DIFFERENTIAL SEA-STATE BIAS - A CASE-STUDY USING TOPEX POSEIDON DATA/

We used selected data from the NASA altimeter on TOPEX/POSEIDON to cal culate differences in range measured by the C and K-u-band altimeters when the satellite overflew 5 to 15 m waves late at night. The range d ifference is due to free electrons in the ionosphere and to errors in sea-state bias. For the selected data the ionospheric influence on K-u range is less than 2 cm. Any difference in range over short horizonta l distances is due only to a small along-track variability of the iono sphere and to errors in calculating the differential sea-state bias. W e find that there is a barely detectable error in the bias in the geop hysical data records. The wave-induced error in the ionospheric correc tion is less than 0.2% of significant wave height. The equivalent erro r in differential range is less than 1% of wave height. Errors in the differential sea-state bias calculations appear to be small even for e xtreme wave heights that greatly exceed the conditions on which the bi as is based. The results indicate that the influence of waves on the i onospheric correction can be ignored. The results also improved our co nfidence in the sea-state bias correction used for calculating the geo physical data records. Any error in the correction must influence K-u and C-band ranges almost equally.