Satellite Communications Terms & Explanations, Study Guides, Projects, Research of Electrical and Electronics Engineering

Download Satellite Communications Terms & Explanations and more Study Guides, Projects, Research Electrical and Electronics Engineering in PDF only on Docsity! Satellite Communications Terms & Explanations: Before we continue here is a look at the terms commonly used in satellite communications. Have a look at them but don't worry if they make no sense, you can come back to them later. Click the link at the bottom to progress to the next Tutorial. ITIP: Use CTRL-F to access the search this page feature of your browser ] ACTS Advanced Communications Technology Satellite. A NASA experimental satellite project to demonstrate the use of the Ka-Band (30/20 GHz) services. A Amplifier A device used to boost the strength of an electronic signal. Amplitude Modulation (AM) The baseband signal is caused to vary the amplitude or height of the carrier wave to create the desired information content. Analog A form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps. An analog signal is responsive to changes in light, sound, heat and pressure. Analog-to-Digital Conversion (ADC) Process of converting analog signals to a digital representation. DAC represents the reverse translation. ANIK The Canadian domestic satellite system that transmits Canadian Broadcasting Corporation's (CSC) network feeds throughout the country. This system also carries long distance voice and data services throughout Canada as well as some transborder service to the U.S. and Mexico. Antenna A device for transmitting and receiving radio waves. Depending on their use and operating frequency, antennas can take the form of a single piece of wire, a di-pole a grid such as a yagi array, a horn, a helix, a sophisticated parabolic-shaped dish, or a phase array of active electronic elements of virtually any flat or convoluted surface. Aperture A cross sectional area of the antenna which is exposed to the satellite signal. Apogee The point in an elliptical satellite orbit which is farthest from the surface of the earth. Geosynchronous satellites which maintain circular orbits around the earth are first launched into highly elliptical orbits with apogees of 22,237 miles. When the communication satellite reaches the appropriate apogee, a rocket motor is fired to place the satellite into its permanent circular orbit of 22,237 miles. Apogee Kick Motor (AKM) Rocket motor fired to circulate orbit and deploy satellite into geostationary orbit. Apstar (Asia-Pacific Star) Name of the Chinese satellite system which carries commercial video services in the region. Arabsat This is the Arabsat Satellite Organization and its is headquartered in Riyadh, Saudi Arabia. It provides regional telecommunications services for the Middle East region. AsiaSat A Satellite system covering the Asia mainland. Asynchronous Communications Stream of data routed through a network as generated, rather than in organized message blocks. Most personal computers send data in this format. (See ATM) Asynchronous Transfer Mode (ATM) This is the new form of super-fast packet switching operating at speeds in the Gigabits/second. Attenuation The loss in power of electromagnetic signals between transmission and reception points. Attitude Control The orientation of the satellite in relationship to the earth and the sun. Audio Subcarrier The carrier between 5 MHz and 8 MHz containing audio (or voice) information inside of a video carrier. Automatic Frequency Control (AFC) A circuit which automatically controls the frequency of a signal. Automatic Gain Control (AGC) A circuit which automatically controls the gain of an amplifier so that the output signal level is virtually constant for varying input signal levels. AZ/EL Mount Antenna mount that requires two separate adjustments to move from one satellite to another; Azimuth The angle of rotation (horizontal) that a ground based parabolic antenna must be rotated through to point to a specific satellite in a geosynchronous orbit. The azimuth angle for any particular satellite can be determined for any point on the surface of the earth giver the latitude and longitude of that point. Itis defined with respect to due north as a matter of easy convenience. B-Mac A method of transmitting and scrambling television signals. In such transmissions MAC (Multiplexed Analog Component) signals are time-multiplexed with a digital burst containing digitized sound, video synchronizing, authorization, and information. Backhaul A terrestrial communications channel linking an earth station to a local switching network or population center. Backoff The process of reducing the input and output power levels of a traveling wave tube to obtain more linear operation. CATV Originally meant Community Antenna