Saturation effects in the Seasat altimeter receiver

Some 10-15 percent of the Seasat altimeter returns over land and inland water, and a larger proportion of sea ice returns, are strongly peaked, in some cases resembling the instrument point target response. Many of these narrow-peaked pulses, from a wide variety of surface types, exhibit well-defined precursors. This paper demonstrates these precursors to be the result of instrument saturation. The response of the altimeter to narrow-peaked pulses is modelled theoretically. The consequences of saturation are shown to be well-defined pre- and post-cursors whose delay depends upon the location of the narrow-peaked component within the altimeter range window, and the clipping of high amplitude events. Seventy-one Seasat pulses exhibiting precursors are examined and their location is shown to coincide with that predicted. It is concluded that, whilst great care must be taken in attempting a quantitative interpretation of narrow-peaked waveforms, their arrival time, and hence measured range, is unaffected by instrument saturation.