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This story appears in the June 8 print edition of Transport Topics.

The accuracy of the Global Positioning System could be compromised as early as next year if the U.S. Air Force cannot meet an ambitious schedule for space and ground infrastructure deployment, according to a new report from the Government Accountability Office.

GAO said the number of satellites in the GPS constellation could fall below the critical 24 required to maintain the current levels of positioning accuracy. GPS currently has 31 satellites in orbit, but many are nearing the end of their operational life and will need to be replaced soon, GAO said.

Representatives of two technology firms that provide the trucking industry with GPS-based services said motor carriers could feel the effects of the problem in the form of spottier signal availability.

In particular, tracking unpowered assets such as trailers and containers could become more difficult.

“There would be a major degradation of the services these technologies offer,” said ������ýira Akbari, chief executive officer of SkyBitz, a Sterling, Va., provider of trailer-tracking systems.

Unpowered asset tracking would be hardest hit, Akbari said, because trailer- or container-mounted tracking devices rely on batteries, limiting the time they can be powered on and search for a satellite signal.

When satellite signals are weak, a typical GPS receiver can take almost a minute from the time it is turned on to secure a position fix.

Skybitz mitigates this delay with a technique known as “assisted GPS,” which uses a ground-based “position server” to process data from GPS satellites and trailer- tracking units, perform calculations that pinpoint the location of tracked equipment and forward the data to Skybitz’s customers.

Assisted GPS is not the only hedge the trucking industry has against a shrinking GPS constellation.

“There are some algorithmic things that you can do so that the user experience isn’t noticeably degraded,” said Alain Kornhauser, founder of routing and navigation software provider ALK Technologies, Princeton, N.J.

An algorithm is essentially a step-by-step checklist that helps software solve a problem. ALK’s PC-Miler truck-routing software uses algorithms to help overcome spotty GPS reception.

“The way we’ve developed all of our software, we anticipate that there is going to be some error in GPS,” Kornhauser said. “We don’t require a perfect GPS position or speed or heading absolutely all the time.”

Other providers of GPS-based services to the trucking industry include Qualcomm Inc. and GE Equipment Services Asset Intelligence. Both companies provide trailer-tracking services using a mix of satellite- and ground-based technologies. They declined to comment for this story.

Meanwhile, in an online forum in late May, Col. Dave Buckman of Air Force Space Command said, “We agree with GAO there’s a potential risk, but GPS isn’t falling out of the sky. We have plans to mitigate risk and prevent a gap in coverage.”

“Going below 24 [satellites] won’t happen,” Buckman added.

Garmin Ltd., which in March launched its first personal navigation device for truckers, also expressed doubt that GPS is in real peril.

“We firmly believe there’s no reason to fear that there will be a significant or serious outage or service interruption,” a spokeswoman for Garmin said.

“From our point of view, it’s kind of a false alarm,” she added.

To maintain current accuracy, the federal government must ensure “at least a 95% probability of maintaining a constellation of 24 operational GPS satellites,” GAO said in its report. The probability of maintaining the requisite 24 satellites could drop below 95% next year and fall as low as 80% between 2010 and 2014, GAO warned.

GPS accuracy would be compromised as a result, because obtaining an accurate position reading requires line of sight to several satellites. Getting a fix on multiple satellites will become more difficult if the GPS constellation shrinks, GAO predicted.

“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,” GAO wrote in its report, citing research from the Department of Defense.

The next generation of GPS satellites, GPS III, will not begin to be launched until 2013, according to Air Force Space Command. Originally, GPS III satellites were to be in orbit by 2011.