The access technique is the method used in satellite communications by which multiple user terminals share the available bandwidth of a satellite communication channel.
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A
The Advanced Communications Technology Satellite (ACTS) was a NASA technology-demonstration spacecraft launched in September 1993 to validate advanced satellite communications concepts. ACTS was the first satellite to...
The Advanced EHF (AEHF) constellation comprises six GEO satellites providing advanced encryption, anti-jam (frequency hopping and steerable nulls), low probability of detection (LPD), low probability of intercept (LPI)...
Aeronautical Mobile Satellite Service is a form of Mobile Satellite Service that provides communications between satellites and mobile Earth stations installed on aircraft. It is used to support communications with aircraft in...
The Air Force Satellite Communications System (AFSATCOM) is not a system in its own right but utilises dedicated channels on other military satellites to disseminate Emergency Action Messages (EAMs). On the FLTSATCOM satellites...
Air launch refers to an orbital launch architecture in which a rocket is carried aloft by a carrier aircraft and released at high altitude before igniting its rocket motor and continuing to orbit. Rather than launching...
Aloha is a multiple-access technique used in satellite communications in which multiple user terminals share a common uplink RF channel to a satellite. ALOHA is a form of asynchronous TDMA in which transmissions are not aligned...
In April 2019, Kuiper Systems LLC, a subsidiary of Amazon, announced Project Kuiper, a broadband internet satellite constellation comprising up to 3,236 satellites, operating in 98 orbital planes in three orbital shells, at 590...
Antenna gain is a measure of how effectively an antenna concentrates radio-frequency energy in a particular direction. In satellite communications, antenna gain is one of the most important antenna parameters because it directly...
Antenna pointing loss is the reduction in antenna gain caused by pointing an antenna slightly away from the intended direction. In satellite communications, it occurs when an Earth station antenna, satellite antenna, mobile...
The antenna subsystem is a critical element of spacecraft design. If the spacecraft antenna can be made sufficiently large, higher antenna gain can be achieved, reducing the amount of RF power that must be generated on board to...
The *apogee* is the point in an Earth-centered satellite orbit at which the satellite is at its maximum distance from the center of the Earth. The opposite point in the orbit, where the satellite is closest to the Earth, is...
An *apogee boost (kick) motor* is the propulsion device or function used to raise a satellite from its initial transfer orbit into its final operational orbit by firing near apogee. The maneuver increases orbital energy to raise...
The *Arab Satellite Communications Organization (ARABSAT)* is an intergovernmental satellite operator established in 1976 by Arab states to provide telecommunications and television broadcasting services across the Arab region...
See: Orbital Parameters
The *ascending node* is the point in an Earth-centered satellite orbit at which the orbital plane crosses the Earth’s equatorial plane from south to north. The opposite crossing, where the orbit passes from north to south, is...
Asia Cellular Satellite (ACeS) was a satellite-based mobile communications system intended to provide voice and data services across Asia, with interconnection to public switched telephone networks. The system was designed to...
In satellite communications, *atmospheric absorption loss* results from the absorption of the Earth-satellite signals as they pass through the Earth’s atmosphere. The value of the atmospheric loss is strongly dependent on...
The *attitude* of a satellite is its orientation in space, usually defined with respect to a reference frame such as the Earth, the Sun, or an inertial coordinate system. Attitude is controlled independently of the satellite’s...
The *Attitude and Orbit Control (AOC) subsystem* is responsible for controlling both the orbit and the attitude of a spacecraft. The orbit defines the path of the satellite through space, while the attitude defines its...
Attitude control is essential on the spacecraft to prevent the satellite from tumbling in space and to ensure that the antennas remain pointed at a fixed point on the Earth’s surface.
In 1967, Australia became the seventh nation to launch a satellite, and the third nation to launch one from its own territory. The Weapons Research Establishment Satellite (WRESAT) was launched into a near-polar orbit on 29...
*Azimuth* is the horizontal angle, measured in the local tangent plane at the Earth station, between a reference direction (usually true north) and the projection of the satellite line of sight onto that plane. Azimuth is...
B
Beam forming is the technique of shaping and directing the radiation pattern of an antenna system so that radio-frequency energy is concentrated in selected directions and reduced in others. In satellite communications, beam...
In many cases, the Earth coverage area from a spaceborne antenna is required to be irregular to focus on high-density population areas, to avoid sparsely populated areas such as deserts and oceans, or to avoid interference with...
In satellite communications, *beam-spreading loss* results from the spreading of the Earth-satellite signals as they pass through the Earth’s troposphere. The atmosphere does not have a constant refractive index profile so that...
Beamwidth is a measure of the angular width of an antenna beam. In satellite communications, it describes how widely an antenna radiates or receives energy around its main pointing direction. Beamwidth is important because it...
A *bent-pipe repeater* is a satellite payload architecture in which the satellite simply receives an uplink signal, amplifies it, translates it in frequency, and retransmits it on the downlink without demodulating or processing...
A bent-pipe satellite is a communications satellite that receives a radio-frequency signal, amplifies it, changes its frequency, and retransmits it without interpreting or processing the information carried by the signal. The...
In a block code, the source data is partitioned into blocks of *k* bits. The encoder translates each block of *k* input bits into *n* output bits where *n*>*k*. The output *n*-bit block is called a codeword. The codes are...
A *broadband amplifier* is an amplifier that provides amplification across a broad band of frequencies. A broadband amplifier is difficult to design because it mut avoid the introduction of harmonics due to non-linear operation.
A *broadband Internet satellite* is a communications satellite designed to provide high-speed Internet access to fixed or mobile users over wide geographic areas. These systems deliver IP-based data services to users who lack...
Broadcast satellite networks transmit broadcast and television signals from a large central Earth station, via a satellite to relatively simple receive-only Earth stations. Broadcast satellite receive stations are either...
See: Satellite Bus.
C
*C-band* is a band of frequencies used for terrestrial radio relay and satellite transmissions in satellite communications. In satellite communications C-band is commonly called 6/4 GHz to illustrate that the uplink frequencies...
A Cassegrain antenna is a dual-reflector antenna configuration in which a parabolic main reflector directs incoming radiation onto a convex hyperboloidal sub-reflector, which in turn redirects the energy to a feed located near...
