Process for detecting mobiles objects by means of passive underwater buoys
The invention is a process relating to the processing of the signals sent by passive buoys dropped from an aircraft so as to compile EGP (Energy Geographic Plot) maps. The process is split into three steps: the first makes it possible to produce an EGP map of the x,y positions of the noise sources, the second makes it possible to associate the velocities V x ,V y with certain designated positions and the third makes it possible to eliminate the noise sources regarded as hampering the readability of the maps.
1. A process for detecting signals coming from mobile objects and received by means of passive underwater buoys to obtain EGP maps, each buoy allowing the formation of N v directive pathways, wherein said signals received by said buoys are processed in the following steps:
a first step to obtain an EGP map of the x,y positions of said objects,
a second step to obtain an EGP map of the velocities Vx,Vy and an estimated spectrum for any object with a position x,y on said EGP map of the positions, said map exhibiting some Doppler ambiguity loci,
a third step to eliminate from the EGP map those of the objects regarded as strong sources of noise.
2. The process as claimed in claim 1 , wherein the first step includes producing an EGP map of the positions by an iteration of operations of detection of maxima applied on a set of the frequency channels constituting an analyzed band, each detection being obtained by comparison of a generalized likelihood ratio with a threshold, each detection being associated with a decisional summary containing the coordinates x,y, the signal-to-noise ratio SNR of each maximum detected the presence probability distribution for each maximum representing a possible target or noise source, and, for each buoy, its relative contribution to the maximum.
3. The process as claimed in claim 1 , wherein the second step include, for each detection on the EGP map of the positions emanating from the first step, in:
selecting the frequency channels that have participated in the detection by using said presence probability distribution datum, merging said data which correspond to these channels giving a global spectrum, deducing the object's spectra seen by each buoy by using relative contribution datum, grouping together homologous frequencies from spectrum-to-spectrum, calculating the average of these homologous frequencies thus giving a spectrum representing the spectral profile of the noise source supposedly placed at the maximum, calculating the differential Dopplers between the buoys on the basis of this spectrum, making it possible to obtain the velocities V x ,V y corresponding to this detection, iterating the procedure for all the detections and, at the end of the analyzed band, associating the velocities V x ,V y with the coordinates x,y so as to obtain an EGP map of the velocities.
4. The process as claimed in claim 3 , wherein the third step includes, for each noise source to be eliminated, said noise source being located on the EGP map of the positions, selecting again said frequency channels, estimating the power of the noise source so as to calculate the theoretic level of the signal originating from the noise source and in subtracting it from the actual level, and iterating this subtraction for each frequency channel so as, at the end of the analyzed band, to recommence the previous steps 1 and 2 again.
5. The process as claimed in claim 1 , wherein steps 1 and 2 are merged by iterating over the frequency channels, before performing the detection; the calculation of the differential Dopplers being taken into account in the detection criterion.