NASA is leading an effort to upgrade the systems that provide location information to GPS satellites.
Before GPS navigation devices can locate a point on Earth, the satellites that make up the GPS constellation must first determine where they are themselves. For that, they rely on a network of sites planted throughout the Earth’s surface.
The difficulty is that the sites don’t sit still because they’re on a planet that isn’t at rest — yet modern measurements require more and more accuracy.
‘In practical terms, we can’t determine a location today and expect it to be good enough tomorrow and especially not next year,’ said Herbert Frey from NASA’s Goddard Space Flight Center.
To meet this need, over the years four types of space geodesy measurements — carried out by a squad of ground stations and satellites — have been developed independently.
The first is GPS itself; then very long baseline interferometry (VLBI); doppler orbitography and radiopositioning integrated by satellite (DORIS); and lastly satellite laser ranging (SLR). A key Goddard innovation is the ‘vector tie’ system that will link together all four measurement techniques.
‘Right now, we have these four independent techniques, and they’re just that: independent,’ said Frank Lemoine, Goddard co-collaborator. ‘Presently, at a particular ground station, the techniques are only tied together by expensive and infrequently performed ground surveys.’
GPS will not be the only beneficiary of the improvements. All observations of Earth from space, whether to measure how far earthquakes shift the land, to map the world’s ice sheets, to watch the global mean sea level or to monitor droughts and floods, depend on the International Terrestrial Reference Frame (ITRF), which is determined by data from this network of designated sites.