Innovation Insights with Richard Langley
While I’m likely preaching to the choir here, GNSS cannot work unless we have an accurate description of the orbits of the satellites and the behavior of their atomic clocks. The accuracy with which this information is provided to a receiver or data processing software is the most important component of the error budget of GNSS positioning, navigation and timing and constitutes most of what is known as the signal-in-space (SIS) range error.
Each GNSS satellite broadcasts a description of its orbit or ephemeris along with the offset of its active clock from the system’s time standard in a navigation message decoded and used by the receiver. These data are predictions of the orbit and clock offset as computed by the system’s ground control segment and uploaded to each satellite. A recent assessment by U.S. Space Systems Command of the GPS SIS error averaged across all active satellites for a one-week period was about 50 centimeters, root-mean-square. While this is entirely adequate for many GNSS uses, it falls short of the required accuracy for high-demanding applications such as surveying, geodesy, atmospheric sensing, reference frame studies and tectonic monitoring. Which is why various organizations both private and public compute very accurate orbits and clocks and provide these to users. These computations, using data from global receiver networks, are very exacting and model the tiniest effects on the (primarily) carrier-phase measurements these receivers provide.
These effects include the offset in the electrical phase centers of a GNSS satellite’s transmitting antenna from the satellite’s center of mass and how that varies with the direction of the signal from the satellite to a receiver on Earth. Furthermore, this behavior must be calibrated and modeled for each radio frequency that the satellite transmits. Another effect that must be accounted for are the perturbations caused by non-perfect yaw-steering of a satellite’s solar panels. These panels continuously track the Sun but they have difficulty keeping up at orbit noon and midnight. Accurate models of the actual yaw angle are very important for high-precision GNSS orbits. As if these model requirements were not enough, the effect of solar radiation pressure on satellite orbits must also be modeled. While they don’t have (rest) mass, photons have energy and this can be imparted to satellites when they impinge on them. While a single photon has a negligible effect, the billions upon billions of photons making up sunlight do have a noticeable effect on a GNSS satellite’s motion and must be accounted for by orbit models.
One organization producing precise orbits for GNSS satellites – arguably the most precise in the world – is the International GNSS Service (IGS), a voluntary federation of more than 200 agencies, universities and research institutions across the globe. Several of these organizations each produce precise orbits, which they submit to the IGS to establish orbit products. One of these organizations is the Navigation Support Office (NSO) at the European Space Agency’s European Space Operations Centre. In this quarter’s Innovation column, a team of NSO engineers discusses how they have improved the orbit modeling of the GPS III satellites by around a factor of two with estimated orbit errors of about 2 centimeters or less. Wizardry? Not really – just rocket science.
Read the full “Innovation” column: New type on the block: Generating high-precision orbits for GPS III satellites.