1 - TECHNICAL FIELD OF THE INVENTION

The object of the present invention is a method for the detection of marine targets. It also has as an object a device suited to implement such a detection method. The essential aim of the present invention is to improve the existing maritime detection systems, in particular by improving on the one hand the capacities for detection of non-cooperative ships, and on the other hand the detection of maritime targets - i.e. boats of all kinds - at greater distances than the current detection distances.

The field of the invention is, in general terms, that of maritime surveillance. In this field, efforts are made to know with precision the status of the maritime traffic, by seeking to register, at each moment, the different boats which are sailing; the registering of the boats at sea is advantageously supplemented by various specific pieces of information concerning each boat, information concerning in particular the type of boat, their course, their speed, their port of arrival and of departure...

To this end, various tools are available to register all of the boats sailing in a given zone.

Thus, various databases are constituted by means of various sources of information; thus, there are databases containing satellite images, military databases, databases providing information on the statuses of entry into port and of leaving port for all of the declared ships, and databases originating from the LRAIS system (Long Range Identification System) . The LRAIS system makes provision that ships are equipped with a transponder suited to constantly communicate the position of the said ship to a terrestrial monitoring system via a satellite; however, quite obviously, the so- called non-cooperative ships (clandestine fishing boats, boats of traffickers...) are not equipped with such transponders or do not activate them, and are therefore not detectable by such a system. Furthermore, the range of the conventional AIS transponders, due to the variable attenuation of the signals propagating on the surface of the sea, are highly noised, and the detection range of the ships by the LRAIS system is consequently greatly reduced, the radio waves which are brought into play being able to reach a maximum of about fifty kilometres.

A satisfactory solution for registering all the boats at sea at a given moment would be a surveillance system by aircraft or by satellite. However, it is economically unrealistic to envisage aircraft, dedicated to the surveillance of boats, flying permanently over maritime zones; and if the surveillance system by satellite is efficient when the ships are in the field of detection of a satellite under consideration, one is also faced with the problem that the maritime zones are not permanently covered by a satellite scanning.

2 - TECHNOLOGICAL BACKGROUND OF THE INVENTION

It has been recently proposed, furthermore, to use radars of the high frequency (HF) radar type to register all the ships which are at sea at a given moment. These radars are characterized essentially by their emission frequency which is typically comprised between 3 megahertz (MHz) and 30 MHz. They can proceed to a permanent surveillance of maritime zones up to 200 nautical km and over an arc of 110° and are disposed in the immediate proximity of the sea. However, such radars are also faced with certain limitations: the waves which they emit propagate on the surface of the sea and they are thus faced with a clutter phenomenon, also termed sea clutter. This phenomenon designates a radar echo observed on the surface of the sea, with the waves being reflected on the sea waves on the surface of the sea and causing a parasite echo. The HF radars thus encounter difficulties in the detection of targets induced by the noise of the sea, these difficulties being all the greater, the smaller the size and the lower the speed of the targets.

3/4 - Aim of the invention / Background of the claims:

The present invention remedies the disadvantages set forth above by a method for the detection of maritime targets, the said method being characterized in that it is implemented on a coastal zone equipped with at least one high frequency radar suited to emit and receive signals for the detection of maritime targets and in that it comprises the following different stages consisting, in the at least one high frequency radar, in:

- receiving and interpreting information relating to a transmission channel status of detection signals in a considered direction; correcting, by means of information relating to the status of the transmission channel, the detection signals received, in the said considered direction, by the at least one high frequency radar, to obtain improved detection signals; and interpreting the improved detection signals to establish, if applicable, the detection of a maritime target . Apart from the main characteristics which have just been mentioned in the previous paragraph, the method according to the invention can present one or more complementary characteristics among the following: the reception and interpretation stage of the information relating to the status of a transmission channel comprises the following different operations consisting in:

