MISSION STS-78 APRS SUCCESS!

Eighteen APRS stations successfully digipeated their position via SAREX and 2 others relayed STATUS, but no POSIT. A total of 65 APRS packets were received. Thirty Nine APRS stations attempted transmissions but indications are that only about 10 APRS stations were making a serious effort and trying every pass. This compares favorably with statistics for conventional SAREX ROBOT activity of 561 stations being heard a total of 1350 times with only 146 getting a successful QSO number. It is estimated that thousands tried...

SEE ALSO: TRAKNET.HTM for a proposal to use the 1200 baud PACSATS for a worldwide amateur mobile status/position reporting network.

APRS AS A HIGHLY EFFECIENT BRIEF PROTOCOL:

The problem with SAREX 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 are required to constitute a successful contact. Of an estimated tens of thousands of uplink stations, only a few hundred are successful. Recognizing the stringent requirements for success using the CONNECTED protocol, provision is also made to recognize those stations which were successful in getting only one packet heard onboard the shuttle. Almost three times as many stations are heard (one successful packet) as are successful in the full two-way connected protocol.

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 packlet 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.

GRID SQUARE POSITION REPORTING: To convey more information than just seeing station callsigns plotted via grid square on the map, provision is made for stations to also include a special Station SYMBOL character in their packet as well. The format is ]$[ at the start of the packet and will cause APRS to use $ as the symbol character ($ can be any of the 94 APRS symbols, see SYMBOLS.HTM). 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.

FORMATS: APRS and APRtrak respond to both the conventional LAT/LONG APRS POSITION reports and to other packets with included Grid-Squares. The exact format of a minimum APRS GridSquare packet is as follows. Obviously the GRID-IN-TO format is the shortest and preferred. These formats convey both your POSIT and your STATUS comments in your

APRS STATUS PACKET:
                                                    comments
     GRID-IN-TO FORMAT:         WB4APR>FM19SX,W5RRR:Hi!...
                                WB4APR>FM19SX,W5RRR:]$[Hi!...
                                                    ^^^ Symbol indicator
                                                        See SYMBOLS.HTM

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-72. During 3 midnight and later passes, over 66 stations successfully uplinked position reports. You can replay this file using the FILE-REPLAY command and select the SPRE file.

DEMONSTRATION: 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. See the TRAKNET.HTM file.

APRS and APRtrak use a special SPACE FORMAT which also configures them for sending their GRID SQUARE Status 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 packet digipeated.
• The alt-SETUP-MODES-AUTOspace command can be used to activate 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 packet at the decayed (15 minute) period (which will miss most 8 minute passes).
• Your shortest packet will be your STATUS. 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 STATUS period to max to minimize QRM since you have already been successful! After 10 minutes, the AUTOspace mode will reactivate for the next pass.

OPERATING TIPS VIA SPACECRAFT DIGIPEATERS:

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 STATUS timer to minimum when the spacecraft is first heard.
• Make your INPUT-MY-STATUS text as short as possible, or none at all.
• Use XMT-STATUS command to force transmissions as needed. Notice that your STATUS 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 10 degrees or so.
• NOTE: The SPACE mode only permits a single SYMBOL character, so only SYMBOLS from the PRIMARY APRS symbol table are usable.

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!

APRS POSITION REPORTING VIA THE 1200 BAUD PACSATS! Although any of the PACSATS can operate in digipeater mode, only the WO-18 ground team has officially invited APRS position reports. Other PacSats with DIGI on could be used as well (AO-16 and ITAMSAT), if there were no objection and digipeater remains on. There are several items that make these satellites very attractive to APRS:

1. They can use ANY 10 watt or better FM XMTR on the uplink!
2. Uplink only requires an OMNI antenna with no pointing (mobile!)
3. ANY TAPR-2 compatible TNC (with an 89 cent mod) can be used on the UPLINK. (see TRAKNET.HTM). The mod is a 7400 chip wired as an XOR between the XMT clock and the data.
4. For vehicle tracking, only a few downlink stations are needed, since they can digipeat the packets onto HF and VHF nets or be linked into live WEB pages...

APRTRAK AND APRS: 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.HTM. APRS still retains a minimum SPACE mode too.