APRS/MIR Test 11 March 1998 Bob Bruninga, WB4APR On 11 March 1998 a special APRS/MIR test was conducted via the packet system on the Space Station MIR. The test was to show possible methods for improving the visibility of MIREX communications to students and schools. The objectives were to: 1) Demonstrate the use of a one-to-all protocol using UI frames for more effective use of the MIR downlink to multiple participating ground stations. 2) Demonstrate the usefullness of spacecraft beaconing their GPS position a few times per footprint to identify their position. 3) Use APRS map displays to display the moving position of the spacecraft and to display the locations of all participating ground stations. 4) Demostrate the use of linked internet groundstations to distribute the live downlink collected from ground stations all over the US to users worldwide in real time. BACKGROUND: The Automatic Packet/Position Reporting System (APRS) is a connectionless protocol using UI frames to communicate information effeciently among a large group of stations in real time. Each station with information transmits, and all stations capture, sort and accumulate the information on a variety of display pages or windows as follows: BULLETINS Lists bulletins in sequence regardless of receipt order MESSAGES Displays messages to and from your station TRAFFIC Displays messages between other stations STATUS Displays the current status of all stations POSITIONS Displays the current position of all stations TELEMETRY Displays any telelmetry packets HEARD LOG Shows number of packets per station per hour, etc Although most people associate APRS with GPS position reporting, a GPS is not required unless a station is moving. Moving stations use a GPS to automatically update their position, instead of requiring manual entry. Otherwise, the primary advantage of APRS is sharing information between the largest possible number of users effeciently over a single shared channel. MIR BULLETINS: The MIR packet radio link is ideal for a one-to-all protocol for distributing BULLETINS, ANNOUNCEMENTS and other information to all users in the footprint. Although MIR currently transmits a few single line bulletins, these are sometimes lost among the hundreds of other ACKS, REJECTS, and BUSY packets in the downlink that sometime overshadow any transfer of information. By using a protocol like APRS to receive the MIR downlink, monitoring stations can capture such properly formatted BULLETINS and ANNOUNCEMENTS of interest to everyone and display them in a user friendly manner. MIR POSITION TRACKING: Since APRS has a full maping capability, it can not only display the location of all ground stations, it can also plot the moving position of a spacecraft with an onboard GPS. But since a hardware upgrade to add a GPS to Mir is not likely, the same effect can be accomplished by a few strategicly placed MIR tracking stations that transmit up a pseudo-GPS position to be digipeated by MIR to all ground stations. Three such stations could provide this service whenever MIR is over the USA. These stations transmit only a single 1/2 sec packet every minute (1% of channel capacity) and the result is an accurately moving MIR ICON on all ground station maps. PARTICIPATING STATION DISPLAY: Since the MIR PBBS is a single user system, it can usually only be accessed by one or two users per footprint under good conditions. Meanwhile, within the same footprint are often dozens if not hundreds of users sending useless and competing CONNECT-REQUEST packets which only reduce channel effeciency. The constant stream of DISCONNECT-BUSY packets in the downlink conveys the number of users participating in the pass, but carries no useful information. If instead of useless connect requests, these same stations simply transmitted a single compressed position report it would not only show who is participating in the pass, but also show the distribution of these stations on the APRS map displays at no additional loading on the channel. The flavor conveyed to the users in this case is a feeling of camaraderie as a participant with others in a pass instead of individuals fiercely competing with each other for the single user access. This in itself is a better attitude to convey to students and the map display is certainly more visually appealing than a scrolling display of disjoint packets. INTERNET INTERLINKED GROUNDSTATIONS: With the worldwide connectivity of the internet, the downlink packets from MIR can be received by groundstations anywhere and made available to all users everywhere. For over a year, the APRServe system has been providing that kind of connectivity to APRS packets genearated nationwide. During the APRS/MIR test it allowed stations even out of the footprint to observe the event. Similarly