What is an amateur radio satellite?
An amateur radio satellite is a little like a repeater in space, a station that relays signal over a broad territory because of the height of the transmitter.
A number of Radio Amateur satellites have been launched, operated, and reached end of life. A number of them are still usable, some even multiple times a day. There are new ones under development almost every year.
Amateur radio satellites will vary by the bands they use/make available, modes, and orbital characteristics.
What is Doppler shift?
Due to the high orbital speed of satellites, the uplink and downlink frequencies will vary during the course of a satellite pass. This phenomenon is known as the Doppler effect. While the satellite is moving towards the ground station, the downlink frequency will appear to be higher than normal and therefore, the receiver frequency at the ground station must be adjusted higher in order to continue receiving the satellite. The satellite in turn, will be receiving the uplink signal at a higher frequency than normal so the ground station’s transmitted uplink frequency must be lower in order to be received by the satellite. After the satellite passes overhead and begins to move away, this process reverses itself. The downlink frequency will appear lower and the uplink frequency will need to be adjusted higher. The following mathematical formulas relate the doppler shift to the velocity of the satellite.
© 2011 AMSAT-UK