[OZAPRS] [aprssig] APRS resolution

[Bob Bruninga]

> I just did some tests, I worked out that APRS 
> uncompressed gives you about 15 meters resolution.
> I also made a tracker output data that gave me 
> 30 cm resolution.

I hope you will use the APRS format for high resolution that is included
in the APRS1.2 addendum.  Not only does it provide resolution down to 30
cm, but it also has two much more important features:

1) each packet also includes the DATUM so that receiving systems know how
to plot such high precision and on what map dataum.  WIthout knowing the
datum, that kind of precision is quite meaningless and can be very wrong.

2) It is fully backwards compatible with ALL APRS clients!  Any existing
system will receive it, and display it to the normal APRS resolution of 60
feet or so.  But any system that can display the higher precision and can
display the same datum can also actually use the higher precision to the
degree intended.

The format is called the !DAO! format.  These 5 bytes of higher precision
may be added to ANY existing APRS format for position, or object, and it
carries the higher precision digits.  The D byte contains the datum.  The
A byte carries the extra lAtitude precision and the O byte carries the
extra lOngitude precision (both in base 91)

Again, using this APRS format for high precision allows it to be 100%
backwards compatible to all existing systems while providing the desired
additional precision which might be useable in some applications.

See the !DAO! format in the APRS1.2 addendum:

  http://web.ew.usna.edu/~bruninga/aprs/aprs12.html

and follow the link for the !DAO! format shown in:

  http://www.ew.usna.edu/~bruninga/aprs/datum.txt

Thanks.
Bob, WB4APR
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