular resolution. The use of additional individual antennas in transmission and/or in reception does not increase the throughput of the data transmitted to the computer, since they are still on a number of constant digitized reception sectors. That offers a significant advantage with respect to a solution in which each additional individual reception antenna would be digitized independently, by generating an additional digital channel for each.
[0105] The increase in the number of encoded virtual channels will however increase the volume of computations performed for the beam-forming by computation. If the computation capabilities are limited, it is possible to limit the distance range processed in order to reduce the volume of the data to be processed. This range limitation is compatible with use at low speed.
[0106] The invention requires at least one active antenna with the possibility of dynamically modifying the configuration of the radiating elements which are activated and of assigning them phase codes. This type of antenna can be employed in surveillance radars, on helicopters in particular. Advantageously, the proximity radar proposed in this invention can, if necessary, use the hardware capabilities of such a radar to perform these functions. The proposed method would in this case be a particular mode of operation of the surveillance radar, which would offer substential advantages since these functionalities would share common hardware, which is highly favourable with respect to the weight, the consumption, the bulk and the cost of the equipment.
[0107] The computer CALC of the radar according to the invention can be produced, for example, on an embedded processor. The processor can be a generic processor, a specific processor, an application-specific integrated circuit (known by the acronym ASIC) or a field-programmable gate array (known by the acronym FPGA). The computation device can use one or more dedicated electronic circuits or a general purpose circuit. The technique of the invention can be implemented on a

reprogrammable computation machine (a processor or a microcontroller for example) running a programme comprising a sequence of instructions, or on a dedicated computation machine (for example a set of logic gates such as an FPGA or an ASIC, or any other hardware module).
[0108] References
[0109][1] Mathieu Cattenoz. MIMO Radar Processing Methods for Anticipating and Preventing Real World Imperfections. Signal and Image Processing. Universite Paris Sud - Paris XI, 2015.
[0110] [2] Goy Philippe. Detection d'obstacles et de cibles de collision par un radar FMCW aeroporte. PhD, Institut National Polytechnique de Toulouse, 2012

CLAIMS
1. Imaging radar system (100, 500) comprising an antenna system (ANT) comprising a plurality of sectors (TX, RX1, RX2, RX3, RX4) each composed of a plurality of activatable radiating elements, the radar system comprising at least one transmission channel and at least one reception channel capable of each controlling a sector, each transmission channel being configured to control a sector (TX) by means of a signal burst having a predetermined recurrence frequency and of which the phase at the origin of each recurrence follows a predefined phase coding law, each reception channel being configured to apply said phase coding law to the active radiating elements (ET1_RX1, ET2_RX1) of a sector (RX1), the phase coding laws applied to the different active radiating elements (ET1_TX, ET2_TX, ET1_RX1, ET2_RX1) of a set composed of a sector controlled by a transmission channel and of a sector controlled by a reception channel, being mutually orthogonal, the system further comprising a computer (CALC) comprising a virtual channel identification module (ID) configured to filter in reception each virtual channel corresponding to the combination of a radiating element (ET1_TX, ET2_TX) of a sector controlled by a transmission channel and of a radiating element (ET1_RX1, ET2_RX1) of a sector controlled by a reception channel.
2. Imaging radar system according to Claim 1, wherein each phase coding law is defined by a different Doppler frequency for each radiating element (ET1_TX, ET2_TX, ET1_RX1, ET2_RX1), each virtual channel being defined by a Doppler frequency equal to the sum of the Doppler frequency associated with a radiating element in transmission and of the Doppler frequency associated with a radiating element in reception, the values of the Doppler frequencies of the radiating elements being chosen such that all of the Doppler frequencies of the virtual channels are evenly distributed in a frequency band [-Fr/2; Fr/2] with Fr the recurrence frequency.
3. Imaging radar system according to Claim 2, wherein the radar system is intended to be embedded on a mobile carrier having a given speed of

movement and the computer (CALC) further comprises an activation module (COD) for the radiating elements and a spectral congestion estimation module (EST) that are configured to determine a maximum number of active radiating elements per sector that observes a constraint of spectral congestion of the virtual channels as a function of the speed of movement of the carrier and of the recurrence frequency of the signals so as to avoid spectral folding.
4. Imaging radar system according to Claim 3, wherein the number of active radiating elements M per sector (TX) controlled by a transmission channel and the number of active radiating elements N per sector (RX1) controlled by a reception channel observe the following constraint: M.N < (AFr/2Vp) in which A is the wavelength of the signals, Fr is the recurrence frequency and Vp is the speed of movement of the carrier.
5. Imaging radar system according to any one of the preceding claims, wherein the phase coding law of the radiating elements follows a modulation of "Doppler Division Multiple Access" DDMA type.
6. Imaging radar system according to any one of the preceding claims, wherein each reception channel comprises a combiner (S1, S2, S3, S4) for summing the signals received by the active radiating elements of a same sector.
7. Imaging radar system according to any one of the preceding claims, wherein each transmission channel comprises a modulator (MOD) configured to frequency-modulate the radar signals on each recurrence.
8. Imaging radar system according to any one of Claims 1 to 6, wherein each transmission channel comprises a radar pulse generator configured to generate a burst of radar pulses at the predetermined recurrence frequency.
9. Imaging radar system according to any one of the preceding claims, wherein each active radiating element (G1_TX, G2_TX) is composed of a plurality of active radiating sub-elements, each transmission channel and each reception channel being configured to apply a predefined beam steering control to the active radiating sub-elements of a same active radiating element.
10. Imaging radar system according to any one of the preceding claims, wherein the computer (CALC) is configured to perform, for each reception channel, a

distance filtering processing (DIST) on the signals on each recurrence and a Doppler filtering processing (DOP) from recurrence to recurrence.