Altimetry, bathymetry and geoid variations at the Gavdos permanent Cal/Val facility

Summarization: The aim of this work has been to examine the relationship of steep bathymetry in the coastal areas around the permanent Cal/Val facility of Gavdos, and their influence on the produced calibration values for the Jason-2 satellite altimeter. The paper describes how changes in seafloor topography (from 200 to 3500 m depth over a distance of 10 km) are reflected on the determined altimeter parameters using different reference surfaces for satellite calibration. Finally, it describes the relation between these parameter trends and the region’s local characteristics. Using 3.5 years of Jason-2 calibration data and careful data analysis, certain features and variations, with permanent signatures at some locations south of Gavdos, have been found in the altimeter bias values. Causes of these variations have been interpreted. Some of these features, for example, at 12 km south of the Cal/Val site seem to be related to the general oceanographic circulation, but others of short wavelength (in the order of 1 km) are because of the insufficient geoid model resolution. Along Pass No. 109, the concealed effect of bathymetry on the geoid has produced a slope of 3.1 cm over 14–21 km from Gavdos. Along the other Pass No.18 and in the range 12–21 km, a slope in the geoid model, caused again by the sharp changes in the topography of seafloor (from 300 to 3200 m depth over a distance of 10 km), have been discovered. Those geoid model imperfections of short wavelength do not change the previously reported bias values. Finally, boat campaigns that took place for verifying the previous features, were also used to determine an altimeter bias of B = +19.6 ± 4.5 cm, for cycle = 80, at 15:14:58 UTC, on 7-Sept-2010, along Pass No. 109. Calibration took place at a distance of about 19 km south of Gavdos, in the open sea while Jason-2 was flying over.Presented on: Advances in Space Researc

First preliminary results for the absolute calibration of the Chinese HY-2 altimetric mission using the CRS1 calibration facilities in West Crete, Greece

Summarization: In this work, absolute calibration of the Chinese HY-2 satellite altimetry mission is carried out, employing Pass No. 280 and the calibration facility, CRS1, located in the Southwest end of the island of Crete, Greece. Satellite Pass No. 280 is descending and follows a ground track almost parallel to the west coast of Crete. It comes close to the coast, at a distance of about 9 km from the CRS1 calibration site, and finally goes away south of Crete. The HY-2 sensor geophysical data records (S-GDR) have been incorporated into the calibration procedures and processing has taken place for cycles No. 54–62, at 20 Hz data rate. Some peculiarities in the HY-2 satellite altimeter data, as delivered and depicted in the I-GDR and S-GDR data, have also been noticed. All calibration results have been determined using a regional, precise and detailed geoid, along with a good knowledge of local ocean circulation and site characteristics and a well-defined sea-surface calibration methodology. The first preliminary results for the HY-2 altimeter calibration have shown that the initial cycles, up to No. 51, display an erratic behavior. After those cycles, the altimeter range bias values seem to be stable and reach a value of B = −45.6 cm ± 4.4 cm, when applying the net instrument corrections as provided in the GDR. If the relativistic effects of the satellite clocks are properly applied for the net instrument corrections, then the altimeter range bias goes down to B = −27 cm ± 3 cm. Also, preliminary cross-over analysis with the SARAL/AliKa and Jason-2 satellites show a bias of B = −23 cm, and B = −28.5 cm, respectively. The performance of the HY-2 on-board radiometer has also been examined in terms of the wet troposphere corrections and shows a mean difference of −1 cm ± 0.1 cm with respect to in-situ GNSS-derived corrections. Finally, the ionosphere path corrections of the HY-2 satellite show a difference of +1 cm ± 1.1 cm, when compared against the GNSS-derived ionosphere values.Presented on