Method for detecting mobile objects with active sonar
AssigneeThomson Marconi Sonar S.A.S.
InventorsYves Doisy, Pierre Metivier
Life of this patent
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Abstract
A process for making it possible to detect moving objects by an active sonar operating by the Dopper effect. The process uses, as a transmission signal, a burst of N pulses encoded so as to present a spectrum having a comb-of-lines structure. In this way, the “signal/reverberation” ratio of the useful signal intensity to the reverberated intensity is increased, thereby increasing the efficiency of the sonar. The process allows objects moving in a reverberating transmission medium to be detected more easily.
Claims13
A process for detecting moving objects by an active sonar moving at a velocity V, in which a signal of duration T is transmitted, which signal is reverberated by the transmission medium, having a spectral spread due to the actual speed of the sonar and this reverberated signal is processed by correlation with a set of frequency-shifted copies of the transmission signal in order to correspond to the set of Doppler shifts capable of affecting the reverberated signal, characterized in that the transmitted signal is broadband encoded in order to present a spectrum having a comb-of-lines structure at successive frequencies fi, the interval separating two successive lines fi and fi+1 of which is a function of the velocity V in order to be at least equal to the spectrum spread by satisfying the formula:
- ( 1 - 2 V c ) f i + 1 - a T ≥ ( 1 + 2 V c ) f i + a T
- where a is an integer between 1 and 2, c is the speed of acoustic waves in water and in that the transmission signal also comprises two pulses at pure frequencies fm and fM intended to make it possible to detect fast moving objects, the echoes from which are located beyond the frequency band occupied by the reverberated signal, both when receding and when approaching, these frequencies being given by the equations:
- ( 1 + 2 V c + 2 [LeftBracketingBar] V c max [RightBracketingBar] c ) f m + 2 T ≤ ( 1 - 2 V c ) f 1 - 2 T and ( 1 + 2 V c ) f 1 + 2 T ≤ ( 1 - 2 V c + 2 [LeftBracketingBar] v c max [RightBracketingBar] c ) f M - 2 T .
- where Vcmax is the maximum velocity of the target.
Dependencies are inferred from the claim text.