Calibration and Validation
This research and collaboration project aims at the calibration and validation (Cal/Val) of the European Sentinel-3 and Sentinel-6 and the Chinese HY-2 satellite altimeters based upon two permanent Cal/Val facilities: (1) the Permanent Altimetry Calibration Facility (PFAC) established by ESA in Crete, Greece and (2) the National Cal/Val facility at the Wanshan islands, South China Sea. Other satellites, such as Guanlan, CryoSat-2, CFOSAT, CRISTAL, etc., may be also supported by these Cal/Val services. Satellites will be calibrated and monitored using uniform, standardized procedures, protocols and best practices and also built upon trusted and undisputable reference standards at both Cal/Val infrastructures in Europe and China. Altimeters will be thus monitored in a coordinated, absolute, homogeneous, long-term and worldwide manner.Calibration of altimeters is accomplished by examining satellite observations in open seas against reference measurements. Comparisons are established through precise satellite positioning, water level observations, GPS buoys and reference models (geoid, mean dynamic topography, earth tides, troposphere and ionosphere) all defined by Cal/Val sites. In this work, final uncertainty for altimeter bias will be attributed to several individual error sources, coming from observations in water level, atmosphere, absolute positioning, reference surface models, transfer of heights from Cal/Val sites to satellite observations, etc. Absolute calibration of altimeters has also been carried out on land with microwave transponders. This is a unique technique that calibrates the altimeter range directly with the transponder. At the moment, the PFAC in Crete hosts the only operational transponder. A second transponder is currently under construction to be installed at the Gavdos Cal/Val facility and will support Sentinel-6 and Sentinel-3 calibrations. If future HY-2 Chinese missions, such as HY-2C, overpass this European Cal/Val facility, and when operationally supported. The project will implement the action plan established by ESA for Fiducial Reference Measurements for Altimetry (FRM4ALT) calibration. At present, the PFAC is the only facility in the world that reports its Cal/Val results along with their FRM uncertainty. Through this project, the procedures, protocols and best practices, developed in the course of the ESAÔÇÖs FRM4ALT project, will be updated, upgraded and followed at both Cal/Val facilities in Europe and China. This proposed collaborative project will contribute to an FRM4ALT calibration of Sentinel-3 and HY-2B missions during their operational phase, but also to upcoming Sentinel-6 and HY-2C and possibly to the Quanlan satellite of China. As such, it will promote the FRM4ALT strategy of ESA to Chinese stakeholders and scientists. The main outcomes of the proposed project will be (1) the standardization of operations and data processing followed at both Cal/Val infrastructures in Europe and China, (2) the exchange of knowledge and training on properly implementing FRM4ALT in practice, (3) the absolute calibration of European and Chinese satellite altimeters along with unified reporting of FRM uncertainty for the produced Cal/Val results, and (4) inter-comparison of the Cal/Val results obtained at the two main Cal/Val facilities and investigation of any of deviations. Calibration and validation activities at the PFAC in Greece are currently supported by two ESA projects namely SeRAC and FRM4S6. Further funding is provided by international space agencies (i.e., CNES, France) and institutional funds (TUC and Space Geomatica). Similar support has been provided by the Chinese government for the Wanshan Cal/Val facility in South China Sea.