- equipping at least one transmission buoy with an emitter, in particular a high frequency emitter; emitting control signals, the characteristics of which are previously determined, from the at least one buoy to a device for the reception and interpretation of the said control signals, associated with the at least one high frequency radar; and determining, from the control signals received by the reception and interpretation device, a buoy filter; and in that the correction stage of the received detection signals comprises the operation consisting in applying the buoy filter to the received detection signals to obtain the improved detection signals. the device for reception and interpretation of the control signals is integrated in the at least one high frequency radar. the method comprises the supplementary operation of equipping the at least one transmission buoy with solar panels to supply the equipped buoy electrically. a functioning of the at least of transmission buoy relative to emissions of the control signals is managed by at least one satellite. the at least one satellite manages the emissions of the control signals from the at least one transmission buoy by asynchronous alternation with the control signals of other transmission buoys. the method comprises the supplementary stages in the at least one high frequency radar consisting in constituting a database with the information relating to at least one transmission channel status of the detection signals; updating the database as soon as a new piece of information relating to the at least one transmission channel is available; for the at least one transmission channel under consideration, using the last information relating to the status of the said channel stored in the database to correct the received detection signals. the method comprises, during the correction stage of the received detection signals, the operation consisting in applying an oceanographic filter on the received detection signals, the said oceanographic filter making a correction, depending on an instantaneous status of the sea, to the received detection signals. the method comprises, during the correction stage of the received detection signals, the operation consisting in applying an ionospheric filter on the received detection signals, the said ionospheric filter making a correction, depending on an instantaneous status of the atmosphere, to the received detection signals. the received detection signals are successively processed by the buoy filter, the oceanographic filter and the ionospheric filter according to a specific order, in at least one correction chain. the method comprises the supplementary operations consisting in: correcting the received detection signals in several correction chains by applying to the received detection signals successively the buoy filter, the oceanographic filter and the ionospheric filter, the order of application of the three filters being different from one correction chain to the other; and comparing the corrected detection signals originating from the correction chains to determine the improved detection signals. the correction operations of the received detection signals are implemented at least in a first high frequency radar and in a second high frequency radar, the first high frequency radar and the second high frequency radar generating respectively first improved detection signals and second improved detection signals, the first improved detection signals and the second improved detection signals being compared in a fusion unit to determine navigation characteristics of the maritime target under consideration. the first high frequency radar and the second high frequency radar are spaced apart at a distance comprised between 50 kilometres and 100 kilometres. the different high frequency radars are managed by a common management system determining in particular the waveforms, and/or the impulse moments and/or the impulse durations and/or the frequencies of the detection signals of each high frequency radar. the method comprises a supplementary validation stage by satellite image, consisting in comparing the detection of a maritime target established following the interpretation of the improved detection signals, and an item of information received from a satellite according to which a corresponding maritime target is in actual fact present, to validate, if applicable, the detection of the said maritime target.

The present invention also relates to a device for the detection of maritime targets suited to implement the method according to the invention.

The invention and its various applications will be better understood on reading the following description and on examining the figures which accompany it.

BRIEF DESCRIPTION OF THE FIGURES

These are only presented as an indication and not all in a restrictive manner with regard to the invention. The figures show: in Figure 1, a diagrammatic representation of different elements occurring in an example implementation of the method according to the invention; and in Figure 2, a diagrammatic representation of the processing of the signals received by the HF radars on an example implementation of the method according to the invention.

5 - DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF

THE INVENTION

Figures 1 and 2 are described together.

In Figure 1 there are represented diagrammatically a coastal zone 101, bordered by a maritime zone 102 in which a maritime target 103 is sailing, of the ship type which is not equipped with a transponder functioning according to the LRAIS system. In the example which is illustrated, the coastal zone is equipped with a first HF radar 111 and with a second HF radar 112, suited to emit towards, and to receive from the maritime zone 102 detection signals Sd. The coastal zone can be equipped with more than two high frequency radars.

