[Xastir] Question about APRS GPS position precision

[Curt, WE7U]

On Sun, 7 Oct 2007, Jim Tolbert wrote:

> I have been told that the limiting factor in position precision is the
> APRS system transmission standard-- that transmissions are limited to
> 1/1000th of a degree or approximately +/- 60 feet.   Is this true?  If
> so, why?

Mic-E format or "standard" APRS format, yes.  In my neck of the
woods it describes a box of about 60' by 40'.  Xastir users can zoom
way in on a station and (perhaps) see a white box.  That describes
the precision they are transmitting.  Stations that are transmitting
NMEA sentences directly that have 3 or 4 digits after the decimal
will display a smaller box.  So will stations that are transmitting
Base-91 packets, which Xastir and OpenTrackers are capable of
sending.  For Base-91 it's something like a 2' by 3' box (in my
area).  Look at any D700/D710/D7A station to see the larger box.
The box in Xastir shows where they _might_ be based on what they are
transmitting, and the box is oriented in the correct direction for
the truncated digits in each hemisphere.

Bob Bruninga came up with yet another method of adding precision,
called DAO.  With this method you add some extra characters to the
comment field that give you the extra precision _and_ specify the
datum for the posit.  Very few APRS clients have implemented DAO at
this time.  Bob likes it because it doesn't make the Kenwood radios
obsolete I think.  As I recall the Kenwoods can handle Base-91 just
fine for posits, but have a problem with Base-91 "Objects" or maybe
it was just Base-91 "Items".


> If not, what is the limiting element in the final display of tracker
> position on Xastir?

*) Accuracy of the GPS position of the tracker.
*) Precision transmitted across the air.
*) Accuracy of your base maps.
*) Registration of your base maps.
*) Datum of your maps vs. what you mapping software can do.
*) Projection of your maps vs. what your mapping software can do.
*) Accuracy of your mapping software in displaying those maps.


> For those of you using Xastir for Search & Rescue, do you run on the
> APRS frequency or a different (quiet) frequency?   What are the
> arguments for each school of thought?   We are going to be running some
> field tests in the near future with a variety of equipment setups....
> does anyone have suggestions of things we should test?  We have a list,
> but new ideas and comments would be appreciated from those that have
> already taken the stumbles<grin>.

I have a lot to say on this subject but have said much of it before
on the SAR_APRS list.  Perhaps read up on it there and then discuss
more about it there?

--
Curt, WE7U: <www.eskimo.com/~archer/>     XASTIR: <www.xastir.org>
  "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!