A satellite modem or sat modem is a modem used to establish data transfers using a communications satellite as a relay by transmit and receive signals from a satellite transponder. A "modem" stands for "modulator-demodulator" for transmission, it modulates digital data signals from a multiplexing device into a carrier frequency for delivery to an up-converter, amplifier and antenna. For receiving, it converts the frequencies from the down-converter into digital pulses for the multiplexor.
There is a wide range of satellite modems from cheap devices for home internet access to expensive multifunctional equipment for enterprise use.