Satellite Glossary

A-Pol This refers to the primary polarization of an antenna or a radio transmission. The orientation, or sense, of polarization is characterized by either being LHCP or RHCP for the circular polarization used with C-Band antennas, or H or V for the linear polarization that can be used with either C-band or Ku-Band antennas. There is a transmit and a receive A-Pol, as well as a transmit and a receive B-Pol, and the A-Pol of the transmitting station always becomes the B-Pol of the receiving station and vice-versa.
Analogue A continuous electrical or radio signal, usually represented simplistically by a sine wave. An analogue signal has the properties frequency, amplitude, and polarization.
Antenna see reflector.
Antenna Illumination Illumination is the distribution of power across the reflector, and it affects gain. Theory dictates ideal illumination is uniform across the reflector and zero beyond its edges. In reality power is high near the center, decreasing steadily off the dish, with some overspill. Distribution of the radio energy depends on the feed horn.
Aperture The surface area of an antenna which is exposed to an RF signal.
Apogee The highest point of travel in flight or an orbit
ATM Asynchronous Transfer Mode A special packet-switch based method of transferring data. The data can be pure data, or digitized video or audio.
Attenuation The power loss between one transmitting source and one receiver due to path losses. Path loss includes signal interference or weakening from atmospheric effects such as rain and storms, radiation surges from the sun or other celestial bodies or dissipation from the massive distances traveled by RF signals.
Attitude Control System
or Station Keeping
The great transmit/receive distance of Satellite communications means Satellite position must be exact to a tenth of a degree or more, since small errors at the Satellite are large at the Earth Station. Axis reaction-momentum wheels stabilize the Satellite. By changing the direction or speed of the wheels a torque can be applied. If this is not enough, thrusters can be fired to change the total momentum of the craft. Unbalance torques and required pointing accuracy are the main constraints on the Attitude Control System (ACS). Unbalances are mainly due to misaligned thrusters or solar radiation pressure.
Axial Ratio This is a function of the two radiated signals in a cross-polarized setup. It represents the misalignment of the sending and receiving antenna, which is the cause for isolation interference.
Azimuth The horizontal angle that an Earth Station antenna must be rotated through to point at a geosynchronous Satellite.
B-Pol The secondary polarization of an antenna. See A-Pol.
Band The frequency which a data communication device operates at. Typically C-Band is 6 GHz for Earth Station transmit, and 4 GHz for Earth Station receive. Ku-Band is 14 GHz Earth Station transmit and 11 GHz receive. Since Satellites relay signals from Earth Station to Earth Station the transmit and receive frequencies are reversed compared to that of an Earth Station. A complete listing of SHF bands is:
L = 350 MHz to 1.55 GHz
S = 1.55 GHz to 5.2 GHz
C = 4.2 GHz to 6.2 GHz
K = 10.7 GHz to 36 GHz
K = 10.7 GHz to 36 GHz
Ku = 10.7 GHz to 15.7 GHz
Ka = 17.7 GHz to 33 GHz
Q = 36 GHz to 46 GHz
V = 46 GHz to 56 GHz
W = 56 GHz to 100 GHz
Bandwidth The frequency range of a data transmitting/receiving device, which dictates how much data per unit of time can flow.
Beamwidth The width of the sent beam, discounting sidelobes, measured in degrees.
Carrier A specific frequency signal which carries data.
Communications Subsystem This is the electronics package on a Satellite dealing with transmit relay and receive of RF signals. On a C-Band system the Satellite Receiver receives a 6 GHz signal at about 10 to the power of -10 watts. The Receiver amplifies the signal by 10 to the power of 6 watts and down converts the signal to 4 GHz. The now 10 to the power of -4 watt signal is passed to the Switching Matrix. This is a devices made of switches, variable attenuators, and multiplexers. The signal exits the Switching Matrix at 10 to the power of -5 watts and then enters a final Power Amplifier. This increases the signal by a further 10 to the power of 6 watts, making the signal 10 watts and 4 GHz. The signal is now sent down to an Earth Station to receive.
Cross-Pol Isolation Since frequency bands are re-used by 90-degree polarization it is important to maintain separation between the signals. This is called isolation. If the two signals aren’t 90 degrees with respect to each other there can be interference, and each signal can lose power. Since the signal which reaches the Satellite antenna is already very weak, any power losses are extremely detrimental. If the isolation is sufficiently low as to be negligible then independent signals may be simultaneously transmitted and received. Because of the excess noise introduced into a system by improper isolation, this is an important property of antennas.
