Re: [PSUBS-MAILIST] Re: UW-DGPS

[Axel Iehle <ai@augusta.cma.fr>]

> On Mon, 20 Mar 2000 10:08:57 -0700 (MST), John Brownlee
> <jonnie@cobweb.scarymonsters.net> wrote:
> 
> > I don't know if they knew in advance about
> > the ability to differentially correct the signal with multiple units on
> > some non-negligible baseline, or the fact that it would be pretty hard to
> > fit a DGPS system in a small, compact system like a missle was
> > discouragement enough.
> 
> As far as I know, the DGPS system is a land-based radio-tower type
> correction, and can (presumably) be shut down if required.
> 
> Later,
> Jon

Jon,
DGPS is not necessarily land-based. It can be used on sea.
DGPS system cannot litteraly be switched off.
Principle is you have a fix receiver at a fix point of known position (OK that 
one cannot be seaborne).
Then a second one at the position (can be mobile) you want to locate.
The idea is that, providing you're not too distant from the fix receiver,
the error induced by the Selective Availability(SA) is similar for both 
receivers.
Thus, if you can extract this error for the fix receiver (using the knowledge 
you have of the real position where you placed it), you can use this error to 
correct the coordinates displayed on the rover-receiver.
Thing is you must use the same constellation of satellites for the two 
receivers, and have to synchronize the readings. Plus the fact that one 
receiver
must compile the info from the other one. All this is done via radio 
transmission between fix and rover receivers.
Now you can also use Radio-DGPS: Some companies provides the DGPS error 
message through the world using large emitting antennae. Some are free some 
are not.
Coverage of these signals is of course not global, plus the fact that the more 
distnat you are from the DPGS emission the worst the correction is. There are 
web sites on that subject (one with radio-DGPS coverage if I'm right), I'll 
dig that out if someone is interested.
True that SA is sometimes turned off. Gulf War is THE example cited everywhere 
for that (it was said that US army didn't have enough military GPS units so 
they provided the GIs with standard GPSs and switched of the SA).
Russians have GLONASS satellite constellation that give another way to get 
precise location. This system has no SA, but is still a little less precise. 
So globally it equals the GPS with SA. But they should improve and this could 
be one of the reasons why it has been said (who said that?) that the SA should 
be definately turned off by 2004... We'll see. I also heard (but take this 
with care as I just read it once and have no other news since then. Maybe 
someone can confirm?) the Europeans were building their own system?

Regards,

Axel