A SOUTHERN-HEMISPHERE VERIFICATION FOR THE TOPEX POSEIDON SATELLITE ALTIMETER MISSION/

As a check on the total system accuracy of the TOPEX/POSEIDON satellit e, a southern hemisphere verification study was undertaken in Bass Str ait (41 degrees S) immediately adjacent to the Southern Ocean, An acou stic tide gauge and two pressure gauges were installed on the southern side of Bass Strait near where descending pass 88 crosses the Tasmani an coast, The tide gauge benchmark height was estimated relative to th e laser tracking station at Orroral Valley and the very long baseline interferometry station near Hobart using Global Positioning System sur veying techniques, In the TOPEX/POSEIDON geophysical data records, fla gs on the altimeter data began to be set 15-20 km from land and little useful altimeter data were available within about 13 km of the coast, The acoustic tide gauge heights were corrected for changes in tidal p hase and amplitude between the coast and the offshore subsatellite poi nt using results from a numerical model, The altimeter correction for atmospheric water vapor calculated from the TOPEX microwave radiometer data and the insitu precipitable water data agreed to 2 cm RMS, with no significant bias, The RMS variability between the insitu sea level estimates and the TOPEX/POSEIDON estimates was less than 3.5 cm, As pa rt of this variability is caused by inaccuracies in the estimates of t he sea level variations between the coast and the subsatellite point, the result indicates that the precision of the TOPEX/POSEIDON sea leve l height measurement system, including the time dependent orbit errors , the ionospheric corrections and the wet and dry tropospheric path de lays, is less than 3.5 cm over Bass Strait, considerably below the ori ginal system specifications of 13 cm. Our estimate of the altimeter bi as (including geographically correlated orbit error) is -25 +/-10 cm, consistent with the results at the NASA (Harvest Platform) and Centre National d'Etudes Spatiales (Lampedusa) calibration sites, However, ac curate estimation of this total altimeter bias is confounded by the la ck of altimeter data close to land and in particular, limitations in t he available local geoid data, The altimeter bias shows no significant drift at this site,