Radar signals must be corrected for path delays encountered in the ionosphere and troposphere. Oceanographic satellites have instruments which can measure the atmospheric effects on the signal path length. These instruments are not useful over the ice sheets, however, so meteorological models are used to correct the altimetric data. These models are usually undersampled in the polar regions.
The ionospheric refraction corrections on the level 2 data are calculated using the International Reference Ionosphere 1990 (IRI90) (Bilitza 1990) model.
The tropospheric refraction corrections on the level 2 data are calculated using the meteorological data generated by the European Center for Medium Range Weather Forecasts (ECMWF). The data used are temporally spaced at 2X daily and spatially at 2 deg. latitude by 2.5 deg. longitude.
The dry troposphere correction is calculated using the mean sea level pressure scaled up to the surface using a global 5 minute topographic grid as follows from:
P=surfp*(1.0-1.1138E-4*seaht) (Pa)
where:
seaht= height from 5 min. topography (m)
surfp= sea level pressure (Pa)
The wet troposphere corrections were calculated using the specific humidity and temperature grids at the 21 defined sigma levels. The method used was a numerical integration of the following integral:
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Curator: Jairo Santana NASA Official: Dr. H. Jay Zwally Last Updated: 12/11/2006 |