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[aprssig] APRS Position Ambiguity

Robert Bruninga bruninga at usna.edu
Tue Aug 21 17:07:25 UTC 2007


> > Doesn't increasing the ambiguity just round 
> > off the position data?   Then it would tend 
> > to jam all the stations into the same corner of  
> > the ambiguity rectangle. 

Yes, that is the problem with many implementations that did not
include the randomizer that was fundamental to the concept.
With APRSdos, the callsign (no SYMBOL) is randomly placed within
the area of ambiguity so there is no precision implied to the
recepient, and so that all the calls in the same vicinity area
are not all jammed into the corner and not all visible.

> The data is truncated.  Rev G of the spec says:
> 
> > Where the exact position is not known, the mm and 
> > hh digits in the latitude and longitude may be 
> > progressively replaced by a (space) character as 
> > the amount of imprecision increases.

It was not intended as a truncation.  The specification of
LAT/LONG in APRS was always intended to be left justified.  If
you had a posit good to the nearest degree (60 miles), then all
you entered was the degrees.  If you had a position assumed to
be to the nearest minute, then you enetered only the degrees and
minutes.  If you knew the position to the nearest tenth of a
minute (0.1 mile), then you entered only those digits.  If you
knew the precise position to the nearest hundreths (60 feet)
then you included that digit. Etc.

This is not a "truncation" it is filling in the LAT/LONG field
with only the digits you know and leaving the others blank
(space).  It is the presence of those SPACE bytes that imply a
position is unknown below that precison and should NEVER be
desplayed in any other way.

Else the integrity of APRS and intention of the sender is
corrupted.

Bob, Wb4APR





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