From: John De Armond
Subject: Re: Good news for GPS users
Date: Wed, 03 May 2000 04:35:57 EDT
Newsgroups: alt.rv,rec.outdoors.rv-travel

"George E. Cawthon" wrote:
> 
> According to the news the military is still reserving (using encryption)
> the still more accurate use of GPS.  Capability of some GPS use
> (differential maybe) is very high.  I remember looking at an article in
> an engineering mag several years ago where they were using GPS to follow
> the movement of one end of the San Francisco Bay Bridge (or whatever the
> official name is). The accuracy was supposed to be in millimeters.

Yes.  This is carrier phase GPS.  A friend of mine who owns a survey
company has a pair of units (at about $30k a pop).  The rated
absolute accuracy is +- 0.1 mm over any distance the two units can
communicate via UHF.  In brief, the way these things work is they
get a rough fix using GPS, get a much better fix using differential
GPS and make the final measurement by measuring the phase difference
of the GPS carrier (and not its data stream) as it arrives at the
two receivers.  Because the wavelength of the carrier is so short,
the less accurate modes must be used to get the fix very close
before using the carrier phase mode.  The UHF link is used to
transport the phase data between the units.

Very impressive units.  He told me that the majority of his business
now is oriented toward millwrighting, that is, laying out and
overseeing the installation of large industrial facilities such as
conveyor belts and lehr lines that require precise alignment.

John