Mobile Satellite Service (MSS)
Mobile Satellite Service (MSS) is a category of satellite communications service that provides radio links between satellites and mobile Earth stations. These mobile terminals may be located on land, at sea, or in aircraft, and are intended to operate while moving or while temporarily stopped. MSS is used where terrestrial communications networks are unavailable, unreliable, congested, or impractical, such as over oceans, in remote land areas, in disaster zones, and along polar, maritime, aeronautical, or expeditionary routes.
The defining feature of MSS is that at least one end of the satellite link is mobile. A handheld satellite phone, a vehicle-mounted terminal, a ship terminal, an aircraft terminal, and a small emergency beacon may all be examples of mobile Earth stations, depending on the application and frequency allocation. MSS therefore differs from the Fixed Satellite Service (FSS), which primarily serves fixed Earth stations at known locations, and from the Broadcasting Satellite Service (BSS), which is intended for broadcast delivery to receiving terminals.
MSS systems commonly operate in L-band and S-band because these frequencies support relatively compact antennas, tolerate rain attenuation well, and are suitable for mobile terminals that may not be precisely pointed. Some MSS systems also use other bands, including portions of the C-band, Ku-band, and Ka-band for higher-capacity mobile platforms such as ships, aircraft, and transportable terminals. Lower-frequency MSS links are well suited to voice, messaging, low-rate data, tracking, and emergency services. Higher-frequency mobile satellite links can support broadband services, but usually require more accurate antenna pointing and are more affected by blockage and rain attenuation.
Mobile satellite systems may use geostationary satellites, medium Earth orbit satellites, or low Earth orbit satellites. Geostationary MSS satellites appear fixed in the sky, which simplifies network control and allows wide regional coverage, but the terminals must have a reasonable view of the satellite and the link has the delay associated with geostationary altitude. LEO MSS constellations reduce path loss and delay and can provide global coverage, including high-latitude regions, but require multiple satellites, handover between satellites, and more complex constellation management.
MSS applications include satellite telephony, short messaging, emergency distress alerting, search and rescue, maritime safety communications, aeronautical safety communications, fleet tracking, remote monitoring, military communications, disaster recovery, and mobile broadband. MSS is especially important for safety-of-life services because it can operate beyond the reach of terrestrial cellular, microwave, fiber, or HF radio networks. In maritime and aviation contexts, mobile satellite links may provide routine operational communications as well as distress and safety functions.
The design of an MSS terminal is shaped by mobility. Terminals must often operate with small antennas, limited transmit power, battery constraints, changing orientation, and partial blockage by terrain, buildings, ship structures, aircraft fuselages, or vegetation. Mobile links must also account for Doppler shift, fading, shadowing, handover, and variations in elevation. For handheld or vehicular terminals, the antenna may be nearly omnidirectional or only broadly directional. For ship and aircraft broadband terminals, stabilized or electronically steered antennas are commonly used to maintain the satellite link while the platform moves.
MSS spectrum is internationally allocated and coordinated because mobile satellite links must coexist with other satellite and terrestrial services. System design must consider frequency sharing, interference protection, licensing, terminal authorization, and service availability in different countries and regions. Some MSS services are global, while others are regional or tailored to particular markets such as maritime, aviation, emergency response, or government users.
In satellite communications, MSS represents the part of the industry concerned with communications to users on the move. It is therefore central to the extension of communications beyond fixed infrastructure and provides connectivity in places where ordinary terrestrial networks cannot readily reach.