This type of system is generally towed from a surface vessel and comprises a fish equipped with an LF emitter which tows a linear receiving antenna furnished with acoustic or hydrophonic sensors. Such a fish and such an emitter are for example described respectively in French Patents published under numbers 2735645 and 2776161. However, the invention may be applied to all types of active sonars. It is well known that an active sonar emits recurrent acoustic pulses and that the echoes received in return are processed to detect and classify any targets.
When an active sonar operates in a zone such that the bottom is insonified, the reverberation which originates from the bottom in essence greatly limits the operational usefulness of the sonar on account of the overly large number of false alarms which appear. This is particularly true for shallow depths.
To reduce nuisance in a reverberating medium, it is known to use emission codes that harness the wide frequency bands, typically an octave, of present-day transducers. These codes possess good distance resolution, hence the large number of alarms that are produced.
It is known to emit at each recurrence, either an HFM (Hyperbolic Frequency Modulation) code, or a BPSK (Binary Pulse Shift Keying) code or an FP (Frequency Pulse) code.
The HFM code is Doppler tolerant: it therefore does not allow measurement of the Doppler induced by a target in motion but, on the other hand, the matched filtering on reception requires only a single copy.
The BPSK code is Doppler intolerant and is used to measure the Doppler; it allows the same detection performance as the HFM code but the matched filtering on reception requires a significant number of copies to carry out the matched filtering on reception, typically a number greater than 200, and hence a correspondingly large processing cost.