DAMA Demand Assignment Multiple Access — DAMA is a technique of Satellite resource manipulation that allows many users on a Satellite to share a limited assignment of transponder capacity. As one user connects to the transponder they draw from a common “pool”. When that user disconnects the transponder space returns to the pool for other users to access.
Decibel dB — A unit of measurement for electrical signals which operates on a logarithmic scale. Never an absolute value, always a ratio.
Digital The alternative method for transmitting electrical signals to analogue. Binary bits characterize digital signals. A digital signal is either a binary 1 or 0, an on or off switch. There are no other digital values. The trend in modern electronics and communications is toward digital, since interference is not an issue unless it is large enough to change a signal from one extreme state to the other.
Earth Segment The stationary section of a Satellite telecommunications system, consisting of the Earth Stations and the network control center.
Earth Station The antenna section of the Earth Segment or a Satellite telecommunications system. An Earth Station consists of a reflector (usually a parabolic dish), feed system to send and receive data, data handling equipment, and mechanical tracking equipment to keep the Satellite within the antenna’s data send/receive area. Parabolic dish antennas come in two main varieties; regular and offset feed. Since a feed originates or collects the data, it has to be at the focus of the parabola, which is in the center of the data stream. This means that it obscures a large portion of data from the dish because of it’s “shadow”. For large dishes the relative amount lost is negligible, but for smaller antennas this “shadow” can have a massive effect on performance. To solve this, the feed is offset, or dropped out of the data stream. This has the added bonus of decreasing sidelobes. Earth Stations are owned by the company receiving the data from the Satellite network, and so need to operate within certain specified parameters, to maintain the network’s stability. Some of these parameters are Sidelobes, Transmit Gain, Cross-Pol Isolation, Axial Ratio, Receive Gain, and G/T.
Elevation A measure of the upward tilt of an Earth Station antenna. An Earth Station pointing toward the horizon has zero elevation, while one pointing straight up has a 90-degree elevation.
E.I.R.P. Equivalent Isotropic Radiated Power — e.i.r.p. is a measure of the power of a transmitter. It includes both the transmitter power and the antenna gain. The e.i.r.p. is the total power that would be needed if the transmitter were to radiate the same level of RF illumination in all directions.
Feed The RF input/output device.
Footprint The surface area of the earth which the Satellite “sees”. No one Satellite can transmit a signal to the entire earth, though theoretically 3 equally spaced apart Satellites in a network could cover the entire earth. The footprint is a roughly square area when represented on a flat-earth map. Global beams cover the 1/3 of the entire surface of the earth, hemi beams cover a single hemisphere, zonal beams cover half a hemisphere, & steerable spot beams using phased array can cover single geopolitical areas.
Frequency A measure of analogue waves (sine waves). Basically a measure of how many “peaks” or cycles from one peak to another pass a given point in every second. Measured in Hertz, Hz.
G/T An antenna property also known as the Figure of Merit. This is a ratio of the amount of power amplification in an antenna to the amount of signal noise. It is measured in dB/K and, since signal noise is a detrimental property, larger ratios are preferred. Earth Station performance is typically indicated in terms of the value of the receive system G/T. The larger this value is, the more sensitive is the receive system and higher link performance is achieved.
Gain The factor of power amplification in an electrical circuit. A signal, which enters a system at 1 watt and exits at 100 watts, has a gain of 100. Since gain is a ratio, it is expressed in decibels.
Geostationary Geosychnronous with an orbital plane in the earth’s equatorial plane (i.e. zero inclination). Satellites in Geostationary orbit are always moving with the rotation of the earth.
Geosynchronous A satellite with an orbital period equal to the earth’s rotation period, and approximately 22,237 miles, or 35,579 Km above the earth’s surface.
H Horizontal — This is the B-Pol transmit polarization on a Ku-band antenna.
High Frequency Radio wave communication occurs at what is considered high frequency. There are three main types of high frequency:
VHF
UHF
SHF
Horn An open-ended waveguide designed to radiate maximum power in one direction. Part of the feed.
Inclination The angle at which the orbital plane of a Satellite is inclined to the equatorial plane of the earth.
Isotropic Radiation Equal radiation in all direction (e.g. the electric field from a point charge, or light radiating from a star)
INMARSAT International Maritime Satellite Organization — An organization created to manage and distribute naval, aeronautical, and mobile land Satellite telecommunications.
