Beam sharpening of delay/Doppler altimeter data through Chirp Zeta Transform

The main goal of a satellite radar altimeter is to measure the height of the reflecting surface scanned by the passage of the instrument overhead. A delay/Doppler altimeter reduces the along-track footprint size by exploiting the coherence of the emitted pulses to synthetize a narrower antenna. Doppler beam formation is then a straightforward procedure that implies application of the discrete Fourier transform to time-domain bursts. Speckle and thermal noise on Doppler echoes are usually reduced using multilooking, i.e., summing incoherent contributions coming from different bursts. The summation of Doppler beams requires precise pointing toward output sample locations, particularly when highly variable topography is effective. However, the procedure of steering after beam sharpening needs a step of interpolation. In this paper, a Doppler beam sharpening and direction toward the sample position are presented which make use of the Chirp Zeta Transform to join the two operations together. This method must be compared with the existing ones to show that a little can be sacrificed to precision for computational gain. To support the discussion, experimental results will be compared with a precise-and so of reference-direct 2-D correlation algorithm over puntiform targets and more complex scenarios.