roger said:The message <[email hidden]>
from "Paul Saunders" <[email hidden]> contains these words:
Quoted message said:Quoted message said:I start all my trips at home, at a known elevation, so I start by
plugging my GPS into the ciggy lighter in the car, wait until it gets a
lock, then calibrate to 71m. By the time I get to the start of the walk
the auto-cal will have settled down.
Quoted message said:That's what I have been doing of late. (Well, almost - height 535 feet,
not 71m). But after driving 100 minutes plus I have still been getting
that large discrepancy between start and finish height on the walk.
Autocal only works when there are enough sats to get a 3D fix. If the
GPSR can only see 3 sats, or if there are more but in a poor config,
then the unit defaults to 2D navigation, i.e., it assumes height isn't
changing and works out horiz position on the basis of that
assumption. Of course autocal doesn't work when satellite lock is lost
anyway.
In other words, if you want autocal to work, i.e., good barometric
altitude tracks, there have to be frequent enough periods of good
enough sat view for GPS altitude to work.
Lots of folk assume that if the GPS can locate you, then autocal must
be working. Not so if you're only getting a 2D fix. Unfortunately many
GPSRs don't tell you if they've defaulted to 2D fixing, and if not,
there's no easy way of finding out, since >3 sats and config too bad
for 3D happens more often than 2D due to just 3 sats. There *is* a
fiddly way of finding out: start the process of trying to manually
calibrate the altimeter to GPS altitude. If it refuses to let you do
so, then it's using 2D position fixing and hasn't got a GPS
altitude. You don't need to complete the process (and possibly reduce
the accuracy of the altimeter) if it *will* permit it, you just need
to see that it would have permitted it.
I'm so glad I forked out the extra money for a GPSR with
autocalibrated barometric altimeter. I suspected it would be more
interesting than a plain vanilla GPSR, but its often apparently
baffling behaviour has provided far more than its money's worth in
terms of interesting puzzles, entertaining experiments, and education!
--
Chris Malcolm [email hidden] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]