A critical aspect in data transmission is the determination of whether the received data is error-free. To do this some form of channel coding (error protection, or error detection and correction) is utilized. The numbers of...
*Code-division multiple access* (CDMA) is a multiple access technique in satellite communications in which each user of a satellite channel is given access to the entire channel all the time, with users separated by giving each...
*Cold welding* is a phenomenon that can occur in a vacuum environment, in which clean metallic surfaces brought into direct contact can bond together due to the absence of intervening gas or contaminant layers. Without adsorbed...
See: Signals Intelligence (SIGINT) Satellites.
Strictly speaking, not only is the Moon the Earth’s first satellite, but it was also used in the early 1950s by the US Navy as the first communications satellite to reflect teletypewriter messages from Washington DC to the west...
The *communications subsystem* is the payload of a communications satellite responsible for receiving, processing, and retransmitting signals between Earth stations. It comprises multiple functional chains known as transponders...
In 1962, the US Congress passed the Communications Satellite Act that authorized the establishment of the Communications Satellite Corporation (COMSAT). COMSAT was established as a publicly owned but government-chartered...
A satellite constellation is a group of satellites designed to operate together as a coordinated system. In satellite communications, a constellation provides coverage, capacity, continuity of service, and network resilience by...
A convolutional code extends the concept of a block code to allow memory from block to block. Each encoded symbol is therefore a linear combination of information symbols in the current block and a selected number of preceding...
On 4 October 1960, the US Department of Defense launched *Courier 1B* *aboard a Thor–Able rocket (Courier 1A had failed at launch on 18 August 1960). Courier was placed into an* elliptical LEO orbit between 396 km and 1,212 km...
*Cross-polarization distortion* is an impairment in a dual-polarized satellite communication system in which a portion of the transmitted signal intended for one polarization leaks into the orthogonal polarization channel. This...
See: Inter-satellite Link (ISL).
A cyberattack in satellite communications is an attack directed against the data, software, or information systems associated with a satellite system, including spacecraft onboard processors, ground-segment infrastructure, and...
*Cyclic codes* are an important sub-class of linear block codes for error detection, where a new codeword in the code can be formed by shifting the elements along by one place and taking one off the end and putting it on to the...
A *cyclic redundancy check* (CRC) is a very efficient error detection algorithm that uses a polynomial function to generate the block check characters. A cyclic redundancy check is often used in conjunction with automatic repeat...
D
See: Space Debris.
The Defense Satellite Communications System (DSCS) was the United States Department of Defense’s primary wideband military satellite communications system for strategic and tactical communications. The program originated as the...
*Demand-assigned multiple access* (DAMA) in satellite communications, is one of the two main techniques for allocating satellite channels to satellite communications users. In fixed-assigned multiple access (FAMA), or...
The *descending node* is the point in an Earth-centered satellite orbit at which the orbital plane crosses the Earth’s equatorial plane from north to south. The opposite crossing, where the orbit passes from south to north, is...
A *digital transparent processor (DTP)* is an onboard satellite payload processor that performs digital signal processing functions—such as channelisation, routing, switching, beamforming, and power allocation—without...
See Prograde Orbit.
The *direct-broadcast satellite service (DBS),* also known as broadcasting satellite service (BSS), refers to satellite networks that transmit broadcast television and radio signals from a central uplink Earth station to a large...
The *Doppler effect* in satellite communications is the change in frequency of an electromagnetic signal that results from the relative speed of the satellite and the Earth terminal. When the orbital parameters of a satellite...
Doppler shift is the change in the observed frequency of a signal caused by relative motion between the transmitter and receiver. In satellite communications, it occurs when a satellite and an Earth station are moving toward or...
A *down converter* translates a carrier frequency from a high frequency to a low frequency. In satellite communications a down converter is used in the transponder to receive the uplink frequency and translate it down in...
The *downlink* in satellite communications is the signal from the satellite transponder to a satellite Earth station, which is transmitted by the transponder and received by the Earth station. In satellite communications the...
E
E-band refers to the portion of the radio-frequency spectrum from approximately 71-86 GHz. It is commonly associated with very high-capacity millimeter-wave links and is often divided into two main ranges, 71-76 GHz and 81-86...
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...
An Earth station antenna is the antenna used at a ground, maritime, aeronautical, transportable, or mobile terminal to transmit signals to a satellite, receive signals from a satellite, or perform both functions. It is one of...
An *Earth-coverage beam* is the radiation pattern produced by an Earth-coverage antenna on a geostationary satellite, designed to illuminate a large fraction of the visible Earth. Such beams have a half-power beamwidth of...
See: Orbital Parameters
On 12 August 1960, NASA, Bell Telephone Laboratories and the Jet Propulsion Laboratory launched a passive reflector, *Echo 1* on a ThorDM-19 Delta rocket into a ~47.2° inclined LEO orbit between ~966 km and ~2,157 km. Echo 1...
Effective Isotropic Radiated Power (EIRP)
See: Signals Intelligence (SIGINT) Satellites.
*Elevation* is the angle between the line of sight from an Earth station to a satellite and the local horizontal plane (horizon) at the Earth station. Elevation strongly influences propagation losses and minimum look-angle...
Ellipso was a proposed satellite communications system based on two coordinated constellations in highly elliptical orbits, known as Concordia and Borealis. Ellipso combined features of geostationary and Molniya-type satellite...
The *Enhanced Polar System (EPS)* is a United States military satellite communications capability intended to replace the *Interim Polar System (IPS)* and to operate as an adjunct to the *Advanced EHF (AEHF)* system. EPS is...
See: Satellite Ephemeris.
Equatorial Launch Australia (ELA) is based at the Arnhem Space Centre, located in Arnhem Land at approximately 12° S latitude, around 40 km east of the township of Nhulunbuy in Australia’s Northern Territory. The site offers...
An equatorial orbit is an Earth-centered satellite orbit whose orbital plane coincides with the Earth’s equatorial plane and therefore has an inclination of 0°.
*Eutelsat OneWeb* is a low Earth orbit (LEO) broadband satellite communications system providing global satellite internet services. Originally developed by OneWeb, the system became part of the Eutelsat Group following the...
