NASA pinning down GPS satellites' location better than ever

Washington, Feb 24 (ANI): Before our Global Positioning System (GPS) navigation devices can inform us where we are, the satellites that form the GPS need to know precisely where they themselves are.

For that, they rely on a network of sites that serve as "you are here" signs planted throughout the world. The catch is, 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 pinpointing where "here" is.

To meet this need, 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 Center in Greenbelt, Md., where the next generation of two of these systems is being developed and built.

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

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

GPS won't be the only beneficiary of the improvements. All observations of Earth from space-whether it's to measure how far earthquakes shift the land, map the world's ice sheets, watch the global mean sea level creep up or monitor the devastating reach of droughts and floods-depend the International Terrestrial Reference Frame, which is determined by data from this network of designated sites.

Earth is a shapeshifter. Land rises and sinks. The continents move. The balance of the atmosphere shifts over time, and so does the balance of the oceans. All of this tweaks Earth's shape, orientation in space and center of mass, the point deep inside the planet that everything rotates around. The changes show up in Earth's gravity field and literally slow down or speed up the planet's rotation.

"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, the head of the Planetary Geodynamics Laboratory at Goddard and a member of the Space Geodesy Project team.

Measuring such properties of Earth is the realm of geodesy, a time-honored 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.

Another technique, Very Long Baseline Interferometry (VLBI), acts as a kind of GPS for Earth. To deduce Earth's orientation in space, and the small variations in the Earth's rate of rotation, ground stations spread across the globe observe dozens of quasars, which are distant enough to be stable reference points.

The key is the painstakingly accurate timing of when the quasar signals arrive.

Keeping tabs on Earth's center of mass is the job of satellite laser ranging (SLR). It measures the distances to orbiting satellites by shooting short pulses of laser light at satellites and measuring the time it takes for the light to complete the round trip back to the ground station.

Another way to measure distances to satellites is with DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), which was built and is operated by the French space agency, known as CNES.

DORIS takes advantage of the Doppler effect, which is at work when an ambulance's siren changes pitch as it's driving towards or away from you.

Like GPS, DORIS requires little hardware on the ground, so its beacons are spread all over the globe, even in areas as remote as the Mount Everest base camp.

Right now, only Goddard and the station in Johannesburg, South Africa, are providing results from all four. NASA wants to change that.

"The plan for the upgraded system is to have at least three, and preferably all four, techniques at every station," said LaBrecque.

"This is one of the keys to achieving the goals of a millimeter of accuracy and a tenth of a millimeter of stability for future measurements."

The Next Generation SLR is also being developed at GGAO, under the direction of Jan McGarry, with the goals of more automated operation and the ability to target satellites in higher orbits.

Another key innovation at Goddard's new station is the "vector tie" system that will link together all four measurement techniques.

But with the vector tie system, which will use a laser to continuously monitor the reference points of each technique, researchers will 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.

"By bringing at least three of the four techniques together in each station, we will get a stronger system overall," said Frey.

"NASA is leading the way in this, building a prototype station that will go beyond our current scientific requirements and serve the satellites of the future," Frey added. (ANI)