VE6AB working AO51
This photo finds me on a somewhat cool afternoon with -23 Celsius temperatures working a pass of AO51 with a handheld Arrow antenna. No! its not the Arctic, but the plains of southern Alberta!
Orbiting 800km above the earth in a sun-synchronous orbit at speeds of about 28,000 kph, AO51 orbits the earth every 90 minutes, so I can expect there to be 2 to 4 good passes over my location every day. Because of the extreme speed and distances there is an effect called doppler shift. It is a similar effect to listening to a car approaching your location along a highway. The sound waves from the car are compressed as the car gets closer (high in frequency) and stretched (lower in frequency) as the car passes and goes further away. It is the same when listening to the LEO (low earth orbit) satellites. For example when listening for AO-51, the downlink frequency being transmitted from the satellite is 435.300 MHz, at AOS (acquisition of signal) and it will first be received on 435.310 MHz when AO-51 comes up over the horizon. When the satellite is overhead at about 800 kilometers (90 degrees) there is no doppler effect and the transmitted frequency is 435.300 MHz. When the satellite is LOS (loss of signal) the frequency will begin to decrease until the received frequency is at 435.290 MHz. So there is a total shift in received signal of 20 KHz.
These frequencies will have to be either programmed into your transceiver or receiver or manually adjusted during the pass. On 145 MHz the Doppler is only about 3.8 KHz, so these is little need to make any adjustments for the uplink. Working the other leo's orbiting the earth is similar to what I have just described,