Calibration of TOPEX/POSEIDON at Platform Harvest

Abstract

We present estimates for the mean bias of the TOPEX/POSEIDON NASA altimeter (ALT) and the Centre National d'Etudes Spatiales altimeter (SSALT) using in situ data gathered at Platform Harvest during the first 36 cycles of the mission. Data for 21 overflights of the ALT and six overflights of the SSALT have been analyzed. The analysis includes an independent assessment of in situ measurements of sea level, the radial component of the orbit, wet tropospheric path delay, and ionospheric path delay. (The sign convention used in this paper is such that, to correct the geophysical data record values for sea level, add the bias algebraically. Unless otherwise stated, the uncertainty in a given parameter is depicted by ±σx, where σx is the sample standard deviation of x about the mean.) Tide gauges at Harvest provide estimates of sea level with an uncertainty of ±1.5 cm. The uncertainty in the radial component of the orbit is estimated to be ±1.3 cm. In situ measurements of tropospheric path delay at Harvest compare to within ±1.3 cm of the TOPEX/POSEIDON microwave radiometer, and in situ measurements of the ionospheric path delay compare to within −0.4±0.7 cm of the dual‐frequency ALT and 1.1±0.6 cm of Doppler orbitography and radiopositioning integrated by satellite. We obtain mean bias estimates of −14.5±2.9 cm for the ALT and +0.9±3.1 cm for the SSALT (where the uncertainties are based on the standard deviation of the estimated mean σx–/y , which is derived from sample statistics and estimates for errors that cannot be observed). These results are consistent with independent estimates for the relative bias between the two altimeters. A linear regression applied to the complete set of data shows that there is a discernable secular trend in the time series for the ALT bias estimates. A preliminary analysis of data obtained through cycle 48 suggests that the apparent secular drift may be the result of a poorly sampled annual signal.

Comparison of Different Techniques for Tropospheric Wet Delay Retrieval Over South America and Surrounding Oceans

“…At the same time, WV reveals very rapid changes both in the temporal and in the spatial domains such that, at present, there are no theoretical models that can reliably predict its behaviour. Retrieving WV content in the atmosphere can be performed in different ways using independent techniques: starting from the more traditional and established ones, such as radiosondes and ground-based microwave radiometers, up to the more recent ones, such as satellite based techniques like satellite radiometers (Christensen et al 1994), Global Navigation Satellite Systems (GNSS) (Bevis et al 1992), Radio Occultation (Kursinski et al 1997) and Numerical Weather Models (NWM). Since each of these techniques presents advantages and limitations, researchers' efforts have been recently focused on comparing the different approaches with the aim of combining them to retrieve WV content with the highest possible accuracy.…”

On the satellite altimeter crossover problem

“…during the 'verification phase'; ALT was on for the remainder. We have used measurements from both altimeters, correcting SSALT ranges for the 15.4 cm relative bias reported by Christensen et al (1994).…”

The use of GPS buoys in the determination of oceanic variables

“…Previous altimeter calibration campaigns have employed GPS buoys with distances to GPS reference sites not exceeding 20 km (Christensen et al, 1994;Born et al, 1994;Key et al, 1999;Zilkoski et al, 1999). In the GRAC experiments, the nearest reference station was at 80 km, which, in principle, makes unreliable the use of the tropospheric solution from the reference site.…”