This correction for the dry gas component of the atmosphere refraction takes into account the path delay in the radar return signal due to the atmosphere. It is by far the largest adjustment that must be applied to altimeter measurement (since its order of magnitude is about 2.3 m) but its temporal variations are low (a few centimetres only).
|fig 1. Amplitude in metres of the dry troposphere correction computed from the ECMWF atmospheric pressures and model during the Jason-1 cycle 223. This map is drawn using the BRAT from the Jason-1 GDR products. A Loess filter (value of 31) is applied to obtain a fully coloured plot.|
The dry tropospheric range correction in units of cm can be approximated by the Saastamoinen formula:
where Po is the sea level pressure in Pascal and j the latitude. Thus, the dry tropospheric range delay is proportional to sea level pressure and also depends on the latitude. A sea level pressure error of 5 mbar corresponds to a range error of about 1 cm.
Since it is not possible to estimate the sea level pressure from space, the dry troposphere correction must be obtained from meteorological models