Television. Independent smaller companies in rural communities would build a large television receiving antenna on a nearby mountain to pick up the weak TV signals from a distant metropolis. These signals were amplified, modulated onto television channels and sent along a coaxial cable strung from house to house. CCITT (now TSS) Comite Consultatif Intemationale de Telegraphique et Telephonique. International body, associated with the ITU, which establishes worldwide standards for telecommunications. Reorganized to include CCIR (radio standards group) and renamed TSS (Telecommunications Standardization Sector). CDMA Code division multiple access. Refers to a multiple-access scheme where stations use spread-spectrum modulations and orthogonal codes to avoid interfering with one another. Channel A frequency band in which a specific broadcast signal is ttansmitted. Channel frequencies are specified in the United States by the Federal Communications Commission. Television signals require a 6 MHz frequency band to carry all the necessary picture detail. CIF Common Intermediate Format. A compromise television display format adopted by the CCITT which is relatively easy to derive from both PAL and NTSC. Circular Polarization Unlike many domestic satellites which utilize vertical or horizontal polarization, the international Intelsat satellites transmit their signals in a rotating corkscrew-like pattern as they are down-linked to earth. On some satellites, both right-hand rotating and left-hand rotating signals can be transmitted simultaneously on the same frequency; thereby doubling the capacity of the satellite to carry communications channels. Clamp A video processing circuit that removes the energy dispersal signal component from the video waveform. Clarke Orbit That circular orbit in space 22,237 miles from the surface of the earth at which geosynchronous satellites are placed. This orbit was first postulated by the science fiction writer Arthur C. Clarke in Wireless World magazine in 1945. Satellites placed in these orbits, although traveling around the earth at thousands of miles an hour, appear to be stationary when viewed from a point on the earth, since the earth is rotating upon its axis at the same angular rate that the satellite is traveling around the earth. CINo or C/kT or C/KTB Carrier-to-noise ratio measured either at the Radio Frequency (RF) or Intermediate Frequency (IF). Coaxial Cable A transmission line in which an inner conductor is surrounded by an outer conductor or shield and separated by a nonconductive dielectric. Codec Coder/decoder system for digital transmission. Co-Location Ability of multiple satellites to share the same approximate geostationary orbital assignment frequently due to the fact that different frequency bands are used. Color Subcarrler A subcarrier that is added to the main video signal to convey the color information. In NTSC systems, the color subcarrier is centered on a frequency of 3.579545 MHz, referenced to the main video carrier. Common Carrier Any organization which operates communications circuits used by other people. Common carriers include the telephone companies as well as the owners of the communications satellites, RCA, Comsat, Direct Net Telecommunications, AT&T and others. Common carriers are required to file fixed tariffs for specific services. Companding A noise-reduction technique that applies single compression at the transmitter and complementary expansion at the receiver. Composite Baseband The unclamped and unfiltered output of the satellite receiver's demodulator circuit, containg the video information as well as all transmitted subcarriers. Compression Algorithms Software that allows codecs to reduce the number of bits required for data storage or transmission. COMSAT The Communications Satellite Corporation (part of Lockheed Martin) which serves as the U.S. Signatory to INTELSAT and INMARSAT. Conus Contiguous United States. In short, all the states in the U.S. except Hawaii and Alaska. Cross Modulation A form of signal distortion in which modulation from one or more RF carrier(s) is imposed on another carrier. CSU Channel service unit. A digital interface device that connects end-user equipment to the local digital telephone loop. CSU is frequently coupled with DSU (see below) as CSU/DSU. CIT Carrier-to-noise-temperature ratio. DAMA Demand-Assigned Multiple Access - A highly efficient means of instantaneously assigning telephony channels in a transponder according to immediate traffic demands. DBS Direct broadcast satellite. Refers to service that uses satellites to broadcast multiple channels of television programming directly to home mounted small-dish antennas. dB The decibel is a logarithmic unit of measurement that expresses a power relative to a reference. dBi The dB power relative to an isotropic