An absolute calibration site for radar altimeters in the continental domain: Lake Issykkul in Central Asia

Abstract

0949-7714International audienceAltimetry missions such as Topex/Poseidon, Jason-1, GFO and ENVISAT have been widely used in the continental domain over lakes, rivers and wetland although they were mostly dedicated to oceanic studies. Knowledge of the instrumental biases is a key issue. Numerous sites have been dedicated to calibration purposes, either in the oceanic domain (Harvest offshore platform in California, Corsica, Bass Strait in Australia) or over lakes (Lake Erie in United States). A new site (Lake Issykkul in Kirghizstan) is proposed for calibration in the continental domain. This lake is covered by past (T/P) and current radar altimetry satellites (Jason-1, T/P, GFO, and ENVISAT). Several in situ water levels and local meteorological variables are available at the site. Located in a mountainous area, it offers an opportunity for calibration far away from all other existing sites and very different environment contexts. Two GPS campaigns have been conducted on the lake in 2004 and in 2005. They consisted of cruises with stations installed onboard a boat following the satellite ground tracks, and onshore settings. This enabled estimating a bias for each altimeter and each tracking algorithm available. Biases obtained for Envisat, GFO, T/P and Jason-1 using the default ocean tracker (respectively, 48.1 +/- 6.6, 7.5 +/- 4.0, 0 +/- 4.3 and 7.0 +/- 5.5 cm) agree with biases published at the other calibration sites. For Jason-1, there is a significant disagreement with results obtained in the ocean field (7 cm instead of 13 cm) but is coherent with bias obtained on the Lake Erie site. Erroneous estimates of the sea state bias correction from non-oceanic-like waveforms is discussed as a possible explanation. Errors in the ionospheric, wet and dry tropospheric corrections for the continental domain are also highlighted and quantified

DAHITI – an innovative approach for estimating water level time series over inland waters using multi-mission satellite altimetry

“…larger lakes) the resulting ranges are less precise than ranges derived from retracking algorithms for ocean applications. It is known that switching retracking algorithms along a single satellite track leads to height offsets (Crétaux et al, 2009). To avoid those offsets, all altimeter measurements of an investigated inland water body are retracked with the same algorithm.…”

The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Quality Assessment and Unique Characteristics of AltiKa Data

“…Lake Issykkul (42)(43) • N, 76-79 • E) is a large lake (6000 km 2 of area extent) located in Kyrgyzstan in Central Asia. Initially used as experimental site, thanks to promising first results obtained with TOPEX/Poseidon [13], the Lake Issykkul became permanent cal/val site for several altimeters in orbit. It has served for Jason-1, Jason-2, and Envisat with results coherent with other cal/val sites over the oceans [14,15].…”

A review of applications of satellite SAR, optical, altimetry and DEM data for surface water modelling, mapping and parameter estimation

“…For river stage estimation and wetlands delineation, problems encountered with satellite altimetry data include incorrect processing of radar echoes over rivers/lakes by satellite trackers, poor temporal resolution, and lack of information within the data about the atmospheric wet vapour content over lakes/rivers (Crétaux et al, 2009). The errors recorded while using altimeter water level data can however be increased by incorrect choice of data, which frequently occurs when dry area data are retained within the data for computing water stages in low-flow seasons (Santos da Silva et al, 2007).…”