Nonlinear frequency modulation chirps for active sonar

Range sidelobe suppression in pulse-compression radars and sonars is conventionally performed using amplitude windowing in either the time or frequency domains. Using peak power limited transmitters, this inevitably causes a degradation in the signal-to-noise ratio available at the receiver output owing to shading or mismatch losses. Nonlinear frequency modulation (NLFM) chirps have been suggested as a solution to this problem and have been successfully implemented in a number of radar systems. Using appropriate NLFM functions it is possible to shape the energy spectrum of a chirp, achieving low-range sidelobes, without the need for any amplitude windowing and hence maintaining maximum efficiency. In the field of active sonar, NLFM chirps have been largely ignored because of their poor Doppler tolerance compared with conventional linear period modulation (LPM) chirps. It is shown that a combination of amplitude windowing and NLFM can be used to achieve a compromise between sidelobe suppression, Doppler tolerance and shading loss. Using a novel family of NLFM functions a significant performance advantage over LPM chirps can be attained over a useful range of target velocities. Experimental results support this argument.