This article is reproduced with the permission of New Scientist for exclusive use by Nova users.

Dead satellites haunt GPS
31 August 2002
From New Scientist Print Edition.
Duncan Graham-Rowe

Enlarge GPS orbits

Defunct global positioning satellites are being junked in graveyard orbits in which they risk colliding with their active sister satellites, according to research commissioned by the US Air Force. The errant spacecraft could pose a threat to the Global Positioning System in decades to come, as well as other satellites in low-Earth and geostationary orbits.

Engineers say the risk of a collision between dead and active GPS satellites is increasing all the time. And even without a collision, the service is in jeopardy as a dead satellite that drifts within a few hundred metres of an active satellite might obstruct its transmissions.

The problem, according to engineers at the Aerospace Corporation, in El Segundo, California, which carried out the research, is the long-term influence of the Sun and the Moon.

When fuel on one of the GPS satellites is about to run out it is tucked away in a "disposal" orbit 500 kilometres above the active GPS fleet of 24 craft, which orbit at an altitude of about 14,000 kilometres. "For a good period of time the [disposal] orbit can remain stable," says Anne Gick, one of the authors of the company's study. "But the Sun and the Moon's gravity perturbs the orbit in the long term." The result is that their orbits become increasingly elliptical (see Diagram).

There are currently 16 retired GPS satellites in disposal orbits, and each year there are two more. To make matters worse, the next generation need an extra launch vehicle stage, and these too will be dumped in the disposal orbits.

Gick, and colleagues Alan Jenkin and Chia-Chun Chao, have a way to reduce the threat from the next-generation craft, which will be much more manoeuvrable than the existing ones. They suggest using their engines to reduce any eccentricities in their disposal orbits from the outset, so it will take much longer - hundreds of years - to become unstable.

But this option isn't available for the majority of the satellites already in service: most are simply not capable of making the necessary manoeuvres. And a spokesman for the GPSProgram Office, also in El Segundo, says the extra fuel used to make a disposal orbit more circular could lower its final disposal altitude.

Those that have already been ditched are also beyond help. "There's nothing we can do about them," Gick says. That's because a satellite's fuel is dumped as soon as it is placed in a disposal orbit - to avoid an explosion in the event of a collision. After the fuel is dumped, there's no way the craft can be moved, the team reveal in the latest edition of the Journal of Spacecraft and Rockets (vol 39, p 532).

In theory, mission planners could have foreseen this problem 30 years ago when GPS was first launched. "But I'm not surprised they didn't know this," Gick says. "It's a very complicated problem."

From issue 2358 of New Scientist magazine, 31 August 2002, page 8

For the latest from New Scientiist visit www.newscientist.com