RE: GPS

For a really basic description of GPS:

http://www.garmin.com/aboutGPS/

Currently civilian GPS is about the same accuracy as military GPS (about
3m) but when SA (Selective Availability) was active the accuracy for
civilians was 100's of m to 1.5 km.  The military turned off SA a few
years ago.

The military reserves the right to add SA whenever they wish as GPS is a
military system which they happen to allow civilians to use.  As it is a
US military system it makes our European Allies a bit nervous and they
are setting up their own GPS satellite network.

Differential GPS depends on known land sites to send out GPS signals -
effectively becoming another GPS satellite, however they are limited by
radio range to be effective.  Such locations would be known or easily
discovered and could be knocked out in a war, since they have to
transmit to be effective.

Who says military research doesn't have civilian applications? :)

--Binhan

> -----Original Message-----
> From: John Sowerby [mailto:sowerbyj@fiu.edu]
> Sent: Tuesday, September 03, 2002 11:41 AM
> To: gzg-l@csua.berkeley.edu
> Subject: Re: GPS
> 
> 
> 
> >I'm not sure what you mean by "open system". The GPS 
> system's always been 
> >pretty open, at least the civilian side.
> 
> When first out, the errors in position using civilian GPS 
> could be quite 
> large, particularly in terms of elevation, all due to the 
> futzing of the 
> signal by the military. Now, even the standard receivers are 
> much better, 
> as the military have relaxed the rules somewhat.
> 
> >>2) Can differential GPS (multi-receiver) defeat or significantly 
> >>attenuate the futzing up signal? (That is, can't 
> differential GPS be used 
> >>in such a way as to amerliorate the effects of signal uncertainty 
> >>introduction)?
> >
> >Yes. However, both the mobile and stationary receivers have 
> to have one or 
> >more satellites in common. The correction is on a 
> satellite-by-satellite 
> >basis, so the more you have in common the better.
> 
> Which is almost a given, as the fixed reference point changes 
> with locality 
> anyway.
> 
> As for usage of GPS, when my father in law to be can plot a 
> course for the 
> yacht using his computer, and the thing can steer itself through the 
> channels in the Port of Miami and down to the Keys, or across to the 
> Bahamas relying on the laptop and the boat's differential GPS 
> system (two 
> receivers and a reference station in South Florida), then the 
> system can no 
> longer be futzed up too badly.
> 
> John.
> 
> 
>