Search

Summarization: This work presents the latest calibration results for the Copernicus Sentinel-3A and -3B and the Jason-3 radar altimeters as determined by the Permanent Facility for Altimetry Calibration (PFAC) in west Crete, Greece. Radar altimeters are used to provide operational measurements for sea surface height, significant wave height and wind speed over oceans. To maintain Fiducial Reference Measurement (FRM) status, the stability and quality of altimetry products need to be continuously monitored throughout the operational phase of each altimeter. External and independent calibration and validation facilities provide an objective assessment of the altimeter’s performance by comparing satellite observations with ground-truth and in-situ measurements and infrastructures. Three independent methods are employed in the PFAC: Range calibration using a transponder, sea-surface calibration relying upon sea-surface Cal/Val sites, and crossover analysis. Procedures to determine FRM uncertainties for Cal/Val results have been demonstrated for each calibration. Biases for Sentinel-3A Passes No. 14, 278 and 335, Sentinel-3B Passes No. 14, 71 and 335, as well as for Jason-3 Passes No. 18 and No. 109 are given. Diverse calibration results by various techniques, infrastructure and settings are presented. Finally, upgrades to the PFAC in support of the Copernicus Sentinel-6 ‘Michael Freilich’, due to launch in November 2020, are summarized. Presented on: Remote Sensing

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 Research

Summarization: Satellite altimetry provides exceptional means for absolute and undisputable monitoring of changes in sea level and inland waters (rivers and lakes), over regional to global scales, with accuracy and with respect to the center of mass of the Earth. Altimetry system's responses have to be continuously monitored for their quality, biases, errors, drifts, etc. with calibration. Absolute calibration of altimeters is achieved by external and independent to satellite facilities on the ground. This is the mainstay for a continuous, homogenous, and reliable monitoring of the earth and its oceans. This paper describes the development of the Permanent Facility for Altimetry Calibration in Gavdos/Crete, Greece, as of 2001 along with its infrastructure and instrumentation. Calibration results are presented for the reference missions of Jason-1, Jason-2, and Jason-3. Then, this work continues with the determination of relative calibrations with respect to reference missions for Sentinel-3A, HY-2A, and SARAL/AltiKa. Calibration results are also given for Jason-2 and Jason-3 altimeters using the transponder at the CDN1 Cal/Val site on the mountains of Crete, with simultaneous comparisons against sea-surface calibration and during their tandem mission. Finally, the paper presents procedures for estimating uncertainties for altimeter calibration to meet the Fiducial Reference Measurement standards. Presented on: Remote Sensing

Summarization: This work presents the first calibration results for the SARAL/AltiKa altimetric mission using the Gavdos permanent calibration facilities. The results cover one year of altimetric observations from April 2013 to March 2014 and include 11 calibration values for the altimeter bias. The reference ascending orbit No. 571 of SARAL/AltiKa has been used for this altimeter assessment. This satellite pass is coming from south and nears Gavdos, where it finally passes through its west coastal tip, only 6 km off the main calibration location. The selected calibration regions in the south sea of Gavdos range from about 8 km to 20 km south off the point of closest approach. Several reference surfaces have been chosen for this altimeter evaluation based on gravimetric, but detailed regional geoid, as well as combination of it with other altimetric models. Based on these observations and the gravimetric geoid model, the altimeter bias for the SARAL/AltiKa is determined as mean value of −46mm ±10mm, and a median of −42 mm ±10 mm, using GDR-T data at 40 Hz rate. A preliminary cross-over analysis of the sea surface heights at a location south of Gavdos showed that SARAL/AltiKa measure less than Jason-2 by 4.6 cm. These bias values are consistent with those provided by Corsica, Harvest, and Karavatti Cal/Val sites. The wet troposphere and the ionosphere delay values of satellite altimetric measurements are also compared against in-situ observations (−5 mm difference in wet troposphere and almost the same for the ionosphere) determined by a local array of permanent GNSS receivers, and meteorological sensors. Presented on: Marine Geodesy

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