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When used for communications, a satellite acts as a repeater. Its height above the Earth means that signals can be transmitted over distances that are very much greater than the line of sight. An earth station transmits the signal up to the satellite. This is called the up-link and is transmitted on one frequency. The satellite receives the signal and re-transmits it on what is termed the down link which is on another frequency.
The circuitry in the satellite that acts as the receiver, frequency changer, and transmitter is called a transponder. This basically consists of a low noise amplifier, a frequency changer consisting a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make sure that any out of band signals such as the transponder output are reduced to acceptable levels so that the amplifier is not overloaded. Similarly the output from the amplifiers is filtered to make sure that spurious signals are reduced to acceptable levels.
The signal is received and amplified to a suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a super-heterodyne radio receiver. As a result the communications satellite receives in one band of frequencies and transmits in another.
In view of the fact that the receiver and transmitter are operating at the same time and in close proximity, care has to be taken in the design of the satellite that the transmitter does not interfere with the receiver. This might result from spurious signals arising from the transmitter, or the receiver may become de-sensitized by the strong signal being received from the transmitter. The filters already mentioned are used to reduce these effects.
- Geosynchronous (GEO) satellite orbit (22,300 miles) rotates at the same speed as the earth’s rotation. Three satellites can cover most of the globe.
- Medium earth orbit (MEO) satellites are closer to users on Earth but 10-18 are required for continuous coverage.
- Low earth orbit (LEO); satellites are closest to users (300-500 miles) but 40-70 satellites are required for full coverage.
According to the Satellite Industry Association (SIA) report, as of June, 2011.
the following statistics were observed;
Operational Satellites by Orbit
- Of the 986 satellites on orbit, 37% are commercial communications satellites
- There are 480 LEO Satellites (49%), 405 GEO Satellites (41%), 65 MEO (7%) and 35 ELI or 4% (Elliptical Orbit)
Operational Satellites by Function
- Commercial Communications 365 Satellites (37%),
- Civil Communications 108 Satellites (11%),
- Military Communications 84 Satellites (9%),
- Meteorological 44 Satellites (4%),
- Military Surveillance and other 89 Satellites (8%),
- Navigation 75 Satellites (8%),
- Remote Spacing 92 Satellites (9%),
- Space Science 59 Satellites (6%),
- other 50 (7%)
The key Satellite Network Elements include;
- Space Segment – Satellite, in geostationary orbit, middle earth orbit, or low earth orbit.
- Ground Segment
- Telemetry, Tracking, and Control (TT&C): used to “fly” the satellite.
- Gateway/Hub: used to manage communication payload
- User Terminals: devices used to connect to satellite network; can be receive-only or transmit; mobile or fixed; a dish, a laptop, or a handheld, depending on application, site, etc.
Satellites function as a cell or Microwave tower in the sky, transmitting data from one point on the earth to another.
Attributes of Satellite Systems
- Large Geographic Coverage interconnecting widely distributed networks, providing broadcasting services over a country, region, or entire hemisphere “last mile” connectivity for telecom services, broadband and video services
- Instant infrastructure, whether for network redundancy or emergency connectivity.
- Mobility, whether for voice, data or broadband or mobile video.
Satellites in Telecommunications
- Commercial satellites are an essential component of the global communications infrastructure
- Satellites carry the world’s media content around the globe
- Satellites deliver television, radio and broadband services directly to consumers
- Satellites offer mobile and portable voice, data and video globally
- Satellite networks link businesses among widely-dispersed locations
- Satellites provide connectivity and network restoration for remote and rural telecommunications operators
- Satellites are critical to disaster recovery and emergency preparedness
- Satellites provide critical communications and network restoration services following natural disasters
- The satellite industry is a subset of both the telecommunications and space industries
- Satellite industry revenues represent 61% of space industry revenues and 4% of telecommunications industry revenues