ITU International Telecommunications Union — The governing and arbitrating body of telecommunications throughout the world. The ITU creates and regulates the standards of telecommunications.
Launch Vehicle The method of delivering a Satellite into orbit. The two methods used are either re-usable re-entry vehicles (like the NASA Space Shuttle or STS), or disposable launchers (like the Arianne or Atlas rocket systems). The Space Shuttle was an option for the IS-VI series Satellites, but most commonly Arianne rockets are used now. Disposable launchers are multiple-stage rockets; in other words they have a main power stage, which drops off when expended, and then other lesser power stages which push the Satellite into the desired orbit. Most launches take place from near-equatorial spaceports. The new Sea-Launch system launches from the middle of the ocean on the equator. Currently research is being done to create a single-stage-to-orbit re-usable re-entry vehicle to replace the Space Shuttle.
LEO Low Earth Orbit — The first Satellites were launched into this orbit, due to launch vehicle limitations. A few thousand to several hundred kilometers altitude.
LHCP Left Hand Circular Polarized — This is the A-Pol for C-Band transmitting Earth Stations.
Link Power Budget The calculation of power levels needed based on losses within the system, known gains, and required output.
Link The connection between Earth Stations and Satellites. The Uplink is the link from the Earth Station to the Satellite, and the Downlink is the other way.
LNA Low Noise Amplifier — This is a pre-amplifier connected to an Earth Station to increase the power of the signal from the Satellites. Signals from Satellites typically reach the earth at a signal strength of around 10-18 watts, so some considerable amplification is needed to make the signal meet the 10-3 watts of the output. Because it is increasing the signal by so much, an LNA needs to be built to extreme tolerances so it doesn’t inject any noise into the signal.
Molniya A Russian Satellite network which followed a highly elliptical, non-Geostationary orbit. It is described by a path of 6.5 degrees inclination to the equator, apogee at 65 degrees north latitude. Every 24 hours each Satellite gives 6-8 hours of coverage to a single area. Using multiple convoy Satellites and switching, constant coverage is achieved.
Multiplexers These devices are part of the Switching Matrix in the Communications Subsystem of a Satellite. They distribute, connect and combine signals for the amplifiers and antennas. Multiplexing is the ability for multiple users to access a single transponder circuit.
Noise Noise is an electrical term for interference, and is due to the random thermal motion of electrons. Therefore it is based on the temperature of the surroundings. For example, a Satellite points at an Earth Station (Earth being roughly 270 Kelvin). The Earth Station points into space (blank space being roughly 80 Kelvin). Occasionally the sun is behind the Satellite and so the Earth Station sees an infinite temperature. A few times a year links suffer station-sun interference.
Parabolic Antenna An assembly consisting of a feed firing or receiving from a parabolic reflector.
Phased Array By using multiple feeds in an array and setting the phase of some of these to a certain amount, there will be high power areas in the beam, and no power areas. By staggering the phases of the feeds & using this vector cancellation, the beam shape can be molded.
Polarization One frequency can be used for both sending and receiving data if the signals are polarized correctly. Since signals are analogue waves they act like light waves. Polarization of light, for example in sunglasses, removes all but one orientation of waves, so that say only vertical waves could be received. In telecommunications polarization works the same way. To communicate two independent signals on one frequency, the signals are cross-polarized. This means that they are polarized at right angles to one another. There are two types of polarization common in telecommunications: Linear and Circular. Linear polarization is used for Ku-Band signals, and Circular is used for C-Band. Polarizing the radio signals allows the same frequency to be used twice over the same coverage area while enabling the Earth Stations and the Satellites to discriminate between the two signals. This effectively doubles the amount of usable bandwidth, i.e. capacity, available.
Power Amplification Due to atmospheric and distance power losses, both the Satellite and the Earth Station need power amplifiers. There are several types:
Travelling Wave Tubes (TWTA)
Solid State Power Amplifiers (SSPA)
Klystron
Power Systems The primary system has to run everything most of the time. Power amplifiers for the RF signals take about ѕ of the total power. The primary system draws from the silicon solar array. Secondary power is the batteries, located on the spacecraft bus. On a spinning Satellite, like the IS-VI, the solar array is connected to the despun bus via slip rings. These are a small power loss, and therefore the solar array power has to be higher than the battery power.
