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On Thu, 4 May 2000 19:17:33 +0100, Trayfors, William wrote:

>It's indeed a wonderful thing that DoD has been instructed to "turn off"
>selective availability, and that civilians now will have access to the same
>GPS signals the military has enjoyed.  This action is long overdue.

The second part is not true. Civilians now have undiddled
C/A service. The Precise Positioning service is not
accessible to civilian receivers at all.


>However, one predictable consequence of enhanced GPS system "accuracy" is
>that some of our fellow mariners, and maybe even an aviator or two, are
>gonna have GPS-assisted groundings!  This will happen because word will get
>around that GPS is now an extremely accurate positioning system and some
>people will use it blindly. There are several reasons to think twice about
>how GPS is used:

This is true of any tool. Someone who can't read a chart
will sometimes hit something. Reading a chart includes
reading the datum legend. If the chart is too old to have a
datum legend, well???

>
>1.  Even if GPS were 100% accurate 100% of the time, there are a lot of
>charts in common use which are referenced to a different datum than WGS-84;
>differences can be      LARGE, e.g., some charts of the Virgin Islands have
>islands "misplaced" by 1/4 mile or more.  I have personal knowledge of a
>custom 70' sloop which was lost on Neckar Island after successfully
>completing a round-the-world voyage with a professional crew; this yacht
>ran aground just 1/4 mile north of the indicated GPS position and, guess
>what....the datum error at that location is 1/4 mile North/South!
>
A perfect example of a chart-reading error.


>2.  GPS isn't available 100% of the time. If system availability and
>accuracy are, say, 99% reliable that means that during the year it will not
>be available or will not be accurate during some 5,256 minutes or 87.6 hours!!

Since the system reached operational status, single sats
have been out on occasion, but the system has never been
unavailable AFIK. I am sure if it had been there would have
been a flurry on this list. Among all location systems, it
is the only one available world-wide for much of the time.

>
>3.  There are numerous other sources of GPS error -- both technical and
>user related -- which can render GPS readings misleading for the mariner.

For technical error, we can check one of the stationary web
site plots. They could now change the scale so we could see
the details. The german site has +/- 120 meters so the new
post SA blob is just that, almost a dot. It looks as if it
was off by 9 meters once in the two hour period. Plus/minus
15 m scales would show it better. The GLONASS plot on the
same chart is much worse now than the GPS. It used to be
better when SA was on.


User errors. Many of us have practiced those with Loran C,
etc long before we could buy GPS. :-)
>
>For most practical navigation uses, I believe the turning off of SA
>will/should have little impact (except, perhaps, GPS-indicated speed will
>be more accurate).   I will continue to believe that GPS readings should be
>treated as just one source of positioning information to be compared with
>all other available sources at all times.  Yes, GPS is the best thing since
>sliced bread.  But, unless you use it intelligently it can cut your finger off!
>

For many practical navigation purposes, GPS may be the only
source of info for many hours at a time. A DR track and
redundant receivers may be the only check available.
Meanwhile, check the webplots, or leave your receiver on at
the dock plotting a track whenver convenient, to get an
idea how the brave new world is working.


Rodney Myrvaagnes                   J36 Gjoa
Senior Editor           Electronic Products
etaoin shrdlu

   

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