The process of sending information towards the spacecraft is called uplink or forward link and the opposite process is called downlink or return link. Uplink consists of commands and ranging tones where as downlink consists of status telemetry, ranging tones and even may include payload data. Receiver, transmitter and a wide-angle (hemispheric or omnidirectional) antenna are the main components of a basic communication subsystem. Systems with high data rates may even use a directional antenna, if required. The subsystem can provide us with the coherence between uplink and downlink signals, with the help of which we can measure range-rate Doppler shifts. The communication subsystem is sized by data rate, allowable error rate, communication path length, and RF frequency.