S

S/N, SNR
Signal to noise ratio

Satellite
Any object in orbit around another. In the context of communications, a geostationary satellite is an artificial satellite carrying a communications payload (amplifiers, frequency translators, etc.) and acting as a radio relay station to facilitate communications between two points on the Earth's surface.

SCPC
Single Channel per Carrier; a communications link that transfers data from one source using one, permanently active carrier, in each direction.

Sidelobe
The radiation from an antenna that is not part of its main beam

Simplex
A system that is only capable of sending data in one direction

Slot
(1) The orbital position of a satellite
(2) The short period of time allowed for a station to transmit in a TDMA system (also timeslot)

SNG
Satellite News Gathering

SNMP
A common system for managing devices on an IP network

Solar alignment event
A brief period during the Autumn and Spring Equinoxes when the satellite, antenna, and sun line up. The antenna on the Earth sees the sun directly behind the satellite making it more difficult to extract the received signal from the increased thermal noise.

Spectrum analyser
A single frequency can be shown as a sine wave if its amplitude is plotted against time, or as a spectral line if the amplitude is plotted against frequency. Plotting a signal in the frequency domain can help to visualise why it is happening in a satellite system. A spectrum analyser will display a picture of signals in the frequency domain and is a vital tool in satellite systems monitoring. Automated systems connecting to the spectrum analyser can continuously monitor power, noise and interference levels.

Spin Stabilisation
A system to stabilise the satellite by rotating its body

Spot beam
A narrowly focussed beam from the satellite, yielding high power / performance over a relatively small coverage area. The spot can often be steered to fine tune the coverage area

Star network
A network where all communication is between a central site and several remote stations. The central site installation is usually large and specialist to give cost savings in the remote site equipment.

Symmetric
Data traffic transmitted is equal to the data stream received. The terrestrial telecommunications network was designed around the assumption of symmetry.

Synchronous transmission
A system where all timing is derived from a single clock source.