If no processing is applied to the detection signals Sd returning towards the HF radars 111 and 112, the signal-to- noise ratio S/N is very unfavourable and does not allow the presence of the maritime target 103 to be determined with certainty. So as to suppress a large portion of the noise received by the HF radars, a plurality of transmission buoys 104 are disposed on the maritime zone 102 between 100 km and 360 km. The buoys 104 are all equipped with a HF emitter 105, suited to emit control signals Oc towards a reception and interpretation device 106 associated with a HF radar. In the example which is represented, a reception and interpretation device 106 is present within each HF radar 111 and 112. Advantageously, the buoys 104 are equipped with solar panels which are suited to generate the electrical energy, from solar energy, which the buoys need in order to function. The buoys are arranged to constitute a grid. This grid is a function of the variability of the status of the sea and of the level of precision which one wishes to obtain. The maximum of grids being that which is fixed by the number of cell distances of the radar processing and the minimum by the variability of the marine currents and by the status of the sea on the zone covered by the radar.

The functioning of the transmission buoys 104 is advantageously managed by one or more satellites 107 which emit signals towards radio modules included in the buoys 104. The satellites 107 thus command the emission sequences of the HF emitters 105 of the buoys 104, determining in particular the emission moments of the HF emitters 105 and the frequency of the emitted signals. Advantageously, the emissions of the control signals Oc by the buoys are realized in an asynchronous manner, with two buoys situated in the same geographical zone, typically extended over about a hundred kilometres, not emitting simultaneously.

Thus, when one of the emitters 105 emits a control wave Oc towards the reception and interpretation device 106, the latter knows, by having previously received this information by the satellites 107, which is the waveform of the expected control wave Oc. By comparing the control wave which is actually received and the expected control wave, the device 106 measures a distortion of the signal under consideration and deduces therefrom information relating to the transmission channel which is used. Generally, the expression "transmission channel" is used to designate the physical space crossed by waves between an emitter, in particular the one situated on a buoy, and a receiver, in particular the one present at the level of a HF radar.

A database 108 is thus constituted with all the information relating to the status of the transmission channels between the different emitters and receivers. The database 108 is advantageously stored within each high frequency radar 111 and 112, and is updated as soon as information relating to the status of the transmission channels is available. This information allows a first filter Fl to be formed, designated a buoy filter, which is used to correct the detection signals received by the high frequency radars.

Thus, when a HF radar receives a high frequency detection signal Sd reflected by a potential maritime target, the filter Fl is applied to the detection signal which is received to obtain a corrected detection signal. The filter Fl is dependent on the direction of origin of the received detection signal by searching in the database 108 the information corresponding to the closest possible transmission channel to the direction of origin of the detection signal. Advantageously, the last stored information is used and is relative to the transmission channel under consideration.

By applying the buoy filter Fl, a large portion of the noise present in the transmission channel, by which the detection signal Sd is received, is eliminated. To further improve the quality of the detection signal received by the HF radars, one or more complementary filters advantageously become involved in the method according to the invention.

Thus, for example, an oceanographic filter F2 is determined. Such a filter is formed from oceanographic prediction tools which allow the status of the sea to be characterized (surface currents, sea wave height, length of repetition wave of the sea wave, speed and orientation of the winds between the sea level and an altitude close to about thirty metres...) at a given instant. The application of the filter F2 on received detection signals allows the precision of the received detection signal to be improved, by eliminating a noise due to the characteristics of the sea . In the same way, in an advantageous form of implementation of the invention, an ionospheric filter F3 is determined. Such a filter F3 is formed from ionospheric sampling tools which measure the noise coming from the ionosphere in the frequency band of the emitter of the HF radar. The application of the filter F3 on received detection signals allows the signal-to-noise ratio of the received detection signal to be improved, by eliminating a noise due to the characteristics of the air passed through by the detection signals .

