Method for detecting and tracking targets using a two-dimensional synthetic antenna array

Abstract

The invention relates to a surface wave radar system comprising a first array (RT) of transmitting elements (Tx) configured to emit surface wave signals along a wavelength λ, a second array (RR) of receiving elements (Rx) configured to receive said signals reflected by targets, a power supply unit for said system, and an ECU for processing said reflected signals, characterized in that: - the first array (RT) of transmitting elements (Tx) comprises a plurality of at least two identical sub-arrays (SRT) of linear transmitting elements (Tx), each extending along a first direction (DT) over a length (LSRT), - the second array (RR) of receiving elements (Rx) is also a linear array extending along a second direction (DR), perpendicular to the first direction (DT). The invention also relates to the use of such a system for the detection and tracking of maritime targets. Figure 1

Description

Title of the invention: Method for detecting and tracking targets using a two-dimensional synthetic antenna array. Technical field

[0001] The present invention relates to surface wave radar systems, and more particularly to target detection and tracking methods using such systems. State of the art

[0002] In the field of maritime surveillance, one of the main challenges is to establish a general maritime situation covering the largest possible area, and to do so permanently.

[0003] Direct-looking radars used for this surveillance function enable target detection with sufficient accuracy for operational use, but at the expense of range over maritime targets beyond the radio horizon. The use of this type of radar over distances greater than fifty kilometers therefore remains impossible due to the shape of the Earth. Its curvature prevents the signal emitted by the direct-looking radar from propagating over long distances at sea level. Consequently, any maritime detection beyond the radio horizon is impossible.

[0004] One of the main consequences is the reduction of the capacity to monitor maritime areas over long distances.

[0005] Surface wave radar systems, in particular high-frequency surface wave radar (HFSWR) systems, have recently been developed to overcome the line-of-sight limitations of microwave radar systems. HFSWR systems exploit a phenomenon known as Norton wave propagation, in which a vertically polarized electromagnetic signal propagates efficiently as surface waves along a conductive surface. HFSWR systems operate in coastal installations, where the ocean forms the conductive surface. The transmitted signal follows the curved surface of the ocean, and a system can detect targets beyond the visible horizon, with a range on the order of 400 km.

[0006] Successful target detection by a surface wave radar system traditionally involves a compromise between many factors, including propagation losses, target radar cross-section, ambient noise, human interference and sea clutter.

[0007] However, surface wave radars (SWR) require high-power HF transmitters to achieve such results. This therefore requires a specific infrastructure, particularly in terms of power consumption.

[0008] Furthermore, surface wave radars (SWRs) are highly affected by meteorological and ionospheric conditions, which make the accuracy of measurements and their coverage unpredictable. Meteorological and ionospheric conditions can indeed reduce the ability to detect targets at long range and the ability to detect small targets.

[0009] Therefore, it has become necessary to improve surface wave radars (SWR) to overcome all or part of the disadvantages mentioned above.

[0010] The inventors have notably discovered that a "two-dimensional" synthetic antenna array should allow for a significant improvement in the performance of surface-wave radars (SWR). "Two-dimensional" means that the transmitting and receiving antennas are deployed along two perpendicular axes, in contrast to conventional surface-wave radars which have a linear antenna deployment.

[0011] The inventors also discovered that increasing the number of synthetic antennas could compensate for the energy requirement and also limit the footprint of the installation.

[0012] Therefore, the present invention relates to a surface wave radar system implementing a two-dimensional synthetic antenna array. Description of the invention

[0013] More specifically, the invention relates to a method for detecting and tracking targets using a surface wave radar system comprising: - A first array of m emitting elements configured to emit signals propagating as surface waves at a wavelength X, said first array extending linearly along a first direction, - A second network of n receiving elements, said second network extending linearly along a second direction, concurrent with the first direction, - A control unit for said system, - A processing unit for said reflected signals,

[0014] Characterized in that the process comprises - A stage in which the m transmitting elements emit signals at a frequency in the HF frequency band, between 3 and 30 MHz, these signals being orthogonal to each other, - A stage in which the n receiving elements receive backscattered signals, - A step involving the digitization of backscattered signals, - A step of separating the digitized signals, leading to the formation of a synthetic network of m*n minimal receiving elements, - A spatial filtering step by forming a minimal m*n beam through calculation, - A step of extracting the detected plots from the data received by the synthetic network of at least m*n receiving elements, said plots including the angular position, distance and speed of the possible target(s) relative to the surface wave radar system.

