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.
Transponder
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. [1]
References
SPIE Paper: system aspects of optical LEO-to-ground links
SPIE Paper: Use of adaptive optics in ground stations for high data rate satellite-to-ground links
SPIE Paper: Three years of optical satellite to ground links with the T-AOGS