fig 1. Envisat’s ‘ICE2’ retracker: Continental ice retracking can also be used successfully on continental surfaces

The continental lands are composed of a variety of surfaces which reflect the signal back with different intensities depending on each surface properties. The altimeter footprint is frequently composed of and contaminated by a multiplicity of surfaces. Thus, waveforms on these surfaces include a wide variety of configurations which are difficult to classify and process. Altimeter data over land must be post-processed in another way that the Brown model because the leading edge of the terrain return waveform deviates from the on-board altimeter tracking gate, causing a significant error in the telemetered range measurement.

Analyses from waveforms over heterogeneous surfaces enable to retrieve several parameters and deduce other interesting characteristics.

Thus, the backscatter coefficient (sigma0) is used to characterize the surface: a low value for mountainous regions and a high value on flat surfaces or wetlands. The backscatter coefficient can also be related to the dates when a surface is completely frozen when the ice breaks up and fast-ice duration. The leading edge width is related to the penetration into the medium and the surface roughness of the target.

fig 2. Jason-2 waveforms on Amazon river: specular waveform (n°319) result of the return signal from very reflective surfaces like water bodies, multipeaked waveforms result of heterogeneous targets in the footprint.

The parameters extracted from the waveforms over continental surfaces are similar to the ones for ice:

  • 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 (or sigma0): gives information on the nature of the surface.
  • 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.

For Ku and S band, the backscatter is low in mountainous regions (e.g. e.g. <7 dB in Ku band, <14 dB in S band) as a direct result of the presence of topographic slopes. For both bands, the backscatter values are high on very flat surfaces, such as deserts, large river basins or wetlands (e.g. >15 dB in Ku band and >20 dB in S band), due to the specularity of the return radar echo.

The leading edge width values are high in desert areas due to the strong penetration of the wave and the dunes generated by the winds. Low values, related to weak penetration, correspond to dense vegetated areas, such as tropical or boreal forests, or to large river basins or flooded regions. In contrast to the backscatter coefficient, the use of only one frequency gives a characteristic signature on continental surfaces, providing good discrimination of forests, deserts, etc.

fig 3. Examples of real waveforms (from the CryoSat-2 SIRAL altimeter), over the isle of Cuba.(Credits ESA)