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vox-rptr.txt
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1996-03-21
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VOX-RPTR.txt INTEGRATING APRS WITH VOICE REPEATERS APRS
rev 3 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. The unit not only reports position and vehicle type,
but also one of 7 canned messages and optionally 4 analog parameters!
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. A new APRS 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) 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. (As an interim fix, any TNC with true DCD can
be used at the repeater). Details of each of these designs follows.
APRS MIC ENCODER: In order to make the APRS LOCATOR SYSTEM practical,
the device 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 about 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 compatibility
* Optionally has 4 analog channels for telemetry
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: Although the electronics could be miniaturized into about
1 cubic inch, the requirement for user access to switches and the
requirement for withstanding the pulling and tugging on the MIC cord
results in a robust dash board box design. The cicruit is powered by the
mic connector and the only external input is via a standard 1/8 inch
phone jack to receive the NMEA data from the GPS unit. This 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!
For initial configuration, the MIC encoder is programmed via its serial
port using any PC running the MIM.EXE program. This program provides
a nominal TNC type user command mode for setting the MIM configuration.
It provides the standard cmd: prompt. Once the MIM is configured, you
use the PERM command to cause the MIM to save the configuration in
EEPROM. The following items can be configured:
MYCall Sets the MIC callsign
MYSymbol Sets the APRS symbol character
VIA digi1,, etc Sets the Unproto digipeater path
TXDelay Sets the key up delay
PERiod Sets the nominal MIC cycle period
POSIT N Sets PSIT period as N * cycle period
TELEMETRY N Sets TELEMETRY period as N * cycle period
BEACON N Sets BEACON period as N * cycle period
BText Sets the Beacon Text
On the APRS MIC ENCODER, there are 8 configuration bits (switches)
that give the operator real-time control over other dynamic MIC choices
as follows:
ON/OFF Used to enable or disable APRS beeps (power switch)
AUTO/OFF Auto will transmit when repeater is silent more than N secs.
DIR/OMNI Used to set Directional or OMNI digipeating paths
LOC/DX Used to set LOCAL or LONG DISTANCE digipeating paths
2 PATH BITS Used to set North/South/East or West routes
3 MSG BITS Used to indicate up to 7 pre-defined messages
Normally the MIC encoder will only send a POSIT if the POSIT timer has
elapsed AND the user has been talking and releases his PTT. In the AUTO
mode, however, after a specified AUTO time period, AND after the
repeater has been silent for the QUIET period, then a position packet will
be initiated and transmitted. The following sketch shows the
recommended front panel for the MIC ENCODER. Notice that thumbwheel
switches are useful for the 3 path and 3 message bits. Otherwise 9
toggle switches could be used...
-------------------------------------------------------------------
| M I K E D X P A T H M S G R A T E O N |
| ----------------- H I G H |
| o o | | | |
| o o O | | | O O |
| o (O) | 7 | 7 | (O) (O) | |
| o o | | | |
| o o LOCAL | | | L O W O F F |
| ----------------- |
-------------------------------------------------------------------
DIRECTIONAL DIGIPEATING: Although the MIC ENCODER can generate an AX.25
packet complete with any number of digipeaters, each digipeater adds 7
bytes to the packet, and for three hops, this almost doubles the length
of the packet! But for backwards compatibility with all existing
networks and TNC's this capability is included and is called the MIC1
mode. But the APRS MIC ENCODER also has the MIC2 mode ready to go.
In this mode, there are NO digipeaters in the path and only 4 bits are
used to tell the digipeater how to route the packet. 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 allows the mobile operator to report his position
forward in the direction he is headed, or back towards where he is
traveling from.
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 DIGIS.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 APRS 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
("Via 146.940")
* It uses the external carrier detect 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 accurate. 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