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- USING APRS FOR SPACE COMMUNICATIONS
-
- NEW: I saw my first APRS packet via the MIR digipeater from W6PNC on 5 April
- and then saw first plotted position from N4YHD on 6 April plus other BCNS from
- N2JNT, KB8DP and KE4EER. To facilitate setting up APRS for best effeciency
- through a space digipeater, there were several changes made in APRS 4.04/5.
- * Added alt-G command to put GridSquare in BCN and POSIT reports (this not
- only puts position info in your BCN, but also saves space and makes the
- position report shorter. It also sets ^O to receive all OTHER packets.
- * Modified alt-S command so that it can be used to reset the DECAY timer
- for position reports to a short period during a satellite pass. As for
- all APRS packets, however, the period will automatically begin to decay
- to double the period after every transmission. This assures that stations
- minimize packet transmissions.
- * See OPERATING section for important tips on operating APRS via MIR.
- * In 4.05 both the BCN and POSIT are identical, so that other stations ALL
- BCNS logs do not fill with alternating BCN/POSITS from adjacent stns.
-
-
- BACKGROUND: The Automatic Packet Reporting System could be a solution to the
- effective use of orbiting terrestrial style packet radio digipeaters in the
- amateur satellite program. To date there have been three AX.25 1200 baud FM
- transponders flown in space. The first was on the Space Shuttle STS-35, the
- second was on the space station MIR, and the third has been via the FM
- transponder mode of AO-21. The problem with a space based digipeater 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 were encouraged to simply insert his/her position as the first few
- characters of his beacon text, everyone within the satellite footprint would
- not only see when he made a successful uplink, but also where he was. 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 GridSquare formats. The exact
- format of an APRS position report is as follows:
-
- Beacon Text: !DDMM.HHN/DDDMM.HHW/CQ comments etc.....
- For example: !3959.11N/07629.12W/Naval Academy Radio Club
-
- Grid Square: [FM18xf] Naval Academy Radio Club...
- @FM18xf] comments... etc (format used by APRS after)
- (1 Aug to indicate APRS stn)
-
- 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 POSIT
- in their beacon text. No changes onboard the shuttle or MIR 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 five 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, I have created a
- track history file that can be replayed using the Ctrl-R command. Simply
- 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 TAGS using the Alt-T command to reduce
- the clutter of callsigns on the display. The replay can be speeded up or
- slowed down by hitting the F or S keys. 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 based
- 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.
-
-
- OPERATING TIPS VIA MIR/SAREX:
-
- To maximize your chances of being seen via the MIR digipeater and to minimize
- unnecessary APRS packets, the following recommended procedures should be used
- during MIR passes. Even under worst case scenarios, APRS stations will still
- generate far fewer packets than other stations attempting to CONNECT to MIR!
-
- * Use ^V to set your VIA path to R0MIR
- * Use alt-G to force your grid square report into the beginning of your BCN
- and your POSIT. The grid square posit only takes 8 characters vice 27.
- This command also does a ^O command to open up APRS to parse all other
- incomming packets instead of just APRS packets. This way, your L-List will
- show all stations digipeated by MIR
- * make your BCN text as short as possible.
- * Use T command to force a transmission as desired - OR -
- * Use alt-S command to reset your POSIT period decay timer. APRS will send
- an immediate packet and then double the period to the next one and so forth.
- This prevents unattended stations from accidentally cluttering the freq.
- * After hitting any APRS command, hit alt-V so you can VIEW all packets on a
- full screen instead of on the normal APRS bottom line.
- * 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 MIR 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 only one BCN and POSIT 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 with MIR. Of
- course your chances are similarly diminished, but if enough APRS stations are
- reporting via MIR, there will still be enough DX position reports getting
- through to make it interesting. If this idea catches on, then maybe all of
- those other stations will STOP trying to CONNECT to MIR and join us! That
- would be a net REDUCTION in MIR QRM!
-
- 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!
-
- WARNING: Versions of APRS prior to 4.04 will only recognize a grid square
- report surrounded with brackets []. But future versions of APRS will use
- @...] to enclose grid squares so that you can distinguish between a grid
- square report comming from an on-line APRS station and one enclosed in
- brackets from a passive TNC BText.
-
- Due to these changes in the way that APRS processes grid square reports,
- all versions of APRS after 4.04 will recognize both formats, but the new
- format should not be transmitted until everyone has upgraded to at least
- 4.04. For this reason, version 4.04 and all later versions have a built
- in timer that will automatically begin using the new format after 1 August
- 1994.
-
- This means that after 1 August, older versions of APRS will not recognize
- APRS generated @...] grid squares. They will still recognize the passive
- [...] grid squares, however. This will probably only effect users of APRS
- via MIR or SAREX where grid squares will be used exclusively because of their
- shorter length.
-
- SAREX NOTE: Until we get the SAREX folks to recognize the value of APRS
- position reports, DO NOT BEACON TO SAREX! If they ever turn the SAREX
- digipeater on, the word will be distributed ASAP via APRS bulletins.
-
- MIR ADDENDUM: Well it was fun while it lasted. Apparently some stations
- that do not understand the benefit of APRS to the overall space/educational
- mission, have sent messages to MIR telling them to shut down the digipeater
- mode. As of today, it has been off for at least a week. Somehow we have got
- to educate the AMSAT community to the advantages of using short APRS
- gridsquare reports for providing good instantaneous visual feedback to MIR
- and SAREX users alike. I do not know of a single person that has been
- exposed to APRS that does not agree that it is an excellent application for
- both SAREX and MIR. But so far, after my articles in the AMSAT journal and
- sending copies of APRS to Houston have been to no-avail. I am NOT insisting
- that APRS should be used on SAREX and MIR, but I am insisting that IT SHOULD
- 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!
-
-