Deep Space Objects Guide Earth's GPS System

[mickw]

That global positioning system (GPS) that guides your car and keeps you on the map is itself guided by the positions of some of the brightest, strangest objects in the universe — quasars.

GPS satellites send signals to a receiver in GPS navigators down on Earth. These in turn calculate your position based on the location of the satellites and your distance to them.

Millions have come to rely on the precision of this system for a multitude of purposes, but to provide accurate position readings, the GPS system itself has to have a point of reference.

"For GPS to work, the orbital position, or ephemeris, of the satellites has to be known very precisely," said Chopo Ma of NASA's Goddard Space Flight Center in Greenbelt, Md. "In order to know where the satellites are, you have to know the orientation of the Earth very precisely."

This can be a tricky prospect because from our perspective on Earth, "everything is always moving," Ma said. For example, Earth wobbles as it rotates due to the gravitational pull from the moon and the sun. And even seemingly minor movements, such as shift in air and ocean currents and motions in the Earth's molten core, all influence the orientation of the planet.

To get a stable reference point, GPS systems have to turn to the denizens of space.

More:   http://www.space.com/scienceastronomy/091103-st-gps-quasars.html