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GPS Signals Have Various Levels Of Quality
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Written by Brian J. Sero   
Monday, 09 March 2009
A military grade GPS receiver isn't necessarily built to withstand shock, vibration, or temperature extremes. Instead, it means the receiver resists electronic jamming and has a special security module to access extremely accurate signals reserved only for the military.

Military GPS signals are on precision frequency, or P Code, explains Bill Walters, sales manager at Trimble Navigation in Sunnyvale, Calif. P Code provides accuracy to within 16 meters. When encrypted, P-Code is called Y Code. P/Y Code is also sometimes called PPS, or Precise Positioning Service.

"PPS offers the highest accuracy," says David C. Moya, strategic product manager at Honeywell's Guidance and Navigation Operation n Clearwater, Fla. "All of the intentional errors introduced by the space segment are removed.

So-called SPS code, short for Standard Positioning Service, retains some errors and is less accurate.

Commercial GPS devices operate off of the L-1 frequency, or Course Acquisition (C/A) Code "The accuracy of C/A Code is slightly degraded by what's called selective availability, explains Bruce Hensel, GPS product manager at the AlliedSignal Aerospace Commercial Avionics Systems division in Olathe, Kan.

Commercially available devices can only provide accuracy of about 100 meters because of the intentional errors introduced into C/A Code. To make up for this, military and commercial users can use differential GPS, which compensates for C/A code errors with ground stations to achieve 5-meter accuracy.

Companies are looking into other ways to ensure high levels of accuracy. One way is through direct access to P/Y Code, which isn't available today.

It takes a receiver a long time to lock onto P-code signals. C/A code was originally a companion code with quick-repeating, low-resolution code that receivers track very quickly to provide crude estimates of time and position, explains Tyler Trickey, marketing manager of navigation products at the Rockwell International Collins Avionics and Communications Division in Cedar Rapids, Iowa, C/A code would read the data and hand it over to P Code.

The problem, however, is C/A code has errors introduced into it and can be jammed either by U.S. users or the enemy. This could be a potentially sticky situation if a military user can't access the P-code because the C/A code is jammed.

Trickey says Rockwell-Collins is developing products to acquire P Code directly without stepping through C/A code. He predicts the company will have a product with direct P/Y-access capability in 1997 or 1998.

Last Updated ( Monday, 09 March 2009 )