source. dBW The ratio of the power to one Watt expressed in decibels. De-BPSK Differential Binary Phase Shift Keying De-QPSK Differential Quadrature Phase Shift Keying.Decibel (dB) The standard unit used to express the ratio of two power levels. It is used in communications to express either a gain or loss in power between the input and output devices. Declination The offset angle of an antenna from the axis of its polar mount as measured in the meridian plane between the equatorial plane and the antenna main beam. Decoder A television set-top device which enables the home subscriber to convert an electronically scrambled television picture into a viewable signal. This should not be confused with a digital coder/decoder known as a CODEC which is used in conjunction with digital transmissions. Deemphasis Reinstatement of a uniform baseband frequency response following demodulation. Delay The time it takes for a signal to go from the sending station through the satellite to the receiving station. This transmission delay for a single hop satellite connection is very close on one-quarter of a second. Demodulator A Satellite receiver circuit which extracts or "demodulates" the "wanted "signals from the received carrier. Deviation The modulation level of an FM signal determined by the amount of frequency shift from the frequency of the main carrier. Digital Conversion of information into bits of data for transmission through wire, fiber optic cable, satellite, or over air techniques. Method allows simultaneous transmission of voice, data or video. Digital Speech Interpolation DSI - A means of transmitting telephony. Two and One half to three times more efficiently based on the principle that people are talking only about 40% of the time. Discriminator A type of FM demodulator used in satellite receivers. Dithering Eutelsat The European Telecommunications Satellite Organization which is headquartered in Paris, France. It provides a satellite network for Europe and parts of North Africa and the Middle East. FCC (Federal Communications Commission) The U.S. federal regulatory body, consisting of five members, one of who is designated chairman, appointed by the President and confirmed by the Senate, which regulates interstate communications under the Communications Act of 1934. FID Ratio of antenna focal length to antenna diameter. A higher ratio means a shallower dish. FDMA Frequency division multiple access. Refers to the use of multiple carriers within the same transponder where each uplink has been assigned frequency slot and bandwidth. This is usually employed in conjunction with Frequency Modulation. Feed This term has at least two key meanings within the field of satellite communications. It is used to describe the transmission of video programming from a distribution center. It is also used to describe the feed system of an antenna. The feed system may consist of a subreflector plus a feedhorn or a feedhorn only. Feedhorn A Satellite TV receiving antenna component that collects the signal reflected from the main surface reflector and channels this signal into the low-noise amplifier (LNA). FM - Frequency Modulation A modulation method whereby the baseband signal varies the frequency of the carrier wave. FM Threshold That point at which the input signal power is just strong enough to enable the receiver demodulator circuitry successfully to detect and recover a good quality television picture from the incoming video carrier. Focal Length Distance from the center feed to the center of the dish. Focal Point The area toward which the primary reflector directs and concentrates the signal received. Footprint A map of the signal strength showing the EIRP contours of equal signal strengths as they cover the earth's surface. Different satellite transponders on the same satellite will often have different footprints of the signal strength. The accuracy of EIRP footprints or contour data can improve with the operational age of the satellite. The actual EIRP levels of the satellite, however, tends to decrease slowly as the spacecraft ages. Forward Error Correction (FEC) Adds unique codes to the digital signal at the source so errors can be detected and corrected at the receiver. Frequency The number of times that an altemating current goes through its complete cycle in one second of time. One cycle per second is also referred to as one hertz; 1000 cycles per second, one kilohertz; 1,000,000 cycles per second, one megahertz: and 1,000,000,000 cycles per second, one gigahertz. Frequency Coordination A process to eliminate frequency interference between different satellite systems or between terrestrial microwave systems and satellites. In the U.S. this activity relies upon a computerized service utilizing an extensive database to analyze potential microwave interference problems