[aprssig] Frequency and Situational Awareness PICS

Robert Bruninga bruninga at usna.edu
Fri Jan 11 18:49:18 EST 2008

Frequency and Situational Awareness.

This is another primary function of APRS (as identified during
Katrina). APRS should show us what operating frequency every
operator is monitoring.

This is 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... But this all can be done right now

1) Install a TNC and 144.39 radio at the repeater site.
2) The TNC will digipeat packets from the repeater input to
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 will exist 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 on and off
for years.  We have had APRS Vicinity Plotting for years..  But
now that we are standardizing on the frequency field in APRS,
this idea is time to be highlighted again...  Here is how it

The Mic-dangle pic generates this brief (or ANY) packet:


This packet is a minimum of only 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 shows that other data can be

The TOCALL of APFxxx-2 identifies it as coming from an APFxxx
device and the -2 is one of the rarely used APRS digipeating
schemes.  It does not take up 7 bytes for WIDE2-2 but works the
same way.  It 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 these 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:


Now since APRS has seen the normal position beacon from the
FFFFFF TNC, it knows the location, and PHG data for that FFFFFF
repeater.  If the client supports APRS Vicinity Plotting, then
it does the following:

1) With Vicinity Plotting it places a special icon (a question
mark) within the 1 mile vicinity of this FFFFFF repeater, and
adds this station to the APRS system.

2) It could optionally build a table for each such FFFFFF
repeater that LISTS each user and the last time he was heard.

If we can look on APRS and see about where everyone is and what
frequency they are monitoring, then APRS is truly a situational
awareness tool as it was intended to be.

See details on the APRS Frequency page:



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