glaces
fig 1. a waveform over ice, and the parameters that can be extracted from it.
(Credits LEGOS/CNRS)

The echo waveform over ice enable to retrieve several parameters:

  • epoch at mid-height: this gives the time delay of the expected return of the radar pulse (estimated by the tracker algorithm), and thus the time the radar pulse took to travel the satellite-surface distance (or ‘range’) and back again.
  • backscatter coefficient, sigma0.
  • leading edge amplitude
  • leading edge width: this is related to the penetration into the medium and the surface roughness of the target.
  • trailing edge slope: this gives information on antenna mispointing, and also on the signal penetration into the medium.

All altimeter data for ice sheets must be post-processed to produce accurate surface elevation measurements. This post-processing is called ‘retracking’ and is required because the leading edge of the ice-sheet return waveform deviates from the on-board altimeter tracking gate, causing an error in the telemetered range measurement.