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 ISS 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 ISS to appear about 3.5 kHz ABOVE ISS's actual transmitting frequency of 145.995 MHz. At the same time, while ISS 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 ISS 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 ISS must tune their receivers several kilohertz ABOVE 145.995 MHz at the time of acquisition of signal (AOS), AND their transmitters several kilohertz BELOW 145.995 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 145.995 MHz at the start of a pass, their uplink signals are received 7 kHz (or 8.5 kHz) ABOVE what the ISS 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 ISS's uplink receiver.
Around the time of closest approach (TCA), which also occurs around
the time of maximum elevation for satellites such as ISS in circular
orbits, the Doppler shift between ISS and the ground station
approaches zero. It then quickly reverses polarity as ISS recedes at
an ever increasing rate away from the ground station. At TCA, it is
safe for ground stations to set their transceivers to 145.995 MHz and
not worry about Doppler shift. As ISS recedes however, stations should
tune their receivers several kilohertz BELOW 145.995 MHz, and their
uplink transmitters several kilohertz ABOVE 145.995 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 ISS 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 ISS, 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 ISS 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 ISS's uplink frequency.
That's why QRM is so damaging and following proper operating procedures is so important when communicating with ISS.
While it may be possible to successfully pass a short connect request packet to ISS 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 ISS in the midst of heavy interference.
In closing, please keep in mind that the Personal Message System on ISS is a single-user BBS with very limited storage capabilities. Only ONE station may connect to ISS at a time. ALL OTHERS MUST WAIT. PLEASE DO NOT connect to ISS unless the PMS is open and transmitting CQ frames. PLEASE DO NOT connect to ISS unless you have a reason for doing so. PLEASE DO NOT use ISS for passing terrestrial packet radio traffic. PLEASE DO NOT use ISS as a digipeater, and PLEASE DO NOT connect to the keyboard port 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 ISS and to determine what mode of communications the Cosmonauts are using on the ISS 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 ISS transmit frequency (145.995 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 145.995 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 ISS 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 ISS 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 ISS along with a few others for comparison:
|Transceivers||Selectivity -6db||Selectivity -60db||Remarks|
|ICOM 228||15kHz||30kHz||Backup radio|
|Yaesu 736R||12kHz||25kHz||Typical Base station|
|Yaesu 2400||12kHz||30kHz||Typical Mobile transceiver|
Remember, only 1 station can connect to the PMS at a time. ALL
others must wait.