APRS Frequency Tracking of all Voice Users             11 Jan 2008
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Frequency and Situational Awareness.

This is another primary function of APRS (as identified during 
the Katrina disaster).  That is, APRS should show us the operating 
frequency of every operator on the air in real time.

This can be a neat club project and a way to get your ENTIRE club 
on the APRS map with Frequency.  Also Pic Project inventors could 
get rich selling these by the thousands.  Even YeaCommWood could 
add these in every radio...

1) Install a TNC and 144.39 radio at the repeater site.
2) The TNC will digipeat packets from the repeater input to 144.39
3) Everyone add a simple PTT "tracker" to their Mic jack

Done.  No GPS, no rig interface, yet on APRS we will see:

- The location of the voice repeater.
- Special surrounding icons of all users using it
- (or a popup table of all users using it)
- Everyone exists in APRS with a frequency
- Everyone's position exists within the PHG vicinity

If they move to another repeater, their frequency AND position 
changes and they show up there.  I have promoted this TNC at
every voice repeater for years.  But now that we are 
standardizing on the frequency field in APRS, this idea is time 
to be implemented.  Here is how it works.

The Mic-dangle pic generates this packet on release of PTT:

    WB4APR>APFxxx-2:...

This packet is a minimum of 16 bytes long.  It needs NO TXD delay.  
This takes less than 0.2 second on the end of a PTT.  Most people 
will not even think of it as a packet, but a simple chirp on the 
end of a voice transmission.  As with all PTT mode APRS PIC's, it 
only does this once every 3 minutes (not every single PTT).
The "..." on the end just shows that the packet can actually
carry any other data as well (though it makes it longer)...

The AX.25 TOCALL is APFxxx-2 where xxx can indicate the type of
device that is generating this packet.  The -2 SSID is one of the 
rarely used APRS generic digipeating schemes.  It does not take 
up 7 bytes for WIDE2-2, yet it means the same thing.  This has 
been defined in APRS since 1994 or so, but not used much.  The
-2 adds no bytes to the basic packet (since the SSID byte is 
there whether used or not).  The TNC added to the voice repeater 
with SSID digipeating enabled does two things:

 1) It has a MYCALL of FFFFFF which means FFF.FFF MHz
 2) It mutes the repeater output for the 0.2 seconds
 3) It digipeats the packet with callsign substitution.
 4) The packet enters APRS on 144.39 as:

WB4APR>APFxxx-1,FFFFFF*:abc

Now since APRS has seen the normal position beacon from the FFFFFF 
TNC, it knows the location, and PHG data for that FFFFFFF repeater.  
Each APRS client that has implemented the APRS Vicinity plotting
feature will do the following:

1) Vicinity plotting will add each such station with a special 
icon (just for this application) in the vicinity of the FFFFFF 
repeater (this is just normal APRS "vicinity plotting").

2) Optionally, with new code, can build a table for each such 
FFFFFF repeater that LISTS each user and the last time he was heard.

CONCLUSION:  If we can look on APRS and see about where everyone 
is and what frequency they are monitoring, then APRS is truly the 
situational awareness tool it was intended to be.  These PIC chips
could be a great club project and could conceivably fit inside
many existing microphones!  My first such in-the-mic APRS tracker
was back in 1994 or so using the MIM chip.  Surely we can do it
better now.

ALL OF THIS CAN BE DONE NOW. 
  Some TNC's did implement SSID routing
  The KPC-3 implements the voice repeater MUTE function
  APRSdos and some others already do VIcinity Plotting
  Most PIC Trackers can be set to BEACON only

Notice that NOTHING IN THE PACKET is used.  That is why any PIC
tracker that can operate in PTT mode can already be used for this
purpose.  Its there... just do it!

Bob, WB4APR