| Satellite | spacecraft placed in a stable orbit around the Earth |
| Satellite link | a radio link between a transmitting earth station and a receiving earth station through a satellite, consisting of an uplink and a downlink |
| Satellite news gathering | technique of using a small transportable earth station to relay news items (usually television) directly from the site where the newsworthy event has occurred to a television studio for incorporation into news programmes |
| Scalar feed | a type of horn antenna feed which uses a series of concentric rings to capture signals |
| Scrambling | see encryption |
| Selection | accessing a particular terminal in a network |
| Serial | a transmission technique in which bits or characters are sent one after another |
| Service area | a geographical area that is served by a specific gateway in a mobile satellite system |
| Service link | the transmission link between a satellite and end-users' mobile transceivers |
| Set top box | decoder (which is often placed on top of a television - hence the name) which decodes and decompresses digital signals to enable them to be viewed on a television; usually also incorporates some form of conditional access |
| Short term fade | a fade whose duration is less than the round trip delay |
| Sideband | the band of frequencies on either side of a carrier produced during the process of modulation |
| Sidelobe | secondary beam of an antenna at an angle to the main beam |
| Simplex | transmission in one direction only |
| Simulcast | simultaneous transmission of an analogue and digital TV channel on the same transponder, each occupying different parts of the transponder bandwidth |
| Slant range | distance between the satellite and the ground station |
| Smart card | a credit card sized electronics key placed in a decoder to allow unscrambling of an encrypted channel or to allow access to a network |
| Soft handoff | in a mobile satellite system the process of transferring a circuit from one beam or satellite to another without interruption of the call |
| Solar cell | device which converts light from the sun into electrical power |
| Solar outage | an increase in sky noise caused by the sun being within the receive antenna beam and which swamps the wanted signal |
| Solid propellant | the rubber or plastic like material used in solid rocket motors; solid propellants contain a fuel (ie aluminium powder) and an oxidiser (ie ammonium perchlorate); solid propellants give a very high thrust for a short time |
| Sound in synch | a TV transmission technique in which the sound channel is broadcast as a compressed signal during the synch pulses in the video signal |
| Space segment | the part of a satellite system consisting of the satellite and the ground support facilities for the satellite including telemetry, telecommand and ranging |
| Space station | ITU terminology for the repeaters and antennas forming a communications payload on a satellite |
| Spillover | satellite signals that fall outside the beam pattern's defined edge of coverage |
| Spin stabilisation | a form of satellite stabilisation and attitude control in which the exterior of the spacecraft is spun about an axis at a fixed rate |
| Splitter | a passive device which divides a signal between two or more paths |
| Spot beam | a narrow satellite antenna beam covering a small geographical area |
| Spread spectrum | a transmission technique which uses a much wider bandwidth than would normally be required |
| Star network | a network configuration in which a major hub is connected to many nodes which are not connected to each other except through the hub |
| Start bit | in asynch transmission, the bit that synchronises the receiver clock to the received data for each received character; it is always a space |
| Stationkeeping | minor orbital adjustments that maintain the satellite within the defined parameters of its orbit |
| Statistical multiplexing | form of multiplexing which allocates time or bandwidth only to active channels |
| Stop bits | bit(s) which terminate the character frame in asynch transmissions |
| Subcarrier | auxiliary signal associated with a TV carrier used for carrying the soundtrack or additional radio channels |
| Sub satellite point | the point on the earth's surface immediately below a satellite |
| Subscriber communicator | small data terminal uses to send and receive packets of data to and from Orbcomm satellites |
| Subscriber identity module | a smart card which carries data about a subscriber and which is inserted into a mobile phone before a call can be made |
| Sun Synchronous Orbit | an orbit in which the satellite retains the same direction in relation to the sun; the satellite, therefore, always passes over the equator at the same location at the same local time; this orbit is generally used by earth observation satellites |
| Supervisory signal | signals used within a network to indicate its status |
| Suppressed carrier | a transmission technique in which one or both sidebands of a modulated carrier are transmitted but the carrier is either totally or partially removed |
| Switch over | transfer of traffic from one part of a network to another, for example in response to a failure in part on the network |
| Synch character | character used to establish and maintain synchronisation between multiplexers or synchronous terminals |
| Synch idle | character (called a synch character) used to fill character space to maintain synchronisation in the absence of data |
| Synchronisation | the process of aligning the timing of electronic signals across a network |
