Investigation of the Ambiguity Function of a Special Kind of Sonar-Signals

PROBLEMLow Frequency Active Towed Array SONAR-Systems (LFTAS) should detect submarines at long distances.Therefore ping periods up to 1 . ..2 minutes are needed.Due to fluctuation effects of transmission and reflection target echoes will appear only irregularly.In shallow water reverberation corrupts the received signal up to long distances (e.g. 30 km).False targets are induced by bottom fixed discrete scatterers.True targets are mostly moving relative to these false targets.By reason of intermittent target contacts and bottom fixed false targets Doppler sensitive signals are needed.which allow to measure range and range-rate simultaneously from every single echo.Doppler-sensitive signals likethe Pseudo Random Noise (PN) signal or the CW signai exhibit a Doppler-resolution capability corresponding to the reciprocal of the signal duration T. For values of T typical for LFTAS this resolution may be too high.Modifying the time-frequency characteristic curve of a conventional LFM-chirp Signal such that the instantaneous frequency is proportional to itm (oz-sweep) will allow to control the Dopplersensitivity of this signal without being confined to the l/T limit mentioned above.White the Doppler-resolution will increase by varying the Exponent o: from a = 1 (for the LFM chirp) to a = 0 (for the CW signal) the range resolution will decrease.So. to choose an exponent a for a given set of signal parameters (centre frequency fa.frequency swing B and signal duration T) means to choose a compromise between rangt.L and Doppter resolution.To support the choice of a proper exponent a the behaviour of the width of the range and the Doppler-resolution cells as functions of the exponent a in dependence upon the signal parameters are examined.Experimental examples are given to demonstrate that the a-sweep enables simultaneous measurement of range and range-rate from a single echo.