How is the surface elevation at each laser footprint determined?

In short, the elevation of the surface at each laser footprint is the height of the spacecraft minus the measured distance to the surface.

A standard parameterization is used to calculate surface elevation for ice sheets, oceans, and sea ice, using the elevation of the maximum peak and no more than two Gaussian functions with a minimum spacing of 30 ns (4.5 m) between Gaussian centers. For land elevations, the centroid of the return signal is used; a maximum of six Gaussians is allowed with 5 ns (75 cm) minimum spacing.

GLAS reports a mean elevation for each ice sheet surface measurement. When multiple peaks are present, the position of the centroid of the maximum peak determines the range. The maximum peak is used because detector saturation causes a small ringing peak that does not represent the surface. This range is corrected for atmospheric delay. The corrected range, satellite position above the ellipsoid, and off-nadir pointing angle are input for calculating surface elevation.

The increment in range between the last and first peaks allows users to recalculate the surface elevation based on the first peak. The number of peaks indicates the surface variability. The range to the centroid of the received waveform, the range from the beginning to end signals, and the skewness and kurtosis of the received waveform are also useful in evaluating surface characteristics.