SPACE.TXT 7.5a USING APRS FOR SPACE COMMUNICATIONS New in 7.5a: The WO-18 satellite is available for digipeating packets. See section below on how this can be used as a worldwide resource for relaying APRS mobile/portable posits. BACKGROUND: Since APRS has such tremendous potential in the effective use of orbiting terrestrial style packet radio digipeaters in the amateur satellite program, a special version of ARS called APRtrak has been donated to AMSAT for use in the amateur satellite program. It is a stripped down version of APRS that is focused on SPACE applications of APRS. See APRtrak.txt. APRS still retains a minimum SPACE mode too. The problem with space based AX.25 FM digipeaters is the total saturation on the uplink channel which makes the use of a normal CONNECTED protocol impractical. For the SAREX robot QSO mode, a total of five successive and successful packet transmissions were required to constitute a successful contact. Of an estimated thousands of uplink stations, only about 250 were successful. Recognizing the stringent requirements for success using the CONNECTED protocol, provision was also made to recognize those stations which were successful in getting only one packet heard onboard the shuttle. Over 700 stations successfully completed single uplink packets. APRS takes advantage of this unconnected, one packet, mode to demonstrate successful uplinks to the shuttle. In addition, however, it capitalizes on the most fascinating aspect of the amateur radio hobby, and that is the display on a map of the location of those stations. Historically, almost every aspect of HAM radio communications has as its root, the interest in the location of other stations. Look at DX maps, countries worked, counties worked, grid squares, mobile chatter; everyone is quite interested in where other stations are. If, instead of every station attempting to CONNECT with the Space Shuttle, all stations simply inserted his/her 6 digit gridsquare into their TNC TO callsign via the SAREX callsign, then, everyone within the satellite footprint would not only see when he made a successful uplink, but also where he was. It takes a total of 128 bytes for a successful SAREX QSO plus 92 bytes for every retry. The APRS GridSquare BEACON only takes 26. This alone could provide an order of magnitude improvement in the number of successful SAREX contacts. Since the shuttle is a rapidly moving object, the locations of successful uplink stations will move progressively along the ground track. The weakest successful stations will almost certainly be immediately below the spacecraft. Stronger and more viable groundstations can show up further to the side of the ground track. If there is a skew in the spacecraft antenna pattern, the pattern of successful uplink stations on the map will clearly make that evident. APRS responds to both LAT/LONG and two Grid- Square formats. The exact format of a minimum APRS GridSquare report is as follows. Obviously the GRID-IN-TO format is the shortest and preferred. GRID-IN-TO FORMAT: WB4APR>FM19SX,W5RRR:... GRID-IN-BEACON FORMAT: WB4APR>APRS,W5RRR:@FM19sx]... @ means APRS is online / means TNC BEACON only To implement this experiment on the next shuttle mission, it would only take a single AMSAT news bulletin to ask all stations to insert their Grid Square in their UNPROTO or BText command. No changes onboard the shuttle or other spacecraft TNC would be required. Those stations that had APRS could then watch the successful uplink stations plotted in real time. Even without real time APRS, a replay of a captured text file containing all the successful uplink packets would still give an excellent map display after the fact. Analysis of antenna pointing anomolies on every orbit could be accomplished with ease. On future missions, the UI beacon frame might completely replace the current CONNECTED robot mode. Without all of the connect requests, acks, and retries at least a ten fold increase in the number of successful uplinks would be realized, and the data exchanged would be more meaningful by a similar factor. To demonstrate the expected results of this experiment, replay the SHUTTLE.HST file and watch the contacts appear as the shuttle moves across the country. You may enhance the demonstration by selecting to see only the Shuttle, STS-99, or by turning off CALLS to reduce the clutter of callsigns on the display. Obviously, in this SHUTTLE.hst file, I assumed that the Shuttle had its TNC connected to a GPS navigation receiver so that it was also beaconing its position once per minute in the APRS format. This capability also demonstrates the practicality of using a space AX.25 digipeater for routine position and status reporting. Imagine a constellation of three AX.25 digipeater satellites all on one FM channel. It would not matter what satellite was in view, or when. Mobile and portable stations could beacon their position once every 5 minutes and be tracked nationwide! Just using 1200 baud AFSK, up to 1000 stations could probably be supported just in the US and have a reasonable chance of getting a position report through at least once every 3 hours! Going