Shortly after Sputnik, on 31 January 1958, the US Army launched the first US artificial satellite, *Explorer 1* . Explorer was a 2-m long, 15-cm diameter, 14-kg cylinder (perigee ≈ 360 km, apogee ≈ 2,550 km), which only operated...
The EHF band is an RF frequency in the band 3-30 GHz, commonly used in satellite communications. The SHF and EHF bands are known as the microwave bands and their wavelengths are short enough to be propagated by highly...
F
The fixed satellite service (FSS) refers to satellite communications services in which signals are relayed between fixed Earth stations at specified locations. These Earth stations are typically large, complex, and relatively...
*Fixed-assigned multiple access* (FAMA), or Pre-assigned Multiple Access (PAMA), in satellite communications is one of the two main techniques for allocating satellite channels to users. In FAMA, each user is allocated a channel...
The success of TACSAT led to the deployment in 1979 of the UHF/EHF *Fleet Satellite Communications System* (*FLTSAT* or *FLTSATCOM*) to support the US Navy and Air Force. Although principally for maritime use, the system was...
The *footprint* of a satellite antenna is the area of the Earth’s surface over which the satellite’s transmitted signal can be received with sufficient strength and quality. The size and shape of the footprint depend on the...
Using *forward error correction* (*FEC*), the satellite communications receiver can correct errors itself without reference to the transmitter. It does so by using additional information transmitted along with the data and...
In satellite communications, *free-space loss* (or free-space path loss) is the major loss suffered by signals in traveling over the Earth-satellite path. The loss is inversely proportional to the square of the distance...
See: Spectrum Coordination, Interference.
*Frequency re-use* is a technique in satellite communications in which the same frequency spectrum is employed multiple times within a system, separated spatially, polarimetrically, or by beam pattern, to increase total system...
*Frequency-division multiple access* (FDMA) divides the bandwidth of a satellite transponder by giving each user a portion of the available bandwidth for their permanent use. FDMA is commonly used in satellite communications...
G
G/T is a receiving-system figure of merit used in satellite communications. It expresses the ratio of antenna gain, G, to system noise temperature, T, and is normally written in decibel form as dB/K. G/T is important because it...
The G60 Starlink constellation—also known as Qianfan ("Thousand Sails")—is being developed by Shanghai Spacecom Satellite Technology under the auspices of the Shanghai G60 Science and Technology Corridor. The long-term plan...
A gateway is an Earth station or network node that connects a satellite system to a terrestrial communications network. In satellite communications, the term is most commonly used for a large fixed Earth station that provides...
A *geosynchronous satellite* has a period of rotation that is an integer multiple of the Earth’s sidereal rotation period. A geosynchronous satellite may have any inclination or eccentricity and therefore does not, in general...
See: Transfer Orbit.
The *geostationary Earth orbit* (GEO) is a special case of a geosynchronous orbit in which the satellite follows a circular orbit in the Earth’s equatorial plane (0° inclination), at an altitude of approximately 35,786 km above...
Gilmour Space Technologies is a venture-backed Australian aerospace company headquartered on the Gold Coast, Queensland, developing low-cost orbital and suborbital launch vehicles and satellite platforms. The company focuses on...
See: Earth-coverage Beam.
The *Global Broadcast System (GBS)* is a military satellite broadcast capability designed to provide high-rate, one-to-many dissemination of data, imagery, and video to deployed forces. The first operational GBS payload was...
*Globalstar* provides global mobile satellite telephony and data services using a constellation of low Earth orbit (LEO) satellites. The original Globalstar constellation, completed in February 2000, consisted of 48 operational...
A *Gregorian antenna* is a dual-reflector antenna configuration in which a concave ellipsoidal sub-reflector is placed beyond the prime focus of a main parabolic reflector. One focal point of the ellipsoid coincides with the...
The ground segment is the part of a satellite communications system that is located on or near the Earth. It includes the Earth stations, gateways, hubs, control centers, network equipment, terrestrial backhaul, monitoring...
See: Earth Station
Guowang (‘National Network’) is China’s primary state-backed broadband satellite constellation. Managed by the state-owned organization China Satellite Network Group, the constellation is expected to comprise approximately...
H
A *Hall-effect thruster* is an electric propulsion device used on spacecraft for efficient, low-thrust orbit raising, station keeping, and attitude or orbital maneuvers. It operates by accelerating ions—typically xenon—using an...
A *high-altitude platform station (HAPS)* is a communications or observation platform operating in the stratosphere, typically at altitudes of about 18–25 km above the Earth’s surface. HAPS systems may take the form of unmanned...
A *high-throughput satellite (HTS)* is a communications satellite designed to deliver substantially greater total system capacity than conventional wide-beam satellites, primarily through the use of multiple narrow spot beams...
A *high-altitude nuclear detonation* is the explosion of a nuclear device at altitudes sufficiently high that blast and ground effects are negligible, but electromagnetic and radiation effects dominate. Such detonations can...
In satellite communications, a *highly elliptical orbit (HEO)* is an Earth-centered elliptical orbit with a large eccentricity, such that the satellite’s perigee (closest point to the Earth) is much lower than its apogee...
Honghu-3 is a proposed Chinese broadband constellation consisting of approximately 10,000 satellites distributed across about 160 orbital planes. Although less publicly developed than Guowang or G60, the project is intended to...
A hub is a central Earth station or network node that controls, aggregates, and routes traffic for a satellite communications network. In many satellite systems, especially VSAT networks and broadband satellite services, the hub...
I
See Intermediate Circular Orbit (ICO) and Intermediate Circular Orbit Satellite System (New ICO Satellite).
The *inclination* of a satellite’s orbit is the angle between the orbital plane and the Earth’s equatorial plane, which corresponds to an inclination of 0°. An orbit with an inclination of 90° is a *polar orbit*, in which the...
See Retrograde Orbit.
See: Defense Satellite Communications System (DSCS).
See: Defense Satellite Communications System (DSCS).
*Inmarsat* provides global mobile satellite telephone and data services using a fleet of geostationary communications satellites operating primarily in the L-, Ka-, and S-bands. Founded in 1979 to provide satellite...