[0015] Various embodiments of the invention are provided, incorporating, according to all their possible combinations, the different optional features set out below.

[0016] Preferably, the m emitting elements emit signals at a frequency in the HF frequency band, between 5 and 30 MHz.

[0017] This frequency range is particularly suitable for the excitation of surface waves and allows far surface wave propagation.

[0018] This working frequency range makes it possible to detect ships generally considered to be stealthy.

[0019] Preferably, the second direction is perpendicular to the first direction.

[0020] This configuration allows for the reproduction of a rectangular synthetic antenna which is optimum.

[0021] Also preferably, the first network of emitting elements comprises a plurality of at least two identical sub-networks of emitting elements, parallel to each other and aligned each along the first direction.

[0022] This configuration allows the size of the synthetic antenna to be increased.

[0023] Also preferably, the second network of receiving elements comprises a plurality of at least two identical sub-networks of receiving elements aligned on the same second direction, the spacing between two adjacent sub-networks of receiving elements being substantially equal to the width of the first network of emitting elements, less half the wavelength of the emitted signals.

[0024] This configuration also allows the size of the synthetic antenna to be increased.

[0025] Also preferably, the spacing between two sub-arrays of emitting elements is substantially equal to the length of the sub-arrays, plus half the wavelength of the emitted signals.

[0026] This configuration allows for the reproduction of a synthetic antenna without gaps.

[0027] Also preferably, the spacing between two adjacent emitting elements is greater than or equal to the wavelength of the emitted signals.

[0028] This configuration allows for decorrelation of the sea clutter and improves the signal-to-clutter ratio.

[0029] Also preferentially, the spacing between two adjacent receiving elements is greater than half the wavelength of the emitted signals.

[0030] This configuration makes it possible to reduce coupling between receiving antennas and to restore a synthetic antenna without gaps

[0031] Also preferably, the emitting elements are longitudinally radiating antennas.

[0032] This type of antenna allows better excitation of the surface wave and limits energy losses to areas not used by the radar.

[0033] Also preferably, the receiving elements are antennas with a folded dipole in a vertical plane.

[0034] This type of antenna allows for better rejection of ionospheric clutter.

[0035] Also preferentially, the second direction along which the second network of receiving elements extends is substantially close to the direction along which a coastline extends.

[0036] Advantageously, the step of separating the digitized signals is a convolution operation between the discrete elements of the first network and the second network.

[0037] This type of operation makes it possible to separate all the emitted waveforms very distinctly and to be able to construct the synthetic antenna.

[0038] Alternatively, the digitized signal separation step can be a Doppler processing operation for waves coded with a DDMA code or a matrix multiplication operation for waves coded with a CDMA code.

[0039] Advantageously, the spatial filtering step is carried out using a beamforming technique.

[0040] This step makes it possible to obtain a good angular resolution of the synthetic antenna lobe and to improve the localization of targets. Brief description of the FIGURES

[0041] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the accompanying drawings in which: - The [Fig. 1] is a schematic representation of a surface wave radar system according to one embodiment of the invention; - Fig. 2 is a schematic representation of the two-dimensional synthetic antenna array, obtained with the surface wave radar system of Fig. 1; - Fig. 3 is an illustration of the steps of a method for detecting and tracking targets using a two-dimensional synthetic antenna array, according to the invention.

[0042] It is understood that the embodiments described below are in no way limiting. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0043] In particular, all the variants and all the embodiments described are combinable with each other if nothing prevents this combination from a technical point of view.

