SPACE.TXT 7.6f USING APRS FOR SPACE COMMUNICATIONS 0.6 APRtrak BULLETIN!: SAREX on STS-78 is available for digipeating APRS packets. Also WO-18 is officially available for APRS position reports. Other PacSats with DIGI on could be used as well (AO-16 and ITAMSAT), if there is no objection... NEW in APRS76f and APRtrak 0.6: Now instead of showing on the map as just a grid square, your Station SYMBOL character will also be transmitted. The format is ]$[ at the start of the packet and will cause APRS to use $ as the symbol character. This format will also force APRS to interpret the TO address as a grid square, even if it is not in SPACE or MScatter mode. BACKGROUND: Since APRS has great potential in the effective use of orbiting 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 with added Spacecraft tracking capabilities. 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 a few hundred 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. Almost three times as many 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 SAREX on the 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. FORMATS: APRS and APRtrak respond to both the conventional LAT/LONG APRS POSITION reports and to BEACONS with included Grid-Squares. The exact format of a minimum APRS GridSquare BEACON is as follows. Obviously the GRID-IN-TO format is the shortest and preferred. These formats convey both your POSIT and your BText comments in your APRS BEACON: comments GRID-IN-TO FORMAT: WB4APR>FM19SX,W5RRR:Hi!... WB4APR>FM19SX,W5RRR:]$[Hi!... ^^^ Symbol indicator See SYMBOLS.txt GRID-IN-BEACON FORMAT: WB4APR>APRS,W5RRR:@FM19sx] Hi!... @ means APRS is online / means TNC BEACON only To implement this experiment on any shuttle mission, the SAREX TNC only needs to have DIGI ON and the word needs to get out to everyone 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 or APRtrak could then watch 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, a many fold increase in the number of successful uplinks might be realized, and the data exchanged would be more meaningful by a similar factor. SPRE EXPERIMENT: The first APRS experiment was during the Uiversity of Maryland SPRE mission on STS-74. During 3 midnight and later passes, over 66 stations successfully uplinked position reports. YOu can replay this file using the FILE-REPLAY command. To demonstrate the expected results of a SAREX flight, 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 and APRtrak use a special SPACE FORMAT which also configures them for sending their GRID SQUARE BEACON via a space digipeater: * First, you must set your UNPROTO path via the space digipeater * Next, use the alt-SETUP-FORMATS-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. * The alt-SETUP-MODES-AUTOspace command can be used to activates an AUTOmatic routine which will reset your packet timers to minimum if the spacecraft is heard. Otherwise your station will continue to only send your posit BEACON at the decayed (15 minute) period. * 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: 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 fewer packets than other stations attempting to CONNECT to SAREX. * Use UNPROTO to set your VIA path to the Space DIgipeater (W5RRR-1) * Select alt-SETUP-FORMATS-SPACE as noted above. * Select alt-SETUP-MODES-AUTOspace if you want APRS to reset your BEACON timer to minimum when the spacecraft is first heard. * Make your alt-BEACON text as short as possible, or none at all. * Use XMT-BEACON command to force transmissions as needed. Notice tha your BEACON contains your compressed grid square posit as well. DO NOT send your full length APRS POSITION packets unless you are mobile. * 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 QRM to your receiver from other local uplink stations, and also minimizes your QRM to them. A ground level antenna is perfectly adequate, 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. AUTOMATIC OPERATION: In AUTOspace mode, your station will transmit 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 AUTOspace MODE, then APRS will listen for the DIGIpeater shown in your UNPROTO path. Once it hears it, it will reset your BEACON timer 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 BEACON timer 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! I hope that other users of SAREX will try APRtrak and realize the reward of a successful digipeated position report. 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. APRS POSITION REPORTING VIA WEBERSAT OSCAR-18 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) Uplink only requires an OMNI antenna with no pointing (mobile!) 3) Rumor says a Manchester encoded modem can be added to MANY TNC's with only a single XOR gate and a little surgery! 4) For vehicle tracking, only a few downlink stations 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...