*Input backoff (IBO)* is the reduction applied to the input power of a radio-frequency power amplifier relative to the input level that would drive the amplifier into saturation. It is used to operate the amplifier in a more...
In 1964, 11 nations signed the agreement establishing the *International Telecommunications Satellite Consortium* (Intelsat), an intergovernmental organization created to operate a global commercial satellite network. Membership...
An inter-satellite link (ISL) is a communications link between two satellites. It allows data, control information, timing information, or network traffic to be passed from one spacecraft to another without first being sent down...
Interference is the unwanted addition of radio-frequency energy to a wanted communications signal. In satellite communications, interference may reduce signal quality, increase the error rate, lower the available...
In satellite communications, an *intermediate circular orbit (ICO)* is a circular Earth-centered orbit at altitudes typically between about 10,000 km and 15,000 km above the Earth’s surface. ICOs were originally proposed for...
New ICO (formerly ICO Global Communications) was a planned global mobile satellite communications system based on a constellation of medium Earth orbit (MEO) satellites manufactured by Boeing. The system was intended to provide...
Intermodulation distortion is a form of nonlinear distortion that occurs in a radio-frequency amplifier when two or more signals are amplified simultaneously and interact to produce spurious signals at frequencies equal to sums...
*The International Telecommunication Union (ITU) is a specialized agency of the United Nations that was established in 1865 to advance international telecommunications (originally telegraph).* For satellite communications, that...
Iridium is a MSS system providing global L-band voice and data communications using a constellation of LEO satellites. Unlike most MSS systems, including Globalstar, Iridium employs on-board digital switching and inter-satellite...
J
*Jamming* is the deliberate transmission of interfering radio-frequency signals to disrupt or deny satellite communications. Jamming may be classified as uplink or downlink jamming, with uplink jamming generally more effective...
K
As the C-band and Ku-bands begin to become congested, significant interest has been generated in Ka-band (30/20 GHz)—also commonly called EHF, particularly in the US—for commercial satellite communications applications. Early...
*Kepler's laws* are three empirical laws of orbital motion formulated by Johannes Kepler in the early 17<sup>th</sup> century, based on observations of planetary motion made by Tycho Brahe. Although originally derived for...
The *Kessler syndrome* (also known as the *Kessler effect*) is a hypothesised scenario in which the density of space debris in LEO becomes sufficiently high that collisions between objects trigger a cascading chain reaction of...
A *klystron* is a high-power vacuum electron-tube amplifier used in satellite communications to generate the RF output power required to overcome the large free-space and atmospheric losses on Earth–satellite links. It operates...
To alleviate congestion in C-band, many modern satellite communications systems operate in higher-frequency bands such as *Ku-band*. Satellites may carry a mix of C-band and Ku-band transponders, with the proportion selected...
See: Amazon Leo.
L
The L-band and S-band occupy the frequency range from approximately 1 to 4 GHz and are sometimes collectively (and loosely) referred to as UHF in satellite communications contexts. These bands are useful for satellite...
*LauncherOne* was a two-stage, all–liquid-fueled orbital launch vehicle developed by Virgin Orbit between 2007 and 2023. It was the first operational air-launched orbital rocket using exclusively liquid propulsion. The vehicle...
LEASAT was a U.S. military UHF satellite communications system built by Hughes Aircraft Company and leased to the U.S. Department of Defense for use by the Navy, Army, and Air Force. It followed the FLTSATCOM and GAPSAT systems...
The *Lincoln Experimental Satellites (LES)*, together with the associated Lincoln Experimental Terminals (LET), were a series of U.S. military research satellites developed to demonstrate and validate mobile satellite...
The *line of apsides* is the line that joins the apogee and perigee of a satellite’s orbit and passes through the center of the Earth.
In satellite communications systems, a *link* is the RF circuit established directly between two satellite communications nodes. In a typical satellite system, the *uplink* is the transmission path from an Earth station to a...
See: Proliferated Warfighter Space Architecture (PWSA).
A link budget is a systematic accounting of all the gains and losses in a radio communications link. In satellite communications it is used to determine whether a signal transmitted from an Earth station to a satellite, or from...
Link margin is the amount by which the predicted performance of a communications link exceeds the minimum performance required for reliable operation. In satellite communications, it is usually expressed in decibels and is one...
To ensure that an Earth station antenna has the center of its main lobe (the antenna *boresight*) pointed accurately toward a satellite, it is necessary to know the angles of *elevation* and *azimuth* (the *look angles*) of the...
The Loopus (Loops in Orbit Occupied by Unstationary Satellite) orbit is a proposed for a communication system based on a constellation of satellites in inclined, highly elliptical orbits. In such orbits, satellites appear...
A *low Earth orbit (LEO)* is an Earth-centered satellite orbit with altitudes typically between about 500 km and 2,000 km above the Earth’s surface. Satellites in LEO are used for a wide range of applications, including...
At a satellite communications transponder, the very weak RF uplink signal is first amplified by a *low-noise amplifier (LNA)* to preserve signal quality. Early LNAs employed devices such as tunnel-diode amplifiers, but modern...
M
A magnetometer is an instrument that measures the magnitude and, in some implementations, the direction of the ambient magnetic field. In spacecraft applications, magnetometers are commonly used for attitude determination and...
*MARISAT* was a U.S. maritime satellite communications system that provided an interim operational capability for mobile users—primarily maritime—prior to the deployment of FLTSATCOM. Three MARISAT gapfiller (GAP) satellites...
Maritime Mobile Satellite Service is a form of Mobile Satellite Service (MSS) that provides communications between satellites and mobile Earth stations located on ships, boats, offshore platforms, and other maritime vessels or...
A *medium Earth orbit* (MEO) is a circular satellite orbit of heights between 10,000 km and 15,000 km above the Earth, in between the inner and the outer Van Allen belts. A medium earth orbit is sometimes called an *intermediate...
See: Constellation.
A mesh network is a communications network in which nodes can communicate with one another through multiple possible paths rather than relying on a single central switching point. In satellite communications, the term is...
When a *meteoroid* enters the Earth’s atmosphere and begins to glow it is called a *meteor*. If the meteor does not burn up during its path through the atmosphere and impacts upon the Earth’s surface it is called a *meteorite*.
