Satellites could navigate by X-ray stars

Future satellites could use X-ray-emitting stars – rather than GPS signals – to get their bearings, according to plans by the US military.
The X-ray navigation technology would be less vulnerable to enemy interference than GPS and could also be used far from Earth to help interplanetary space probes keep track of their positions.
Satellites in low-Earth orbit locate themselves in space by using signals from higher-orbiting Global Positioning System satellites. US defence agencies are concerned that enemies of the states could try to jam GPS signals or otherwise interfere with the system.
If GPS suddenly became unavailable, it could cause satellites to leave their proper orbits. "The satellite will just slowly over time drift out of its orbital slot," says David Cavossa, executive director of the Satellite Industry Association in Washington DC, US. "It could start to get closer to other satellites and eventually it could hit another satellite."
Unique frequency
The desire for alternative navigation systems has prompted the US Defense Advanced Research Projects Agency (DARPA) to investigate using naturally occurring X-ray beacons for satellite navigation. The beacons would be rapidly spinning stars, called X-ray pulsars, which emit regular pulses of X-ray light as they spin.
Each pulsar has a unique frequency and location in the sky. Locking onto any one of them would tell the satellite which direction is which in the sky.
The signals from several pulsars can be exploited to determine a spacecraft's position and velocity in a somewhat similar way to GPS. The pulsar signals are ideal for this, because their stability rivals that of atomic clocks.
"It's very difficult to jam an X-ray source," says John Goldsten of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, US. "It might bridge the gap if we were to lose GPS for a period of time."
A further useful element of the system, from a defence point of view, is that X-ray detectors are not easily "blinded" by lasers – which might be aimed at them maliciously – or by being pointed accidentally at the Sun, unlike visible-light cameras used to track regular stars.
"There are all kinds of threats to navigation systems – not just GPS," says DARPA's Darryl Pines, manager of the X-ray Navigation and Autonomous Position Verification (XNAV) programme. "What we're trying to do is develop a system that provides the Department of Defense with a navigation capability as a backup."
Beyond the solar system
The system might work as more than just a backup, however. Satellites are typically tracked from the ground by radar, but using X-ray navigation, they would be able to determine their own position without relying on the ground.
And unlike GPS, it would also work beyond Earth orbit, Pines says: "This would be for anywhere in the solar system: low-Earth orbit, highly elliptical orbits, interplanetary, or even beyond our solar system." For interplanetary space probes, X-ray navigation would be at least an order of magnitude more accurate than current techniques which rely on tracking from Earth, he says.
Tests of prototype sensors are in progress at NASA's Goddard Space Flight Center, Goldsten says. The results will be presented for review in September, when DARPA will decide whether to continue the development of XNAV. The second phase of the project could begin as early as November 2006 and would see an X-ray sensor built for testing in 2009 aboard the International Space Station.
An overview of the XNAV programme was presented at a conference on small satellites in Logan, Utah, US on 16 August by David Becket of Ball Aerospace, which is also helping to develop the technology.