[0044] In the figures and in the rest of the description, elements common to several figures retain the same reference. Detailed description of the FIGURES

[0045] Figure 1 shows a surface wave radar system forming a two-dimensional synthetic antenna array, according to one embodiment of the invention.

[0046] The fundamental principle of a synthetic network is to create by construction with a limited number of transmitting and receiving antennas, an equivalent network comprising one transmitting antenna and a multitude of receiving antennas.

[0047] Detection systems that use the so-called "MIMO" (multiple input, multiple output) functionality, which relies on the use of several antennas on both the transmitter and receiver sides, have significant constraints regarding antenna design and placement. In these systems, due to the need for finer resolution and higher transmit / receive gain, the trend is to integrate more antennas into an array for beamforming and the reception of low-level signals.

[0048] When several transmitting and receiving antennas are co-located, they can act together to form what is called a virtual (or synthetic) antenna network.

[0049] The virtual network is not a true array of antennas; it is a mathematically equivalent object that describes the behavior of the antenna array. An important part of constructing an antenna array that enables MIMO functionality, including spatial multiplexing, consists of designing the arrangement of the virtual antennas within a virtual network.

[0050] When a set of transmitting and receiving antennas in an array operate together to transmit and receive a signal, they act as an equivalent antenna array, called a virtual array. The actual gain of the antenna in transmit / receive mode is equal to the gain of the virtual array when it operates only in transmit or receive mode.

[0051] In short, when several elements work together, regardless of the beam formation mode, the received beam will have a higher directional gain and better angular resolution.

[0052] In the present case, the transmitting antenna of the virtual network is a single, non-directional antenna. It covers an angular sector of 120°. On the other hand, for reception, a large virtual antenna is reconstructed, which will have a higher directional gain and better angular resolution.

[0053] The number of virtual elements in a planar antenna array with m transmitting elements TX and n receiving elements NRX is: m*n.

[0054] This number is important because it is associated with the maximum resolution of the network.

[0055] When operating as a MIMO network, the angular resolution of the entire network is related to the angular resolution dRx of a single antenna as follows:

[0056] « _______________________ p number of subnets transmission*n

[0057] Antenna gain refers to the angular resolution of the system, and it results from the superposition of electromagnetic waves received by the antenna array.

[0058] In the case where a large number of antennas are used, the angular resolution reaches less than 1°.

[0059] In the calculation of a virtual network, the locations of the virtual antenna elements are calculated. These are calculated using a convolution operation between the discrete elements constituting the network.

[0060] Alternatively, the digitized signal separation step can be a Doppler processing operation or a matrix multiplication operation.

[0061] When constructing antenna arrays for beamforming in MIMO mode, the locations of the individual antennas must be specified. The antennas in beamforming arrays are normally spaced by multiples of half the wavelength.

[0062] Thus, according to the principle of the invention, the surface wave radar system comprises a first RT array of Tx transmitting elements configured to emit surface wave signals along a wavelength X, and a second RR array of Rx receiving elements configured to receive said reflected signals. The surface wave radar system includes a UP control unit, comprising a power supply, to control the emissions of the Tx transmitting elements of the first RT array, and a UTS processing unit for the signals received by the Rx receiving elements of the second RR array.

[0063] The first RT array of Tx transmitting elements comprises a plurality of at least two identical SRT sub-arrays in terms of Tx antenna composition, these sub-arrays being further linear in the sense that each extends along a first direction DT over a length LSrT-

[0064] As shown in the embodiment of [Fig. 1], the first RT network comprises 3 SRT sub-networks of 6 Tx transmitter elements each.

[0065] The second RR network of Rx receiving elements is also a linear network extending along a second direction DR, perpendicular to the first direction DT.

[0066] Advantageously, the second RR network of Rx receiving elements comprises a plurality of at least two identical SRR sub-networks of Rx receiving elements.