When a *meteoroid* enters the Earth’s atmosphere and begins to glow it is called a meteor. If the meteor does not burn up during its path through the atmosphere and impacts upon the Earth’s surface it is called a *meteorite*.
*Meteoroids* are between 100 µm and 10m in diameter and move at velocities of 20–27 kms<sup>-1</sup>; micro-meteoroids are <100 µm and are largely made of cosmic dust. While the concentrations of both are low, their velocity is...
*Military satellite communications* refers to the use of satellite systems to provide secure, reliable communications for military and defense users, supporting command and control, intelligence dissemination, situational...
*Military Strategic, Tactical and Relay (Milstar) satellites* are a U.S. military satellite communications system developed to provide highly secure, survivable communications for strategic and tactical users, particularly in...
A *Mission Extension Vehicle (MEV)* is an on-orbit servicing spacecraft designed to dock with an existing satellite and assume responsibility for station-keeping and attitude control, thereby extending the operational life of...
*Mobile satellite internet* refers to satellite-based Internet access provided to users who require connectivity while moving or operating beyond the reach of terrestrial broadband networks, such as cellular, fiber, or DSL...
Mobile Satellite Service (MSS) is a category of satellite communications service that provides radio links between satellites and mobile Earth stations. These mobile terminals may be located on land, at sea, or in aircraft, and...
The *Mobile User Objective System* (MUOS) is a US military narrowband satellite communications system developed as the successor to the UHF Follow-On (UFO) constellation. MUOS comprises five geostationary satellites, including...
The *Molniya* satellite orbit is a highly elliptical Earth orbit designed to provide long-duration visibility at high latitudes, particularly over the Northern Hemisphere. It typically has an orbital period of one-half of a...
*A multibeam antenna* i*s* antenna systems designed to generate multiple simultaneous beams from a single antenna aperture. In satellite communications, multibeam operation is used to create higher-gain spot beams, enable...
*Multiple access* in satellite communications refers to the techniques that allow multiple users or nodes to share a common transmission medium, such as a satellite transponder, in an orderly and efficient manner...
Multiple Channel per Carrier (MCPC) is a satellite communications arrangement in which several independent carriers share the same satellite transponder or channel. Each carrier may carry a separate voice, video, data...
N
*Non-orthogonal multiple access (NOMA)* is a multiple-access technique in which multiple users share the same time–frequency resources simultaneously by allowing controlled non-orthogonality between their signals. User...
A non-synchronous orbit is any orbit in which a satellite’s orbital period is not synchronized with the rotation period of the Earth. In satellite communications, the term is most often used to distinguish satellites that move...
A *non-synchronous satellite* is a satellite whose orbital period is not equal to the Earth’s sidereal rotation period and therefore does not remain fixed relative to the Earth’s surface. As a result, the subsatellite point...
O
O3b is a MEO satellite network operated by SES that provides low-latency broadband connectivity and mobile backhaul, originally branded around serving the ‘other three billion’ people without high-speed access. The...
*Onboard processing* refers to the use of signal processing functions implemented within the satellite payload to manipulate communications signals beyond simple amplification and frequency translation. These functions may...
See: Eutelsat OneWeb
Open Cosmos is not deploying an independent global LEO constellation. Instead, it is one of the industrial partners participating in Europe's Infrastructure for Resilience, Interconnectivity and Security by Satellite (IRIS²)...
The first Optus satellite was AUSSAT 1 launched in 1985 and a total of 10 satellites have been launched, with Optus 10 being the most recent launch in September 2014. The five current Optus satellites are :
*Orbcomm* provides global monitoring, tracking, messaging, and industrial Internet of Things (IoT) services using a low-Earth orbit (LEO) satellite constellation. The original Orbcomm system employed first-generation satellites...
The orbit of a satellite is its path through space—in satellite communications, the orbit is the path through space around the Earth as the barycenter. The satellite’s orbit is dictated by Kepler's Laws.
Orbital altitude is the height of a satellite above the surface of the body it is orbiting. In satellite communications, the term normally refers to the height of a satellite above the Earth’s surface. It is therefore sometimes...
See: Orbital Parameters.
See: Orbital Altitude
Although a very large number of parameters may be defined, at least six are required to fully describe the *motion and instantaneous position* of a satellite in an orbit. These six classical Keplerian orbital elements are...
The *period* of an orbit is the time taken by a satellite to complete one full revolution around the Earth. The orbital period depends primarily on the size of the orbit, increasing with the distance of the satellite from the...
Orbital radius is the distance from the center of the body being orbited to the satellite or other orbiting object. In satellite communications, it normally refers to the distance from the center of the Earth to a communications...
Orbital velocity is the speed a satellite must have to remain in orbit around a central body such as the Earth. In satellite communications, the term usually refers to the speed of a communications satellite as it travels around...
*Outgassing* is the release of gases trapped within materials or components when they are exposed to the vacuum of space. Gases absorbed or trapped during manufacture can expand and escape in orbit, potentially leading to...
*Output backoff (OBO)* is the reduction in the output power of a radio-frequency power amplifier relative to its saturated output power. It is a measure of how far below saturation the amplifier is operated in order to maintain...
The nearly 10,000 LEO satellites will eventually fall back toward the atmosphere or will be deliberately de-orbited as part of their disposal strategy to burn up on re-entry. On entering the atmosphere, satellites leave behind a...
P
*P-band* refers to radio frequencies approximately in the range 0.2–1.0 GHz. In satellite communications, practical operation is limited to frequencies above roughly 100 MHz, as lower frequencies are reflected or absorbed by the...
A *parametric amplifier* is a low-noise amplifier used in satellite communications transponders to amplify very weak RF uplink signals. It operates by transferring energy from a high-frequency pump signal to the desired RF...
The *perigee* is the point in an Earth-centered satellite orbit at which the satellite is at its minimum distance from the center of the Earth. The opposite point in the orbit, where the satellite is at its maximum distance from...
A phased array is an antenna system formed by arranging multiple radiating elements with regular spacing and controlling the relative phase (and often amplitude) of the signals applied to each element. By appropriate phase...
