US: NASA is leading an effort to build a prototype station that will go beyond current scientific requirements and serve the navigation satellites of the future, according to Herbert Frey, Head of the Planetary Geodynamics Laboratory at Goddard (in the US) and a member of the Space Geodesy Project team. With the ‘vector tie’ system, the agency will upgrade and interlink GPS, very long baseline interferometry (VLBI), satellite laser ranging (SLR) and, doppler orbitography and radiopositioning (DORIS).
Frey explained, “In practical terms, we can’t determine a location today and expect it to be good enough tomorrow.” Before GPS navigation devices can locate a point on earth, the satellites that make up the GPS constellation 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 are on a planet that is not at rest — yet modern measurements require more and more accuracy.
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 VLBI, DORIS lastly SLR. ‘Vector tie’ system will use a laser to continuously monitor the reference points of each technique, and let the researchers know exactly where a station’s GPS, VLBI, SLR and DORIS sit relative to each other at all times, allowing them to better correct one of the last sources of error in the terrestrial reference frame.
Source: NASA
