Swiss Clock Model Improves Satellite Orbits

Guidance and Navigation Satellite System (GNSS) satellites use the precise measurement of the travel time it takes signals emitted by the satellites to be recorded by the receivers. This one-way concept requires two clocks to measure the signal propagation time – one in the satellite and one in the receiver. As such, clocks are often the primary payload of GNSS satellites.

Contrary to the very precise models used to determine the orbits of GNSS satellites and Low Earth Orbiting (LEO) satellites, the models used for clock corrections currently ignore the physical characteristics of the satellite transmitters and receivers that drive the clock itself.

 The on board and ground station clocks are vital to estimating a satellite’s orbit and for its applications. For all precise applications of GNSS, from precise positioning for geodetic applications to precise orbit determination, all of the clocks contributing to the tracking dataset have to be synchronised. This synchronisation has to be performed to within a few picoseconds.

To get the time right, a large number of parameters must be estimated, including orbit parameters, station coordinates, site-specific troposphere parameters, phase ambiguities and environmental factors.

Yet instead of using models, clock corrections are estimated as independent parameters for each satellite and receiver or, regularly, are eliminated.

A TDE activity with Universität Bern Astronomical Institute, in Switzerland, has developed three clock models to estimate how including the corrections for both satellite and ground station clocks could influence or improve satellite orbits.

The three models are for precise orbit determination of GNSS satellites, precise orbit determination of LEO satellite and the ground stations.

The activity reviewed the clocks on board Galileo satellites, and then investigated different clock models to identify which ones were optimal for the different types of satellite clocks.

Overall, the activity showed that applying clock modelling is beneficial to all missions as it can help improve the estimated orbit.

For GNSS satellites, clock modelling was beneficial for both dynamic and kinematic orbit determination and in all processed scenarios, the activity saw an improvement of the estimated orbits. For both ground stations and LEO receiver clocks, modelling improved the kinematic orbit by a factor of 2-3.

T710-402GN closed in November 2019, all documentation received by November 2020.