that arise between organizations using the same microwave band. As the same C-band frequency spectrum is used by telephone networks and CATV companies when they are contemplating the installation of an earth station, they will often obtain a frequency coordination study to determine if any problems will exist. Frequency Reuse A technique which maximizes the capacity of a communications satellite through the use of specially isolated beam antennas and/or the use of dual polarities. Gain A measure of amplification expressed in dB. GE Americon This is a large U.S. corporation providing satellite systems for domestic communications. Has ownership in some international satellites. Geostationary Refers to a geosynchronous satellite angle with zero inclination. so the satellite appears to hover over one spot on the earth's equator. Geostationary Transfer Orbit This orbit is in the equatorial plane. This type of orbit has an elliptical form, with a perigee at 200 km and an apogee at 35870 km. Geosynchronous The Clarke circular orbit above the equator. For a planet the size and mass of the earth, this point is 22,237 miles above the surface. Gigahertz (GHz) One billion cycles per second. Signals operating above 3 Gigahertz are known as microwaves. above 30 GHz they are know as millimeter waves. As one moves above the millimeter waves signals begin to take on the characteristics of lightwaves. Global Beam An antenna down-link pattern used by the Intelsat satellites, which effectively covers one-third of the globe. Global beams are aimed at the center of the Atlantic, Pacific and Indian Oceans by the respective Intelsat satellites, enabling all nations on each side of the ocean to receive the signal. Because they transmit to such a wide area, global beam transponders have significantly lower EIRP outputs at the surface of the Earth as compared to a US domestic satellite system which covers just the continental United States. Therefore, earth stations receiving global beam signals need antennas much larger in size (typically 10 meters and above (i.e.30 feet and up). Gregorian Dual-reflector antenna system employing a paraboloidal main reflector and a concave ellipsoidal subreflector. Globalstar A mobile satellite system that deployes a network of 48 satellites to create a global voice and data service. This system is backed by Qualcomm, Loral, and Alcatel. GIT A figure of merit of an antenna and low noise amplifier combination expressed in dB. "G" is the net gain of the system and "T" is the noise temperature of the system. The higher the number, the better the system. Guard Channel Television channels are separated in the frequency spectrum by spacing them several megahertz apart. This unused space serves to prevent the adjacent television channels from interfering with each other. Half Transponder A method of transmitting two TV signals through a single transponder through the reduction of each TV signal's deviation and power level. Half-transponder TV carriers each operate typically 4 dB to 7 dB below single-carrier saturation power. Headend Electronic control center - generally located at the antenna site of a CATV system - usually including antennas, preamplifiers, frequency converters, demodulators and other related equipment which amplify, filter and convert incoming broadcast TV signals to cable system channels. Heliosynchronous Orbit At an altitude of 600 to 800 km and situated in a quasi-polar plane. The satellite is permanently visible from that part of the Earth in sunlight. Heliosynchronous orbits are used for Earth observation or solar-study satellites. HEO Highly Elliptical Orbit. This is type of orbit used by the Russian Molniya Satellite system. It is also referred to as Extremely Elliptical Orbit (EEO). Hertz (Hz) The name given to the basic measure of radio frequency characteristics. An electromagnetic wave completes a full oscillation from its positive to its negative pole and back again in what is known as a cycle. A single Hertz is thus equal to one cycle per second. High Frequency (HF) Radio frequencies within the range of 3,000 to 30,000 kilohertz. HF radio is known as shortwave. High-Power Satellite Satellite with 100 watts or more of transponder RF power. Hour Angle Steering direction of a polar mount. An angle measured in the equatorial plane between the antenna beam and the meridian plane. Hub The master station through which all communications to, from and between micro terminals must flow. in the future satellites with on-board processing will allow hubs to be eliminated as MESH networks are able to connect all points in a network together. Hughes Galaxy A domestic U.S. satellite system which provides a range of telecommunications services. IBS INTELSAT Business Services. Margin The amount of signal in dB by which the satellite system exceeds the