GPS primer

Don at Don at
Fri Aug 26 10:04:23 CDT 1994

Hi Tom and Netters,

        As far as Global Positioning System (GPS) goes, the best resolution in
history was obtained during Desert Storm.  Strange as it may seem, there is a
bit of history to the story ...

        The Department of Defense (DOD) is the primary funding agency for GPS,
and GPS was orginially intended to be used as guidance systems for the nuclear
warheads deployed on submarines.  Because the satellites will transmit data to
anyone with a receiver, the DOD decided to "scramble" their signals by turning
on Selective Availability (SA).  SA is basically a randomization algorithm that
plays with the atomic clocks of the satellites, and only DOD personnel have the
"codes" to unscramble it.  What it effectively does is through your position off
by 100-300 meters, and because it is randomly affecting all the satellites in
orbit, you have no way of knowing if the satellites you are using to take a
position are being affected ...  (that way none of us civilian terrorists can
send a missle down a silo).

        As Desert Shield/Storm was brewing, the last of the 26 satellites were
being deployed.  GPS was becoming FULLY functional.  The problem was that the
DOD did not have enough military issue receivers that could "decode" the SA.
In order to solve this problem (so they could fight) they bought as many
civilian receivers as possible.  However, they had a problem, the civilian
receivers could not "decode" SA.  To solve this, the DOD turned off SA for a
very small window of about 4 months.  They of course turned it back on as soon
as they could.

        Civilians that began using GPS during Desert Storm/Shield were obviously
mad when SA came back on because all their new GPS locations were in question.
However, there is a technique called Differential Correction (DC) where if
you have a known location (know by standard surveying techniques) and that
known location is receiving the same satellite information that a rover is, you
can back-track and "fool" the SA and figure out your position.  (basically you
follow the randomization of the known point and extrapolate to the point
out in the field).  Using this technique, you can fully resolve the GPS.

        Of course, this brings in another problem.  You can by GPS receivers and
you can BUY GPS receivers.  Right now, the better the resolution, the more
money.  It is totally related to the solid state engineering to the unit.  Using
DC you can get measurements plus or minus a cm, sub-meter, 3-5 meters and
obiviously higher ... It is totally dependent upon the equipment that you own as
well as the "base station" (the recevier tracking at the known location).

        Well, in a nutshell, that is the basics on GPS.  It is a wonderfull tool
but as of now is quite expensive (the best thing to do is look around for a
base station (they have an effective range of 2-300 miles) and then get a
receiver and get your points DC by the folks with a base station).

        For those wondering, the University of Arkansas has a base station and
makes its GPS points freely available via an electronic BBS.  We also teach
a two day class in the fundamentals of GPS and techniques of GPS.  Any one
that would like to register for the class or has more questions, please feel
free to contact me.  I hope this kind of clears up a few things about GPS.

                Don Catanzaro
                dgc at

Center for Advanced Spatial Technologies
Ozark 12
University of Arkansas
Fayetteville, AR  72701

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