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From: smb@ulysses.att.com (Steven Bellovin)
Newsgroups: sci.military
Subject: Re: Global Positioning of Scud launchers
Keywords: Global Positioning, Navstar, Scud
Message-ID: <1991Feb9.031056.1850@cbnews.att.com>
Date: 9 Feb 91 03:10:56 GMT
References: <1991Feb8.014206.18215@cbnews.att.com>
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From: smb@ulysses.att.com (Steven Bellovin)

In article <1991Feb8.014206.18215@cbnews.att.com>, cl@lgc.com (Cameron Laird) writes:
> I've come across a claim that:
> 1.  Iraq bought some number of US Global Positioning
>     System units within the last decade;

Dunno.

> 2.  everyone, including, in particular, hostiles, has
>     access to communications from Navstar satellites;

See below.

> 3.  automated positioning information of the sort GPS
>     provides helps missile launching crews speed their
>     set-up.

Dunno, but it does seem likely.

The GPS satellites transmit two different signals.  One is intended for
general civilian use, and receivers are commonly available.  I have no
idea whether or not there are export restrictions on these receivers,
but I'd doubt it.

There's a second signal, for military use; this signal is transmitted
encrypted.  It allows calculation of a much more precise location.
Additionally, the civilian signal's accuracy can be degraded even
further by the Pentagon, thus -- in theory -- denying an enemy precise
navigational information.

Unfortunately, it turns out not to work that way.  For one thing, the
military had too few secure GPS receivers; they've been forced to buy
large numbers of civilian receivers to support Desert {Shield,Storm}.
Thus, they can't degrade the civilian signal any more or they
themselves would be hosed.  Besides, it turns out that if you have
another receiver located in a known location, or an additional source
of info such as LORAN, you can do some arithmetic and calculate your
position to a very high degree of accuracy.  Thus, neither the
encrypted signal nor selective degradation have proved to be
particularly useful

(Source:  a recent NY Times article, which has unfortunately been fed
to the Recycler.)


		--Steve Bellovin