New Cosmic Map Guides GPS

Millions of people use the perks of the Global Positioning System (GPS) every day, but few of us take a moment to consider how this exceptional invention works. Most people know that it establishes the position of a GPS receiver, which may be installed in a car, for example, based on the time it takes for beams of data emitted by satellites to reach it, and then return. But have you ever wondered how GPS satellites know where they are?

Astronomers explain that, naturally, there are no lines drawn in outer space that could help the satellites figure out their position. In order to provide us with highly accurate data, the orbital transmitters also need to know where they are, with a very narrow margin for error. The way this orientation is accomplished is by specialized instruments in the spacecraft measuring its position away from a number of celestial objects, which are used as landmarks.

“For GPS to work, the orbital position, or ephemeris, of the satellites has to be known very precisely. In order to know where the satellites are, you have to know the orientation of the Earth very precisely,” scientist Dr. Chopo Ma, an expert at the NASA Goddard Space Flight Center, in Greenbelt, Maryland, says. Only a few objects can be used as reference points for the GPS, the scientist adds. “For the extremely precise measurements needed for things like GPS, stars won't work, because they are moving too,” Ma explains.

The International Celestial Reference Frame (ICRF), completed in 1995, was the first map to feature celestial objects as reference points. It combined a large number of images of more than 600 targets in the sky, to which all GPS satellites reported themselves. Working together with experts affiliated with the International Very Long Baseline Interferometry Service for Geodesy and Astrometry (IVS), and also with the International Astronomical Union (IAU), Ma spent three years focusing on updating the aging ICRF with the most recently available data.

The new map, called ICRF2, contains measurements of more than 3,000 quasars, and became the new, IAU-recognized reference frame for the GPS system in August 2009. “A significant challenge was modeling all these disturbances in computers to take them into account and reduce the noise, or uncertainty, in our position observations.” Ma says. The new observations were also used to infer data about stars around us, in terms of origins, evolution and age.