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The new GPS accuracy: what the U.S. military isn't saying
Highly accurate GPS signals have been available to the public for years -- so the new "unscrambled" transmissions may have a deeper military significance

The new GPS accuracy: what the U.S. military isn't saying
May 2, 2000

GPS signals are used by civilians to pinpoint locations of planes, boats and cars -- but the military probably uses it for much more
GPS signals are used by civilians to pinpoint locations of planes, boats and cars -- but the military probably uses it for much more
On Monday, the White House announced that the Global Positioning System (GPS) - the satellite transmissions used to pinpoint the location of aircraft, sea and land vehicles and even people worldwide - would become doubly accurate for the average person. Monday night marked the removal of selective availability (SA), a process that altered the signals received by civilian GPS users, making positioning accurate only to within 100 metres, while the military still had access to the undegraded signal accurate to within 20 or 30 metres. The "unscrambling" is being hailed as a big step forward for rescue operations and aviation safety.



Differential GPS -- accurate up to one metre -- has been available for years
Differential GPS -- accurate up to one metre -- has been available for years
It sounds like a great advance -- but it's not exactly what it appears. Differential GPS -- corrected GPS signals accurate to within one to three metres -- has always been available to the public. In fact, civilian users are likely to still require the corrected signals, SA or no SA. But the decision to allow an undegraded signal to be received indiscriminately implies that the United States military has found new, secret ways to overcome use of the GPS by hostile forces.

"Since SA was implemented there have been differential systems capable of giving one to three metre accuracy," points out Peter H. Dana, a GPS systems designer in Georgetown, Texas, who has consulted for government and businesses for the past two decades. "So it seems clear that SA wasn't implemented for 10 years to prevent people from getting one- to three- metre accuracy. One would speculate there were more sophisticated reasons for implementing it."

SA works, for the most part, by intentionally dithering the clocks in the 24 satellites that make up the GPS system. The clocks are steered by an algorithm, known to the control station and military receivers and transmitted in encrypted code over the satellite signals.



The corrected GPS signals are already used in aviation, where it really matters
The corrected GPS signals are already used in aviation, where it really matters
"It's a very specific pseudo-random pattern," says Dana. "It's constantly changing and doesn't average to zero except over many hours."

But according to Dana, while this intentional degradation does make GPS accuracy drop from about 30 metres to about 100 metres, DGPS is available - and necessary - to just about everyone who uses the system.

"There aren't very many applications that fall into that odd category where 30 metres is helpful and 100 metres is not," Dana points out. "Beacon DGPS systems provided by the Canadian and U.S. Coast Guards can be used with a $300 or $400 (U.S.) box by anybody. Except for the box, those are free services. There are also private services that transmit communication satellite corrections to large areas of North America, South America...[They] can be used by anyone and only cost $1,000 to $2,000 per year, so they are not exclusive systems at all."



GPS signals can also be used to communicate encrypted messages to military forces -- without interception
GPS signals can also be used to communicate encrypted messages to military forces -- without interception
But SA was supposed to be some kind of security measure - and removing SA doesn't benefit anyone - what's all the hoopla about? Well, according to Dana there may be some isolated situations where the greater accuracy will help save a life. For example, if a person calling 911 had a GPS receiver in his cell phone, and there were no buildings to block the GPS signal (the receiver must have a clear view of at least four satellites to compute its position), emergency crews might be able to find him quicker.

And the removal of SA also highlights the other, more sinister uses one could make of the precise time and frequency information provided by the GPS signal - and the fact that the U.S. military is, for some reason, no longer concerned about them.

"There are some applications DGPS doesn't help you with [when there is SA], such as remote synchronization of crpytographic equipment," Dana muses. "If you communicate with your forces through high speed encrypted communication, you might prevent the U.S. military from overhearing you. It might have been possible [without SA] for terrorists or some paramilitary organizations to communicate using GPS without the Department of Defence being able to intercept them.

"Or they may have been able to evaluate the bias and accuracy of the inertial navigation systems of submarine launch missiles very quickly. When launched, the missiles are perturbed by the water and when they get in free air you have to rate those inertial sensors somehow -- you can do that very nicely, very quickly, with undegraded GPS."

Dana says those of us without access to the rationale underlying SA in the first place can only speculate as to why these possible hostile uses of the GPS are no longer a concern.

"It is possible in the meantime that the Department of Defence either has seen that threat disappear, or has found other ways to counter it, or doesn't care anymore," he suggests.

For more about how the GPS works, visit the University of Texas Global Positioning System Overview page.




Story by:
Tamar Simon

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