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[DPRG] Further Quandaries on GPS
Subject: [DPRG] Further Quandaries on GPS
From: John Abshier
jabshier at kc.rr.com
Date: Tue Jan 29 13:00:27 CST 2008
I made similar plots to Dr. Anderson's. A thing to note is that the errors
are higher correlated. This means that averaging a small number of readings
does not help. I have reached the following conclusions about using
consumer grade GPS's. They will not keep you on a sidewalk or road. They
will not take you to a RoboMagellan cone, but will get you close provided
you have a clear view of the sky. At most locations, only one of the two
WAAS satellites will be in view and it may often be blocked. I cannot see
it from my house. If I were using it for contests 1, 2 and 3 (travel on the
order of 100 feet), I would calculate distance and heading from the contest
data. When the robot runs the contest, I would read the current reported
location as its starting position, calculate end coordinates from distance
and heading and use those two locations to navigate. This is not intended
to say that GPSs are useless. Odometry error grow without bound. GPS
errors have a fixed bound. Poor odometry over "short" distances or good
odometry over "long" distances can use a GPS to cap the error.
The discussions on filtering versus classification and use of a Kalman
filter are interesting with respect to GPS errors. As mentioned above, GPS
errors are correlated. But they often have large jumps when the satellites
in use change. How should a filter handle the large jumps in position?
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