## Science

## Science

The satellite emits a radar pulse with a spherical wavefront at
time T

_{T} which reflects from the surface and is received
at time T

_{R}. The range-to-surface or altimeter range
measurement is then calculated:

Range=c(T_{R}-T_{T})/2

where c = the speed of light

The altimeter product of primary interest to scientists is the
Altimeter range measurement,
which, when subtracted from a precise satellite orbit referenced
to a mathematical ellipsoid gives a measure of the surface
height above that reference ellipsoid. Subtracting the geoid height
which is a good approximation of sea level from the surface height
yields a surface height with respect to sea level.

The altimeter measures the range using an onboard
tracker,
This instrument receives and filters the return signal into time bins of varying resolution. The output from the filter, referred to as the waveform,
gives information on the surface characteristics within the range
window. The altimeter maintains acquisition by keeping the signal
within the range window. The tracker predicts the range by centering
the waveform at the pre-designated tracking gate. The onboard system
can predict precise ranges for the normally-distributed ocean
surfaces. Typical ocean waveforms
have a sharp ramp and slowly declining trailing edge, where the
mid-point of the ramp is centered at the tracking gate.

Over ice surfaces, the performance of the radar altimeters
differs significantly from the performance over oceans due to the
higher slopes of the ice surface, variations in the surface
reflection and penetration of the radar signal in the snow surface,
and generally irregular surface geometry. Waveforms over
sea ice and ice sheet
are very different than the typical ocean waveforms.
Information about the ice sheet surface properies can be obtained
by examination of the waveforms.
The range correction (typically a few meters) to account for
variations of the waveform shape and positioning of the waveform
in the window of range gates is obtained by a procedure known as
Retracking