On Sat, 16 Aug 2008, Gerry Creager wrote:
In general terms, you've got GPS accuracy (repeatability vice a known,
surveyed point) on the order of 6 meters. This has been the same ever since
DoD turned off selective availability in 2000. There are times when it's
better, times when it's worse, mostly due to satellite constellation
geometry.
If you use DGPS, RTK or WAAS, you should see an improvement of about (not
precisely) an horder of magnitude. Lots of receivers have WAAS now.
The simplest possible way to put it: DAO or Base91-Compressed
packets allow for more precision but GPS, unless augmented, will not
be accurate enough to make use of it. Unaugmented GPS _will_ fill
in the extra digits but those extra digits will be worthless.
This is a better situation than standard APRS or Mic-E packets where
the precision of the packet is not enough to represent what standard
GPS'es are capable of doing since Selective Availability was turned
off years ago. Standard APRS or Mic-E packets give about 40' x 60'
resolution in my part of the world.
Use a WAAS unit and have a clear view of the southern sky so you get
a good WAAS lock: You'll be able to take advantage of the extra
precision that Base91-Compressed or DAO gives you. Assuming that
the APRS software makes proper use of these types of packets. As
soon as you lose your low-angle view of the southern sky due to
obstructions you are back to 6 meter precision, so WAAS is useless
in a lot of terrain. If you're in a fairly flat area with a good
southern view then it can work while mobile/portable quite well.
The datum of the GPS is also important if you're trying to actually
use the extra precision: Set the GPS to WGS84 datum. DAO can send
the datum along as well but Xastir doesn't currently have support
for translating the received datum into WGS84 for display.
We can also get into my old argument about actually using the extra
precision with accurate maps and hand-placing objects (not using
GPS), but that's a very specific case and not a common use.
--
Curt, WE7U. archer at eskimo dot com
http://www.eskimo.com/~archer
Lotto: A tax on people who are bad at math. - unknown
Windows: Microsoft's tax on computer illiterates. - WE7U.
The world DOES revolve around me: I picked the coordinate system!"
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