Doppler Shift Tips During a MIR Contact

(from John Magliacane)

The Mir operating hints that have been carried in SpaceNews over the past several weeks have received wide acclaim from all corners of the world. It is hoped that these tips will help make contacts with Mir more practical and rewarding for all those involved. Many thanks go to G. Miles Mann, WF1F and Dr. Dave Larsen, N6CO, for contributing their sage advice to those recent articles. As a follow-up, I thought I'd throw my hat into the ring and offer some comments on the effect that Doppler shift plays in communicating with a simplex communications satellite such as the Amateur Radio station on Mir.

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 approching 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 145.985 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 145.985 MHz at the time of acquisition of signal (AOS), AND their transmitters several kilohertz BELOW 145.985 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 145.985 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 145.985 MHz and not worry about Doppler shift. As Mir recedes however, stations should tune their receivers several kilohertz BELOW 145.985 MHz, and their uplink transmitters several kilohertz ABOVE 145.985 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. Incidentially, 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 acknowledgement 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.

Many of us have transceivers that are "channelized" (tuned in discrete steps of several kilohertz each). This means you cannot make any fine tuning adjustments to your receiver or transmitter frequency. Most Mobile/HT transceivers are limited to the smallest frequency change of 5 kHz or 2.5 kHz channel steps. Doppler shift will cause the Mir transmit frequency (145.985 MHz) to appear as if it is 3.5 kHz higher in frequency. If you tune to 145.555 MHz, you may improve you reception (for a 5 kHz rig). Consult your transceiver's operating manual for information on establishing "odd-splits" and program in the following consecutive frequencies into your transceiver's memories:

For transceivers with 5 kHz steps:

As you may have noticed, I do not recommend adjusting you uplink frequency for the 5 kHz transceivers. This is because you may have better results if you leave your receiver tuned to 145.985 MHz. The Doppler shift is only at the +3.5 kHz setting for a short period of time at AOS prior to an overhead pass. Shortly after AOS, the downlink signal will drift lower, and in five minutes or less when the spacecraft is overhead, the Doppler shift will be zero for a brief period of time. The downlink signal will then drift lower, down to -3.5 kHz over the next five minutes.

The receiver on Mir can work ground stations when the ground station is off frequency by as much as 3 kHz. This is because the receiver on Mir has a relatively wide passband. In some situations, a wide receiver is good, and we are lucky to have such a receiver on Mir. If the receiver on Mir had better selectivity, it would actually make it harder for ground stations with channelized transceivers to access Mir. The following are specifications for the transceiver on Mir along with a few others for comparison:

Remember, only 1 station can connect to the R0MIR-1 PMS at a time. ALL others must wait.

Miles Mann, WF1F
Dr. Dave Larsen, N6CO