GPS fuzziness

Thayer thayer at FMNH785.FMNH.ORG
Fri Aug 26 10:15:05 CDT 1994


The accuracy of coordinates obtained with GPS (Geographic Positioning System)
receivers depends on the kind of unit used (about 3 broad classes) and
whether the US Defense Department has "Selective Availability" turned on.
Usually SA *is* on now; in the past, relatively few of the Navstar satellites
used for GPS were capable of SA, but now nearly all the functioning ones are.

The user should thus assume now that SA is always on.  Under those conditions,
any GPS unit operating on its own (=autonomous GPS:  push a button, get a
reading without use of a second unit or base station) can only be assumed to
be accurate to 100m in the horizontal plane (i.e., lat-long or UTM, etc.),
and worse in the vertical (elevation).  The accuracy may at times be slightly
better than that, but there is no way of telling and the accuracy varies (I
think more or less randomly) over short periods of time because of the nature
of the error introduced by SA.

Military units (available to the US military and a few other users) are able
to get around SA in autonomous GPS use by receiving a different part of the
satellites' code.  For civilians, that's not possible, and differential GPS
is the only way to get more accurate positioning.  Differential GPS is using
a roving unit in conjunction with either a broadcasting base station at a known
fixed location (=real-time differential GPS) or after-the-fact computation &
correction using data saved at the same time from the same satellites
(=post-processing differential GPS). Much greater accuracies are attainable
even by civilians using these methods (which require more than a $500 hand-held
GPS receiver): survey GPS can get sub-centimeter accuracies with the
appropriate equipment and techniques.

With regard to Desert Storm and GPS, SA was in fact turned OFF during that
time, because the US military did not yet have enough military grade (i.e.,
SA-evading) GPS receivers and had to give the troops off-the-shelf civiian
units made by several manufacturers.  (The manufacturers, of course, took
full advantage of that later by advertising various units as having been
used in Desert Storm!)  Since then, the military has gotten lots more
decoding units and SA has been on pretty continuously, although the Defense
Dept will never really say for sure when it's on.

I hope this will help clear up some of the confusion.

Margaret Thayer       thayer at fmnh.org
 Zoology-Insects, Field Museum
 Chicago IL 60605, USA   phone 312-922-9410 ext. 838




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