Satellite communications connections consist of unidirectional transmission paths. This, by definition, makes them simplex connections. To create a full-duplex path, you need two communication paths on differnt frequencies. Speaking from the perspective of the ground station, you need frequency for transmission and one non-interfering frequency for receiving. When transmitting, you use a modulator. When receiving you use a demodulator.
The modulator takes data (a voice or computer file) and changes some aspect of the radio frequency wave to indicate a certain value. The majority of digital systems transmitting on satellites use what is called Phase Shift Keying (PSK). A waveform has peaks and troughs. Phase shift keying alters where the peaks and troughs appear. A modulator that 'modulates' a radio signal using phase shift keying (PSK) will change the phase of the waveform. Standard Quadrature Phase Shift Keying shifts radio waveforms 90, 180 or 270 degrees out of phase.
The demodulator extracts the data that was originally modulated into the radio signal. It recognizes the alterations to the radio signal and uses those changes to produce a stream of binary data for use with computers.