Satellite ground stations are large installations for high-capacity networks and their locations are obvious. They are very fragile, so can be readily destroyed by conventional forces, terrorist activity or missiles. Physical...
See: Antenna Pointing Loss.
A *polar orbit* is a satellite orbit in which the orbital plane passes close to the Earth’s north and south poles, corresponding to an inclination of approximately 90°. Polar orbits are most commonly used by LEO satellites and...
*Polarization* describes the orientation and time variation of the electric field vector of an electromagnetic wave as it propagates through space. In satellite communications, common forms of polarization include linear...
*Polarization loss* in satellite communications is the reduction in received signal power that occurs when the polarization of a received electromagnetic wave is misaligned with the polarization of the receiving antenna. This...
*Power flux density (PFD)* is a measure of the power from a radio transmitter incident on a surface, expressed as power per unit area, typically in watts per square meter (W m<sup>–2</sup>) or in decibel watts per square meter...
The *Power Subsystem* generates, stores, controls and distributes electrical power to the other subsystems on board. Most of the power (approximately three-quarters) is required for the communications subsystem. How much power...
*Pre-assigned multiple access (PAMA)*, also known as fixed-assigned multiple access (FAMA), is a method used in satellite communications to allocate channel resources to users. In a PAMA system, each user is permanently assigned...
A *prograde* (or *direct*) orbit is one in which a non-synchronous satellite moves in the same direction as the Earth’s rotation, corresponding to an inclination of less than 90°. A *retrograde* (or *indirect*) orbit is one in...
In 1963, the US Air Force conducted *Project West Ford* (originally *Project Needles*) , an experiment to create an artificial orbiting communications medium. Nearly 500 million hair-thin copper dipoles, each 1.78 cm long and...
The *Proliferated Warfighter Space Architecture (PWSA)* is a US Department of Defense space program led by the Space Development Agency (SDA) to deploy a large, resilient constellation of low Earth orbit (LEO) satellites to...
Propagation delay is the time taken for a signal to travel from a transmitter to a receiver. In satellite communications, it is mainly determined by the distance the electromagnetic wave must travel through space. Since radio...
Q
Q-band refers to the portion of the radio-frequency spectrum from approximately 33-50 GHz and is commonly classified within the SHF/EHF transition region. In satellite communications, Q-band is mainly associated with...
R
A radome is a protective enclosure placed over an antenna to shield it from the environment while allowing radio-frequency energy to pass through with minimal loss and distortion. The word is formed from radar and dome, but...
See: Weather Loss.
Russia has initiated development of a low-Earth orbit broadband constellation known as *Rassvet*, (Russian for ‘dawn’) intended as a domestic counterpart to Starlink. The system, developed by Bureau 1440, is designed to provide...
Rayleigh fading is a form of small-scale signal fading caused by multipath propagation in which multiple replicas of the same signal arrive at the receiver with different delays and phases, producing constructive and destructive...
The *Reed-Solomon code* (RS code) is a class of non-binary block error-correcting codes belonging to the Bose–Chaudhuri–Hocquenghem (BCH) family, in which coding is performed on symbols composed of multiple bits rather than on...
A *regenerative payload* is a satellite communications payload architecture in which the received uplink signal is demodulated, decoded, and processed onboard the satellite before being re-encoded, remodulated, and transmitted...
A regenerative satellite is a communications satellite that receives an uplink signal, processes it on board, and then retransmits a newly generated downlink signal. Unlike a bent-pipe satellite, which mainly amplifies...
*Relay 1* was an early 78-kg spin-stabilized satellite communications system launched by NASA on a Thor-Delta B rocket on 13 December 1962 into an elliptical orbit between 1,300 km and 7,500 km with 47.5° inclination and an...
A *retrograde* (or *indirect*) orbit is one in which the satellite moves opposite to the Earth’s rotation, corresponding to an inclination greater than 90°, and exhibits an apparent westward motion relative to the Earth’s...
See: Orbital Parameters
S
A satellite antenna is the part of a satellite communications system that transmits or receives radio-frequency energy between the satellite and Earth stations, other satellites, or other spacecraft. In a communications...
Satellite broadband refers to the provision of high-speed Internet access using communications satellites to deliver data connectivity to fixed or mobile users. It is used to serve customers in remote, rural, maritime...
*Satellite broadcasting* refers to satellite communication systems that distribute broadcast content, such as television and radio programs, from a central transmitting Earth station to a large number of receive-only Earth...
The satellite bus is the platform that supports and sustains the payload of a satellite. In a communications satellite, the payload is the equipment that performs the communications mission, such as antennas, receivers...
See: Constellation.
The *satellite environment* encompasses the physical and electromagnetic conditions encountered by satellites during launch and throughout their operational life in space. Most satellites operate within cislunar space, the...
A satellite ephemeris is a set of data that describes the predicted position and velocity of a satellite as a function of time. In satellite communications, an ephemeris allows Earth stations, user terminals, gateways, tracking...
*Satellite frequencies* are chosen to support specific satellite communications applications. Satellite frequencies extend from P-band to Ka-band and further. The higher the satellite frequencies, the wider the bandwidth and the...
*Satellite internet* refers to broadband satellite internet services offered in a number of countries. The most common configuration is for internet downloads to be carried over the satellite, with a back channel via telephone...
*Satellite navigation* is the use of signals transmitted by navigation satellite constellations to determine position, velocity, and time. These systems operate by measuring the propagation time of precisely timed radio signals...
The *satellite pager* is a device that receives paging messages delivered via a satellite communications system, providing one-way messaging services to subscribers over wide geographic areas, including regions beyond the reach...
Satellite radio provides access to radio channels broadcast from a satellite. Because of the greater height of the satellite, satellite radio has a much greater range than terrestrial radio stations.
A satellite system comprises a number of *satellite subsystems*. The major satellite subsystems are: Communications Subsystem, Structural Subsystem, Antenna Subsystem, Power Subsystem, Thermal Control Subsystem, Tracking...
A *satellite system* is a communication system based on a satellite as a repeater. The satellite system has two major segments: the space segment and the ground segment. Each of these segments comprises a number of systems and...
Satellite technology encompasses the range of technologies required to design, launch, operate, and sustain communications satellites and their transponders. The emergence of the satellite communications industry depended on the...