[0067] As shown in the embodiment of [Fig. 1], the second RR network of Rx receiving elements comprises 4 SRR sub-networks of 8 Rx receiving elements each.

[0068] Thus, as shown in [Fig.2], with the installation of 18 transmitting elements Tx, a synthetic antenna array RR' is obtained, equivalent to 576 receiving elements R'x, and one transmitting element T'x. This array RR' actually comprises 6 sub-arrays SRR' of receiving elements R'x, spaced apart by a distance ESrr' equal to the spacing between two successive transmitting elements Tx of the same sub-array SRT.

[0069] Preferably, to optimize the formation of the synthetic network, the ESRr spacing between two sub-arrays SRR of receiving elements Rx is substantially equal to the width 1SrT of the first array RT of emitting elements Tx, less half the wavelength of the emitted signals.

[0070] Preferably, also to optimize the formation of the synthetic network, the ESRT spacing between two SRT sub-arrays of Tx emitting elements is substantially equal to the LSRR length of the SRR sub-arrays, plus half the wavelength of the emitted signals.

[0071] Advantageously, the spacing between two emitting elements Tx, of the same sub-network is greater than the wavelength of the signals emitted by the emitting elements Tx.

[0072] Similarly, the spacing between two receiving elements Rx of the same sub-network is greater than half the wavelength of the signals emitted by the emitting elements Tx.

[0073] By way of example, as shown in the embodiment of [Fig. 1], the distance between two Tx transmitting elements of an SRT sub-array is on the order of 50 m. This makes it possible to maintain a spacing greater than half the wavelength (30 m for the frequency of 5 MHz) and to preserve a sufficient margin for the design of the Tx transmitting element.

[0074] This architecture therefore fixes the dimensions of the second RR array of Rx receiving elements. In the embodiment of [Fig.1], each SRR sub-array of eight Rx receiving elements has an LSR length R of 210m, which corresponds to the ESRT spacing of 240 m between two SRT sub-arrays of Tx transmitting elements minus half a wavelength.

[0075] Each SRR subnetwork is separated from the next subnetwork by the total width of the first RT network, i.e. 480m.

[0076] The second RR network is composed of 32 Rx receiving elements, separated into groups of eight, over a total distance of 2400 meters.

[0077] As an alternative, the SRT sub-networks of Tx transmitting elements may include between 4 and 8 transmitting elements to ensure wide coastal surveillance.

[0078] Similarly, the SRR sub-networks of Rx receiving elements can comprise between 6 and 10 receiving elements, to also ensure wide coastal surveillance.

[0079] Preferably, the Tx emitting elements are longitudinally radiating antennas, these antennas being capable of emitting linearly polarized waves (Endfire in English).

[0080] The optimization of the Tx transmitter elements is directly related to their operating frequency. The Tx transmitter elements are optimized for a particular frequency, and in particular to operate at several frequencies (in this case from 5 to 10 MHz).

[0081] Preferably and by way of example, the Rx receiving elements are antennas with a folded dipole in a vertical plane, so that their directivity allows the impact of ionospheric disturbances to be minimized.

[0082] Ionospheric returns are significant when the electromagnetic wave is normal to the Earth's magnetic field lines. By definition, ionospheric disturbances propagate along the magnetic field lines, and their intensities are stronger perpendicular to the magnetic field lines.

[0083] The Rx receiving elements are therefore designed to have maximum gain in the axis corresponding to the lines of the Earth's magnetic field, and minimum on the orthogonal axis in order to attenuate the ionospheric returns received.

[0084] The Rx receiving elements must have a large bandwidth, that is to say, they must be able to receive signals over the entire range envisaged (in this case from 5 to 10 MHz) without modification.

[0085] The surface wave radar system according to the invention, and in particular that of [Fig. 1], can be used for the detection and tracking of maritime targets. In this configuration, the second RR array of Rx receiving antennas extends along a second direction substantially close to the direction formed by the coastline.

