CoAuthors

Craig Donlon (European Space Agency, The Netherlands); Demetrios Matsakis (Formely with the US Naval Observatory, Time Reference, USA); Constantin Mavrocordatos (European Space Agency, The Netherlands); Zuheir Altamimi (IGN (National Geographic Institute) Geodesy Research Laboratory, France); Costas Kokolakis (Technical University of Crete, Greece); Achilles Tripolitsiotis (Space Geomatica, Greece)

Time is the fundamental measurement in satellite altimetry and the key parameter in building and keeping up a long-term, consistent, and reliable record for monitoring changes in sea level. Over the years, different time scales, although interconnected, have been used in altimetry but also in satellite positioning. This leads to unclear and sometimes ambiguous definitions of time and orbit coordinates as well as of their transformations between various reference and measuring systems.

Altimetry satellites, like Sentinel-3, CryoSat-2, Jason-3, HY-2A/-2B/-2C, IceSat-2, etc., observe and practically realize ranges by measuring time differences between transmission and reception of an electromagnetic wave (either radar or laser at present). Similar principles apply for global navigation satellite systems and for their terrestrial reference systems (TRS) on which all are linked and tied together. Yet, the “meter” (scale) of any TRS is also defined by time.

This work seeks to establish a standard reference system for the parameter “time” and for the “coordinates” in satellite altimetry. A new strategy of Fiducial Reference Measurements for Altimetry has been thus introduced to achieve reliable, long-term, consistent and undisputable satellite altimetry observations and products via calibration based upon metrology standards. The new strategy is also to include reference systems of coordinates, precise orbit determination, time tagging and range measurements in satellite altimetry. It has been carried out by the European Space Agency in an effort to reach uniform and absolute standardization for satellite altimetry. This principle delivers a common language of communication (1) for building trust on the scientific data we produce, (2) for presenting the correct information to the public, and (3) for supporting us to make the right decisions for Policies regarding the Earth and its Oceans.