to 9600 baud FSK would support almost 8000 users. APRS uses a special SPACE mode to configure for sending its GRID SQUARE BEACON via a space digipeater: * The alt-SETUP-MODES-SPACE command places your Grid Square in the TO address of your TNC. It also sets CONTROLS-OTHER on so that you can see other packets. It sets up a congratualtions BEEP-MSG when it sees your BEACON digipeated. And it also activates an ARMing routine which will reset your packet timers to minimum if the spacecraft is heard, so that you send several BEACONS randomly once the spacecraft is heard. * Next you must set your UNPROTO path via the space digipeater * Your shortest packet will be your BEACON. Although your lat/long POSIT, MESSAGES and OBJECTS are still active, they are not encouraged. As usual, all packet periods will automatically begin to decay to double the period after every transmission. This assures that stations minimize packet transmissions. * Since only the SPACECRAFT will be digipeating, APRS will detect any of your packets that are digipeated and will announce your success with some BEEPS. It also resets your BEACON period to max to minimize QRM since you have already been successful! OPERATING TIPS VIA SPACECRAFT DIGIPEATERS: First, be sure that APRS position reporting is being authorized via the particular spacecraft. To have a good chance of being seen via the SPACE digipeater and to minimize unnecessary QRM, use the following procedures. Even under worst case scenarios, APRS stations will still generate far fewer packets than other stations attempting to CONNECT to SAREX! * Use UNPROTO to set your VIA path to the Space DIgipeater (W5RRR) * Use alt-SETUP-MODES-SPACE as noted above. * If unattended, APRS will listen for the digipeater specified in your UNPROTO path, and will reset your BEACON timer to minimum when the spacecraft is first heard. * Make your BEACON text as short as possible, or none at all. * Use XMT-BEACON command to force transmissions as needed * Use the APRS VIEW screen so you can VIEW all packets on a full screen * Use your lowest 2m antenna (preferably on the ground). This minimizes any QRM to your receiver from other local uplink stations, and also minimizes your QRM to them. A ground level antenna should work perfectly well, since it can still see the sky, and the SPACECRAFT is so far away on the horizon and has such high doppler that you will NOT make it anyway at elevations below 20 degrees or so. UNATTENDED OPERATION: If you use a low antenna as described above and leave APRS unattended, you will be transmitting your normal packets about once every 15 minutes. This is less than one-half of one percent (0.5%) of the number of packets generated by other stations trying to connect to the spacecraft. If you have set space mode, then your system will listen for the DIGIpeater shown in your UNPROTO path. Once it hears it, it will reset your BEACON timers to minimum and also set a random number of seconds up to 12 before your first packet is transmitted. As long as you continue to hear the digipeater callsign, your timers will stay at minimum and your starting time to the first packet will continuously be reset to a random number under 12. Since APRS is on a 5 second timing cycle, you have a 5/12 or 42% chance of transmitting in each window as long as the digipeater is being heard. This gives you an average of about 1 packet per 10 seconds which is still less than what a connected station would be doing... If this idea catches on, then maybe all of those other stations will STOP trying to CONNECT to the spacecraft and join us! That would be a net REDUCTION in QRM to on the uplink! Imagine the fun that the cosmonauts and astronauts will have if they carry a lap-top computer so they can see everyone on their maps! NOTE: Even if you only see a GRIDSQUARE from a station, you can tell if he is running APRS by the @ vice [ at the start of the report. APRS uses this distinction so it can tell who is an on-line APRS station and one which is only coming from a passive TNC BText. I am NOT insisting that APRS should be used on SAREX, but I AM insisting that IT SHOULD at least BE CONSIDERED as a viable and useful addition! The net effect would be FEWER packets on the uplink, and more meaningful packets on the downlink! I wrote APRtrak and gave it to AMSAT so that I have no monitary interest in this facet of SPACE communications. Now I can PUSH APRS FOR SPACE FUN! APRS POSITION REPORTING VIA WEBERSAT OSCAR-18 For the last few weeks (March 96) WEBERSAT has beeen placed in digipeater mode and AMSAT enthusiasts have been encouraged to use it. There are 3 significant items that make this very useful as an APRS position reporting satellite: 1) It can use any 25 watt FM XMTR on the uplink! 2) The uplink only requires an OMNI antenna with not pointing 3) Rumor says a Manchester encoded modem can be added to some TNC's with only a single XOR gate an a little surgery! 4) For vehicle tracking, only a few receivers are needed, since they can digipeat the packets onto HF and VHF nets Receiving the BPSK downlink takes a separate BPSK satellite modem, but many hams already have these...