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From: nathan moser (no email)
Date: Thu Nov 04 2004 - 13:23:36 EST
On Nov 4, 2004, at 7:53 AM, Wally wrote:
> There was an excellent article last spring in a
> British boating publication about the vagaries of GPS
> - including the fact that there are moments when the
> system is off substantially by hundreds of yards, that
> there are holes in the coverage, that there are in
> fact numerous problems with it. That the satellites
> are getting old and not being replaced in a timely
> fashion, and that the triple built in redundancy is
> approaching or past its planned useful life...
I'd be interested to see that article! It's true that the accuracy of
"the system" is dependent upon the accuracy of the known orbits of the
satellites and the accuracy of the clocks aboard, and that errors in
these factors can't be known or corrected for by a GPS receiver.
However, there is a constant ground monitoring effort, and when errors
are detected, they're corrected by uplink to the satellite from ground
control... I guess the question is how quickly they're corrected and
how often errors occur. I don't know.
What gets people into trouble most often, is the inaccuracy inherent in
trying to receive faint signals from satellites under varying
conditions. Too many people assume they're always working with that +/-
13 ft accuracy that they saw on a fine day two months ago when there
were 10 satellites in the visible sky. Some chartplotters do a good job
of representing the current inaccuracy with a 'circle of uncertainly'
drawn around the current position to graphically illustrate that they
may be safely in the channel, or about to run up on that shoal 200 feet
away. Other (older) chartplotters require the user to go to a different
page to view the current accuracy in numerical form.
And of course the charts shown on your GPS aren't necessarily a
god's-eye view of reality. There are errors that can creep in when
translating a paper chart into electronic form, and the paper charts
used in the first place may not have been updated in quite a while.
> home. There are way too many boaters out there who
> can't say the same - and we do them no favour by not
> saying, clearly and loudly, that they must learn how
> to walk with charts and a compass before they run with
> a gps.
Rather like people who only know arithmetic through what the calculator
tells them, so when they fat-finger an extra digit while entering 128 *
64, they don't question the result of 81920.
I like how you added the "and a compass" part. I've heard a few people
say "I have paper charts!" when what they mean is they have a chart and
know how to plot the lat-long values they get from their GPS onto the
chart. If you don't have power (and those 'backup' handheld receivers
can chew through AA batteries pretty quickly!), your GPS is nothing
more than a very poor anchor.
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