From: Sean Phelan <sean@multimap.com>
Subject: TECH: Re: GPS in helmet tracking?
Date: Wed, 03 Apr 1996 20:05:56 +0100
Message-ID: <3162CC14.3CD@multimap.com>
Organization: Multi Media Mapping


From: Sean Phelan <sean@multimap.com>

Steve Delanghe wrote:
> > My understanding is that the military have
> > much higher resolution systems, but we (the general public) cannot
> > access them.
> 
> I believe the military systems are called *differential* GPS. AFAIK,
> the GPS info is routed to a local receiver in a known location, which
> compares the GPS info with a detailed local map, calculates the
> difference (don't ask me how), and relays the amended info to the
> mobile receiver.

Hm.  I think that the original poster is right about the (U.S.) 
military, and Steve is right about how differential GPS works.  

But... the US military gets high accuracy without differential GPS. 
They have the full-function kit, whereas us civilians can only buy 
equipment that has the SA errors (Selective Availability?).  
Civilain kit is fine for nautical navigation and most other 
applications, but will not enable you to target a cruise missile.

Many more details of SA and other GPS stuff at 
http://www.inmet.com/~pwt/gps_gen.htm and elsewhere on the web.

Cheers
Sean