The wet troposphere correction is the correction for the path delay in the radar return signal due to cloud liquid water and water vapour in the atmosphere. It is calculated from radiometer measurements and meteorological models.
Its order of magnitude is about 0 to 50 cm with an annual cycle amplitude of up to 20 cm.
It is usually computed over oceans with simultaneous radiometric measurements (see the Jason-1 Microwave Radiometer (JMR)). However, such radiometric measurements generally fail over land or near the coasts and may be superseded by a correction computed from meteorological model outputs (GDP+, NCEP or ECMWF models).
In addition to its limited geographical coverage, this correction suffers from the fact that the altitude of the water body is not taken into account: the atmosphere column reaches the sea level everywhere and is thus too thick!
|fig 1. Location of the global set of GNSS coastal and island stations (black dots) used in the GPD+( GNSS-derived path delay Plus algorithms) computations. The background picture is the map of the standard deviation of the wet tropospheric correction, in metres, computed from two years of ECMWF model fields. (Credits University of Porto)|
An error of 10 m in the thickness of the atmosphere column corresponds to an error of 1 mm in the correction. Consequently, the correction is often overestimated for non-ocean areas. When the surface altitude with a DEM (Digital Elevation Model) is taken into account in the meteorological models, the correction is more accurate.
Therefore, the precise estimation of the altitude of the reflecting continental surface appears as the key factor for an accurate computation of the propagation delay through the troposphere. Some projects are underway to improve this wet troposphere correction near the coasts and over land using the altitude value deduced from the altimetric measurement itself.
- Desportes, C., Obligis, E., Eymard, L., On the wet tropospheric correction for altimetry in coastal regions, in Geosc. and Remote Sensing, Vol.45, Issue 7, Part 1,2139-2149, 2007.
- Mercier, F., Zanife, O.Z, Improvement of the Topex/Poseidon altimetric data processing for hydrological purposes, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006.