Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!rochester!rutgers!att!cbnews!budden@manta.nosc.mil
From: budden@manta.nosc.mil (Rex A. Buddenberg)
Newsgroups: sci.military
Subject: Re: Spoofing GPS signals?
Message-ID: <6104@cbnews.ATT.COM>
Date: 1 May 89 03:01:53 GMT
Sender: military@cbnews.ATT.COM
Organization: Naval Ocean Systems Center, San Diego
Lines: 32
Approved: military@att.att.com



From: budden@manta.nosc.mil (Rex A. Buddenberg)

GPS comes in two flavors for users.  P and c/s codes
(can't decode the abbreviations at the moment).  P-code
requires the user to have a crypto key to properly
receive.  In brief, the GPS satellites 'lie' about
their position (last few bits of a word) and you can't
correct the lie without this code.  Because it is the
last few bits, the rest of the word is OK and c/s users
get service, but at a reduced accuracy level.  

The p-code also suffices quite nicely as an anti-spoof
measure.

If you are in the local area, you can circumvent any
spoofing problems by using differential GPS.  Including
the intentional spoofing due to p-code.  Requires
a reference receiver on a surveyed location as well
as the user receiver.  The diff-GPS ref receiver can tell
how much the satellites are 'lying' because he knows where
he really is.  This degree of dishonesty is then communicated
by local means to the user who then corrects his GPS
signals accordingly.  

Same differential principle works quite nicely to correct
for Loran propagation errors.

Rex Buddenberg
(usual disclaimers -- I work in USCG headquarters and nav
systems is part of my job.)