Orbit

Orbit

To convert the measured range to a sea surface height, it is necessary to subtract the orbit height of the satellite relative to the reference ellipsoid (a mathematical model of the Earth's shape used as a reference surface for height measurements). The orbit height corresponds to the orbital position of the centre of mass of the satellite. The distance between the satellite centre of mass and the altimeter antenna, and any changes of this as the centre of mass moves as onboard fuel is used up, are dealt with in the external group calibration. 

The orbit is determined from an orbit trajectory model that is corrected by independent observations of the satellite motion through an assimilation process to ensure consistency with the physics of the trajectory model. On S6-MF, the independent observations come from two systems: a GNSS based system (the GNSS-POD multi frequency receiver), which measures the position of the satellite relative to GPS and Galileo GNSS satellites, and the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) system, which is a set of ground radio beacons that broadcast omnidirectionally and continuously at stable frequencies. The receiver onboard receives the DORIS signal and measures the Doppler shift. This process relies on an accurate onboard ultrastable oscillator, which also provides a reference frequency for the altimeter. S6-MF also has a laser retroreflector array (LRA), which supports independent ground-based laser tracking from laser ranging stations.  

Measurements are made in reference to the ITRF (international terrestrial reference frame), which provides a highly accurate, stable and consistent coordinate system for the Earth's surface in a geocentric coordinate system. This reference is made at a particular time, and degrades with respect to truth thereafter.  The orbit height above the ellipsoid is determined via the ITRF. 

Orbit Uncertainties

To come ...