VOX-APRS.txt         INTEGRATING APRS WITH VOICE REPEATERS              APRS
rev 2                      THE APRS LOCATOR SYSTEM               PROPRIETARY
                          Copyright 1993,4,5 WB4APR

     As the mobile GPS phenomenon continues to grow, there is a much more
cost effective method to provide mobile vehicle tracking, than to require
every mobile to have an additional digital radio and second antenna
costing over $300!  The mobile position reports should be integrated into the
EXISTING vehicle two-way radios by transmitting the position report in a
brief tone burst at the end of a voice tramnsmission, or when the repeater  
is not being used for voice.   With this scheme, no
additional hardware is required in the vehicle, other than a hand-held GPS
unit.

     By transmitting a position report at the end of a voice transmission,
not only is this a period of dead time due to the almost universal courtesy-
beeps found on amateur repeaters, but the tone burst can be easily muted out
at the repeater receiver, so that the other mobile users DO NOT HEAR it! 
An automatic mode is also included that will
initiate packet position reports if the repeater is not otherwise in use
for voice.  Since the one-second packets are keyed up from scratch, the
normal repeater kerchunk filter can be modified so that the repeater 
transmitter will not be keyed up even though the packet is received by
the repeater and is digipeated onto the digital channel.

     At the voice repeater receiver, a TNC picks
off the position report and digipeats it out onto the dedicated APRS digital
frequency for mobile position reporting.  Since all of these voice repeaters 
digipeat onto the same digital position
reporting channel (usually 145.79 nationwide) then anyone monitoring the APRS
frequency will see ALL mobile position reports from ALL GPS mobiles on ALL
frequencies!


HARDWARE:  To implement this APRS LOCATOR SYSTEM, there are two critical
elements needed.  First is the APRS GADGIT (GPS Audio Digital Interface Thing)
to handle the integration of
the mobile GPS data into the users mobile microphone connector.  Second is
the TNC at the repeater site that handles the digipeating of the
position reports heard on the voice repeater inputs over to the
APRS digital channel.  Details of each of these designs follows.


APRS GADGIT:  The APRS GADGIT integrates all of the needed functions into
a single compact package to modify a conventional stand-alond-tracker for
operation via voice radio:

  Appends position packets on the END of voice transmissions
  Only appends packets at the minimum rate set in the TNC
  Limits length of transmissions
  Limits number of packets to ONE
  Initiates packets if repeater output innactive for 30 seconds 


APRS REPEATER NODE:  This special TNC NODE is designed to be intgrated into
every standard amateur voice repeater.  The TNC performs a number of special
functions to fully implement the APRS LOCATOR SYSTEM:

   * It uses Data Carrier Detect to destinguish between voice and data
   * It has a MUTE circuit to mute the repeater audio during posit
     reports 
   * It digipeats all position reports from the repeater receiver to the
     dedicated APRS digital channel
   * It has a second DCD audio input for the APRS packet channel for
     true CSMA effeciency, but this need only be an audio COR detector.
  

Notice that although the REPEATER TNC only listens on the
voice repeater input and only transmits on the digital APRS packet frequency,
it also listens to the APRS packet channel to
avoid collisions.

PERFECT DCD OR MUTE CONSIDERATIONS:

        Since the ultimate acceptance of the POSIT-PACKET on voice
repeaters will be determined by the minimization of the BRAAAAAAP sound
on the repeater output, the DCD and subsequent muting of the repeater
transmitter are very important.    Since most voice
repeaters have simple analog delay lines of up to 50 ms to eliminate the
squelch tail, the actual MUTE decision can be made as late as 50 ms after the
initial DCD, and still be able to mute the packet from the repeater output! 
The APRS DCD circuit provides a low pass filter and clamp circuit to 
eliminate any false triggers.  With todays microprocessor repeater
controllers, the difficulty is in determining a way to connect this filtered
DCD signal to the controller.  There are two objectives:

   1)  Audio mute.  Whenever there is DCD, the audio to the repeater
       transmitter should be muted

   2)  The repeater KERCHUNK timers should be modified so that a single
       packet will NOT key up the repeater output.  To do this, the 
       DCD signal can be used to cancel the effect of the normal repeater
       key-up detector.




