Ever tried to contact the Cosmonauts on the Mir space station? In this issue of SpaceNews, G. Miles Mann presents some useful hints and friendly advice to those who would like to improve their communications success with the Mir spacestation.
Working the Soviet space station Mir on packet is easier than you may think. I have been able to connect to the Mir Packet BBS on a regular basis with a very modest 2 meter base station and from my car packet station, but you must be very careful not to cause interference (QRM) to other amateurs who are either connected, or attempting to establish a connection with the Mir space station.
The Russian space station is in orbit approximately 200 miles above the earth. The inclination of the orbit is 51 degrees. This means that Mir flies over Amateur stations North and South of the equator up to 51 degrees latitude. Stations from the USA and southern Canada should have no problems hearing Mir during a good orbit. The space station typically comes in range 6-8 times per day for up to 10 minutes per pass. A good computer satellite tracking or orbital prediction program fed with accurate Keplerian orbital data elements for Mir should be used for best communication results.
The Space Station Mir is running an ICOM-228A with voice and packet communications on 145.550 MHz simplex. Power output is 5 or 25 watts with a dual band dipole mounted outside the space station. By comparison, the NASA Space Shuttle uses 5 watts with a dipole mounted to the inside of a thick radiation resistant window.
On most of the passes, the Mir BBS will be connected to a ground station. If you try to connect to the Mir BBS, you will get the message "R0MIR-1 Busy". If this happens, STOP calling the Mir BBS until you see the message "- Logged off" going to the station who is currently connected to Mir's BBS. Remember, ONLY ONE STATION CAN CONNECT TO THE MIR BBS AT A TIME! If you try to connect to the Mir BBS while it is busy, the following will result:
C-Connect request, D-Disconnect request, DM-Disconnect mode, UA-Unnumbered
Acknowledge, UI-Unconnected Information frame or I(n)-Information frame (n=0-7).
This is a sample of suggested TNC parameters for a KAM. Your actual parameters may vary:
Set your terminal program to save all data to disk. This will help you look back at what was captured after the Mir pass is complete, and even if you are unable to connect to Mir, you can read what other people were sent.
This is a sample of a station logging off from Mir:
R0MIR-1>WF1F/V [05/01/95 03:50:57]: <<I1>>: - Logged off
R0MIR-1>WF1F/V [05/01/95 03:50:57]: <<D>>:
*** DISCONNECTED [05/01/95 03:50:57]
R0MIR-1>CQ/V [05/01/95 03:50:59]: <<UI>>: - Logged off
"NOW YOU MAY BEGIN CALLING MIR, C R0MIR-1"
Look for the "<<D>>" Disconnect frame. This is different from the "<<DM>>" Disconnect Busy message. If the station connected to Mir goes our of range, then an idle timer will force a log out after 8 minutes. The Mir PMS will send 15 <<D>> messages, then MIR resets and sends out the desired "CQ, <<UI>>" packet (UI = Unconnected Information frame).
The Russian crews have asked users NOT to use their Personal Message System as a maildrop between terrestrial stations. If you think having a monoband FM packet satellite or a simplex digipeater in space is a good thing, then build one. Please try not to download Mir's help file. The commands are similar to most Personal Message Systems available on packet radio TNCs. A copy of the help file is reproduced below:
|B(ye) B [CR]||disconnects you from PMS.|
|H(elp) H [CR] or ? [CR]||displays this help file.|
|J(log)||J [CR]||displays a list of callsigns heard (optional date/time)|
|K(ill)||K n [CR]||deletes message number n (only to/from your callsign).|
|KM(ine)||KM [CR]||deletes all READ messages addressed to your callsign.|
|L(ist)||L [CR]||lists the 10 latest messages.|
|M(ine)||M [CR]||lists the 10 latest messages to/from your callsign.|
|R(ead)||R n [CR]||reads message number n.|
|S(end)||S (callsign) [CR]||begins a message addressed to (callsign).|
|Subject: max 28 characters ending with [CR].|
|Text: End each line with [CR].|
|End message by typing /ex [CR] or CTRL-Z [CR] at the beginning of a new line.|
|SR(eply) SR n [CR]||Sends a reply to message n prompting only for text.|
|V(ersion) V [CR]||displays the software version of the PMS system.|
Doppler shift is nothing new in satellite communications, but it takes on an interesting "twist" when a simplex communications link is employed. Doppler shift is the apparent frequency change observed as the result of the motion of either the transmitter or the receiver in a communications link. Both the satellite's motion and the Earth's rotation contribute to Doppler shift, with the satellite's motion playing the larger role. Doppler shift's magnitude is determined by the rate of motion, while its polarity is determined by the direction of motion. The polarity is also determined by whether the transmitter or the receiver is in motion.