for schools, it gives easy access even to individuals and classes with no amateur radio equipment. THE APRS/MIR TEST: Since the Mir packet system has been operating well the last few weeks, Dr. Larson of the MIREX group suggested the APRS Mir test be conducted as soon as possilbe since precession was taking Mir passes earlier every day and it would soon be out of view during school hours. He authorized the APRS/MIR test on only two orbits on the 10th of March. Unfortunately the MIR packet system went off the air on these two orbits, so the test was extended to the next few orbits over the USA. The test was limited to the USA only because it had the largest numbers of existing APRS ground stations ready to test in sufficient numbers to fully load the system. But the delay further complicated matters since the next orbits over the USA were between 0230 AM and 0400 AM local time. As a result, the test was extended for a full 5 orbits to allow testers to choose a pass and still get some sleep. With the short notice of the test, and since the more effecient SPACE mode of APRS had not been used or tested since the STS-74 and 78 SAREX and SPRE Missions over two years ago, APRS users were told to just operate normally on the MIR frequency but to shorten their position comments where possible. The SPACE mode compresses the station's position report into his gridsquare to save 16 bytes per packet. Due to the typical 10 minute duration of a pass, they were told to transmit their position reports once every 3 to 4 minutes to get a reasonable probability of success per orbit. To make Mir appear to move on all groundstation maps, three special tracking-uplink stations beaconed the moving position of MIR via the MIR digipeater. One from California using the callsign MIR-6, one from Michigan using MIR-8, and one in Maryland using MIR-3 to match their callsign areas. West coast stations would see the moving MIR-6, midwest stations would see the incoming MIR-6 change to a MIR-8 and then east coast stations would see the moving ICON on their maps change to a MIR-3. Although MIR was not programmed to transmit any special Bulletins, several ground stations transmitted such BULLETIN, STATUS and MESSAGE packets. Over 65 separate such packets were successfuly captured during the test. On average these packets were transmitted successfully twice (110 times). The advantage of using the few specially authorized uplink sites to digipeat bulletins instead of MIR means that the bulletins may be updated instantly on the ground and then digipeated to ALL users at once without requiring an upload to the BBS nor hundreds of individual downloads. Only bulletins originated by the MIR crew would need to be entered by the crew. Each of the participating stations transmitted their brief position packet once every 3 to 4 minutes. These packets were far less frequent than the usual once-every-10-seconds of all of the usual CONNECT- REQUEST packets normally which congest the uplink. Each time one of these packets was successfully digipeated by MIR, all user map displays in the footprint would be updated with colorful Icons showing all stations locations. During the test 202 station position reports were displayed averaging about 40 per pass. To inject the downlink from Mir into the Internet, a few of the normal APRS I-Gates tuned their radios from the normal APRS frequency to the Mir downlink frequency. These Mir packets were intermingled with the normal stream of APRS packets into the APRServe Internet system. Although they would be seen on the main www.aprs.net maps they would be hard to distinguish from the usual 1000 to 1200 or more APRS stations on the air. To provide a unique display of the APRS/Mir packets alone, a special WEB page was designated to filter out only the APRS/MIR packets and display them spearately to users. During the day of the event there were over 11,000 hits on the server system representing a peak load of 150 simultaneous users and as many as 1000 users on the special MIREX page. STATISTICS: In raw numbers, the following table compares the APRS packets during the test to the other packets observed on the downlink. These statistics were mostly derived from observations on the east coast (Maryland) plus the APRS packets logged elsewhere. PASS TOTAL PBBS PBBS BUSY R0MIR R0MIR APRS APRS PKTS PKTS USERS REJ PKTS USERS PKTS USERS ----- ------- ----- ----- ---- ----- ----- ---- ----- 0740z 229 88 3 18 30 1 89 44 0910z 153 46 3 7 14 0 102 41 1050z 122 47 3 5 11 0 75 51 1220z 170 96 3 15 35 4 72 39 1400z 188 94 5 12 41 3 53 27 Of the 202 APRS stations, 6 were associated with schools and 5 were mobiles inclusing one Naval Academy boat, and one railroad car. Individual APRS packets were also reported by one station in each of Tiawan, South