1 Tracking method associating a passive radar with other sensors The invention concerns the general field of radar tracking and of merging detection information produced by a plurality of sensors of different kinds. It concerns in particular improving the tracking capabilities of a passive radar (RP) thanks to the use of tracking information obtained from external sensors (for example active radars). Multi-radar, or more generally multi-sensor, data merging strategies are based on merging detection information generated by each sensor in a cartesian frame of reference common to all the sensors. In principle, these strategies take no account of the bistatic character of the measurements provided when one of these sensors is a passive radar. They thus correspond to sub-optimum merging solutions for this type of sensor. Information delivered by different sensors may be merged either at the individual detection (merging of plots) level or at the tracking (merging of tracks) level. It is known that the first of these strategies is that which provides the better performance, at the cost of greater complexity than the second. Moreover, given that for a passive radar system to be able to deliver consistent cartesian tracks (i.e. tracks sufficiently pertinent to be merged with data coming from other sensors), it is necessary for the detected targets to be detected simultaneously at a plurality of the bistatic bases that constitute its configuration (ideally at least three bases), a track merging type approach runs the risk of proving detrimental. It should further be noted that the formation of tracks in a passive radar system is preceded by various processing steps aiming in particular to associate with each other radar plots (i.e. detections) formed on the basis of a plurality of bistatic bases. Such a configuration thus offers possibilities, not yet exploited, of merging detection information coming from different types of sensors in a manner different from that generally employed. The present invention aims to define a merger scheme making best possible use of the tracking internal architecture of a passive radar to incorporate therein exogenous information (coming from other sensors) likely to improve tracking quality and/or to accelerate tracking. 2 One object of the invention is to propose a solution enabling merging of multi-sensor data in the most efficacious manner possible in the particular situation where one of the sensors is a passive radar system. In other words, the object of the invention is to improve the rendition of an airborne situation by merging optimally detection of targets by a passive radar and those coming from other sensors. To this end the invention consists in a data processing system for merging detection information produced by a passive radar system with detection information coming from an external sensor, the passive radar system including a set of bistatic bases, each bistatic base being defined by a receiver and an opportunistic transmitter, the transmitter including a plurality of transmission channels to which correspond reception channels in the receiver, the receiver producing a bistatic plot, or RP bistatic plot, for each detection produced for a given channel, and transmitting these plots to the system, the RP bistatic plots transmitted being grouped as a function of the channel in which the corresponding detection was produced, the external sensor transmitting to the system temporally chained plots, forming external tracks or SE tracks, assigned an identifier. The method of the invention includes at least: - first SE tracking means for transforming each SE plot delivered by the external sensor into a set of bistatic plots synchronized to the measurement times of the passive radar RP, or SE bistatic plots, each SE bistatic plot being associated with a given bistatic base, and into an SE cartesian plot; - merging means for merging, for each of the bistatic bases concerned, the RP bistatic plots produced for the various channels relating to that base and the SE bistatic plots associated with that base to form merged plots, a merged plot being assigned a set of attributes determined by the nature of the RP and/or SE bistatic plots merged to form the plot concerned; - second tracking means for producing, for each bistatic base, bistatic tracks of different types by temporal chaining of merged plots, a bistatic track being assigned a set of attributes determined by the nature of the latest merged plot to be associated with it; the merged plots used to fomri a bistatic 3 track being assigned an attribute indicating that they have been used to this effect; -cartesian third tracking means for producing, for all bistatic bases formed on the basis of the same receiver, RP or RP+SE cartesian tracks and SE cartesian tracks, these cartesian tracks being produced on the basis of the RP merged plots produced by the second bistatic tracking means and the SE cartesian plots produced by the SE first tracking means, a cartesian track being assigned a set of attributes determined by the nature of the plots, RP and/or RP+SE merged plots and SE cartesian plots, that contribute to its formation and its maintenance. In one embodiment of the method of the invention the SE tracking means are configured to transform each SE plot delivered by the external sensor into a set of bistatic plots or SE bistatic plots synchronized to the measurement times of the passive radar RP, the coordinates of each SE bistatic plot being obtained by projecting the coordinates associated with the SE plot concerned into the frame of reference associated with the bistatic base concerned. In one embodiment of the method of the invention the SE tracking means are further configured to produce, from the plots produced by the external sensor, SE cartesian plots the attributes of which are defined relative to a given cartesian frame of reference, each SE cartesian plot being assigned the identifier of the SE track to which the SE plot at the origin of the SE cartesian plot concerns contributed. In one embodiment of the method of the invention the merging means produce and transmit to the bistatic tracking means merged bistatic plots divided into three categories: - RP merged plots constituted only on the basis of RP bistatic plots produced on the basis of each channel of the radar system, the attributes of each RP