[0086] To do this, the first RT network of Tx emitting elements emits surface wave type signals, orthogonal to each other, following a frequency in the HF frequency band between 3 and 30 MHz and more particularly between 5 and 30 MHz.

[0087] As previously explained, a synthetic antenna array corresponds to a set of virtual antennas, each of which corresponds to a pair consisting of a real transmitting antenna and a real receiving antenna.

[0088] Thus, with the 18 real transmitting antennas and the 32 real receiving antennas of the surface wave radar system of [Fig.1], 576 (18x32) different pairs of synthetic antennas can be created.

[0089] During step 100 in which the m transmitting elements Tx emit signals following a frequency in the HF frequency band, and during step 200 in which the n receiving elements Rx receive backscattered signals, it is then necessary to differentiate the signals from the transmitting antennas at the level of each receiving antenna, in order to separate them during the phase of processing the received signals in order to virtually recreate the signal from a pair of transmitting / receiving antennas.

[0090] The choice of the waveform set is important for the development of a network synthetic antennas are used because it is necessary to identify and separate the transmission from each Tx antenna at the level of each of the receiving antennas. Therefore, a set of orthogonal waveforms is required for this purpose.

[0091] As a reminder, signals are said to be orthogonal when they are perpendicular to each other in complex space, which means that their dot product is zero. This orthogonality is used to separate signals backscattered by targets.

[0092] Orthogonality is created directly during the development of the emitted signals and is exploited during the digital processing of the backscattered signals.

[0093] For the emission step 100, several waveform generation methods exist, including

[0094] - The TDMA (Time Division Multiple Access) method which consists of activating the transmitters one after the other, or to simultaneously emit a single waveform (e.g. a chirp, i.e. a linear frequency ramp signal) with a time offset on each transmitter;

[0095] - The FDMA (Frequency Division Multiple Access) method which consists of transmitting on disjoint frequency supports;

[0096] - The CDMA (Code Division Multiple Access) method which consists of issuing phase codes having orthogonality properties between them;

[0097] - The DDMA (Doppler Division Multiple Access) method, which consists of shifting the frequencies of each transmitter vary from one source to another.

[0098] In order to limit sensitivity to ionospheric disturbances, the inventors opted for an elaboration of the emission waveforms based on the DDMA format, that is to say with simultaneous emission from each transmitter Tx at different frequencies and using chirps with a different phase shift at each draw.

[0099] The signals backscattered by a target, and therefore received during step 200 by the receiving antennas, are analog, and must therefore be digitized.

[0100] The digitization step 300 follows, which consists of converting the analog signals into signals usable by digital processing. The aim is to sample the signals at regular intervals and convert them into representative digital values. Each signal sample is represented as a complex sample composed of a real part and an imaginary part, which makes it possible to capture both the amplitude and the phase of the signal at each instant.

[0101] Next, the digitized signals are processed by the UTS processing unit, by means of a number of steps which consist of analyzing and extracting useful information from the signals received by the Rx receiving antennas.

[0102] These steps are represented in [Fig.3].

[0103] First, DDMA demodulation and Doppler filtering are performed, which consists of distributing the 32 signals from the receiving antennas, digitized as complex samples, over a Doppler frequency band typically corresponding to the target velocity. The waveform repetition rate on the surface-wave radar system can be, for example, 100 Hz, with a Doppler frequency band of 100 Hz. This frequency band contains 20 sub-bands of 5 Hz each.

[0104] This frequency band is the same for the 32 signals received by the Rx receiving antennas. On each of the Rx antennas, the presence is found in 18 of the 20 sub-frequency bands of 5 Hz, signals corresponding to the backscattering of signals emitted by each Tx transmitting antenna.

[0105] Each of the 32 input signals undergoes identical processing to divide the Doppler frequency band into 20 sub-bands. 18 of these sub-bands each contain a signal from one of the Tx transmitting antennas, each of these signals being centered on a different Doppler frequency, i.e., the frequency used by each transmitting antenna.

