[aprssig] Omni DF support

[Robert Bruninga]

> Is it still only APRSdos and Xastir that support Omni DF'ing? 
> 
> I'm going to be working with some radios that are 
> frequency-agile and have a RSSI output.  I want to try 
> implementing an omni DF query that'll cause all
> capable stations to QSY briefly to a specified frequency, 
> take a RSSI measurement, and return to the APRS channel 
> to report their results.

Yes!  That was part of the original APRS omni-dfing idea.  Glad
to see someone thinking along those lines.  The format for an
OMNI-DF report is similar to any other station or object.  What
makes it a DF report is its SYMBOL type and the DFS format (the
packet data after the LAT/LONG is the same whether it is a
station or an object.  The DFS is identical to PHG, except that
the POWER byte is replaced with the DF Signal Strenght
(Subjective evaluation from 0 to 9).

DDMM.hhN/DDDMM.hhW\DFSshgd comments......

It is in the comments field where the specific 'thing' being
DF'ed should go.  This should be identified by its FREQUENCY.
And since we are trying to standardize on the first 10 bytes of
any comment field as being FREQUENCY, then this makes the format
be:

DDMM.hhN/DDDMM.hhW\DFSshgd MMM.KKKMHz ...comments if any...

So all of the reporting formats are standardized and well
documented.  Now the Signal Strength Query was not in the
original spec, but I have proposed the following format:

 ?DFS? FFF.fffMHz, LAT, LON, RR

This is a Query for a DF signal report on the frequency FFF.fff
MHz
within RR miles of the given lat,lon.

A DFS Query will ONLY trigger a response only if all of these
conditions are met:

 - The remote DF station has enabled it for ! (auto) or * Prompt
 - The remote DF unit is within RR mile range of DDMMN/DDDMMW
 - A previous DFS response is more than RR seconds old.

Notice that I made the response repeat time proportional to
distance.  This seems to make sense as a way to prevent channel
overload from someone sending too many queries.  If someone is
down to 10 miles, then there may only be one DF unit in the
area, and so 10 second queries might be OK.   If it is a 100
mile search area, then one request every 100 seconds might be
OK.  I'm beginning to think we made need some additional limits.
A california repeater with 100 mile range, and a 100 mile DF
request could cause lots of auto-df stations to respond...

For more on all of the APRS Dfing techniques (left out of most
APRS clones) see:

http://www.ew.usna.edu/~bruninga/dfing.html

De Wb4APR, Bob