The only information I found was on a Civel Air Patrol Website. The discussion was on APRS (Automated Position Reporting System). I copied and pasted the information below. Maybe you can make heads or tails out of the info but it's all greek to me.
Copied from a Civil Air Patrol site:
ARNAV FORMAT: Aeronautical manufacturers ignore the NMEA spec (for boats) and have no formal standard for GPS. Several use a format called ARNAV, which is the same as the KING format. APRS parses ARNAV in two ways. First, it plots any received packet that contains the RAW ARNAV data in a packet beginning with STX. It can also parse RAW ARNAV data comming in the same port as the TNC without the packet header. For this to work, the station must be in SPM or HSP modes and be validated for GPS. And the GPS and TNC must be set to the SAME baudrate (4800 usually). One user says he has to operate both at 2400 baud to make it work reliably.
The ARNAV data begins with an STX, has lots of data lines, and then ends with an ETX. Each line of data has a single leading character that indicates what the remaining data on the line represents. APRS will parse out the following fields:
AN dd mmhh (North Latitude in degrees and minutes to the hundredths BW ddd mmhh (West Longitude in degrees and minutes to the hundredths Cccc (Course) Dsss (Speed) ..... (other fields for E,G,I,J,K,L,M,Q,d,e, and v are given Wxxxxx N dd mmhh W ddd mmhh +aaaa (Waypoint where: xxxxx is its name ) LAT LONG as shown aaaa is altitude in ft)
Notice, that APRS will not only place the aircraft on the map, but it will also generate a symbol for the WAYPOINT and place it in the APRS system as well. The WAYPOINT symbol is a circle. The ARNAV station will be a standard airplane. Contact me for info on how to change the default SSID symbol definitions if necessary.
IMPLEMENTATION: Since the data begins with an STX but has numerous carriage returns in the middle, there is no way to make any of the HAM TNC GPS modes work on this data. PacComm has a comercial TNC that will. Instead, you have to set the TNC into the UI MODE (unconnected CONVERSE) and just let it transmit all of the data as it streams in the serial port. Therefore:
FOR NOW, THIS WILL ONLY WORK WITH OLDER ARNAV PRODUCTS WHICH HAVE A USER DEFINED PERIODICITY. NEWER 5000 series products output at a 1 second rate which (just like the NMEA standard) is too fast for a 1200 baud shared packet channel.
NOTE: PACCOMM has an ARNAV parser built into their commercial TNC, but newer ARNAV devices output almost a 400 character NAV message. So although the data will be transmitted, it is quite innefficient for a shared channel when you only need 20.
ALTERNATELY, wire up a 555 circuit that only passes the ARNAV data to the TNC for two seconds out of every N period.
Here is how to set up a TNC to transmit ARNAV:
Set up the aircraft TNC to be permanently in the UNPROTO-CONVERSE mode. In the Paccomm, set UI MODE ON. Or buy the DRSI APRS rom.
Set COMMAND $1B. This changes the COMMAND mode character from its normal control-C to be the ESCAPE character. Actually, you can set the COMMAND character to any other character, just NOT ^C.
Set the SENDPAC character to $03 (^C) instead of $0D (Carriage Return) so that the packet is not transmitted until the ARNAV ETX character ($03) comes along.
Set your ARNAV device to output data once every 30 to 60 seconds or so, depending on channel activity.
If you cannot change the ARNAV periodicity, set up a 555 chip to only send the data to the TNC for 2 seconds out of every N period. In this case, you must also set CPACTIME ON, so that the TNC will go ahead and send its transmit buffer even if it does NOT get the normal ETX char. With CPACTIME ON, the TNC will wait for 1 second after the last incomming character and then go ahead and send the data, even if the 555 oscillator cut off the incomming data.
