Satellite Terminology

Here’s some basic information to get you started with satellite operations…with my thanks to the many different sources from which it has been gathered around the Internet including AMSAT and Stephen Holmstead, N7TQL, for use of the information contained in the HAM Satellite FAQ.


  • AMSAT – It is a registered trademark of the Radio Amateur Satellite Corporation, a non-profit scientific/educational organization located in Washington, DC that builds and operates Amateur Radio satellites.
  • Attitude – Position of a craft in space; determined by the inclination of its axis to a fixed reference point on the Earth.
  • Apogee – Point in an Earth orbit where an orbiting body is farthest from the earth.
  • Argument of perigee – The polar angle that locates the perigee (point where the satellite is CLOSEST to the earth) point of a satellite in the orbital plane; drawn between the ascending node, geocenter, and perigee; and measured from the ascending node in the direction of satellite motion.
  • Ascending node – The point on the ground track of the satellite orbit where the sub-satellite point (SSP) crosses the equator from the Southern Hemisphere into the Northern Hemisphere. Traveling from South America up (ascending) to North America.
  • Azimuth – Direction (side-to-side in the horizontal plane) from a given point on Earth, usually expressed in degrees. North = 0 or 360 degrees; East = 90 degrees; South = 180 degrees; West = 270 degrees.
  • Beacon – Most satellites have a fixed Morse beacon at the lower end of the satellites band-pass transponder. This is useful to detect when the satellite has crossed the horizon and is in range for operation. It can also be used to determine Doppler shifts.
  • BBS – An electronic Bulletin Board System which users can use to leave and retrieve messages.
  • Cosmic Speed – Five miles per second, the velocity required to put a satellite in Earth orbit; called first cosmic speed.
  • Drag – Air resistance to a body in flight.
  • Descending node – The point on the ground track of the satellite orbit where the sub-satellite point (SSP) crosses the equator from the Northern Hemisphere into the Southern hemisphere. Traveling from Canada DOWN (descending) towards South America.
  • Doppler effect – Apparent frequency change of waves which results when the source and recipient of the waves move toward, and then away, from each other. A shift in frequency caused by satellite movement toward or away from your location. When the satellite is coming toward you, the Doppler shift decreases the frequency. When the satellite is going away from you, the frequency increases. Like the waves from the ocean piling up in front of a storm.
  • Downlink – The frequency which the satellite transmits to the Earth for reception by stations on Earth.
  • Eccentricity – The orbital parameter used to describe the geometric shape of an elliptical orbit; eccentricity values vary from e = 0 to e = 1; where e = 0 describes a perfect circle and e = 1 describes a straight line. The amount a spacecraft deviates from a circular orbit.
  • Ephemeris – Table indicating the computed positions of celestial bodies from day to day or at regular intervals throughout the year.
  • Elliptical Orbit – Those orbits in which the satellite path forms an ellipse with the Earth at one focus.
  • Epoch – The reference time at which a particular set of parameters describing satellite motion (Keplerian elements) are defined. The particular time the elements were produced.
  • Equatorial orbit – Earth orbit with a plane near or identical to that of the equator.
  • Geocenter – The center of the Earth.
  • Geostationary orbit – A satellite orbit at such an altitude (approx. 22,300 miles) over the equator that the satellite appears to be fixed above a given point. The satellite must also travel in the direction of the earth’s rotation. The satellite remains in the same spot day after day, year after year.
  • Ground Station – A radio station, on or near the surface of the earth, designed to transmit or receive to/from a spacecraft.
  • Groundtrack – The imaginary line traced on the surface of the Earth by the subsatellite point (SSP).
  • Inclination – The angle between the orbital plane of a satellite and the equatorial plane of the Earth.
  • Keplerian Elements – The classical set of six orbital elements numbers used to define and compute satellite orbital motions. The set is comprised of inclination, Right Ascension of Ascending Node (RAAN), eccentricity, argument of perigee, mean anomaly and mean motion, all specified at a particular epoch or reference year, day, and time. A decay rate or drag factor is usually included to refine the computation.