When Mir is approaching your ground station, the spacecraft appears to be moving towards you at a high rate of speed. This motion causes the signal received from Mir to appear about 3.5 kHz ABOVE Mir's actual transmitting frequency of 145.550 MHz. At the same time, while Mir races toward your ground station, it receives signals from your uplink transmitter about 3.5 kHz ABOVE your actual transmitting frequency. No Doppler shift compensation is performed on the Mir spacecraft. All compensation must all be handled by individual ground stations.
In order to compensate for the effects of Doppler shift, ground stations wishing to communicate with Mir must tune their receivers several kilohertz ABOVE 145.550 MHz at the time of acquisition of signal (AOS), AND their transmitters several kilohertz BELOW 145.550 MHz. This involves transmitting and receiving on separate (split) frequencies approximately 7 kHz apart. Most ground stations use transceivers that transmit on the same frequency on which they receive (simplex), and if they tune their transceivers 3.5 kHz (or 5 kHz if their equipment tunes in 5 kHz steps) above 145.550 MHz at the start of a pass, their uplink signals are received 7 kHz (or 8.5 kHz) ABOVE what the Mir space station receiver is tuned to. The chances of establishing a radio contact under these conditions are extremely remote, even if there are no other ground stations competing for Mir's uplink receiver.
Around the time of closest approach (TCA), which also occurs around the time of maximum elevation for satellites such as Mir in circular orbits, the Doppler shift between Mir and the ground station approaches zero. It then quickly reverses polarity as Mir recedes at an ever increasing rate away from the ground station. At TCA, it is safe for ground stations to set their transceivers to 145.550 MHz and not worry about Doppler shift. As Mir recedes however, stations should tune their receivers several kilohertz BELOW 145.550 MHz, and their uplink transmitters several kilohertz ABOVE 145.550 MHz to compensate for the increasing Doppler shift. Failure to properly compensate for the Doppler shift on the uplink would, again, result in the ground station's signal falling outside the passband of the narrowband FM receiver on Mir shortly after TCA. As Doppler shift increases, so must the split between the ground station's transmitting and receiving frequencies. As DOWNLINK signals drift DOWN in frequency during a pass, UPLINK signals must drift UP. T/R offset is zero at TCA, and maximum at AOS and LOS.
The effects of Doppler shift play a greater role in packet radio communications than they do in FM voice communications. 1200 baud packet radio communications (1 kHz shift AFSK on a narrowband FM carrier) occupy a much greater signal bandwidth than does FM voice, 1200 baud Manchester encoded FSK, or even 9600 baud FSK packet. If an uplink signal isn't properly centered within the passband of the FM receiver on Mir, it will be received with severe distortion if it is received at all. Tests have shown that a 1200 baud FM packet signal must be strong enough to produce at least 25 dB of receiver quieting for an acceptable bit error rate performance. Along the same lines, if other stations are competing with one another on the uplink frequency, it would take a signal strength at least 25 dB above the SUM TOTAL OF ALL OTHER SIGNALS on frequency to be received reliably on Mir because of the capture effect of FM. If just one bit of a packet frame is received in error, then the entire packet frame is damaged and must be re-sent by the transmitting station until it is received cleanly without error. Incidentally, a signal 25 dB (300 times) stronger than the sum total of all others is an ASTRONOMICAL signal level even if only a few other ground stations are colliding with one another on Mir's uplink frequency. That's why QRM is so damaging and following proper operating procedures is so important when communicating with Mir.
While it may be possible to successfully pass a short connect request packet to Mir and receive a connection acknowledgment in between interference bursts, it's much more difficult to go much beyond that and transmit longer information <I> frames to issue PMS commands and send a message to Mir in the midst of heavy interference.
In closing, please keep in mind that the Personal Message System on Mir is a single-user BBS with very limited storage capabilities. Only ONE station may connect to Mir at a time. ALL OTHERS MUST WAIT. PLEASE DO NOT connect to Mir unless the PMS is open and transmitting CQ frames. PLEASE DO NOT connect to Mir unless you have a reason for doing so. PLEASE DO NOT use Mir for passing terrestrial packet radio traffic. PLEASE DO NOT use Mir as a digipeater, and PLEASE DO NOT connect to the keyboard port (R0MIR) unless it is in active use by a Cosmonaut wishing to make a contact. PLEASE LISTEN carefully to Mir's downlink frequency to avoid causing interference to other stations who may already be in contact with Mir and to determine what mode of communications the Cosmonauts are using on the Mir spacestation.