minimum levels required for operation. Master Antenna Television (MATV) An antenna system that serves a concentration of television sets such as in apartment buildings, hotels or motels. Medium-Power Satellite Satellite generating transmit power levels ranging from 30 to 100 watts. Megahertz (MHz) Refers to a frequency equal to one million Hertz, or cycles per second. Microwave Line-of sight, point-to-point transmission of signals at high frequency. Many CATV systems receive some television signals from a distant antenna location with the antenna and the system connected by microwave relay. Microwaves are also used for data, voice, and indeed all types of information transmission. The growth of fiber optic networks have tended to curtail the growth and use of microwave relays. Microwave Interference Interference which occurs when an earth station aimed at a distant satellite picks up a second, often stronger signal, from a local telephone terrestrial microwave relay transmitter. Microwave interference can also be produced by nearby radar transmitters as well as the sun itself. Relocating the antenna by only several feet will often completely eliminate the microwave interference. Modem (modulator/demodulator) A communications device that modulates signals at the transmitting end and demodulates them at the receiving end. Modulation The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal. Modulator A device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders. Modulators are also used by CATV companies to place a baseband video television signal onto a desired VHF or UHF channel. Home video tape recorders also have built-in modulators which enable the recorded video information to be played back using a television receiver tuned to VHF channel 3 or 4. Molniya The Russian domestic satellite system which operated with highly elliptical satellites which overlooked the high latitudes of the territories of the USSR. MPEG The Moving Pictures Experts Group, the television industry's informal standards group. MPEG-2 The agreed standard covering the compression of data (coding and encoding) for digital television. MPEG-2 MP@HL Main Provile at High Level - The agreed much higher bit-rate system adopted to provide high definition television in wide screen format. Multiple Access The ability of more than one user to have access to a transponder. Multiple System Operator (MSO) A company that operates more than one cable television system. Multipoint Distribution System (MDS) A common carrier licensed by the FCC to operate a broadcast-like omnidirectional microwave transmission facility within a given city typically carrying television signals Multicast Multicast is a subset of broadcast that extends the broadcast concept of one to many by allowing "the sending of one transmission to many users in a defined group, but not necessarily to all users in that group." Multiplexing Techniques that allow a number of simultaneous transmissions over a single circuit. Mux A Multiplexer. Combines several different signals (e.g. video, audio, data) onto a single communication channel for transmission. Demultiplexing separates each signal at the receiving end. NAB National Association of Broadcasters. NASA (National Aeronautics and Space Administration) The U.S. agency which administers the American space program, including the deployment of commercial and military satellites via a fleet of space shuttle vehicles. NASDA National Space Development Agency of Japan. NCTA National Cable Television Association. Noise Any unwanted and unmodulated energy that is always present to some extent within any signal. Noise Figure (NF) A term which is a figure of merit of a device, such as an LNA or receiver, expressed in dB, which compares the device with a perfect device. NTIA The National Telecommunications and Information Administration is a unit of the Department of Commerce that address U.S. government telecommunications policy, standards setting and radio spectrum allocation. Nutation Damping The process of correcting the nutational effects of a spinning satellite which are similar in effect toa wobbling top. Active nutation controls use thruster jets. NTSC - National Television Standards Committee A video standard established by the United States (RCA/NBC} and adopted by numerous other countries. This is a 525-line video with 3.58-MHz chroma subcarrier and 60 cycles per second. OFTEL The Office of Telecommunications of the United Kingdom government. This unit a part of the Department of Industries regulates telecommunications in the United Kingdom. Orbital Period The time that it takes a satellite to complete one circumnavigation of its orbit. Packet Switching Data transmission method that divides messages into standard-sized packets for greater efficiency of routing and transport through