Advantageously, in the invention, the detection signals received by each HF radar are processed in a correction chain, or filtering chain, constituted by three filters such as the buoy filter Fl, the oceanographic filter F2 and the ionospheric filter F3. In a correction chain, the signals Sd are applied successively to the different filters of the chain. Each HF radar can comprise several correction chains which are distinct from each other according to the successive order of application of the filters on the signals Sd, the order being different from one correction chain to the other. In the example illustrated in Figure 2, for each HF radar under consideration, three correction chains, referenced respectively 211, 212 and 213 for the HF radar 111, and 221, 222 and 223 for the HF radar 112 comprise the three filters Fl, F2 and F3 and process the received signals Sd concomitantly. Thus for the HF radar 111, the signals Sd are processed at the same time in the correction chain 211 successively by the filters Fl, F2 and F3; in the correction chain 212 successively by the filters F3, Fl and F2; and in the correction chain 213 successively by the filters F2, F3 and Fl. A similar correction is applied to the detection signals Sd received by the HF radar 112. As the order of convolution of the filters is not commutative in the mathematical sense, it is important to be able to test the result coming from the combination of the three filters. Out of each correction chain i (i varying from 1 to 3), a corrected detection signal Sd _± is obtained. The different corrected detection signals Sdl, Sd2 and Sd3 originating respectively from the different correction chains of a HF radar are thus compared in a selection unit 231 of the HF radar under consideration. The selection unit 231 determines the most probable corrected detection signal, considering known signal processing calculation rules. The most probable corrected detection signal therefore corresponds to an improved detection signal Sda. The HF radar 111 thus provides a signal Sdal, and the HF radar 112 in turn provides a signal Sda2.

An evaluation unit 232 of the improved detection signal then determines whether the improved detection signal can be interpreted as revealing the presence of a maritime target by the direct evaluation of the signal-to-noise ratio of the target. For each detection signal revealing the presence of a marine target, the unit 232 determines a path (PI; P2) comprising a speed and a course relative to the revealed marine target. This path is transmitted to a fusion unit 233 connected to one or more HF radars to validate the presence of a maritime target.

In a particular form of implementation, the fusion unit 233 verifies that several radars detect the same target by fusing the paths transmitted from the different radars into a single path selected according to the precision of its speed and its associated course. According to a variant embodiment, the fusion of the paths is made by weighting the different speeds and associated courses. The unit 233 validates the obtained detection of a maritime target by comparing the received paths (PI; P2) or the fused path to information received from a satellite (107) according to which a corresponding maritime target is or was actually present at a precise location. According to a variant, the detection information of maritime targets coming from the satellites 107 is also used to adapt and/or modify the waveforms of the HF radars according to the detection results obtained by considering the available satellite images. Indeed, with the presence of a detected maritime target giving access to the size of the boat, its course and its speed allow the information received from the radars to be correlated. In the absence of HF radar detection, a manual analysis of the data of the radars is carried out to draw improvements therefrom on the different parameters of the radar (frequency, PRF, integration time, width of the signal, waveform) .

The fusion unit 233 comprises or is connected to a monitoring system of the LRAIS type so to determine whether the detected ship comprises an activated transponder suited to constantly communicate the position of the ship. If the ship is detected without an associated transponder, it is considered to be non-cooperative by the operators of the system.

Advantageously, in the invention, when several HF radars are implemented, they are managed by a single managing system which determines the characteristics of the detection signals of the radars so as to limit the interference between them. The common managing system determines in particular the waveforms, and/or the impulse moments and/or the impulse durations and/or the frequencies of the detection signals (Sd) of each high frequency radar. Thus, advantageously, two adjacent HF radars - the average distance between two radars being between 50 and 100 kilometres - do not emit simultaneously.

The device according to the invention is constituted by the elements necessary for implementing the method according to the invention with, according to various embodiments, in particular one or more HF radars 111, 112, a fusion unit 233 between the HF radars; the transmission buoys 104; and one or more satellites 107. Each HF radar in the device comprises one or more correction chains including one or different filters, such as the buoy filter Fl, the oceanographic filter F2 and the ionospheric filter F3, a selection unit 231 and an evaluation unit 223.