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 TNC, digital radio, and second antenna
costing over $400!  The mobile position reports should be integrated into the
EXISTING vehicle two-way radios by transmitting the position report in a very
brief tone burst at the end of a voice tramnsmission.  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!  If
the tone burst is about 0.25 seconds, then it will be virtually transparent
to voice repeater operation.  The new APRSc (Compressed) format achieves a
complete position report, course, speed, and digipeater information in about
30 bytes including header, vice the 90 or more bytes in a normal APRS
position report.

     At the voice repeater receiver, a special APRS REPEATER NODE TNC picks
off the position report and digipeats it out onto the dedicated APRS digital
frequency for mobile position reporting using a path derived from the bits in
the TO-SSID.  In addition, it appends the repeater frequency onto the end of
the position report so that digital users can see where the packet
originated.  If all voice repeaters digipeated 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 an APRS MIC ENCODER to handle the integration of
the mobile GPS data into the users mobile microphone connector.  Second is
the specialized APRS REPEATER NODE that handles the digipeating of the
compressed position reports heard on the voice repeater inputs over to the
APRS digital channel.  Details of each of these designs follows.


APRS MIC ENCODER:  In order to make the APRS LOCATOR SYSTEM practical, the
packet TNC for injecting the position report into the microphone audio had to
meet several design constraints:

   * Must interface to UNMODIFIED radios via the MIC connector
   * Must compress position report into less than 0.25 seconds
   * Must Fail Safe so that MIC can always be used
   * Must be low enough in power so that it does not load the MIC power pin
   * Must have easily selectible user parameters
   * Must be very small for easy mounting and portability
   * Must accept the readily available NMEA output from GPS receivers
   * Must use standard AX.25 for interim compatibility

The result is a 1200 baud position report compressed to 32 bytes including
beginning and ending FLAGS.  This equates to about 250 ms, including
CALLSIGN, DIGIPEATER PATHS, and a minimum message capability, and the packets
are still receivable on ANY AX.25 TNC.


PACKAGING:  The initial APRS MIC ENCODER design is about the size of a match
box.  This permits it to be TAPED to the MIC cord for convenient mounting and
for easy access to the configuration switches.  The circuit interfaces to the
MIC connector and has a miniature 1/8 inch phone jack to receive the NMEA
data from the GPS unit.  Not only is this arrangement the ultimate in
simplicity, but it also makes the entire mobile vehicle position reporting
system as portable as the microphone!  Simply move the MIC from vehicle to
vehicle, and as long as the radios are compatible at the MIC connector, then
the vehicles are GPS ready!

     On the APRS MIC ENCODER, there are 12 user configuration switches to
permit the choice of operational mode, APRS symbol, digipeater path, and a
brief message indicator as follows:

   ON/OFF      Used to disable APRS beeps on non configured voice repeaters
   AUTO/OFF    Auto will transmit when repeater is silent more than N secs.
   DIG/VOX     Configures for digital channel or voice repeater use
   1 Minute    Used to add 1 minute to the standard 30 sec period
   2 minute    Used to add 2 minutes to the standard 30 sec period
   DIR/OMNI    Used to set Directional or OMNI digipeating paths
   LOC/DX      Used to set LOCAL or LONG DISTANCE digipeating paths
   N/S         Used to set North/South digipeater directional routing
   E/W         Used to set East/West digipeater directional routing
   MSG1        Message bit 1 for user defined messages
   MSG2        Message bit 2 for user defined messages
   MSG3        Message bit 3 for user defined messages

On the back of the encoder module are 4 jumpers for selecting from the 16
different APRS mobile vehicle symbols.  In the AUTO mode, after DOUBLE the
normal transmission period, AND if the repeater has been silent for at least
this same extended period, then a position packet will be transmitted.


DIRECTIONAL DIGIPEATING:  Incorporated into this APRS LOCATOR SYSTEM is a new
directional digipeating algorithm.  Since the standard AX.25 digipeater field
is omitted from the APRS compressed format, only a few bits are used to
encode digipeating instructions to the APRS REPEATER NODE.  These bits
indicate whether the position report is to be digipeated omnidirectionally or
directionally, a short or long distance, and in what direction.  This
directionality concept is useful for reducing QRM for the long distance
traveler that wants to be seen several hops ahead or behind his direction of
travel.  He sets the bits in the forward direction if he wants to alert those
ahead that he is comming, or he sets them in the backward direction if he
wants to inform his home area of where he has gone. 

OMNIDIRECTIONAL DIGIPEATING:  If the mobile has selected OMNI, then his
position reports will be radiated outward in all directions.  The new FLOOD-N
algorithm (described in WIDE-N.txt) describes this function.  

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 has true DCD to destinguish between voice and data
   * It has a MUTE output (DCD) to mute the repeater audio during posit
     reports which must be 99.9% immune from falsing on voice
   * It digipeats all position reports from the repeater receiver to the
     dedicated APRS digital channel
   * It implements the APRSc Directional Digipeating algorithm
   * It implements the APRS FLOOD-N digipeater algorithm for OMNI packets
   * It appends ADDText (usually the rptr freq) to the end of all packets
     ("Voice on 146.940") 
   * It must have 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 APRS REPEATER NODE function only listens on the
voice repeater input and only transmits on the digital APRS packet frequency,
it must also have a secondary DCD listening to the APRS packet channel to
avoid collisions.  This special APRS node function is NOT involved in any
further routing on the APRS digital channel (I mean that it does NOT serve as
a general purpose APRS digipeater on the digital channel).  All it does is to
insert the appropriate directional or OMNI digipeater path and digipeat the
packet.  This distinction, of course, is only a functional distinction, since
APRS digipeater functions can be co-located, or even built into the same NODE
box as long as dual digital receiver channels are maintained.


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.  The APRS NODE must therefore provide a
separate MUTE signal that is 99.99% percent accurrate.  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! 


NOTES:   Notice that the APRS REPEATER NODE will also work on the digital
channel!  In other words, the APRS REPEATER NODE algorithms can also be
running simiultaneously in all of the APRS DIGI's so that the APRS compressed
format will be picked up directly on the digital channel.  These original
packets are distinguishable because they DO NOT have an original DIGI field.
Once a NODE processes them and adds the DIRECTIONAL or WIDE routing, they
will be forwarded as usual.  Notice that the NODE hardware can actually do
both functions as long as dual digital receive channels are provided.
       

APRS PROPRIETARY                                           APRS PROPRIETARY

