Absolute calibration of Jason radar altimeters from GPS kinematic campaigns over Lake Issykkul

This study presents results of calibration/validation (C/V) of Jason-1 and Jason-2 satellite altimeters over Lake Issykkul located in Kyrgyzstan, which was chosen as a dedicated radar altimetry C/V site in 2004. The objectives here are to estimate absolute altimeter biases and to quantify the altimetry instrument error budgets for lakes studies, including errors associated with the atmosphere media delay corrections of the radar signal, and with the different retracking mode used over "nonocean" surface. The C/V is conducted using various equipments, for example, GPS local network, moving GPS along the satellites tracks over the Lake Issykkul, in situ level gauges and weather stations. The absolute bias obtained for Jason-1 and Jason-2 from field campaigns conducted in 2008, 2009 and 2010 are 96 +/- 40 mm and 162 +/- 42 mm, respectively. The bias calculated for Jason-1 is in close agreement with estimates done at other dedicated C/V sites in the ocean, but the Jason-2 bias still differs by 1-2 cm with ocean C/V estimates. The relative bias (Jason-2 minus Jason-1) deduced from measurements during the tandem mission is estimated at 81 +/- 10 mm. The accuracy of Jason-1 and Jason-2 for the determination of the lake level variations as compared to the historical in situ gauges of the Lake Issykkul is 33 and 31 mm RMS, respectively. The bias between the two modes of retracking available on Jason-2 Geophysical Data Records (GDRs) has been estimated over the two first years of Jason-2 data over the Lake Issykkul. We found an average value of 234 +/- 40 mm, which is in good agreement with results obtained in a previous study on the Envisat satellite (Cretaux et al. 2009). Lake level studies using Jason-2 or Envisat must therefore take this relative bias into account if both retracked altimetry heights are used