A Satellite-to-Ground Link (SGL) describes the communication between a satellite and a ground station. If the ground station is transmitting the signal up to a satellite it is an uplink signal. Similarly, if the satellite is transmitting a signal down to a ground station it is a downlink signal. A basic satellite communication network is formed when one ground station uplinks a signal to a satellite which then in turn downlinks the signal to a second ground station.
Uplink and downlink signals are usually transmitted at different frequencies. After the satellite receiver obtains the uplink signal, it is the role of the transponder to down-convert the signal to the downlink frequency and then amplify the signal before it is transmitted to the receiving ground station.