Date: Fri, 12 May 2000 19:38:42 -0400
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: GPS
In-Reply-To: <B541AEB1.1399%happycampers@picusnet.com>
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At 12:51 5/12/2000, Dave C wrote:
>GPS is "acceptably accurate" above 11 mph. Below 11mph its not reliable due
>to SA. The moving average David refers to is usually an option that can
>usually be toggled on or off. The displayed speed is a calculation between
>the last automatic position fix and current position measured every few
>milliseconds.
I was talking based on experience with low-speed units intended for
sailboat use, with averaging periods of 30 seconds or more. Wouldn't apply
at all if they're now working as you describe here.
>I think David's referring to the (VMG) Vector Made Good function which is
>different than the displayed speed over ground (SOG). It calculates speed at
>which you approach a destination regardless of whether one is on course. If
>one is one direct course then VMG and SOG will be the same.
I wasn't, but again I was talking about a long averaging period...
>Some one indicated that there could be 100's of meters of error. Not so, In
>the US, Selective Ability (SA) employed by the owners fo the system (DoD)
>for civilian receivers is between 15m and 100 meters (which is hundreds of
>feet) horizontally and between 100m-156m vertical.
>100m supposedly happens no more than 1:20 position fixes and then there is a
>much larger error of 300m no more than 1:1000 position fixes.
"Selective Availability" was officially turned off about a week or two
ago. For one thing, there is now a second GPS-type system fully deployed
by guess who -- you can even get receivers that work with both systems. A
major incentive, though (I think) is that with SA turned off, the Coast
Guard can make a much better case for discontinuing the LORAN-C system
which has very high maintenance costs. LORAN-C has an absolute accuracy
specified as better than +/- 0.1 nautical miles at least 95% of the time --
however its repeatability is similar to, maybe just a touch better than GPS
without SA, and drastically better than GPS with SA. USCG promised to keep
LORAN-C running at least until 2010, but they would really like to renege
on that, especially since they don't have to pay for GPS at all. They have
even gone to the extent of building differential transmitters in many many
places so that DGPS will work.* But with SA turned off they can tear those
down and forget the whole thing.
*Differential GPS involves placing a GPS receiver at a precisely known
location and generating difference signals (i.e. between the received
location and the actual location) which are transmitted by regular ?UHF?
?HF? dunno... transmitters to properly equipped GPS receivers in the
vicinity. This of course makes the receiver much more expensive, and only
works when in range of a transmitter. Basically a hack job...
>BUT,newer GPS units are adept at reducing this further through an averaging
>algorithim. The GPS knows how much error is induced and over time averages
>these to reduce the amount of error by about half.
Which of course suffers somewhat from the fact that the SA disturbances are
carefully arranged to be unpredictable...
> If you want even more
>accuracy then opt for DGPS Differential GPS which needs radio signal to
>correct fro the error of SA to around 15 meters.
Not as good as LORAN...usually. And a heap more expensive for the
user. Granted, not having to haul around a great big whip antenna is nice
(LORAN-C operates at 100 kHz, getting down toward the audio band).
>In practice, civilian GPS users should enjoy accuracy of 50m or less for
>more than 50% of the time.
Which is utterly unacceptable for coastal navigation in fog etc,
particularly because of the fliers. 50% of the time is as bad as 0% of
the time under these circumstances, and +/- 50 meters repeatability is
lousy. But with SA turned off, the GPS system is suddenly a lot more
competitive, and I expect to see renewed pressure to shut down LORAN-C.
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"