[aprssig] APRS DF reporting

[Robert Bruninga]

> > (with DF code in it).  The APRS software then parses out the
DF
> > info and the interleaved GPS info and then when appropriate,
> > sends the combined single packet containing the DF unit's:
> > LAT/LONG/CSE and SPEED
> > DF bearing 
> > Signal quality 
> > Range of the bearing line 
> 
> My only question here is what the range is supposed to mean.  
> How can you
> know the range, if you don't know the transmitter power?

The human who is Dfing knows it or has the best guess on it.  In
APRSdos, it is automatically assigned based on the range-scale
of  the map he is currently viewing.  If he is driving around
and looking at a 16 mile range scale map, then he probably is
not expecting the FOX to be beyond that range.  When he is
zoomed into the 1 miel range scale and is clearly zooming in on
the fox, then the bearing lines are transmitted as only 1 mile
long lines.  

This is very important because distant viewers may be looking at
any number of range scales.  We don't want him looking at a 128
mile long DF bearing line on his map implying the fox is in West
Virginia because he is looking at a state map of virginia, while
the mobile is only 5 blocks east of the FOX and has a west
heading.

Back in 1993 when we added Dfing to APRS, the very first use of
it made it obvious that SCALE must be included in the
transmitted information or on receipt the informaiton is
useless.  

EXAMPLE:  I was on a fox hunt with APRSdos and my Doppler DF.  I
started about 8 miles from the fox.  As I got closer and closer,
and due to limited exits on interstates and limited turns on
highways, My final several miles were all right turns as I
spiraled in on the FOX.  Now if you are zoomed into the few mile
scale, my DF bearing lines looked perfect as they always pointed
inward to the fox as I spiraled in.  BUT TO SOMEONE ON any map
scale larger than about the 8 mile range scale, my infinately
long bearing lines looked like random OUTWARD bound bearing
lines in all directions of the compass!

People in other parts of the state assumed it was garbage
(because it was to them since they were not zoomed in....).  So
since 1993, APRS has always included the DF operators estimate
of the LENGTH of his bearing line based on his best estimate.
If it is a fox hunt in your conuty, then the rules of the hunt
will place a limit on the rnage.  Start there.  But as one gets
closer, he needs to indicate that he is getting closer and that
his lines should be shorter on everyone's maps.  

Of course it is an estimate, but again, we don't want someone
that is 3 miles from a fox to be sending out a bearing line that
appears on everyone's display that is infinately long no matter
what scale map they are on...

Bad data is worse than no data... Etc...

Bob, Wb4APR