Hi,

     I've started working on a "just for fun" project involving an
autonomous vehicle using a GPS receiver (Garmin GPS35) and one or more PICs.
The goal is to give the PIC a destination and then have it monitor the
vehicle's displacement through the GPS receiver and then correct it's
heading in real-time. The vehicle being a solar powered boat it will be
rather slow...

     My problem is the following. "Domestic"  GPS receivers (as opposed to
dynamic GPS or military GPS) have a granularity of approximately 30-50 feet.
The boat may take anywhere from 10 seconds to a minute to travel that
distance depending on winds, currents and so on. The GPS needs a
displacement in order to find a heading, so even in the best of conditions
the PIC will only get the boat's heading every 10 seconds or so. So the
PIC's sample/calculate/correct refresh rate can be anything between 0.1Hz
and 0.016Hz. Many things can happen during that rather long period, the boat
could get turned around by a wave, it could hit something and deviate etc...
Under these conditions, I'm afraid the PIC's corrections will always be
erroneous and it will either end up in a sort of feedback loop zigzagging
(overshooting) or taking a very long time to correct small heading
discrepancies instead of keeping a straight line.

     Is there any way you can think of to shorten the sample period to let's
say 1 second without adding too much expensive hardware (gyroscopes,
accelerometers and such) ? Perhaps some kind of linear regression algorithm
or a PID loop?

 TH
 thorswil@videotron.ca

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