*Radar Satellite Altimetry in Geodesy - Theory, Applications and Recent Developments DOI: http://dx.doi.org/10.5772/intechopen.97349*

RMSE (Root Mean Square Error) of 2 cm and orbit accuracy estimated at around 2.5 cm (see [6, 9]). Later improvements of the TOPEX/Poseidon data processing based on its dual-frequency altimeter estimates of sea-surface height resulted in an overall precision expressed with root-sum-of-squares (RMS) of about 4 cm [6], which today is an expected accuracy of altimeter data from different satellite missions and can get up to RMS of 2 cm for open ocean altimetry [9]. The advances in orbit determination were due to the development of the DORIS satellite tracking system. DORIS was developed by CNES to determine the satellite orbits with centimeter accuracy from a network of 60 ground stations settled worldwide [15].

At present, several satellites are providing measured altimeter data:


Sentinel-6 satellite mission is currently in its commissioning phase, i.e., in the calibration/validation phase. **Figure 3** presents Sentinel-6 sea-level anomaly derived from 'Short Time Critical Level 2 Low Resolution' data, overlaid on a map showing similar products from the other Copernicus altimetry missions: Jason-3, Sentinel-3A, and Sentinel-3B (for details and original research, please see [17]). The background image is a map of sea-level anomalies from satellite altimeter data provided by the Copernicus Marine Environment Monitoring Service for 4 December 2020. The data for this image were taken from the Sentinel-6 products generated on 5 December 2020. Being in its commissioning phase, the measurements obtained by the Sentinel-6 are promising [17].

The characteristics of previous and current satellite missions are given in **Table 1**.

Surface Water Ocean Topography (SWOT) mission is planned to be launched primarily to enable terrestrial water monitoring. The mission is a joint project of NASA, CNES, the Canadian Space Agency, and the UK Space Agency. It is expected to operate in Ka-band with a 0.86 cm radar wavelength [18].
