What is a satellite modem?
A satellite modem (modulator-demodulator) is the core signal processing unit at the user terminal or hub station that converts between digital network data and the radio frequency signals carried by the satellite link. On the transmit side, it applies digital modulation (QPSK, 8PSK, 16APSK, 32APSK, 64APSK depending on link conditions), forward error correction (FEC) encoding, and spectral shaping before the signal is upconverted and amplified. On the receive side, it downconverts, demodulates, and decodes the satellite signal, correcting errors using FEC.
Key functions
Modern satellite modems implement: Adaptive Coding and Modulation (ACM) — dynamically adjusting the modulation order and FEC rate based on measured signal quality to maximise throughput during clear sky and maintain connectivity during rain fade. DVB-S2X compliance for the forward link and DVB-RCS2 for the return link in standard VSAT architectures. Quality of Service (QoS) management to prioritise voice and real-time applications over bulk data. TCP acceleration — satellite-specific protocol enhancements (Performance Enhancing Proxies, PEP) that compensate for the high latency of GEO links, improving TCP throughput by pre-acknowledging data segments.
Major manufacturers
Hughes Network Systems (HughesNet Jupiter modems), Comtech Telecommunications, iDirect (now part of ST Engineering), Gilat Satellite Networks, and Newtec (now ST Engineering) supply the majority of commercial VSAT modems. Each implements a proprietary air interface for the return link channel access scheme (TDMA, SCPC, or MF-TDMA) while using DVB-S2/S2X as the open standard for the forward link.