Reflector This is the part of the Earth Station Antenna which “bounces” the RF signal into the feed. Most typical reflectors are parabolic dishes, and are usually referred to as dishes or antennas. Originally antennas were made large, on the order of 32 meters in diameter, but there is a trend toward smaller and smaller diameter antennas. Since most operating parameters for Earth Stations are functions of the diameter of the antenna, it is the most important parameter.
RF Radio Frequency — The type of radiation which can be turned into useful patterns for communication.
RHCP Right Hand Circular Polarized — this is the B-Pol for C-Band transmitting Earth Stations.
Rx The abbreviation for Receive. All Satellites and Earth Stations have a receive channel where signals from another source are intercepted and passed along.
Sensors These measure the position and/or acceleration of the Satellite, and transmit this information to the ground controllers via the beacon signal.
Sun Sensors
Earth Sensors
Star Trackers
RF Sensors
Gyroscopes
Sidelobe Power Sidelobe power is usually insignificant compared to the main beam, but may transmit interference to other Satellites & microwave relay stations on the 4-6 GHz bands. Physical obstructions such as buildings and pits can help decrease interference. Inserting a weak inverse-phase signal into the main signal will weaken the main beam and decrease sidelobes. Usually the antenna has to confirm to FCC or CCIR standards (32-25 log Theta or 29-25 log Theta).
Sidelobe A local maximum off the center peak on a graph of antenna gain in dBi against the angle from the center of the reflector. The graph is three dimensional, and theoretically symmetrical in two dimensions, about the center of the reflector. Large sidelobes for Earth Stations cause interference with Satellites adjacent to the receiving Satellite.
Space Segment The mobile section of a Satellite telecommunications system, consisting of the Satellite and the launch vehicle that delivers the Satellite into orbit.
Spacecraft Bus This is the term used to describe the chassis or main structural element of the Satellite. All parts of the Satellite root themselves to the bus in some way. On a spinning Satellite the bus is de-spun, so as not to create problems with mass and inertia. The bus also aids precise alignment of RF components and thermal control. The dimensions and specifications of the bus are decided by many constraining factors. The Launch Vehicle requires a certain size and mass payload, the ACS requires a certain mass distribution for motion, the thermal control systems require certain conduction properties, as well as an ease of construction required on the ground and a rigidity required under the pressures of launch stresses.
Station Keeping see Attitude Control.
TDMA Time Division Multiple Access — This is a method of dividing one carrier between many users, by individually handling several signals from different Earth Stations.
Thermal Control Due to the lack of air in space, heat has trouble dissipating to the environment, and tends to remain with the object which generated it. Thermal control on a Satellite helps keep the batteries in a narrow temperature range. If the batteries become too hot they begin to lose charge or the ability to hold charge, and if they become too cold they fail to produce charge when required. Thermal control also helps keep the liquid thruster fuel from freezing, and keeps the amplifier circuits from melting. The amplifiers produce a great deal of heat, which is shunted via the thermal control system and the bus to the fuel and battery areas.
Transponder A combination receiver, frequency converter, and transmitter package, physically part of a communications Satellite. Communications Satellites typically have between 12 and 24 onboard transponders although the INTELSAT VI at the extreme end has 50.
TTC&M Telemetry, Tracking, Control & Monitoring — This is a collection of Satellite systems which require active human interaction on the ground. They direct the Satellite’s motion and relay important information back to the controllers.
Telemetry Gathers Satellite component status data, processes it into the desired form, modulates the RF beacon signal and transmits it to the ground controllers.
Tracking Thruster control (station keeping) is controlled by digital programming or analogue time-control. Position and direction are calculated from the RF beacon signal.
TVRO Television Receive-Only — This type of antenna is mainly used for personal use, and can only receive video and audio data, it can not transmit.
Tx The abbreviation for Transmit. All Satellites and Earth Stations have a transmit channel except TVRO antennas.
V Vertical — This is the A-Pol transmit polarization for a Ku-Band antenna.
VSAT Very Small Aperture Terminal — This is a newer type of antenna, usually between 3 and 1 meters in diameter. Anything smaller is a USAT (Ultra Small Aperture Terminal).
Waveguide Metal tubing used to carry RF signals in the SHF bands between the components of an antenna system. Waveguide can be rectangular, elliptical or circular in cross-section and can be smooth or corrugated in its surface. The internal dimensions of waveguide are dictated by the wavelength of the frequencies to be carried. At SHF frequencies solid conductors cannot be used to carry RF signals due to transmission losses arising from a phenomena known as ‘skin effect’.