a network. PAL - Phase Alternation System The German developed TV standard based upon 50 cycles.per second and 625 lines. Parabolic Antenna The most frequently found satellite TV antenna, it takes its name from the shape of the dish described mathematically as a parabola. The function of the parabolic shape is to focus the weak microwave signal hitting the surface of the dish into a single focal point in front of the dish. It is at this point that the feedhom is usually located. PBS (Public Broadcasting System) A domestic USA television and radio broadcast network. Perigee The point in an elliptical satellite orbit which is closest to the surface of the earth. Perigee Kick Motor (PKM) Rocket motor fired to inject a satellite into a geostationary transfer orbit from alow earth orbit especially that of a STS or Shuttle-based orbit of 300 to 500 miles altitude. Period The amount of time that a satellite takes to complete one revolution of its orbit. Phase Alternation System (PAL) A European color television system incompatible with the US NTSC television system. Phase-Locked Loop (PLL) A type of electronic circuit used to demodulate satellite signals. Polarization A technique used by the satellite designer to increase the capacity of the satellite transmission channels by reusing the satellite transponder frequencies. In linear cross polarization schemes, half of the transponders beam their signals to earth in a vertically polarized mode; the other half horizontally polarize their down links. Although the two sets of frequencies overlap, they are 90 degree out of phase, and will not interfere with each other. To successfully receive and decode these signals on earth, the earth station Simplex Transmission Capability for transmission in only one direction between sending station and receiving station. Single-Channel-Per-Carrier (SCPC) A method used to transmit a large number of signals over a single satellite transponder. Single Sideband (SSB) A form of amplitude modulation (AM) whereby one of the sidebands and the AM carrier are suppressed. Skew An adjustment that compensates for slight variance in angle between identical senses of polarity generated by two or more satellites. Slant Range The length of the path between a communications satellite and an associated earth station. Slot That longitudinal position in the geosynchronous orbit into which a communications satellite is "parked". Above the United States, communications satellites are typically positioned in slots which are based at two to three degree intervals. SMATV (Satellite Master Antenna System) The adding of an earth station to a MATV system to receive satellite programs. SNG Satellite news gathering usually with a transportable uplink truck. Snow A form of noise picked up by a television receiver caused by a weak signal. Snow is characterized by alternate dark and light dots appearing randomly on the picture tube. To eliminate snow, a more sensitive receive antenna must be used, or better amplification must be provided in the receiver (or both). Solar Outage Solar outages occur when an antenna is looking at a satellite, and the sun passes behind or near the satellite and within the field of view of the antenna. This field of view is usually wider than the beamwidth. Solar outages can be exactly predicted as to the timing for each site. Sparklies A form of satellite television "snow" caused by a weak signal. Unlike terrestrial VHF and UHF television snow which appears to have a softer texture, sparklies are sharper and more angular noise "blips". As with terrestrial reception, to eliminate sparklies, either the satellite antenna must be increased in size, or the low noise amplifier must be replaced with one which has a lower noise temperature. Spectrum The range of electromagnetic radio frequencies used in transmission of voice, data and television. Spillover Satellite signal that falls on locations outside the beam pattern's defined edge of coverage. Spin Stabilization A form of satellite stabilization and attitude control which is achieved through spinning the exterior of the spacecraft about its axis at a fixed rate. Splitter A passive device (one with no active electronic components) which distributes a television signal carried on a cable in two or more paths and sends it to a number of receivers simultaneously. Spot Beam A focused antenna pattern sent to a limited geographical area. Spot beams are used by domestic satellites to deliver certain transponder signals to geographically well defined areas such as Hawaii, Alaska and Puerto Rico. Spread Spectrum The transmission of a signal using a much wider bandwidth and power than would normally be required. Spread spectrum also involves the use of narrower signals that are frequency hopped through various parts of the transponder. Both techniques