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russotto · 11-06-2003, 10:49 AM

The artificial margin of error (Selective Availability) for civilian GPS was discontinued in May 2000. Civilian units still only have access to the C/A code, which is nominally accurate to 15 feet. The military gets the P(Y) code as well, which is nominally accurate to 1 foot. Fancy civilian units can still use the L2 signal to correct for ionospheric interference; this gets you more accuracy, as do the better antennas on the fancy units. Wide area differential GPS (which is available on really cheap units) can also get you down to the 1 foot range if you can get the signals. Survey-type differential GPS (still civilian) can get you to the sub-cm range, but not in real time, and it requires two units, one with an accurately-known position.

11-06-2003, 10:49 AM	#15
russotto Professor Join Date: Jan 2001 Posts: 1,788	The artificial margin of error (Selective Availability) for civilian GPS was discontinued in May 2000. Civilian units still only have access to the C/A code, which is nominally accurate to 15 feet. The military gets the P(Y) code as well, which is nominally accurate to 1 foot. Fancy civilian units can still use the L2 signal to correct for ionospheric interference; this gets you more accuracy, as do the better antennas on the fancy units. Wide area differential GPS (which is available on really cheap units) can also get you down to the 1 foot range if you can get the signals. Survey-type differential GPS (still civilian) can get you to the sub-cm range, but not in real time, and it requires two units, one with an accurately-known position.