A southern hemisphere verification for the TOPEX/POSEIDON satellite altimeter mission

Abstract

As a check on the total system accuracy of the TOPEX/POSEIDON satellite, a southern hemisphere verification study was undertaken in Bass Strait (41°S) immediately adjacent to the Southern Ocean. An acoustic tide gauge and two pressure gauges were installed on the southern side of Bass Strait near where descending pass 88 crosses the Tasmanian coast. The tide gauge benchmark height was estimated relative to the laser tracking station at Orroral Valley and the very long baseline interferometry station near Hobart using Global Positioning System surveying techniques. In the TOPEX/POSEIDON geophysical data records, flags 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 phase and amplitude between the coast and the offshore subsatellite point 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 part of this variability is caused by inaccuracies in the estimates of the sea level variations between the coast and the subsatellite point, the result indicates that the precision of the TOPEX/POSEIDON sea level height measurement system, including the time dependent orbit errors, the ionospheric corrections and the wet and dry tropospheric path delays, is less than 3.5 cm over Bass Strait, considerably below the original system specifications of 13 cm. Our estimate of the altimeter bias (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, accurate estimation of this total altimeter bias is confounded by the lack of altimeter data close to land and in particular, limitations in the available local geoid data. The altimeter bias shows no significant drift at this site.

The Harvest Experiment: Calibration of the Climate Data Record from TOPEX/Poseidon, Jason-1 and the Ocean Surface Topography Mission

“…Data from dedicated Jason calibration sites at Corsica (Bonnefond et al 2003) and Bass Strait (Watson et al 2003) showed similar behavior. Ironically, these initial estimates of the Jason-1 SSH bias nearly matched early estimates of the TOPEX SSH bias Born et al 1994;Ménard et al 1994;White et al 1994). More contemporary estimates of the Jason-1 SSH bias are roughly 40 mm smaller (e.g., Willis 2009;Bonnefond et al 2010), owing primarily to evolutions in the altimetric data products.…”

“…With data from additional overflights and improved GPS-based determinations of the platform geocentric height and velocity, Haines et al (1996) reported a TOPEX bias of 125 ± 20 mm at the conclusion of the three-year primary mission. Also identified in early results from other dedicated calibration sites (Ménard et al 1994;White et al 1994;Murphy et al 1996), the bias was determined to be the expression of an error in the software used to produce the TOPEX data for the mission scientists. (O. Zanife, P. Vincent, and P. Escudier of CNES are credited with discovering the software error.)…”

Tidal corrections for TOPEX altimetry in the Coral Sea and Great Barrier Reef Lagoon: Comparisons with long‐term tide gauge records

“…Numerous studies have compared tidally corrected SSH data with tide gauge records [e.g., Mitchum , 1994; Nerem et al ,1994; Le Provost , 1994; Koblinsky , 1999]. Actual calibration studies [ Born et al , 1994; Christensen et al , 1994; Ménard et al , 1994; White et al , 1996; Exertier et al , 2000] seek to compare absolute levels from TOPEX at, or carefully extrapolated to, the calibration site, and have demonstrated RMS differences ranging from to about 1.5 to 3.5 cm. Studies using non‐coincident coastal and island tide gauges, pressure gauges or inverted echosounders, and dynamic heights computed from temperature sensors [ Katz et al , 1995; Mitchum , 1994; Picaut et al , 1995; Verstraete and Park , 1995; Woodworth et al , 1996; Murphy et al , 1996] generally achieve RMS differences ranging between about 4 and 12 cm, depending upon location, with correlations ranging from 0.4 to 0.9, but typically 0.6 to 0.7.…”

A new facility to enhance Australian GPS-geodetic research

“…It must be emphasised that the consortium already has an excellent working relationship, as evidenced by joint publications (eg. Ananga et al, 1995;King et al, 2000;Morgan et al, 1996;Phillips et al, 1998;Tregoning et al, 1998a;White et al, 1994), a number of collaborative grants supported by the Australian Research Council (eg. Featherstone and Rizos 1997 RIEFP grant;Morgan, Coleman and Rizos 1993 large grant;Coleman and Morgan 1998 large grant), as well as other informal collaborative projects.…”