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Agencies debate the health of GPS system

Air Force insists there will be no degradation, despite critical government report on satellites

The General Accounting Office has warned that worn-out satellites may not be replaced quickly enough to keep the global positioning system operating at current levels. The Air Force, however, which maintains GPS, says it is confident it can keep the system working fine.

Concerns have been raised over the ability of the Air Force to maintain the full constellation of GPS satellites at its current level.

“No, GPS will not go down,” says Col. Dave Buckman, of the Air Force Space Command, who addressed the topic in a May 20 Twitter news conference — a first for AFSPC. Buckman says GAO is right that there is a “potential risk” of some GPS degradation, but it is “very unlikely.”

“We have 30-plus satellites on orbit now,” he says. “We’ll launch another in August ’09 and again in early 2010. Going below 24 won’t happen.” And if it does, GPS managers have “proven procedures to optimize the constellation to mitigate impact,” he says.

The critical GAO report says deployment of the first of the latest generation of GPS satellites in November is three years behind schedule. It is moving ahead at more than twice the $729 million originally budgeted because of technical problems, oversight issues, and contractor mergers and acquisitions, GAO says.

It says the Air Force plans to compress the development time for the next generation of satellites by three years so it can begin their deployment in 2014 is too optimistic. It bases that contention on recent experience and the fact that the next phase involves a lot of modernization to improve the system’s performance, accuracy and security.

The government is investing $5.8 billion in the system between this year and 2013. The Department of Defense “predicts that over the next several years many of the older satellites in the constellation will reach the end of their operational life faster than they will be replenished, thus decreasing the size of the constellation from its current level and potentially reducing the accuracy of the GPS service,” the GAO report says. “It is uncertain whether the Air Force will be able to acquire new satellites in time to maintain current GPS service without interruption. If not, some military operations and civilian users could be adversely affected.”

The GPS system currently has 31 satellites in orbit. Earlier generations of satellites had a theoretical life expectancy of 7-1/2 years, but most lasted twice that long. New-generation satellites have a theoretical life expectancy of 11-1/2 years. The system is designed so that a GPS signal is picked up by four satellites that fix the position of the signaling device by measuring the different distances to the satellites. The system is designed to provide a 95 percent probability of maintaining a minimum 24 satellites in orbit. GAO predicts an 80 percent probability at times from 2010 and 2014 and as low as 50 to 80 percent from 2018 to 2020 — odds the Air Force disputes.

A Delta II launch vehicle carries a Global Positioning System satellite into orbit.

“I have high confidence we will continue to sustain at least the 24 satellites required to maintain our current performance standard,” says Gen. C. Robert Kehler, commander of the Air Force Space Command, in a statement released by the command center at Peterson Air Force Base, Colo. “Let me state emphatically: Since we declared full operational capability in 1995, the Air Force has maintained the constellation above the required 24 GPS satellites on orbit. In fact, we have achieved sub-3-meter accuracy. … At this point, we foresee no significant loss of service in the future, near or far.”

The Air Force has adopted measures to extend a satellite’s life — for instance, shutting down subsystems when unneeded to reduce power consumption from solar panels, which often are the first part to go bad, according to the GAO.

If GPS does degrade because of a smaller satellite constellation, civil and commercial users could seek an alternative in Europe’s Galileo system, which should be deployed by the middle of the next decade, the GAO says.

One decision that still must be made is if and when to put search-and-rescue instruments aboard the next generation medium-Earth-orbiting GPS satellites, which would expand the constellation of satellites equipped with gear to receive an emergency alert and locate the distress beacon that sends it. According to the GAO, the current search-and-rescue capability using low-Earth-orbiting and high-Earth orbiting geostationary satellites is “expected to degrade by 2017 and completely fail by 2020,” unless the system is augmented or replaced. The AFSPC says putting SAR instruments on the new generation of GPS satellites could cut the time to fix the location of a distress beacon from a maximum of 47 minutes using the current system to 12 minutes — with a full constellation of GPS satellites.

If the Department of Defense does not include SAR on its new generation of GPS satellites, U.S. distress beacon users “may be dependent on foreign systems which already include, or have plans to include, [SAR] in their satellite navigation systems,” the GAO says.

The agency has recommended that the secretary of defense appoint a single authority to oversee the development of the GPS system and keep it on track.

This article originally appeared in the August 2009 issue.