GPS Error
Guy,
I am an engineering student, and I just finished a couple of courses in
GPS, so I will try to give you and explanation of your error.
First, since the GPS became officially operational a short time ago,
selective availibility has been in effect. With SA on, the DOD "guarantees"
that your calculated 2D position will be correct within +- 100 metres, about
95% of the time. Your error may be a result of that 5% that does not occur
very often.
Second, your reception of the GPS signal is subject to the effects of
multipath or in laymans terms, refletions. This means that instead of the
signal reaching your reciever directly, it is reflected off of some object and
then to your reciever, thus giving an incorrect range to the satellite. I have
heard of errors up to 20 metres or so due to multipath, but anything is
possible.
Third, the accuracy of your position is very much dependent on the
satellite geometry. You can check how good the satellite geometry is by
looking at the PDOP (positional dilution of precision) value that the reciever
reports. The smaller the value, the better the geometry (should be lower than
3).
Hope that sheds some light on your problem. If you are interested in GPS, see
"Satellite Geodesy", 1993, by Seeber, or GPS World magazine.
Rob