Date: Wed, 8 May 2013 22:45:56 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
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From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: GPS speed accuracy
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At 04:20 PM 5/8/2013, george jannini wrote:
> My Garmin Sat Nav receivers display navigational accuracy between 7
>feet and infinity (lost in the woods) I can put two of them side by each on
>the dash, the 2004 model says I'm within 7 feet, but a 2008 model with a
>much more sensitive chip is always at least double that.
If you seriously want a device that will output Doppler data with its
track data so you can analyze speed very closely after an event, the
Locosys GT-31 is your baby, and it's not even expensive. Real-time
Doppler speed in a GPS is either completely unavailable or costs
stupid money. Furuno has a depth sounder that will do it for
$80,000, but that's not a GPS.
However based on readings I've taken in the last couple hours, a GPS
unit relying on a single pair of position fixes to determine speed
could have an error of at least 4 mph for a single reading taken at
60 mph. However the great bulk of the time it would be 1 mph or
less. I've found one (marine) unit that quotes an RMS (Root of the
Mean of the Squares) figure of about +/- .35 knots, meaning the
reading will be within that most of the time (66%? I forget). I
think that +/- half a statute mile per hour the great bulk of the
time would be fair for automotive GPS.
There is always jitter in the signal. If you want to see it, set
your receiver to track movement with a recording interval as often as
possible and leave it in one place for an hour or two. Many
automotive-type GPS units may not show this because they are
programmed to latch onto the nearest road and if they're within a
couple hundred feet of a road will show you as being on it. To get
them to display jitter I suspect taking them far from any roads or
trails would work. I've got my Garmin GPSmap 76 collecting some data
now, with WAAS enabled, using an external antenna. With claimed
accuracies mostly from 6.8 to 7.6 feet, I'm finding each successive
fix is somewhere between 0.5 feet and 1.6 feet different from the
previous one. So far, after an hour or so of tracking, the various
excursions could be covered by a fifty-foot circle.
Well now. Even as I finished writing that, our friend took a hike to
the ESE at a maximum of 4.5 feet between fixes with claimed accuracy
of 10 feet. It would now take a 100 foot circle to cover his
perambulations. He then went roughly due north and is gradually
heading WSW to slowly get back to his previous haunts; but he seems
to have taken up residence for the moment about 35 feet west of them
and is starting a new position cluster there. 1 to 2.2 feet between fixes.
Three different runs of roughly 1,000, 2,000, and 3,000 seconds each
produced a track that could be covered by a 100-foot circle, with
most of the readings occurring within +/- 25 feet of the "center of
mass". I do not have any way to determine the absolute positional
accuracy of the fix but the center seems to be within fifty feet of
the true position.
Next, there is a factor called HDOP, or Horizontal Dilution Of
Position (also VDOP but it's of less importance to us). This is a
figure of merit for loss of accuracy in the system caused by the
geometrical configuration of the various satellites visible to the
receiver at a given time. Therefor at certain times of day in your
location, the position accuracy of the system will be degraded
because of the need to work with small angles (same reason the
altitude measurement is only specified to within +/- 100 feet, the
vertical angles are much smaller than the horizontal ones). I've
been sitting here watching the HDOP go up and down between about one
and four, and watching the GPS claimed accuracy roughly tracking it
between 8 and 23 feet.
Third, there is a systematic error introduced by the GPS receiver and
its software. If you take five different GPS units of different
models and manufacturers and place them in the same location for
several days, logging the positions they report, you will find that
some of them have quite a bit of variation compared to others. We
would say the second sort are more precise. However if you then
average the logged positions, you will probably find that no one of
the five gives the exact same position, and that a unit with large
variations nonetheless may give a more accurate position over time
than one with small minute-to-minute variations.
Fourth, at least with automotive GPS, if the device loses signal I
believe it will attempt to perform dead reckoning for a little while
in the hope that it will see some satellites before you notice there
was a problem. Not sure whether marine units do this as well but I'm
inclined to think they do.
Fifth, the displayed is always averaged over the last several
readings, which will reduce the effect of outliers. On my marine
unit I can select the averaging period down to a minimum of two
seconds, or I can let the GPS decide for itself.
All these things affect the accuracy of the speed reading you see on the GPS.
Now some more geometry, that of the land. If you jump off a cliff,
will the GPS show you accelerating to your date with Wile E. Coyote,
or will it show zero? That would depend on how fancy the software
people wanted to get. However at the very least the accuracy would
be degraded because the altitude measurement is inherently
inaccurate. I would lay (small amounts of) money on the units not
compensating for slopes. This could be tested by an extremely adroit
skier with a GPS and a radar gun. ;-)
Yours,
David
