In this unit we are going to speak about satellite communications. Satellites are useful for connecting to remote areas, or when you want to broadcast video or data with minimal infrastructure.
A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals between a source and a receiver. To do so it uses an electronic device called transponder. A satellite usually has several transponders. Communications satellites are used for television, telephone, radio, internet, and military applications. There are over 2,000 communications satellites in the Earth s orbit, used by both private and government organizations. They are used for ship-to-shore maritime communications, Internet access for remote, rural classrooms, or vital data and communications for petroleum operations, for example.
There are two major classes of communications satellites, Passive and Active. Passive satellites only redirect (usually via reflection) the signal coming from the source towards the direction of the receiver. With passive satellites, the reflected signal is not amplified before it returns to Earth, so it is received usually very weak, and so you will need big antennas. Active satellites, on the other hand, amplify the received signal before re-transmitting it to the receiver on the ground. The problem is that they have higher power needs and their active life is shorter.
Most communication satellites use microwave frequency bands for their services, as their propagation characteristics are suitable for this application. There are four radio frequency bands used for civil communication and military satellite services. The lower frequencies have less bandwidth and need bigger antennas, but perform better under adverse weather conditions. The bands, going for lower to higher frequencies, are called C, X, Ku and Ka.
To an observer on the earth, a satellite in a geostationary orbit appears motionless, in a fixed position in the sky. This is because it revolves around the earth at the earth's own angular velocity (360 degrees every 24 hours in an equatorial orbit), you can see it in the slide. To do so it must be located in a a circular orbit 35,786 kilometers (22,236 mi) above the Earth's equator and following the direction of the Earth's rotation. A geostationary orbit is useful for communications because ground antennas can be aimed at the satellite without them having to track the satellite's motion. This is relatively inexpensive. In applications that require a large number of ground antennas, such as DirecTV distribution, the savings in ground equipment can more than outweigh the cost and complexity of placing a satellite into that orbit, that is far from the earth The concept of the geostationary communications satellite was first proposed by Arthur C. Clarke. In October 1945 Clarke published an article titled "Extraterrestrial Relays" in the British magazine Wireless World.[7] The article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus, Arthur C. Clarke is often quoted as being the inventor of the communications satellite and the term 'Clarke Belt' employed as a description of the orbit
The footprint of a communications satellite is the ground area that its transponders offer coverage, and, together with the transponder power, determines the antenna dish diameter required to receive each transponder's signal (for the same power, the bigger the footprint, the bigger the antenna, as the signal will be spread over a larger area and less power will reach the antenna). There is usually a different map for each transponder (or group of transponders), as each may be aimed to cover different areas.
Low Earth Orbit (LEO) is a circular orbit between 160 and 2000 km above the earth's surface that has a period (time to revolve around the earth) of about 90 minutes. Because of their low altitude, the satellites using it are only visible from within a radius of roughly 1,000 kilometers from the sub-satellite point. In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity. Low-Earth-orbiting satellites are less expensive to launch into orbit than geostationary satellites and, due to proximity to the ground, do not require the same signal strength (recall that signal strength falls off at the square of the distance from the source) and they have less latency (that is, the time required by the radio wave to reach the satellite and come back). Thus there is a trade off between the number of satellites and their cost.
when it comes to delivering full time communications service over a given region, LEO requires very large constellations of small satellites, what can translate in higher costs of deployment and operation.
Allocating frequencies to satellite services is a complicated process which requires international coordination and planning. This is carried out under the auspices of the International Telecommunication Union (ITU). To facilitate frequency planning, the world is divided into three regions: Region 1: Europe, Africa, what was formerly the Soviet Union, and Mongolia Region 2: North and South America and Greenland Region 3: Asia (excluding region 1 areas), Australia, and the southwest Pacific Within these regions, frequency bands are allocated to various satellite services, although a given service may be allocated different frequency bands in different regions. Some of the services provided by satellites are: Fixed satellite service (FSS) Broadcasting satellite service (BSS) Mobile satellite service Radionavigation-satellite service Meteorological-satellite service Amateur-satellite service
To receive satellite transmissions any user needs a proper receiver. Depending on the distance to the satellite and the transmission power, larger or smaller antennas are needed. For Geostationary satellites the most used antenna is the parabolic dish, which has to be aimed to the desired satellite. The ground stations used for two way satellite communications with small parabolic antennas (less than 3 meters and usually around 1 m) are called VSAT (for very small aperture terminal) and have data rates usually ranging from 4 kbits/sg to 4 Mbits/sg There are also satellite based mobile communications in which antennas track the satellites with mechanical systems in real time while the ground or sea station is moving. The same system can be used if constellations of moving satellites are used.