Altimetry satellites are able to measure the distance between the satellite and the surface of the Earth. This distance is called range.
Altimetry satellites transmit a radar signal to the Earth. This signal is reflected by the Earth's surface and the satellite receives the reflected signal. The time elapsed between transmission and reception of the radar signal is the key parameter in calculating the distance between the satellite and the ground surface.
In Sentinel-3, the SRAL instrument measures this elapsed time.
Figure 1: Range, Altitude and Sea Surface Height Measurements
Precise orbit altitude is needed to calculate the range. The Sentinel-3 instruments, GNSS and DORIS, retrieve the orbit altitude. The orbit altitude is the distance between the satellite and an arbitrary reference surface (the reference ellipsoid or the geoid).
The scientific community is usually interested in the surface height in relation to this reference surface (the reference ellipsoid or the geoid) instead of being referenced to the position of the satellite. The surface height can be approximately derived from range and altitude using the following equation:
Surface Height = Altitude - Range
The complete calculation of surface height should also include all corrections due to environmental conditions. Examples of these corrections are atmospheric propagation corrections (ionosphere and troposphere) and geophysical corrections (tides and atmospheric pressure loading).