How do GPS satellites know where they are?

Washington -  Our Global Positioning System (GPS) tells us exactly where we are, down to a few metres, but how do satellites that make up the GPS themselves know where they are?

They have to rely on a network of sites that serve as "You Are Here" signs planted throughout the planet. The catch is, the sites don't sit still because the planet is moving all the time, yet modern measurements require more and more accuracy in pinpointing where "here" is.

Accordingly, NASA is helping to lead an international effort to upgrade the four systems that supply this crucial location information. NASA's initiative is run by Goddard Space Flight Centre, Greenbelt, US, where the next generation of two of these systems is being developed and built.

"In practical terms, we can't determine a location today and expect it to be good enough tomorrow-and especially not next year," says Herbert Frey, who heads the Planetary Geodynamics Lab at Goddard.

And Goddard, in partnership with NASA's Jet Propulsion Lab in Pasadena, Calif., is bringing all four systems together in a state-of-the-art ground station, according to a NASA statement.

"NASA and its sister agencies around the world are making major investments in new stations or upgrading existing stations to provide a network that will benefit the global community for years to come,"

says John LaBrecque, Earth Surface and Interior Program Officer at NASA Headquarters.

Measuring such properties of Earth is the realm of geodesy, a time-honoured science that dates back to the Greek scholar Eratosthenes, who achieved a surprisingly accurate estimate of the distance around

the Earth by using basic geometry.