History and Origins of Communications Satellites
– Arthur C. Clarke published the concept of communications satellites in geostationary orbits in 1945.
– Sputnik 1, launched in 1957, was the first artificial Earth satellite.
– Passive satellites were the first communications satellites, reflecting signals.
– Project SCORE, launched in 1958, was the first purpose-built satellite for relaying communications.
– Echo 1, launched in 1960, was the first artificial satellite used for passive relay communications.
– The launch of Sputnik 1 marked the beginning of the Space Age.
– The United States Naval Research Laboratory initiated the Communication Moon Relay project in 1951.
Early Experiments with Active and Passive Satellites
– Communications Moon Relay aimed to create a long communications circuit using the Moon as a passive relay.
– Active satellites amplify and retransmit signals, while passive satellites only reflect signals.
– Project SCORE used a tape recorder to store and retransmit messages, including a Christmas greeting from U.S. President Dwight D. Eisenhower.
– Courier 1B, launched in 1960, explored the possibility of a global military communications network.
– Echo 1, launched in 1960, was an aluminized balloon satellite used for passive relay communications.
International Commercial Satellite Projects
– The Communications Satellite Corporation (COMSAT) was created in the United States in 1962.
– International commercial satellite projects aimed to develop satellite communications.
– COMSAT played a key role in the development of international satellite communications.
– The Intelsat consortium was formed in 1964 to provide global commercial satellite communications.
– International commercial satellite projects expanded the use of communications satellites worldwide.
Satellite Orbits
– Communications satellites primarily have three types of orbits: geostationary orbit (GEO), medium Earth orbit (MEO), and low Earth orbit (LEO).
– GEO is located 22,236 miles (35,785km) from Earth’s surface and allows satellites to appear stationary in the sky.
– MEO satellites are closer to Earth, with altitudes ranging from 2,000 to 36,000 kilometers (1,200 to 22,400mi).
– LEO satellites are about 160 to 2,000 kilometers (99 to 1,243mi) above Earth’s surface and require a larger number of satellites for continuous coverage.
– LEO and MEO satellites offer reduced latency and lower power requirements compared to GEO satellites.
Satellite Constellation and Other Orbit Types
– A satellite constellation is a group of satellites working together.
– Iridium and Globalstar systems provide satellite phone and low-speed data services using constellations.
– The Iridium system has 66 satellites with an orbital inclination of 86.4°, providing global service availability.
– SpaceX’s Starlink aims to provide global satellite internet access coverage.
– Some constellations, like CASCADE and Orbcomm, use a store-and-forward method for discontinuous coverage.
– Molniya orbit satellites were launched to address visibility issues in extreme northerly latitudes.
– Polar orbits, used for meteorological operations, are Sun synchronous and cross the equator at the same local time each day.
– Plans and initiatives exist to bring dedicated communication satellites beyond geostationary orbits, such as LunaNet and Mars Telecommunications Orbiter.
– Communications satellites consist of communication payload, engines, station keeping subsystem, power subsystem, and command and control subsystem. Source: https://en.wikipedia.org/wiki/Communications_satellite
A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. Many communications satellites are in geostationary orbit 22,300 miles (35,900 km) above the equator, so that the satellite appears stationary at the same point in the sky; therefore the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track the satellite. Others form satellite constellations in low Earth orbit, where antennas on the ground have to follow the position of the satellites and switch between satellites frequently.
The radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points. Communications satellites use a wide range of radio and microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.
