Why doesn't my modem work?

A "modem" stands for "modulator-demodulator". A satellite modem's main function is to transform an input bitstream to a radio signal and vice versa. There are some devices that include only a demodulator (and doesn't comprise any modulator) that are also referred to as "satellite modems". These devices are used in satellite Internet access (in this case data are transferred in other direction through a conventional PSTN modem or an ADSL modem).