  • Kepler, Johannes – (1571-1630) German astronomer who determined the periods of revolution of the planets. Created “Kepler’s laws” which read: 1) The path of every planet in its motion about the sun forms an ellipse, with the sun at one focus (see Elliptical orbit). 2) The speed of a planet in its orbit varies so that a line joining it with the sun sweeps over equal areas in equal times. 3) The squares of the planets’ periods of revolution are proportional to the cubes of the planet’s mean distances from the sun. Kepler was very active in astronomy throughout his whole life. He was once the astrologer and astronomer to Rudolph II of Bohemia. He also corresponded with Galileo and Tycho Brahe while at the university of Graz, Austria. In 1600 Kepler became Tycho’s assistant in Prague. Kepler’s laws removed all doubt that the earth and planets go around the sun. Later Newton used Kepler’s laws to establish his law of universal gravitation. Kepler also invented the present-day form of the astronomical telescope.
  • Mean anomaly – An angle that increases uniformly with time, starting at perigee, use to indicate where a satellite is located along its orbit. MA is usually specified at the reference epoch time where the Keplerian elements are defined. For AO-10 the orbital time is divided into 256 parts, rather than degrees of a circle, and MA (sometimes called Phase) is specified from 0 to 255. Perigee (closest to Earth) is therefore at MA = 0 and apogee (farthest away from Earth) is at MA = 127.
  • Mean motion – The Keplerian element (a number) to indicate the complete number of orbits a satellite makes in one day. (ie. 14.00374 orbits per day)
  • Mode A – This mode requires a 2M SSB/CW transmitter and a 10M SSB/CW receiver and supports CW and voice.
  • Mode B – This mode requires a 70cm SSB/CW transmitter and a 2M SSB/CW receiver and supports CW and voice. Some satellites also support RTTY and SSTV in this mode.
  • Mode K – This mode requires a 15M SSB/CW transmitter and a 10M SSB/CW receiver and supports CW and voice. This mode is unique in that it can be done with a simple HF rig.
  • Mode JA – This mode stands for J Analog and requires a 2M SSB/CW transmitter and a 70cm SSB/CW receiver and supports CW and voice.
  • Mode JD – This mode stands for J Digital and requires a 2M FM transmitter and a 70cm SSB/CW receiver and supports packet.
  • Mode S – This mode requires a 70cm SSB/CW transmitter and a 2.4Ghz SSB/CW receiver and supports CW and voice. Many people use a 2.4Ghz to 2M converter with a 2M SSB/CW receiver instead of buying a 2.4GHz SSB/CW receiver.
  • Mode T – This mode requires a 15M SSB/CW transmitter and a 2M SSB/CW receiver and supports CW and voice.
  • Nodal period – The amount of time between two successive ascending nodes of a satellite orbit.
  • Orbital elements – See Keplerian elements.
  • Orbital plane – An imaginary plane, extending throughout space, that contains the satellite orbit.
  • OSCAR – Orbiting Satellite Carrying Amateur Radio.
  • Pass – An orbit of a satellite.
  • Passband – The range of frequencies handled by a satellite translator or transponder.
  • Perigee – The point in a satellite’s orbit where it is CLOSEST to Earth.
  • Period – The time required for a satellite to make one complete revolution about the Earth.
  • RAAN – Right Ascension of Ascending Node. The Keplerian element specifying the angular distance along the celestial equator, between the vernal equinox and the ascending node of a spacecraft. This can be simplified to mean roughly the longitude of the ascending node.
  • Repeater – This closely resembles a land-based repeater. It listens for signals on one frequency and retransmits it on another frequency. All satellite repeaters (and transponders) are full duplex, meaning you can (and should) listen to you signal on the downlink (with headphones) while you are transmitting.
  • Satellite pass – Segment of orbit during which the satellite “passes” nearby and in range of a particular ground station.
  • SSP – Subsatellite point. Point on the surface of the Earth directly between the satellite and the geocenter (center of the Earth).
  • Telemetry – Radio signals, originating at a satellite, that convey information on the performance or status of onboard sub-systems. Also refers to the information itself.
  • Transponder – A transponder is a band-pass repeater, a device onboard a satellite that receives radio signals in one segment of the spectrum (band), amplifies them, translates (shifts) their frequency to another segment of the spectrum and retransmits them. It accepts a range of frequencies on the input and retransmits the entire range on the output. All offsets within that range are preserved.
  • Uplink – The frequency at which signals are transmitted from ground stations to a satellite. The signal you transmit to a satellite.





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