See: Satellite Internet.
*Satellite-switched frequency division multiple access (SS-FDMA)* is a multiple-access architecture in which multiple narrowband FDMA uplink carriers are dynamically routed onboard the satellite to different downlink beams or...
*Satellite-switched time division multiple access (SS-TDMA)* is a multiple-access architecture in which uplink TDMA bursts transmitted from different Earth stations are dynamically switched onboard the satellite to selected...
*Scintillation loss* in satellite communications is the degradation in received signal power caused by rapid, random fluctuations in signal amplitude and phase arising from variations in the refractive index of the Earth’s...
On 18 December 1958*,* the United States launched *SCORE (Signal Communicating by Orbiting Relay Experiment)* the world’s first artificial communications satellite capable of relaying terrestrial transmissions. SCORE, launched...
Sea-based orbital launch refers to the placement of satellites into Earth orbit from a mobile maritime platform, rather than a fixed land-based spaceport. The most significant and fully realised example of this concept was Sea...
See: Orbital Parameters
See: Orbital Parameters
The Service Life Enhancement Program (SLEP) was an upgrade initiative applied to the final four of the fourteen DSCS III satellites to extend operational capability and increase system capacity. The SLEP modifications included...
*Signals intelligence satellites* are spacecraft designed to intercept, collect, and analyse electromagnetic emissions from sources of interest for intelligence purposes. SIGINT is commonly divided into communications...
*Single channel per carrier (SCPC)* is a satellite communications transmission technique in which a single information channel is modulated onto each radio-frequency carrier. Each carrier occupies its own portion of transponder...
As part of the Australian National Broadband Network (NBN) scheme, NBN Co Limited operates two Sky Muster satellites in GEO orbit, each providing 101 Ka-band spot beams for a combined capacity of 185 Gbps. The satellites provide...
The Skynet series of military communications satellites is operated for the United Kingdom’s Ministry of Defence (MoD), providing strategic and tactical communications for the British Armed Forces, UK intelligence agencies...
*Slant range* is the straight-line distance between an Earth station and a satellite, measured along the line of sight. It depends on the Earth station location, the satellite orbit, and the elevation, and varies with time for...
A *software-defined satellite* is a communications satellite whose functionality, performance, and service configuration can be substantially modified after launch through software updates rather than fixed hardware changes. In...
Satellite communications systems may be affected during intense periods of solar activity such as during solar flares. For example, solar activity is believed to have totally disabled AT&T’s Telstar 401 satellite in January...
The final stage of the transponder includes a power amplifier such as a TWTA or a transistor *solid-state power amplifier* (SSPA). SSPAs are tending to replace TWTAs for low-to-medium power requirements due to their better...
Southern Launch is a launch service provider headquartered in Adelaide, South Australia on the Eyre Peninsular. Southern Launch facilities include the Whalers Way Orbital Launch Complex, and the Koonibba Test Range, a suborbital...
In addition to potential impact with meteoroids, spacecraft are also at risk from the nearly 130 million pieces of man-made space debris. There are some 128 million pieces <1 cm, approximately 900,000 from 1–10 cm, and 34,000...
Space Machines Company (SMC) is developing space transportation infrastructure which will augment the capabilities of launch companies such as SpaceX, Rocket Lab and Gilmour Space Technologies, to provide flexibility in...
The space segment is the part of a satellite communications system that is located in space. It normally includes the communications satellites themselves, their payloads, spacecraft platforms, antennas, transponders or...
There are three types of space-weather storms: geomagnetic storms, radiation storms and radio blackouts. There have been a considerable number of large solar storms since they were first observed in 1859—the Carrington event. In...
See: Space Segment.
*SPADE* (SCPC, PCM, Demand-assigned Equipment) is a demand-assigned satellite communications access technique developed for use in international satellite networks to provide efficient allocation of satellite channels for...
*Spatial Division Multiple Access (SDMA)* is a multiple-access technique that separates users in the spatial domain, allowing multiple signals to be transmitted simultaneously on the same frequency band and time resources by...
*Spectrum coordination* is the regulatory and technical process by which proposed satellite and terrestrial radio systems are assessed and mutually agreed to operate in a manner that ensures interference to existing and planned...
A *spot beam* concentrates the radiated power in a small geographic area. A spot beam might have a diameter of up to a few hundred kilometers.
The first man-made satellite, *Sputnik 1* , was launched by Russia on 4 October 1957*.* Sputnik was an 84-kg, 58-cm sphere launched into an orbit between 231 km and 942 km. Although not used directly for satellite...
Starlink is a satellite internet constellation operated by SpaceX providing satellite Internet access to most of the Earth using a constellation that will eventually comprise over 40,000 small LEO satellites.
*Station keeping* is the process of using onboard propulsion to maintain a satellite within its assigned orbital position and orientation limits throughout its operational life. In satellite communications systems, station...
Stratolaunch which uses a similar launch method to LauncherOne, but instead releases a hypersonic (greater than Mach 5) launch vehicle called Talon instead of Virgin Orbit’s more traditional rockets. Stratolaunch uses a bespoke...
The *Structural Subsystem* provides the framework for mounting the other satellite’s subsystems as well as the interface with the launch vehicle. It must meet the payload volume and mass constraints of the launch vehicle as well...
In a vacuum, metals and semiconductors tend to sublimate; that is solids convert into a vapor, causing the loss of structural material from the satellite and its components. Additionally, fluids tend to vaporise so normal...
Submarine-based orbital launch refers to the placement of a satellite into Earth orbit using a launch vehicle fired from a submerged submarine. In this approach, a missile is ejected vertically through the water column...
The *subsatellite point* is the point at which a line between the satellite and the center of the Earth intersects the Earth’s surface.
A Sun-transit outage occurs due to the particularly strong interference that results when the geometry of an orbit is such that the beam of an Earth station antenna is pointed directly at the Sun. The days on which maximum solar...
A switched satellite is a communications satellite that dynamically connects uplink and downlink channels through onboard switching rather than using fixed, predetermined signal paths. In a switched satellite, signals received...
On 14 February 1963, NASA and the US Department of Defense launched Syncom 1 , the first satellite planned for a geosynchronous orbit and the first to employ range and range-rate tracking. Although Syncom I was lost during...
