Differential GPS Yields Military Quality Accuracy
|
|
|
|
|
|
|
Written by Bradley S. Melara
|
|
Monday, 09 March 2009 |
|
Differential GPS (DGPS) receivers are making a name for themselves by providing accuracy close to the military's super-precise P Code.
This new approach uses a ground station to enhance GPS C/A Code to enable GPS users without access to military P Code to achieve accuracy of less than five meters. Accuracy of this caliber is good enough even for precision aircraft instrument landing approaches.
To maintain security -- especially to deny potential enemies access to highly accurate GPS signals -- the U.S. Defense Department degrades its P code GPS signals, which are available only to authorized military users, and makes these degraded signals available to everyone as 100-meter-accurate C/A code.
Honeywell's Business and Commuter Aviation Systems Division in Phoenix, Ariz., offers a DGPS system for FAA Special Category I aircraft precision instrument approaches, which the FAA is set to certify for use by this spring, says David C. Moya, strategic product manager at Honeywell.
Military transport and tanker pilots want an FAA-certified DGPS precision instrument landing system "because most of those aircraft fly in commercial airspace," Moya says. "They are interested in a military solution that's consistent with what's available in the commercial world."
Commercial users are far ahead of the military in using DGPS, says Tyler Trickey, marketing manager of navigation products, at the Rockwell International Collins Avionics and Communications Division in Cedar Rapids, Iowa. Perhaps the primary reason: the military already has precision 10-meter accuracy with its P Code and has little need for DGPS, Trickey points out.
Trickey says he anticipates that will have a bigger military role in the future to be compatible with civil aviation. "The FAA is looking at a satellite-based differential system that's going to be broadcast all over North America, and probably in five to ten years, they're going to replace their instrument landing systems with local differential systems," Trickey says. "Military equipment will need to accommodate that."
Civil aviation officials emphasize the accuracy and integrity of DGPS. They demand virtually no system downtime -- .99999 availability -- which DGPS receivers can only achieve by detecting and excluding invalid satellite data in real time. That's done with a technique called Receiver Autonomous Integrity Monitor, better known as RAIM
This approach, however, requires a receiver with more than five channels. A 12-channel receiver, called all-in-view, can track all satellites above the horizon to calculate an accurate navigational solution. "This all-in-view technology will be inserted into the military marketplace as well," Moya says.
|
|
Last Updated ( Monday, 09 March 2009 )
|
Vehicle GPS Unit