[0106] Doppler frequency filtering therefore makes it possible to eliminate data that does not contain a target.

[0107] At this stage of the processing we begin to perceive on each doppler / distance representation, the appearance of useful echoes, that is to say the echoes of the targets to be identified, but also the various disturbances due to the external environment (interference, ionospheric disturbances, sea clutter...).

[0108] This is followed by a pulse compression step in order to increase the distance resolution of the measurement as well as the signal-to-noise ratio.

[0109] The FMCW (Frequency Modulated Continuous Wave) continuous frequency modulation technique can be used in which the frequency of the emitted signal is modulated linearly in time.

[0110] This signal is then reflected by a target and received by the receiving antennas. The frequency difference between the emitted and received signals is used to determine the distance between the radar system and the target. Furthermore, by analyzing the phase change of the received signal, the target's velocity can also be measured.

[0111] Antennas used in the HF frequency band are generally weakly directional, meaning that they emit a non-negligible amount of energy in all directions, even the one in which they are not pointing. Furthermore, the transmitting antennas are not very far from the receiving antennas.

[0112] As a result, the receiving antennas also measure the signal called the "direct path signal" coming directly from the transmitting antennas.

[0113] This direct path can interfere with the backscattered signal from the targets, which can make their detection and precise position analysis difficult. It is therefore important to identify it.

[0114] The processing of the direct path also allows for monitoring the proper functioning of the transmitting and receiving antennas in real time.

[0115] Step 400 of separating the digitized signals is then carried out, leading to the formation of a synthetic network RR' of m*n receiver elements R'x. This step is a convolution operation between the discrete elements of the first network RT and the second network RR. This amounts to constructing a synthetic network of m*n virtual receiver elements Rx', each of which is obtained with the scalar products of the sum of the backscattered and received signals for the real receiver Rx considered, with the signal S,j backscattered by the real transmitter considered Tx.

[0116] In the example of Figures 1 and 2, the signal formulations are obtained The following are backbroadcasts from the virtual network:

[0117] ■ SR'x(l,6) SR'xfcô) SR'x(96,6) ' SR'x(l, j) SR'x(fc, j) SR'x(96, j) ■ SR'x(l,l) SR'x(^ 9 SR'x(96,l) ■

[0118] For the receiving subnetwork no. 1:

[0119] Sæx(k',j)=Srx(kk)<*> Six(ij)

[0120] with: - ke [1;8], where 8 is the number of Rx receptor elements in a receptor subnetwork SRR, - I [1 ;6], where 6 is the number of Tx transmitting elements in an SRT transmitting subnetwork, - i.e. [1;3], where 3 is the number of SRT transmitting subnets, k'=8.(il)+k

[0121] For any receiving subnetwork:

[0122] SR'x(k',j)=SR(u.k^^ STx(ij) - ke [1;8], where 8 is the number of Rx receptor elements in a receptor subnetwork SRR, - I [1 ;6], where 6 is the number of Tx transmitting elements in an SRT transmitting subnetwork, - i.e. [1;3], where 3 is the number of SRT transmitting subnets, - ue[l;4], where 4 is the number of SRR receiver subnetworks, - k'=8.(il)+k+(ul).(3x8), 3 corresponding to the number of subnetworks transmission and 8 the number of antennas in a receiving subnetwork.

[0123] This is followed by step 500 of spatial filtering, beamforming or pathforming (FFC), which is a signal processing technique used in antenna and sensor arrays for the directional transmission or reception of signals. This is achieved by combining the elements of a controlled antenna array The phase is designed so that in certain directions, signals interfere constructively while in other directions the interference is destructive. This method also improves the signal-to-noise ratio by sending all the energy in one direction or receiving all the energy from a specific direction.

[0124] With the surface wave radar system, using FFC, 241 beams can be created to cover a surveillance sector (±60°). Each beam will have an azimuth of approximately 0.5° and 7° in elevation. To also cover the elevation area, a minimum of 6 elevation beams are planned for each position. This results in a total of 241 beams to cover 120° in azimuth, multiplied by a minimum of 6 beams to cover 40° in elevation.

