Satellite radio, quite simply, is a non-terrestrial microwave transmission system utilizing a space relay station. Satellites have proved invaluable in extending the reach of voice, data, and video communications around the globe and into the most remote regions of the world. Exotic applications such as the Global Positioning System (GPS) would have been unthinkable without the benefit of satellites.
Contemporary satellite communications systems involve a satellite relay station that is launched into a geostationary, geosynchronous, or geostatic orbit. Such satellites are called geostationary satellites. Such an orbit is approximately 36,000 km above the equator as depicted in Figure. At that altitude and in an equatorial orbital slot, the satellite revolves around the earth with the same speed as of that the speed of revolution of earth and maintains its relative position over the same spot of the earth's surface. Consequently, transmit and receive earth stations can be pointed reliably at the satellite for communications purposes.
The popularity of satellite communications has placed great demands on the international regulators to manage and allocate available frequencies, as well as the limited number of orbital slots available for satellite positioning are managed at national,regioi1al and international levels. Generally speaking, geostationary satellites are positioned approximately 2° apart in order to minimize interference from adjacent satellites using overlapping frequencies.
Such high frequency signals are especially susceptible to attenuation in the atmosphere. Therefore, in case of satellite communication two different frequencies are used as carrier frequencies to avoid interference between incoming and outgoing signals. These are:
Uplink frequency: It is the frequency used to transmit signal from earth station to satellite. The uplink signal can be made stronger to cope better with atmospheric distortion. The antenna at transmitting side is centered in a concave, reflective dish that serves to focus the radio beam, with maximum effect, on the receiving satellite antenna. The receiving antenna, similarly, is centered in a concave metal dish, which serves to collect the maximum amount of incoming signal.
Downlink frequency: It is the frequency used to transmit the signal from satellite to earth station. In other words, the downlink transmission is focused on a particular footprint, or area of coverage. The lower frequency, used for the downlink; can better penetrate the earth's atmosphere and electromagnetic field, which can act to bend the incoming signal much as light bends when entering a pool of water.
Broadcast: The wide footprint of a satellite radio system allows a signal to be broadcast over a wide range Thereby; any number (theoretically an infinite number) of terrestrial antennae can receive the signal, more or less simultaneously. In this manner, satellites can serve a point-to-multipoint network requirement through a single uplink station and multiple downlink stations,
Recently, satellites have been developed which can serve a mesh network, requirement, whereby each terrestrial site can communicate directly with any other site. Previously, all such communications were required to travel through a centralized site, known as a head end. Such a mesh network, of course, imposes an additional level of difficulty on the network in terms of management of the flow and direction of traffic.
Classification of Satellites
Depending on the height of the Earth at which the satellites are, these can classify as:
• Geostationary Satellites (GEO).
• Medium Earth Orbit Satellites (MEO).
• Low Earth Orbit Satellites (LEO).
GEO satellites (geostationary Earth orbit):
They are satellites that fly at high altitude (35,000 km), and due to their period, they appear to remain motionless in the sky.
The first GEOs had a single space beam that illuminated about a third of the earth called footprint. Currently, these beams can concentrate on small geographic areas (small beams).
A recent advance in the world of satellites is the development of low-cost micro stations called VSATs (Tiny opening terminals). These terminals have 1-meter antennas running their downlink at 19.2 kbps and the descending to 512 kbps. Direct broadcast satellite television uses this technology.
The micro stations do not have enough energy to communicate with each other (via satellite) VSATs are required with a central station with a large antenna and a powerful amplifier to relay traffic between VSATs.
They are satellites of medium Earth orbit (between 5000 and 15000 km) between the two Allen belts. It takes 6 hours to go around the Earth, they are smaller than GEOs, and they have less footprint. Examples: The 24 GPS satellites (Global Positioning System).
Low Earth orbit satellites (below the two Allen belts) with a height of 750 km. Due to their speed of movement, large amounts of them are required to cover the Earth. They have a lower round trip delay (msec) and offer telecommunications services worldwide through handheld devices that communicate with the satellite.
Iridium: Iridium satellites form six necklaces around the Earth with a satellite every 32 degrees latitude, therefore 66 satellites.
