Olin Lathrop wrote:
> The deliberate error was removed some years ago.  Today DGPS corrects for
> systemic errors from to the satellites bouncing up and down due to
> gravitational anomolies and other errors that are unpredictable or can't be
> corrected for without a large database beyond the capability of low cost
> receivers.

I belive we have Regan to thank for that, btw.  Must have been between naps.

Selective Availability is turned off now, but the military reserves the 
right to enable it again without warning.  The military also taught the 
sats to turn SA on only when over certain areas, such as, well, whoever 
we're illegally bombing today.  However, practically, they can't enable 
it globally without planes crashing.

This is one reason the FAA helped sponsor WAAS, which does three things:

	They act like normal, civilian-grade L1 transmitters.

	They transmit a DGPS-like signal.

	They transmit atmospheric correction information.

So, with WAAS, a large part of the atmosphere is "fixed" -- the 
propagation delay through parts of it is given to a receiver in 
real-time, so it can correct in real-time.

For more info on WAAS, see http://gps.faa.gov/FAQ/index.htm

However, it is still fairly vague on resoloution of this data since it 
would be impossible to transmit (say) 1 meter details fast enough to matter.

There are some receivers that use the civilian band to get a rough fix, 
and then do some tricks with the military band to get a more accurate 
fix.  Since the L1 band is unencrpted, and the L2 band is the military 
encrypted one, I don't know what sort of tricks they can pull with it, 
but who knows.

I believe the military receivers use both L1 and L2, probably using L1 
for course position and L2 for fine-tuning.  The one military unit I saw 
also had a "kill switch" -- hitting two specific but badly marked 
buttons at once would wipe the decryption key from the unit.

--Michael
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