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GPS Accuracy Could Start Dropping in 2010

We've all become accustomed to using the Global Positioning System, or GPS, whether via our iPhones, car navigation devices, handheld GPS units, or even watch-based devices like the Garmin Forerunner series. The GPS system went into full operation in December of 1993, was declared a dual-use (military and civilian) system by President Bill Clinton in 1996, and in 2000 had "Selective Availability" removed to increase accuracy for civilian uses. It relies on a constellation of between 24 and 32 medium Earth orbit satellites, some of which have been operating for nearly 19 years. Unlike other national satellite navigation systems, GPS serves the entire world and is maintained by the United States Air Force 50th Space Wing.

So far, so good, but TidBITS reader Mike Craymer, a geodesist who studies the size, shape, and temporal variations of the Earth, recently alerted me to a report about a possible future problem with the accuracy of the GPS system. Mike and his team at Natural Resources Canada use very high-end GPS receivers and special data processing techniques to measure the motions of the Earth with an accuracy of about 1 mm per year in their work defining and maintaining the coordinate systems used in North America and in contributing to the global coordinate system used by GPS. Needless to say, Mike is very interested in GPS maintaining its high level of accuracy.

The problem is that, at the end of April 2009, the U.S. Government Accountability Office released a report expressing concern about the Air Force's modernization and maintenance of the GPS system. Constant replacement and upgrading of satellites is necessary, especially with hardware that's been operating in space for almost two decades.

The GAO's report draws attention to problems that the Air Force has had in working with contractors to build and launch GPS satellites within cost and schedule goals. Some of the problems stem from government acquisition methods that didn't provide for enough oversight, and added requirements that resulted in cost and schedule overruns.

The GAO also lays some of the blame on a series of industry mergers (Boeing buying Rockwell, Boeing merging with McDonnell Douglas, Boeing buying Hughes Electronics Corporation's space and communications businesses) that resulted in moving the GPS work repeatedly and losing knowledgeable workers.

Delays in launching new satellites - the next one is scheduled for a November 2009 launch, almost 3 years late - could be problematic if the older hardware starts failing. The GAO has calculated - using reliability curves for each operational satellite - that the probability of keeping a 24-satellite constellation in orbit drops below 95 percent in 2010, and could drop as low as 80 percent in 2011 and 2012. And if the Air Force doesn't meet its goals for the next-generation GPS III satellites, the probability drops to around 10 percent in 2017. (The GPS III satellites bring with them new features, including new military and civilian signals for greater accuracy, particularly in urban environments, plus higher power for current civilian signals, which will help existing GPS receivers.)

Even if the satellite constellation drops below 24 satellites, that doesn't mean GPS service will fail altogether. It does mean that the level of accuracy that both military and civilian users have become accustomed to - which is actually higher than promised - may degrade significantly.

The GAO has made recommendations for addressing the problems it found in the handling of the GPS system, most notably a single authority to oversee development. Apparently, while the Air Force is in charge of the satellites and ground control, various other branches of the military develop their own user equipment, which makes for coordination problems as the technology is updated and improved.

Another solution may come in the form of international cooperation. Although the GPS system is available worldwide, the European Union has a proposed global navigation satellite system called Galileo in the works, currently due to come online in 2013. Galileo is intended to be a largely civilian system that wouldn't operate at the whim of the U.S. government in time of military conflict, although a 2004 compromise makes it possible both for the United States to block the Galileo frequencies and for the two systems to interoperate in the future. Galileo also aims to provide greater accuracy than GPS, and when combined with the next-generation GPS III satellites, could improve accuracy even further.

Russia and China also have satellite navigation systems, and there have been discussions with the Russian government about making the Russian GLONASS system compatible with both GPS and Galileo. No formal announcement has emerged from those talks, but in 2007, GLONASS was opened up for civilian use. GLONASS has a checkered history, coming online in 1995 but falling into disrepair soon after, due to Russian economic troubles. Russia committed to restoring the system in 2001, with a full 24-satellite constellation (necessary for global coverage) in operation by 2010.

From the standpoint of normal people in the United States, there's nothing to do except wait and watch, and, if the opportunity presents itself, make sure your elected representatives are aware of the situation. If you think about how essential GPS-based services have become to society at large in the years since Selective Availability was abolished, you can imagine how much more important those capabilities will become in the next decade.

Personally, I can't imagine that the Air Force would let GPS accuracy drop, especially given the military's reliance on it for everything ranging from helping soldiers find their objectives in the dark to accurate targeting of missiles and other precision-guided munitions. If the GAO's suggestions (with which the Department of Defense agrees) don't resolve the problems faced by the GPS system, the Air Force may end up spending far more money to patch the system on an interim basis.


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