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[aprssig] Balloon DFing on APRS (test lines)

Bob Bruninga bruninga at usna.edu
Fri Mar 2 17:00:27 UTC 2012

> I'm not really familiar with DF activities. 
> Can you dummy up a set of reports and send it to me 
> so I know what sort of thing to expect?  

OK I just put 3 DF reports on the map.  WB4APR-3, DF2 and DF3.

They all have approximate lengths of about 500 miles and intersect out over
the ocean.  Hopefully people with clients that plot DF's will see them.

Notice the NRQ value is 997.  N is no longer used (so make it 9).  Then
Range value of 9 means about 512 miles LENGTH of the bearing line.  The 7 is
a quality factor.  8 would be a perfect bearing line.  7 is less precise on
down.  A value of 2 would mean your bearing is within 45 degrees.  3 would
mean 22 degrees, 4 wouild mean 11 deg, 5 is 6 deg, 6 is 3 deg, 7 is 2 deg
and 8 is 1 deg.  Tho those are not formally defined.

In the original APRSdos I could not do colored wedges, so I just made the
dottedness of the line represent the quality of the bearing.  For modern
programmers, a nice shaded transparent wedge with variable width would be
real nice.

The RANGE I do not ask my users to enter, but I generate that from the RANGE
SCALE of the map they are using when they enter the DF object.  If they are
zooomed into the to 2 mile range scale map when they add the object to the
map, then the length of the line is 2 miles.  If they are zoomed out to the
512 mile range scale and add a DF line (as I have done), then the RANGE
digit is the max of 9, indicating it is 512 miles long.

So I guess working backwards.... on the RANGE digit,

9 = 512
8 = 256
7 = 128
6 = 64
5 = 32
4 = 16
3 = 8
2 = 4
1 = 2

I just noticed that the APRS spec does not seem to go into this detail,
unless I missed it...

Bob, Wb4APR

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