Absolute calibration of satellite altimeters, during their lifetime, by external and independent facilities, is a prerequisite for a continuous, homogenous and reliable monitoring of the earth, its oceans and climate change. These Cal/Val facilities ensure that altimetry observations are 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-center reference system, maintained over a long period of time. 

Up to now, absolute calibration of satellite altimetry is mainly provided by permanent ground calibration and validation (Cal/Val) facilities. These are located either offshore or on land (using microwave transponders) exactly under (absolute direct) or adjacent to satellite’s ground track on nearby coasts, to ensure monitoring of uncontaminated satellite observations (absolute indirect). Relative calibration of satellite measurements is also performed using multi-mission crossover analysis between reference altimeters and other missions (relative direct).