T
Encouraged by the success of the Syncom DSCS I and LES experiments the US DoD deployed a large 730-kg satellite designed for UHF tactical use. TACSAT-1 was launched by a Titan-3C on 9 February 1969 into a synchronous orbit at...
*Telemetry* refers to data collected by sensors onboard a satellite to monitor the status and performance of its subsystems, including parameters such as pressure, temperature, voltage, and current. This housekeeping and...
Telesat originally proposed a constellation of approximately 298 satellites, but the current Lightspeed design comprises 198 Ka-band satellites equipped with optical inter-satellite links. Initial deployment is expected to begin...
On 10 July 1962, AT&T launched *Telstar 1* , which was the first satellite to provide repeated transmission without delay. Telstar was the first satellite to provide real-time repeater transmission of voice, TV, and data. The...
The *thermal control subsystem* maintains a spacecraft and its onboard equipment within allowable temperature limits throughout all mission phases. Unlike on Earth, where heat may be lost through convection, conduction, and...
The *thrust subsystem* comprises the propulsion elements used to place a satellite into its final operational orbit and to maintain or adjust that orbit throughout the mission. It typically includes an apogee boost (or apogee...
Thuraya is a mobile satellite communications operator established in 1997 and based in the United Arab Emirates (UAE) . It operates as a subsidiary of Yahsat and provides mobile satellite services across Europe, the Middle East...
*Time-division multiple access* in satellite communications is a mechanism for sharing a channel among a number of satellite communications users, whereby each user has access to the whole channel bandwidth for a small period of...
The Tracking and Data Relay Satellite System (TDRSS) is a space-based communications relay network operated by NASA to provide continuous communications, tracking, and data acquisition services for the International Space...
The *Tracking, Telemetry, and Command* (TT&C) Subsystem is the spacecraft subsystem responsible for monitoring, controlling, and maintaining a satellite throughout its mission. It comprises three functional elements.
A *transfer orbit* is an intermediate orbit used to move a satellite from its initial injection orbit after launch to its final operational orbit. In satellite communications missions, the most common example is the...
A *transponder* is a functional unit within a communications satellite that receives an uplink signal, translates it in frequency, amplifies it, and retransmits it on a downlink frequency. A satellite communications payload...
A *traveling-wave tube amplifier (TWTA)* is a high-power vacuum electronic amplifier widely used in satellite communications transponders to provide the RF output power required for the downlink. TWTAs are generally preferred...
See: Orbital Parameters
A *Tundra* orbit is a highly elliptical geosynchronous Earth orbit characterized by an inclination of approximately 63.4°, an eccentricity typically in the range 0.2–0.4, and an orbital period of one sidereal day (about 1,436...
A tunnel-diode amplifier is a form of low-noise amplifier (LNA) used in satellite communications transponders to amplify very weak RF uplink signals. It exploits the negative-resistance characteristic of a tunnel diode to...
U
*UHF communications* refers to radio communications conducted in the UHF band, typically spanning frequencies from about 300 MHz to 3 GHz. In satellite communications, UHF links are valued for their robust propagation...
The US Navy sponsored UHF Follow-on (UFO) replaced FLTSATCOM and LEASAT, providing almost twice as many channels and some 10% more power per channel, and was compatible with the terminals in service. The first UFO failed on...
An *up converter* is a frequency-translation device that shifts a signal from a lower frequency to a higher frequency. In satellite communications systems, an up converter is used to translate a modulated baseband or IF signal...
The uplink in satellite communications is the transmission path carrying a signal from an Earth station to the satellite. In an uplink, the transmitter is located at the ground station and the receiver is located in the...
The user segment is the part of a satellite communications system that is used directly by customers, subscribers, operators, or end users to access the satellite service. It includes the user terminals, antennas, modems, indoor...
See: User Segment.
*USNS Kingsport* was the world’s first dedicated satellite communications ship. Originally launched as a cargo vessel, she was converted by the US Navy and commissioned on 24 September 1961 as a satellite communications test...
V
V-band refers to the portion of the radio-frequency spectrum from approximately 40-75 GHz and is commonly classified within the extremely high frequency (EHF) range. In satellite communications, V-band is of interest for very...
The *Van Allen radiation belts* are regions of enhanced charged-particle radiation surrounding the Earth, formed by the trapping of energetic protons and electrons within the Earth’s magnetic field, known as the magnetosphere...
A *Very Small Aperture Terminal (VSAT)* is small Earth stations used for satellite communications, typically operating in C-band and Ku-band (and increasingly Ka-band), and characterized by the use of relatively small-diameter...
Viasat is a United States–based provider of satellite broadband and communications services for residential, enterprise, aeronautical, maritime, and government users. The company specialises in high-capacity geostationary...
Virgin Orbit's LauncherOne is a two-stage orbital launch vehicle in development from 2007 to 2020. It is the first all liquid-fueled air-launched orbital rocket (21m long), designed to carry small payloads of up to 300 kg into...
W
W-band refers to the portion of the radio-frequency spectrum from approximately 75-110 GHz. It lies in the millimeter-wave region and extends above the frequencies commonly used for most present-day commercial satellite...
The *Weapons Research Establishment Satellite (WRESAT)* was Australia’s first satellite and marked Australia becoming the seventh nation to launch a satellite, and the third to do so from its own territory. WRESAT was launched...
*Weather loss* in satellite communications is the attenuation of Earth–satellite signals caused by hydrometeors in the atmosphere, including rain, hail, ice, fog, cloud, and snow. As the signal propagates through the atmosphere...
Wideband Global Satellite (WGS) system is a United States Department of Defense geostationary satellite communications constellation providing high-capacity X-band and Ka-band services for military and allied users. Formerly...
See: Weapons Research Establishment Satellite (WRESAT).
X
X-band refers to the portion of the radio-frequency spectrum from approximately 8–12 GHz and is commonly classified within the SHF range. In satellite communications, X-band is used extensively for government and military...
Z
A zonal beam is satellite antenna beams designed to cover a region of the Earth’s surface that is smaller than full Earth coverage, typically less than about 40% of the visible Earth. They provide an intermediate coverage option...