On Mon, 3 Apr 2017 17:48:21 -0400, rickman  declaimed the
following:


>I believe there have been a number of significant air accidents when the
>navigation devices were not set up correctly or failed.  I recall a
>sailing accident that cost several lives when the GPS constellation was
>in a very poor configuration giving a very high error, on the order of a
>tenth mile or so.  The ship hit the rocks and the obstinate captain who
>wouldn't listen to the warnings of those who saw the rocks was killed
>along with some others.

 That degree of error probably also meant this was the days of
"Selective Availability" -- when even a good constellation produced error
circles of 100 feet or more (or is that reported in meters on my units --
need to check the manual). But near major ports, even in the days of SA,
there are differential GPS transmitters -- which do need extra receiver
equipment...

 Since SA was turned off, most GPS units are good for 30 feet (meters),
and with WAAS/EGNOS, 10 feet (meters) or better is common.

 WAAS/EGNOS transmit on the regular (civil) GPS frequencies, providing
correction data for poor quality constellations. Differential GPS provides
very detailed correction but only near the DGPS transmitter (basically,
DGPS sends the difference between the GPS constellation results at that
port and the surveyed location of the port). Most current production GPS
decode WAAS (two satellites for west and east CONUS) or EGNOS (Europe).

--
 Wulfraed                 Dennis Lee Bieber         AF6VN
    wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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