In satellite operation, being on the right frequency is complicated by a phenomenon called "Doppler Shift" or "Doppler Effect." It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842, and is the change in frequency of a wave caused the motion of the wave source in relation to an observer. In other words, a satellite in space is moving so fast that it distorts the frequencies at which it transmits and receives. The direction and magnitude of this shift in frequencies varies depending on whether the satellite is approaching or receding, its distance from the observer, and the original frequency of the wave being distorted. Thus, the Doppler Shift will be different for every user of the satellite at any given moment.
FM Satellite Doppler Correction
FM, due to its inherent characteristics, is rather forgiving about being precisely on frequency. But Doppler Shift becomes more pronounced as frequency increases. So for FM satellites, we can usually get by ignoring the Doppler Effect in the 2 meter band, but we often need to make some corrections in the 70cm band. For example, FM satellite AO-91 transmits down toward earth (the downlink) on 145.960 MHz. In most instances, one may tune their receiver to that frequency and hear the satellite just fine. However, AO-91 receives signals coming up from the ground (the uplink) at 435.250 MHz. A station transmitting on that frequency will probably only be successful in being heard through the satellite when it is near its closest approach to that station -- when the Doppler Shift is near minimum.
At the beginning of the satellite's pass over that station, as the satellite is approaching, the operator will have much greater success by tuning to a lower frequency -- perhaps about 435.240 MHz. Once the Doppler Effect has made its impact, that transmitted signal will appear to the satellite's receiver to be very nearly on frequency (435.250). As the satellite moves a bit closer, but is still approaching, the operator will be more successful raising the transmitted frequency a bit -- perhaps to 435.245 MHz -- because the Doppler Effect will have become somewhat less pronounced as the satellite draws closer. Similarly, as the satellite begins to recede from the station, the operator will keep tuning upward -- perhaps to 435.255. And as the satellite nears the horizon, still going away, the operator might tune up still more to 435.260 MHz.
Conversely, FM satellite SO-50 transmits its downlink at about 436.795 MHz while listening for uplink signals at 145.850 MHz. In this case, the operator may tune the transmitter to 145.850 MHz and be quite successful being heard through the satellite throughout its pass. But in order to receive SO-50 well, the operator will have to begin the pass (at AOS, or Acquisition of Signal) by tuning the receiver at bit higher -- perhaps to 436.805 or so. The operator will then tune the receiver downward in frequency, passing the satellite's actual transmitted frequency of 436.795 near the Time of Closest Approach (TCA), and continuing down perhaps as low as 436.785 at Loss of Signal (LOS), when the satellite disappears below the horizon once again.
This may all seem very confusing, but it is easy to remember that one only needs to adjust the 70cm frequency -- no matter whether that happens to be the uplink or the downlink -- and to recall that uplinks move up, and downlinks move down!
Linear Satellite Doppler Correction
Linear satellites make use of CW and SSB modes, and these modes are less forgiving of errors in frequency. If a received SSB signal is tuned off frequency by even a few hundred Hertz, it will begin to become unintelligible. As frequencies change due to the Doppler Effect, they move by much more than this amount, particularly at UHF frequencies.
Because the Doppler shift depends on the geometrical relationship between the station location and the satellite’s path and the ground stations are in diverse locations. If each operator sets his transmitter (or receiver) at the beginning of a QSO and then doesn’t touch it, they will not stay on the same frequency. They will end up diverging because the various locations have various Doppler shift profiles.