COMMUNICATIONS SATELLITE

A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency. Beside communications satellites, there are other types of satellites namely:

• Weather satellites - provide meteorologists with scientific data to predict weather conditions and are equipped with advanced instruments.
• Earth observation satellites - allow scientists to gather valuable data about the earth's ecosystem.
• Navigation satellites - these satellites are able to provide a person's exact location on Earth to within a few meters

These satellites are placed in different orbit ( the path that a satellite follows as it revolves around earth ) depend on what type of satellites they are. For communications satellites they are placed in the geostationary orbit. Why? Because firstly, one satellite can cover almost 1/3 of Earth's surface, offering a reach far more extensive than what any terrestrial network can achieve and secondly, communications require the use of fixed antennas. Since geosynchronous satellites remain stationary over the same orbital location, users can point their satellite dishes in the right direction, without costly tracking activities, making communications reliable and secure. GEO satellites are proven, reliable and secure - with a lifespan of 10-15 years

Satellite Architecture
Communications data passes through a satellite using a signal path known as a transponder. Typically satellites have between 24 and 72 transponders. A single transponder is capable of handling up to 155 million bits of information per second. With this immense capacity, today's communication satellites are an ideal medium for transmitting and receiving almost any kind of content - from simple voice or data to the most complex and bandwidth-intensive video, audio and Internet content.

Frequency Bands and Beams
Satellites transmit information within radio frequency bands. The frequency bands most used by satellite communications companies are called C-band and the higher Ku-band. Over the next several years, the use of a higher frequency band known as Ka-band is expected to increase. Modern satellites are designed to focus on different ranges of frequency bands and different power levels at particular geographic areas. These focus areas are called beams. Intelsat offers four beam types i.e. Global which covering almost 1/3 of Earth's surface, Hemi which covering almost 1/6 of Earth's surface, Zone which covering a large landmass area and Spot which covering a specific geographic area.

What is Installed on The Ground?
All communications with a geostationary satellite require using an earth station or antenna. Earth Stations may be either fixed (installed at a specific location) or mobile for uses such as Satellite News Gathering (SNG) or maritime applications. Antennas range in size, from large telecommunications carrier dishes of 4.5 to 15 meters in diameter, to VSAT antennas which can be as small as under one meter, designed to support services such as Direct to Home TV (DTH) and rural telephony.

The antenna, itself, will generally be connected to equipment indoors called an indoor unit (IDU), which then connects either to the actual communications devices being used, to a Local Area Network (LAN), or to additional terrestrial network infrastructure.

Network Topologies
Depending on the application, satellites can be used with different ground network designs or network topologies. At its simplest, satellite can support one-direction or two-direction links between two earth stations (called respectively simplex transmission and duplex transmission). More complex communications needs can also be addressed with more sophisticated network topologies, such as star and mesh. Some of the options available for satellite networks are :

• Simplex Transmission - applications for simplex services include broadcast transmissions such as TV and video services, radio services.
• Point-to-Point Duplex Transmission - applications for duplex services include voice telephony transport, data and IP transport (especially in asymmetric configurations), corporate networks, TV and Broadcast program contribution and distribution.
• Point-to-Multipoint Transmission (May be simplex or duplex, symmetric or asymmetric) - applications for point-to-multipoint services include corporate networks, including VSAT services and business television, video and broadcast distribution including Direct-to-Home Internet services.
• Mobile Antenna Service - applications for mobile antenna services include satellite news gathering, special event backhaul and broadcasting and maritime services
• Star Network - applications for Star Networks include corporate networks, distance learning
• Mesh Network - applications for Mesh Networks include national and international telephony and data networks, rural telephony