Notes About Aircraft, ACARS, and Balloons

Civil Air Patrol
and the
APRS Interface to ARNAV Serial Data Format

NEW in 786: APRS now has several special NUMBERED symbols that can display 36 different alpha-numeric overlay characters each. With the original 2 aircraft symbols, now 74 total aircraft can be assigned individual symbols. Then turn CALLS off to reduce map clutter! See sections below.
Also a Glider symbol was added. But use the NUMBERED TRIANGLE symbol if large numbers of Gliders will be using APRS.

NEW in 782: APRS can now use the MAPS-OVERLAY-WRECKS command to overlay all of the known aircraft crash sites. This is valuable during SAR missions.

NEW: There is now a special APRSair.exe which can parse and display ACARS data and posits. ACARS is the internaltional Airline Packet Data system operated on 131.55 MHz. See ACARS.HTM.

IMPORTANT!!! OPERATING APRS FROM AIRCRAFT AND BALLOONS: This is UNLIKE any other APRS, since you will hear hundreds of stations and hundreds of stations will hear you. It is imperative that you understand the following:


* UNPROTO PATH Since everyone will hear you direct, you do not need any digipeaters. But having one WIDE doesnt hurt too badly. ABSOLUTELY DO NOT USE MORE THAN ONE HOP and DO NOT GO VIA RELAY!!! That keys up hundreds of TNC's over thousands of square miles. Recommendation:

Use UNPROTO APRS VIA WIDE and everyone on VHF will see you -OR- use UNPROTO APRS VIA GATE (to be seen on HF too)

* FULLDUPLEX - If you plan on operating airborne on 145.79 or the main APRS freq in your area, you must consider that your TNC will hear hundreds of stations, and the channel may be constantly busy! This means your TNC may never transmit due to its built in collision avoidance scheme. To avoid this, set the TNC to FULLDUPLEX ON so that it will ignore the DCD circuitry. Do this with due consideration to others on the channel since you will now NOT be listening before you transmit.


Use UNPROTO APRS VIA GATE,WIDE. Not only does this allow the packets to be GATED by dual-port TNCs to HF, but it also allows anyone with a standard TNC to GATEWAY your packets over to the APRS frequency by receiving your packets on one frequency and then digipeating them on another. In this case, the TNC alias is set to GATE so that it will digipeat the packet, and the WIDE on the end makes sure that after it gets to the APRS freq that it then hits the local WIDES.

APRS 3D ALTITUDE DISPLAY: The Y-AXIS command displays a 3D perspective with the horizon infinity at the screen center. The 2D map center appears near the bottom 1/3rd of the screen in 3D and Altitude is displayed along the right screen edge. Altitude is obtained from the string "/A=XXXXXX" appearing in any POSIT. ALtitude is automatically extracted from the standard NMEA GGA sentence or from the special $PGRMZ and $PMGLB sentences, and or it can be manually entered. Note that the field MUST be exactly 6 characters with leading zeros. The ALTITUDE scale along the right side of the 3D screen is calibrated to the original 2D center of the map. THis means that the scale will always be correct, but the true 3D visual perspective my be awkward or confusing for stations that are near or far to the south or north of the 2D map centerline.

REPLAY the BALLOON and AIRCRAFT.hst files to see the effects. Notice that as the aircraft flies to the south (toward you), it will always stay at the 5000 foot altitude regardless of perspective. At some ZOOM ranges, as it departs the airport or returns, it will actually appear in perspective to have a negative altitude because the SYMBOL is always going to be displayed at the CORRECT altitude according to the scale rather than in perspective.

HOW TO MAKE AN AIRCRAFT SYMBOL: For stand-alone-trackers, you must indicate the aircraft symbol character in either of two ways: 1) Set the TNC SSID to -7. i.e. W3ADO-7 OR 2) Make the first three characters of the BText be: [Macro error: Can't compile this script because of a syntax error.] ..... where the /$ represents the following symbols: /' small aircraft ' crash site /g glider /^ jet aircraft ^ big aircraft #^ Numbered aircraft 0-9 or A-J #n Numbered Triangle

In the case of #2, the aircraft symbol will not begin to display as an aircraft in APRS until the receiving station receives a BText first. Until then, the object will appear as a dot.

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:

  1. 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.

  2. 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.

  3. 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.

  4. Set your ARNAV device to output data once every 30 to 60 seconds or so, depending on channel activity.

  5. 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.



FOR DETAILS, CONTACT THE ARIZONA WING OF THE CAP which is doing performance testing on the APRS system.

SAR GRIDS: APRS can overlay the 15x15 minute Search & Rescue (SAR) grids used by the CAP. The four quadrants of these grids correlates exactly with the readily available USGS 7.5 minute maps. These grid squares have a standard numbering system within each sectional chart. Where ever there is an overlap, the western most map takes precedence. APRS accomplishes this ordering by the sequence of maps in CAPGRID.DAT. Also, ALBUQUERQUE must be first. Except for the overlaps, most charts are listed alphabetically.

If the cursor is in an overlap area, the proper grid numbers will be seen. If you are just to the side of the overlap area, then APRS will use the numbering scheme that applies to the exact grid found at the cursor. This may place the "wrong" numbers in the adjacent overlap area temporarily. If you are unsure, zoom out to see where the overlaps are, or move your cursor to the WEST of the area of interest to force the correct sequence.


TO DISPLAY CAP CRASH SITES, use the MAPS-OVERLAYS-OTHER command and enter any of the following files for your area:


These files are separately distributed as CRASHES.ZIP and should be loaded into your APRSSYSTEM directory. COPY any one of them to the filename of WRECKS.POS and the MAPS-OVERLAYS-WRECKS will bring it up quickly.

TRACK HISTORIES: Back at the SAR headquarters, track histories can be processed offline by periodically doing a FILES-SAVE to save the latest track history to file and to a floppy. This disk can then be taken to another computer for analysis. APRS will automatically do a save whenever 199 positions have been received. After each save, memory is cleared except for the last position of all stations.

GPS UNITS: I decyphered the format of the quantity of black box GPS receivers that were donated to National CAP. They are Motorola's and can be switched from the proprietary binary format to NMEA with a simple command. I wrote the MOTOROLA.BAS program that makes it easy to reset the GPS units and to send them the NMEA timing requirements.

REGISTRATION: Since most CAP comms personnel are also HAMs, secondary CAP registrations are discounted to only $9 each if included with a HAM registration. CAP-only registrations are the same as HAM registration. Quantity pricing of calls in groups of 10 or more is also available see F1(HELP)-V.

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Mail comments/corrections on content to Bob Bruninga and on HTML formatting to Steve Dimse