Precise measurements of the surface topography of the ocean, and of continental waters are made through satellite altimetry missions all over the globe. An orbiting satellite emits electromagnetic waves to the surface of the Earth. Then the satellite observes and processes the reflected signals, their time of arrival, and their properties. From these altimetric observations, the range from the satellite to the earth surface, as well ocean wave heights and wind measurements can be determined precisely. Combined with precise satellite location data, altimetry measurements yield sea-surface heights that are used to estimate sea-level changes, the circulation of the ocean, significant wave heights, sea ice freeboard and a variety of other parameters of relevance to Earth science and climate change.
Accurate monitoring of sea-level changes is of fundamental value and crucial in understanding the ocean’s influence on our weather patterns and long-term climate changes of our planet. Satellite altimetry provides the only means for monitoring changes in sea level, unequivocally, over regional to global scales with [mm/yr] accuracy and with respect to the center of mass of the Earth. Current and future altimetric missions (i.e., such as Jason-2, Jason-3, CryoSat-2, HY-2, Sentinel-3, Jason-CS/Sentinel-6, etc.) extend nowadays the usage of their products to new ocean and earth applications by incorporating novel measuring techniques (i.e., Delay Doppler, wide swath), different operating frequencies and unique sensors.
To establish a continuous, homogenous and reliable monitoring of the ocean and its changes, altimetry observations have to be free of errors and biases, uninterrupted, but also tied from one mission to the next in an objective and absolute sense. Altimetry system’s responses have to be, thus, continuously monitored and controlled for their quality, biases, errors, drifts, although relations among different missions have to be established on a common and reliable earth-centered reference system, maintained over a long period of time.
This project of Fiducial Reference Measurements for Altimetry (FRM4ALT) sets the ground for the establishment for such a site for satellite altimetry in Gavdos and West Crete, Greece to "establish and demonstrate SI-traceability of Fiducial Reference Measurements and their use for satellite derived altimeter calibration and validation".
The concept of Fiducial Reference Measurements has been established by the European Space Agency as: "The suite of independent ground measurements that provide the maximum Return-On-Investment for a satellite mission by delivering, to users, the required confidence in data products, in the form of independent validation results and satellite measurement uncertainty estimation, over the entire end-to-end duration of a satellite mission."