Africa, Australia, Japan and Hawaii. Although APRS users were encouraged to only send their position report until the total load on the channel could be assessed, there were still 65 Bulletins and Messages that were successfully relayed. Since the test was conducted over multiple passes, the 202 successful stations actually represent 104 different stations. SUCCESS RATE: Of these 104 different stations, all were running 10 watts or more, but two were successful at 5 and 7 watts. Of the ten stations reporting a lack of success, 3 were running only 1 watt, and two were running 4 and 9 watts. One was only transmitting once an hour, and one was transmitting once every 5 minutes. Another was using an inside antenna. Two were running 10 watts and 50 watts but beacon rate was unreported. If you consider a 10 watt baseline and proper setup with a 3 to 4 miunte rate, then the success rate appears to be near 98%. Although there may be many more stations that did not report their lack of success, these numbers clearly show that the channel was not saturated nor congestion limited. In fact, during this test a nominal 3 Mir PBBS users logged onto the BBS per pass, but typically only one was successful at uploading and downloading any traffic. Actually this is rather typical on most passes during user hours, but in the middle of the night, as in this test, these BBS users would have expected a less congested channel. LESSONS LEARNED: 1) Due to the short lead time to the test, there was insufficent time to educate all users to use the short Gridsquare or compressed SPACE mode, and to remove unnecessary verbage from their position reports. For this reason there were many quite verbose packets. A 30% improvement could be expected here. 2) Similarly, the SPACE mode was not used. This further reduces bandwidth by automaticlly canceling all further transmission as soon as one's own packet is successfuly digipeated. With the 3 to 4 minute cycle time used and without the automatic cancelation on success, there were probably double the number of packets transmitted than were actually required. 3) Non APRS stations were recommended to use the very efficient grid square method of reporting position by placing their gridsquare in the UNPROTO TOCALL or their packets. Unfortunately, this does not work on Kantronics TNC's which always send their BText UI frames to the callsign of BEACON. THus their packets conveyed no position information. 4) For the pseudo moving MIR-6, MIR-8, and MIR-3 uplinked posits, these should have all used the same ICON name of just MIR. Although the numbered MIR Icons made it possible to easily see which uplink station was responsible for the ICON, the changing calls meant that as the MIR-6 and Mir-8 uplink stations passed out of range, these posits were stuck on the map at their last uplinked position and stopped moving. By using the same Icon name of Mir, then the single Icon would have continued to move as long as there was at least one uplink station in range. 6) The uplinked MIR positions were more static than dynamic since only about 20 were recorded on all 5 passes. At one per minute, this shows only a 20 % success rate for the possible 100 pass minutes. Higher power or tracking antennas may be required. CONCLUSIONS: The test was successful in meeting all of the original objectives. The short notice and early morning hours helped to reduce the number of participants to about 104 stations. We think this number is representative of the nominal number of schools that could be authorized to simultaneously participate in future such Mir experiments. A total of 12 stations associated with schools and students were reported. One station even displayed 5 APRS stations while operating with a whip antenna inside a motel room on travel. The test demonstrated the value of using a UI frame one-to-all packet protocol to improve the delivery of information to all ground stations. Further, the test demonstrated the value of a few special MIREX ground stations to uplink the moving Mir position reports and to relay real-time MIREX bulletins and announcements that can be received by all stations in the footprint including receive-only school stations. Finally, the test showed the value of multiply internet connected ground stations for not only providing a continuum of data from the downlink across the whole country, but also for providing WEB access to students and schools outside of the footprint or without amateur radio equipment. You may see a replay of the event by using APRS to replay the file MIRTEST.HST or by visiting www.aprs.net/mirex.html. All of the APRS stations want to thank the MIREX team and also those normal Mir BBS users who were inconvenienced by this test, for this opportunity to conduct this important experiment.