merged plot being determined on the basis of the attributes of the RP that gave rise to it; - RP+SE merged plots constituted on the basis of RP bistatic plots and an SE bistatic plot, the attributes of each RP+SE merged plot being 4 determined from the attributes of the RP bistatic plots and SE bistatic plots that gave rise to it; - SE bistatic plots that have not been merged being transmitted as they stand. In one embodiment of the method of the invention the bistatic tracking means are configured to produce bistatic tracks divided into two categories: - RP bistatic tracks produced only on the basis of RP merged plots, each RP bistatic track being constituted by all RP merged plots used to initialize it and maintain it; - RP+SE bistatic tracks produced on the basis of RP merged plots and SE merged plots, each RP+SE bistatic track being constituted by all the +SE merged plots used to initialize and maintain it. The bistatic tracking means are also configured to identify merged plots not used to produce bistatic tracks. In one embodiment of the method of the invention the cartesian tracking means include: - means for updating the cartesian tracks already formed by attempting to associate those tracks with the plots transmitted by the bistatic tracking means and with SE cartesian plots (19) transmitted by the SE tracking means; - means for initializing RP cartesian tracks and initializing RP+SE and SE cartesian tracks, this initialization being effected on the basis of plots not associated by the preceding association means, - means for maintaining RP and RP+SE cartesian tracks already formed; - means for maintaining SE cartesian tracks already formed; the means for maintaining RP and RP+SE cartesian tracks already formed including means that merge RP and RP+SE tracks already formed and new RP and RP+SE tracks provided by the initialization means. According to the invention, the cartesian tracks are assigned a cartesian identifier, the cartesian tracking means include means that assign an identifier to each new cartesian track initialized and that manage over time 5 the list of identifiers already assigned by reassigning identifiers assigned to tracks that have been eliminated to new tracks that have been initialized. In one embodiment of the method of the invention the cartesian tracking means further include means for testing the consistency of the RP+SE cartesian tracks formed, these means determining the likelihood of the associations effected and delivering three groups of tracks: - RP+SE cartesian tracks for which consistency is established, - RP or SE requalified cartesian tracks, these requalified tracks being derived from RP+SE cartesian tracks for which consistency has not been established. In a preferred embodiment the consistency test consists in comparing the current position associated with the RP+SE track concerned and the result of a location operation effected on the basis of the bistatic distances of the RP merged plots associated with that track. The features and advantages of the invention will be better appreciated thanks to the following description, which discloses the invention via a particular embodiment taken by way of nonlimiting example and refers to the appended figures. Figure 1 is a general block diagram showing the various means constituting the system of the invention and the data exchanged by the various means. Figure 2 is a block diagram showing the data exchanged by the means responsible for tracking plots transmitted to the system by an external sensor. Figure 3 is a block diagram of the processing effected by the means responsible for tracking plots transmitted to the system by an external sensor. Figure 4 is a block diagram showing the data exchanged by the means responsible for merging bistatic plots originating from the various channels of the same bistatic base and the means responsible for effecting bistatic tracking on the basis of these merged bistatic plots. 6 Figure 5 is a blocl< diagram showing the processing effected by the means responsible for constituting cartesian tracks from merged bistatic plots and formed bistatic tracks. The following description explains the general structure of the system of the invention for a surveillance system, taken by way of nonlimiting example, including a passive radar (RP), also known as a passive coherent locator (PCL), constituted of a receiver and using a plurality of opportunistic transmitters, and a complementary sensor, or external sensor (SE). Thus a passive radar system may be associated with an automatic dependant surveillance broadcast (ADSB) technology, a primary radar, a secondary radar or a lidar, for example, configured to deliver tracks in a standard geographical frame of reference of the World Geodetic System 1984 (WGS84) type. In the embodiment described, each opportunistic transmitter transmits signals on a plurality of frequency channels, each channel corresponding to a defined band of frequencies. The passive radar system (RP), here composed of a single receiver, is associated with a single external sensor. However, it is of course possible to design a system of the invention suited to a more restricted set-up in which each opportunistic transmitter transmits on only one channel; similarly, it is also possible to design a larger system adapted to a set-up including a multi-receiver passive radar and a plurality of external sensors. The scope of the invention is thus not limited to the generic structure explained by way of example. As shown in figure 1, the system 11 of the invention includes a plurality of processing channels 12 each including means 13 for merging bistatic plots and bistatic tracking means 14. Each of the processing channels 12 delivers the results of the processing carried out to one of the global tracking means 15 or cartesian tracking means, the role of which is to construct and to maintain cartesian tracks on the basis of merged bistatic plots produced by each of the processing channels. According to the invention, these cartesian tracks are defined in a common cartesian frame of reference, for example a frame of reference centered at the position of the receiver of the passive radar (RP) system. 7 According to the invention, the various functions executed by the various processing channels and those effected by the cartesian tracking means may be effected by any appropriate l