US Air Force waves off warnings about GPS accuracy


The US Air Force is playing down a Government Accountability Office (GAO) report released last month that warns that the accuracy of Global Positioning System (GPS) signals could deteriorate in the next few years because of delays in satellite launches.

The Associated Press reports the GAO cautioned that there is a risk that launches of new satellites will not keep pace with the wear and tear on the GPS.

That could mean that the accuracy and reliability of hundreds of millions of civilian and military GPS devices — including everything from “buddy finder” cell phone applications to guided bombs — could degrade until new satellites are in orbit, the GAO adds.

The next generation of GPS satellites, dubbed IIF, has been beset by launch delays and budget overruns.

Contractor Boeing said the delays were due to design changes necessary to ensure that the satellites would last. The work is now done, and the first IIF is slated to launch in November, nearly three years behind schedule.

The GAO, the investigative arm of Congress, said the chief risk is that the following generation of satellites, IIIA, will be delayed in a similar fashion. Lockheed Martin Corp. is building that series, and the first are scheduled to launch in 2014.

Though it’s now in widespread civilian use, GPS was originally developed for the military and it’s still managed by the Air Force.

The Air Force’s mission is to maintain a “constellation” of 24 working satellites, virtually ensuring that at any time there are at least four in the sky above any point on the Earth.

That’s the minimum number needed for a GPS device to compute its location by measuring the slightly different amounts of time it takes for radio signals to reach it from each satellite.

The satellites don’t work forever: A few launched in the early ’90s are still in operation, but most have shut down.

The delayed launch of the IIF series means that for a few years, satellites could be failing faster than they’re being replaced.

Lt. Col. Tim Lewallen, deputy director of GPS at Air Force Space Command at Peterson Air Force Base in Colorado, agreed with the GAO’s conclusion that there is a risk that service could degrade. But he said the risk is very small.

There’s substantial spare capacity in the system, Lewallen said.

There are 30 working satellites in orbit, plus three older satellites that could be reactivated. Another satellite, based on the previous-generation technology, is due to go up in August. The $30 billion system has never been larger and more accurate, Space Command says.

There are also ways to extend the useful life of current satellites if some of them break down, Lewallen said.

For instance, the Air Force could shut down other functions of the satellites to conserve power for GPS signals. These secondary capabilities of the satellites are mostly secret, but one that has been made public is that they carry sensors to detect nuclear explosions.

Per Enge, a professor of aeronautics and the director of the GPS Research Laboratory at Stanford University, said the Air Force had developed good stopgap measures, but they won’t work forever.

“No one can complain or state that the sky is falling right now,” Enge said. At the same time, delays in launch schedules and funding are difficult to account for and it’s possible that new satellite models could have problems that aren’t discovered until they are in orbit.

Enge would like to see the nation commit to maintaining a higher minimum number of satellites, perhaps 30.

“The GPS constellation is skinny compared to what it should be,” he said. “The most important thing is that we keep funding GPS and don’t take it for granted.