From: isdale@illusioninc.com (Jerry Isdale)
Subject: Re: TECH: GPS in helmet tracking?
Date: Fri, 5 Apr 1996 11:22:29 -0800



 Steve (zulu@cix.compulink.co.uk) wrote...
>I believe the military systems are called *differential* GPS.

The Military GPS is not Differential GPS (DGPS). It uses an encryption
scheme to get more accurate data from the satellites.  It can get
about 10meters CEP (circular error probablity), I think, perhaps
better. The DGPS systems can get centimeter or better accuracy,
depending on how long you let them sit.

DGPS uses data collected from a high quality receiver at a known
location.  This data (the constellation of satellites seen w/errors
from known location) tagged with date/time and can be used to post
process the field roaming GPS data later or broadcast to systems to do
real time DGPS.  The US Coast Guard and other services provide DGPS
broadcasts in many areas.  Many DGPS receivers have the built in
processing to do on-board correction.  Several of the PCMCIA GPS cards
are included in this bunch.  'Course you need to have two antennas
(one for GPS one for FM reception of Diff info), possibly supplying
your own diff radio receiver.

Jerry Isdale
Illusion Engineering, Inc.
2660 Townsgate Rd., Suite 530, Westlake Village, CA  91361-2714
Phone:          (805) 371-4530        FAX:    (805) 371-4533
Internet:       isdale@illusioninc.com