Evaluation of the TOPEX/POSEIDON altimeter system over the Great Lakes

Abstract

The TOPEX/POSEIDON altimeter measurement system is evaluated for the first 46 repeat cycles (September 23, 1992–December 23, 1993) using tracks over the Great Lakes. The temporal variations in lake level are removed from the altimeter measurements using in situ lake level measurements, thus permitting the performance of the altimeter system to be assessed. For the NASA altimeter, the root‐mean‐square (RMS) scatter of the residuals is 3.95 cm using all the tracks over the lakes. However, some of the scatter in this result is probably due to lake tides or seiche, which can amount to a few centimeters amplitude near the ends of the lakes. When the seven best tracks are used, which cross the center of the lakes where tides/seiche effects are minimal, the RMS error is reduced to either 2.9 or 3.0 cm, depending on whether the Centre National d'Etudes Spatiales (CNES) or NASA orbit is used. This places an upper limit on the error budget of the altimeter system, excluding ocean tides and inverse barometer effect. There are several short‐period variations in the residuals. The most pronounced is a 55‐day period, with a 1‐cm amplitude, which we believe is (at least in part) due to orbit error. When the model‐derived wet tropospheric correction is substituted for the TOPEX microwave radiometer correction, the RMS error increases significantly, possibly resulting in an annual cycle of a few centimeters. Evaluation of the ionospheric correction indicates that the dual‐frequency correction provides an average improvement of 0.85 cm over the Doppler orbitography and radiopositioning integrated by satellite (DORIS) correction. Although there are insufficient data to directly assess the CNES altimeter, the relative bias between the altimeters is estimated to be either −14.3 or −15.6 cm (NASA altimeter measuring short), depending on whether the DORIS or dual‐frequency ionospheric correction is applied to the NASA altimeter.

Contribution of the TOPEX NASA Radar Altimeter to the global monitoring of large rivers and wetlands

“…Assuming the river stage data are relatively accurate, the comparative NRA rms over the river/wetlands is, however, poor compared to that obtained by validation for lakes (3-4 cm rms [Morris and Gill, 1994;Birkett, 1995]) and is poorer than expec.ted for wetland/river regions contributing two or more waveforms (section 3.3). Access to more global river data, particularly from gauges within a few kilometers of the satellite pass, for further validation exercises is highly desirable.…”

Multi-Decadal Monitoring of Lake Level Changes in the Qinghai-Tibet Plateau by the TOPEX/Poseidon-Family Altimeters: Climate Implication

“…2016, 8,446 those over oceans. The waveforms at spots 1-8 and spots 27-33 are irregular due to the land's rugged surface and its large slope (>2°).…”

Present-Day Lake Level Variation from Envisat Altimetry over the Northeastern Qinghai-Tibetan Plateau: Links with Precipitation and Temperature

“…Satellite radar altimetry has been used to study water elevation changes over lakes (Morris and Gill 1994;Birkett 1995;Lee et al 2010), rivers (Birkett 1998;Birkett et al 2002;Kouraev et al 2004;Berry et al 2005), and wetlands Lee et al 2009). Hwang et al (2005) generated decadal water level time series using TOPEX/Poseidon (T/P) altimeter data over six inland lakes in China and investigated their links to precipitation and El Niño Southern Oscillation (ENSO).…”