[0125] The Constant False Alarm Rate (CFAR) detection principle refers to an algorithm used to isolate the backscattered signal from a target from significant background noise, jamming or interference.

[0126] The role of the TF AC is to determine the power threshold above which each received signal can be considered as likely originating from a real target; the signal is then called a backscattered signal or radar echo. If the threshold is too low, more targets are detected, but the number of false alarms increases. Conversely, if the threshold is too high, there are fewer false alarms, but fewer likely targets are detected. In most cases, the threshold is chosen to be compatible either with a specific false alarm rate or with a time t between two false alarms.

[0127] Following the calculation of TFAC per averaged cell, we therefore obtain 576 signals on which the potential targets have been isolated from ambient noise and according to the tolerated false alarm rate.

[0128] Doppler processing makes it possible to detect and measure the speed of moving objects. The Doppler phenomenon occurs when the emitted wave is reflected off an object. In this case, the frequency of the signal reflected by a moving object varies very slightly compared to the frequency of the signal emitted by the radar. Doppler processing therefore consists of perceiving and analyzing these frequency changes to determine the speed of moving objects relative to the radar.

[0129] On each of the beams, targets are detected and isolated. The received signals corresponding to these targets are called radar echoes. The extraction function allows useful information to be extracted from the received radar echoes.

[0130] For each identified target, the Doppler frequency, phase, and amplitude information of the corresponding signal are retrieved. After calculation, this allows us to obtain information about the location of each target, namely:

[0131] - the angular position relative to the radar (azimuth / elevation);

[0132] - the distance from the radar;

[0133] - speed.

[0134] For each detection, this analysis will allow the definition of an elementary plot which corresponds to the position of the target at a time t. As the integration time (Ti) is 100s, each plot is dated with the mid-hour of the 100s block considered.

[0135] The purpose of tracking is to follow the movement of the markers over time. This makes it possible to determine the trajectory of the monitored objects and to estimate their acceleration and direction.

[0136] By combining the markers from several 100-second blocks, according to their respective positions, a track is formed. Since all targets present in a 100-second time block are dated with the same time, the tracks each have a different number to differentiate them.

[0137] In parallel, the surface wave radar system according to the invention also makes it possible to evaluate the state of sea clutter, the state of ionospheric clutter and external noise levels.

[0138] The level of the sea clutter uses the estimation of the root mean square height h of the waves, obtained with the empirical formula established by Maresca.

[0139] The sea state can be considered stable for about twenty minutes, so this assessment will be made based on distance and azimuth, on average doppler / distance images estimated over ten to twenty minutes.

[0140] Ionospheric clutter is found over distances between 100 and 400 km, at speeds that can be quite high. It is less stable than sea clutter, which is why its calculation is based on distance and azimuth, using average Doppler / distance images estimated over five minutes.

[0141] To evaluate it, it is sufficient to look for levels on the order of ten dB above the noise, for Dopplers of more than twice the frequency of the Bragg lines.

[0142] The external noise is obtained, for each integration time (Ti), from the Doppler / distance images. For each azimuth, the median of the squared amplitude of the image gives the average noise, disregarding high levels (targets, clutter). This noise level is accessible to the user via real-time visualization.

[0143] In summary, surface wave radar (SWR) systems according to the invention make it possible to:

[0144] - Measure wave height over a geographical area;

[0145] - To produce surface wind direction maps over a geographical area (drawing of vectors);

[0146] - Create surface wind speed maps in m / s over an area geographical;

[0147] - Identify and track targets.

[0148] Despite lower azimuth and range accuracy compared to other systems, the use of surface wave radar (SWR) systems according to the invention in an operational context offers numerous advantages:

[0149] - Detection and tracking of targets beyond the horizon,

[0150] - Large range;

[0151]

[0152]

[0153]

[0154]

[0155] - Doppler accuracy; - Ability to detect targets concealed by masks; - Suppression of stealth; - Ability to detect surface and aerial targets. Of course, the invention is not limited to the examples just described.