Globalstar: It is an alternative design for Iridium. It based on 48 LEO satellites that use a different switching scheme. Iridium retransmits satellite calls on satellite, while Globalstar does so through a terrestrial network. The use of large antennas in-ground stations can produce a strong signal and receive a weak one. They are used, for example, for low power phones.
Teledesic:Iridium intended for phone users who are in places with extreme conditions.
Teledesic intended for Internet users in the world desiring bandwidth. It was conceived in 1990 by Craig MacCaw and Bill Gates.
Its objective is to offer a 100 Mbps uplink and 720 Mbps downlink through small and fixed VSAT antennas that ignore the telephone system. It consists of 30 satellites in the Ka-band, using packet switching in space. They planned to start in 2005.
General Properties of Satellite Communication
• Configuration: Satellite communication systems consist of antennae and reflective dishes, much as a terrestrial microwave. The dish serves to focus the signal from a transmitting antenna to a receiving antenna. The send (receive) dishes that make up the earth segment are of varying sizes, depending on power levels and frequency bands. They generally are mounted on a tripod or other type of brace, which is anchored to the earth, pad or roof or attached to a structure such building. Cables connect the antennae to the actual transmit (receive) equipment. The terrestrial antennae support a single frequency band, for example, C-band, Ku-band or Ki-band. The higher the frequency bands the smaller the possible size of the dish. Therefore, while C-band TV dishes tend to be rather large, Ku-band DBS (Direct Broadcast Satellite) TV dishes tend to be very small. The space segment dishes are mounted on a satellite, of course. The satellite can support multiple transmit (receive) dishes, depending on the various frequencies which it employs to support various applications, and depending on whether it covers an entire footprint or divides the footprint into smaller areas of coverage through the use of more tightly focused spot beams· Satellite repeaters are in the form of number of transponders. The transponders accept the weak incoming signals, boost them, shift from the uplink to the downlink frequencies, and transmit the information to the earth stations.
• Bandwidth: Satellites can support multiple transponders and, therefore, substantial bandwidth, with each transponder generally providing increments in bandwidth.
• Error Performance: Satellite transmission is susceptible to environmental interference, particularly at frequencies above 20 GHz. Sunspots and other types of electromagnetic interference affect satellite and microwave transmission. Additionally, some satellite frequency bands, for example, C-band needs careful frequency management. As a result of these factors, satellite transmission often requires rather extensive error detection and correction capabilities.
• Distance: Satellite is not considered to be distance limited as the signal largely travels through the vacuum of space. Further each signal travels approximately 36,000 km in each direction.
• Propagation Delay: Geostationary satellites, by virtue of their high orbital altitude, impose rather significant propagation delay on the signal. Hence, highly interactive voice, data, and video applications are not effectively supported via two-way satellite communications.
• Security: is with all microwave and other radio systems, satellite transmission is inherently not secure. Satellite transmission is especially vulnerable to interception, as the signal is broadcast over the entire area of the footprint. Therefore, the unauthorized user must know only the satellite and associated frequency range being employed. Security must be imposed through encryption (scrambling) of the signal.
• Cost: The acquisition, deployment, and rearrangement costs of the space segment of satellite systems can be quite high in several million dollars. However, the satellite can be shared by a large number of users, With each user perhaps connecting a large number of sites. As a result, satellite networks often compare very favorably with cabled systems or microwave systems for many point-to-multipoint applications.
• Application: Satellite applications are many and increasing rapidly as the traditional voice and data services have been augmented Traditional international voice and data services have been supplanted to a considerable extent by submarine fiber optic' cable system.
Traditional applications include international voice and data, remote voice and data, television and radio broadcast, maritime navigation, videoconferencing, inventory management and control through VSATs, disaster recovery and paging. More recent and emerging applications include air navigation, Global Positioning Systems (GPS), mobile voice and data because of Low Earth Orbit Satellites (LEOs), Advanced Traffic Management Systems (ATMS), Direct Broadcast Satellite (DBS) TV, Integrated Digital Services Network (ISDN), interactive television, and interactive multimedia.