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From: karn@thumper.bellcore.com (Phil R. Karn)
Newsgroups: rec.boats,sci.space.shuttle
Subject: Re: NAVSTAR
Message-ID: <1365@thumper.bellcore.com>
Date: 25 Oct 88 18:14:19 GMT
Article-I.D.: thumper.1365
References: <3551@phri.UUCP>
Reply-To: karn@thumper.UUCP (Phil R. Karn)
Organization: Bell Communications Research, Inc
Lines: 21


SATNAV does *not* use GPS satellites. They use the older Navy TRANSIT
navigation satellites operating in relatively low polar orbits. A SATNAV
receiver measures the Doppler shift of a satellite as it goes by, and
determines position by fitting the measured curve against a predicted
one based on a position estimate.  You have to wait for a satellite pass
to get a fix, you can't move much during the pass, and accuracy is
typically several Km.

GPS in the C/A (Clear Access) mode has been demonstrated to have
accuracies of 30m or better. This is not much worse than GPS in the
(encrypted) military mode. The original design specified that the
military mode would be 10x better, but the actual ratio is more like 3
or 5:1, depending on receiver.

GPS uses L-band spread spectrum. The military mode has the advantage of
a 10x faster chip rate (giving ten times better theoretical resolution)
plus two separate carriers to allow for ionospheric propagation delay
correction. The C/A mode allows access to only frequency.

Phil