Claims

Demands

1. A method for detecting and tracking targets using a surface wave radar system comprising: - a first array (RT) of m transmitting elements (Tx) configured to emit signals propagating as surface waves at a wavelength X, said first array extending linearly along a first direction (DT), - a second network (RR) of n receiving elements (Rx), said second network extending linearly along a second direction (DR), concurrent with the first direction (DT), - a control unit (CU) of said system, - a signal processing unit (SPU) for said reflected signals, characterized in that the process comprises - a step (100) in which the m emitting elements (Tx) emit signals at a frequency in the HF frequency band, between 3 and 30 MHz, these signals being orthogonal to each other, - a step (200) in which the n receiving elements (Rx) receive backscattered signals, - a step (300) of digitizing the backscattered signals, - a step (400) for separating the digitized signals, leading to the formation of a synthetic network (RR') of at least m*n receiving elements (R'x), - a spatial filtering step (500) by computationally forming at least m*n beams, - a step (600) of extracting the detected plots from the data received by the synthetic network (RR') from the at least m*n receiving elements (R'x), said plots including the angular position, distance and speed of the possible target(s) relative to the surface wave radar system.

2. Method for detecting and tracking targets according to claim 1, characterized in that during step (100), the m elements Transmitters (Tx) emit signals at a frequency in the HF frequency band, between 5 and 30 MHz.

3. Method for detecting and tracking targets according to claim 1 or 2, characterized in that the second direction (DR) is perpendicular to the first direction (DT).

4. A method for detecting and tracking targets according to any one of the preceding claims, characterized in that the first array (RT) of emitting elements (Tx) comprises a plurality of at least two identical sub-arrays (SRT) of emitting elements (Tx), parallel to each other and each aligned along the first direction (DT).

5. A method for detecting and tracking targets according to claim 4, characterized in that the second array (RR) of receiving elements (Rx) comprises a plurality of at least two identical sub-arrays (SRR) of receiving elements (Rx) aligned on the same second direction (DR), the spacing (ESRR) between two adjacent sub-arrays (SRR) of receiving elements (Rx) being substantially equal to the width (1SRt) of the first array (RT) of transmitting elements (Tx), less half the wavelength of the emitted signals.

6. A method for detecting and tracking targets according to claim 5, characterized in that the spacing (ESRT) between two sub-arrays (SRT) of emitting elements (Tx) is substantially equal to the length (Lsrr) of the sub-arrays (SRR), plus half the wavelength of the emitted signals.

7. A method for detecting and tracking targets according to any one of the preceding claims, characterized in that the spacing between two adjacent emitting elements (Tx) is greater than or equal to the wavelength of the emitted signals.

8. A method for detecting and tracking targets according to any one of the preceding claims, characterized in that the spacing between two adjacent receiving elements (Rx) is greater than half the wavelength of the emitted signals.

9. A method for detecting and tracking targets according to any one of the preceding claims, characterized in that the emitting elements (Tx) are longitudinally radiating antennas.

10. A method for detecting and tracking targets according to any one of the preceding claims, characterized in that the elements Receivers (Rx) are antennas with a folded dipole in a vertical plane.

11. A method for detecting and tracking targets according to any one of the preceding claims, characterized in that the second direction (DR) along which the second array (RR) of receiving elements (Rx) extends is substantially close to the direction along which a coastline extends.

12. Method for detecting and tracking targets using a surface wave radar system, according to any one of the preceding claims, characterized in that the step (400) of separating the digitized signals is a convolution operation, a Doppler processing operation or a matrix multiplication operation, between the discrete elements of the first array (RT) and the second array (RR).

13. Method for detecting and tracking targets using a surface wave radar system, according to any one of the preceding claims, characterized in that the spatial filtering step (500) is carried out using a beamforming technique.

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