Earth Station
An Earth station is the ground-based part of a satellite communications link. It consists of the antenna, radio-frequency equipment, transmission and reception electronics, control systems, and supporting infrastructure needed to communicate with one or more satellites. Earth stations may be used to transmit signals to a satellite, receive signals from a satellite, or perform both functions. In satellite communications terminology, the link from the Earth station to the satellite is the uplink, while the link from the satellite to the Earth station is the downlink.
Earth stations range from very large gateway or teleport antennas serving many users to small terminals used for private networks, broadcasting, maritime communications, aeronautical communications, or broadband access. A large fixed Earth station may include a parabolic reflector antenna several meters to tens of meters in diameter, high-power amplifiers, low-noise receiving systems, redundant power supplies, tracking systems, monitoring equipment, and network interfaces. At the other end of the scale, a small very small aperture terminal (VSAT) may use an antenna of less than 1 m to a few meters in diameter and may be installed on a building, vehicle, ship, aircraft, or temporary field site.
The basic function of an Earth station is to convert information between the user or network interface and the radio signal used by the satellite link. On transmission, voice, video, data, or internet traffic is formatted, modulated, up-converted to the appropriate radio frequency, amplified, and radiated by the antenna toward the satellite. On reception, the weak signal arriving from the satellite is collected by the antenna, amplified by a low-noise amplifier, down-converted, demodulated, decoded, and passed to the user equipment or terrestrial network.
The antenna is usually the most visible part of an Earth station. For fixed satellite services (FSS), parabolic reflector antennas are common because they provide high gain and narrow beams. High gain is needed because satellite links involve very long path lengths, especially for geostationary satellites. Narrow beams also help reduce interference by concentrating transmitted power toward the intended satellite and limiting reception from unwanted directions. In some modern systems, particularly mobile or low Earth orbit (LEO) systems, electronically steered phased-array antennas may be used instead of mechanically pointed dishes.
Earth station performance is often described using parameters such as antenna gain, effective isotropic radiated power (EIRP), receiver noise temperature, and the gain-to-noise-temperature ratio, usually written as G/T. For receiving systems, low-noise performance is critical because the signal arriving from the satellite may be extremely weak. For transmitting systems, amplifier power, antenna gain, linearity, and spectral purity are important because excessive power, distortion, or poor frequency control can interfere with other users.
Earth stations must normally be coordinated and licensed because they share radio spectrum with other satellite and terrestrial services. This is particularly important in bands such as C-band, where satellite links may operate near or within frequency ranges also used by terrestrial microwave links. Earth station siting, antenna pointing, elevation, shielding, and frequency planning are therefore important aspects of system design.
Earth stations may be classified in several ways: fixed or mobile; transmit-receive or receive-only; gateway, hub, user terminal, or telemetry, tracking and command station; and by frequency band, such as C-band, Ku-band, Ka-band, or X-band. Whatever the size or application, the Earth station is the essential terrestrial interface between the satellite network and the users, networks, or control centers it serves.