produce low levels of interference Between the users. They also provide security in that the signals appear as though they were random noise to unauthorized earth stations. Both military and civil satellite applications have developed for spread spectrum transmissions. SSMA Spread spectrum multiple access. Refers to a frequency multiple access or multiplexing technique. SSPA Solid state power amplifier. A VSLI solid state device that is gradually replacing Traveling Wave Tubes in satellite communications systems because they are lighter weight and are more reliable. Stationkeeping Minor orbital adjustments that are conducted to maintain the satellite's orbital assignment within the allocated "box" within the geostationary arc. Subcarrier A second signal "piggybacked" onto a main signal to carry additional information. In satellite television transmission, the video picture is transmitted over the main carrier. The corresponding audio is sent via an FM subcarrier. Some satellite transponders carry as many as four special audio or data subcarriers whose signals may or may not be related to the main programming. Subsatellite Point The unique spot over the earth's equator assigned to each geostationary satellite. Superband The frequency band from 216 to 600 MHz, used for fixed and mobile radios and additional television channels on a cable system. Synchronization (Sync) The process of orienting the transmitter and receiver circuits in the proper manner in order that they can be synchronized . Home television sets are synchronized by an incoming sync signal with the television cameras in the studios 60 times per second. The horizontal and vertical hold controls on the television set are used to set the receiver circuits to the approximate sync frequencies of incoming television picture and the sync pulses in the signal then fine tune the circuits to the exact frequency and phase. T1 The transmission bit rate of 1.544 millions bits per second. This is also equivalent to the ISDN Primary Rate Interface for the U.S. The European T1 or E1 transmission rate is 2.048 million bits per second. T3 Channel (DS-3) In North America, a digital channel which communicates at 45.304 Mbps. Teleconference An electronic multilocation, multiperson conference using audio, computer, slow-scan, or full-rate video systems. Teledesic The name of the U.S. proposed LEO satellite system that would deploy 840 satellites for global telecommunications services. Telstar The AT&T Corporation has maintained its trademark for the Telstar name and currently operates its domestic satellite system under the Telstar name. Terrestrial TV Ordinary "over the air" VHF (very high frequency) and UHF (ultrahigh frequency) television transmissions which are usually limited to an effective range of 100 miles or less. Terrestrial tv transmitters operate at frequencies between 54 megahertz and 890 megahertz, far lower than the 14/2 and 6/4 billion hertz (gigahertz) microwave frequencies used by satellite transponders. Three-Axis Stabilization Type of spacecraft stabilization in which the body maintains a fixed attitude relative to the orbital track and the earth's surface. The reference axes are roll, pinch, and yaw, by nautical analogy. Threshold Extension A technique used by satellite television receivers to improve the signal-to noise ratio of the receiver by approximately 3 db (50%). When using small receive-only antennas, a especially equipped receiver with a threshold extension feature can make the difference between obtaining a decent picture or no picture at all. Thruster A small axial jet used during routine stationkeeping activities. These are often fueled bydrazine or bi- propellant. In time ion-engines will probably replace such thrusters. TI - Terrestrial Interference Interference to satellite reception caused by ground based microwave transmitting stations. Transfer Orbit A highly elliptical orbit which is used as an intermediate stage for placing satellites into geostationary orbit. Transmitter An electronic device consisting of oscillator, modulator and other circuits which produce a radio or television electromagnetic wave signal for radiation into the atmosphere by an antenna. Transponder A combination receiver, frequency converter, and transmitter package, physically part of a communications satellite. Transponders have a typical output of five to ten watts, operate over a frequency band with a 36 to 72 megahertz bandwidth in the L, C, Ku, and sometimes Ka Bands or in effect typically in the microwave spectrum, except for mobile satellite communications. Communications satellites typically have between 12 and 24 onboard transponders although the INTELSAT VI at the extreme end has 50. Transponder Hopping A single TDMA equipped earth station can extend its capacity by having access to several down-link