David McDonald wrote:
>
> It's true!!! Bloody amazing!!!
> * On my Garmin 12 I'm getting an estimated position error as low as 4
> metres!
> * When I am stationary, it gives my speed as zero!
> * Extremely rapid changes to the readout of speed and track
> * and amazingly, even the altitude readout is both stable and correct.
> Wow!
My own experiments confirm that SA has been turned off. On first lock,
the EPE (Estimated Position Error) was 9 metres, with only 3 satellites
- better than I ever remember seeing it even with 12 satellites locked
in! I imagine that the time to lock on to the satellites will be
reduced now, too.
I'm running VisualGPS (http://www.apollocom.com/VisualGPS/default.htm),
a program that takes NMEA data from the GPS and performs long-term
averages on the position and elevation. In two previous runs of over
24 hours, the position track showed the classic "drunken walk" of the
SA error - a blob of approximately 50 metres radius, with some longer
runs caused by multipath reflections etc. The altitude plot was
similarly random, but with an even greater error (rule of thumb:
altitude error is 150% position error.) The altitude values changed
so much that they were practically useless!
With SA turned off, the plots are *much* different. I haven't let it
run for 24 hours yet, but even after thirty minutes, it looks great!
Both the position and altitude plots still show random deviations, but
the errors are *much* smaller. The position has varied so little that
the resolution limitation of NMEA data is clearly visible. (with SA,
the "random walk" error swamped this after only a few minutes.) The
altitude values still aren't great, but I would consider them to be
usable; if altitude accuracy is important to you, stick with a (properly
calibrated) barometric altimeter.
If you had doubts about the usefulness of GPS before, take another
look!
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Paul Taylor Veni, vidi, tici -
I came, I saw, I ticked.
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