Glossary of Satellite Communication Terms
Acquisition of Signal(AOS):
The time at which a radio signal is first heard from a satellite, usually just after it rises above the horizon.
Anomalistic Period:
The elapsed time between two successive perigees of a satellite.
Apogee:
The point in a satellite's orbit where it is farthest from earth.
Argument of Perigee:
The polar angle that locates the perigee 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 a satellite orbit where the sub-satellite point(SSP) crosses the equator from the Southern Hemisphere into the Northern Hemisphere.
Beamwidth:
The angular width of the main lobe of an antenna radiation pattern as measured between predetermined points on the lobe, usually the half-power (-3 dB) points.
Circular Polarization:
A special method of radio energy emission where the
electric and magnetic field vectors rotate about the central axis of radiation. As
viewed along the radiation path, the rotation directions are considered to be
right-hand-circular(RHCP) if the rotation is clockwise and left-hand-circular(LHCP) if the
rotation is counterclockwise.
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.
Downlink:
The frequency of which radio signals originate from a satellite 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 (0) describes a circle and (1) describes a straight line
Elliptical Orbit:
Those orbits in which the satellite path describes an ellipse with the Earth at one focus.
Elevation:
The angle above the local horizontal plane, usually specified in degrees. (0 degrees = plane of the Earth's surface at your location; 90 degrees = straight up, perpendicular to the plane of the Earth).
Epoch:
The reference time at which a particular set of parameters describing satellite motion (Keplerian elements) are defined.
Faraday Rotation:
Rotation of the plane of polarization of an electromagnetic wave when traveling through a magnetic field. In space communications, this effect occurs when signals traverse the ionosphere.
Frequency Division Multiple Access(FDMA):
Multiple access being achieved
through frequency band discrimination; either in the ground receiver, in the case
of a wide band frequency translating repeater, or in the satellite, in the case of
a frequency channelized satellite.
Geocenter:
The center of the Earth.
Geostationary Orbit:
A satellite orbit at such an altitude (approximately 22,300 miles) over the equator that the satellite appears to be fixed above a given point.
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 sub-satellite point (SSP).
Inclination:
The angle between the orbital plane of a satellite and the equatorial plane of the Earth.
Increment:
The change in longitude of the ascending node between two successive passes of a specified satellite, measured in degrees West per orbit.
Keplerian Elements:
The classical set of six orbital element numbers used
to define and compute satellite orbital motions. The set is comprised of inclination,
Right Ascension of the Ascending Node(RAAN), eccentricity, argument of perigee, mean
anomaly, and mean motion, all specified at a particular epoch or reference year, day,
and time. Additionally, a decay rate or drag factor is usually included to refine the
computation.
Mean Anomaly:
An angle that increases uniformly with time, starting at perigee.
Used to indicate where a satellite is located along its orbit. Mean Anomaly is usually
specified at the reference epoch time where the Keplerian Elements are defined.
Mean Motion:
The Keplerian Element that indicates the complete number of orbits a satellite makes in a day.
Nodal Period:
The amount of time difference between two successive ascending nodes of a satellite orbit.
Orbital Plane:
An imaginary plane, extending throughout space, that contains the satellite orbit.
Pass:
An orbit of a satellite.
Passband:
The range of frequencies handled by a satellite translator or transponder.
Perigee:
The point in s satellite's orbit where it is closest to Earth.
Period (of a satellite):
The time required for a satellite to make one complete revolution about the Earth (see Anomalistic Period and Nodal Period).
Repeater:
Equipment which receives, amplifies, and retransmits signals in order to maintain adequate signal strength and intelligence characteristics.
Satellite Pass:
Segment of orbit during which the satellite "passes" nearby and in range of a particular ground station.
Saturation:
A condition that exists when a further change in the input produces no additional output (a saturated amplifier).
Sidereal Day:
The amount of time required for the Earth to rotate exactly 360 degrees about its axis with respect to the "fixed" stars. The Sidereal Day contains 1436.07 minutes (see Solar Day).
Solar Day:
The Solar Day contains exactly 24 hours (1440 minutes). During the Solar Day, the Earth rotates slightly more than 360 degrees (see Sidereal Day).
Spin Modulation:
Periodic amplitude variations resulting from the rotation of a satellite's antennas about its axis, rotating the antenna peaks-and-nulls.
Sub-satellite Point(SSP):
Point on the surface of the Earth directly between the satellite and the geocenter.
Telemetry:
Radio signals, originating at a satellite, that convey information on the performance or status of onboard subsystems. Also refers to the information itself.
Transponder:
A device onboard a satellite that receives radio signals in one segment of the spectrum, amplifies them, translates their frequency to another segment of the spectrum and retransmits them.
Uplink:
The frequency at which signals are transmitted from ground stations to a satellite.
Window:
Overlap region between acquisition circles of two ground stations referenced to a specific satellite. Communications between two stations is possible when the sub-satellite point is within the window.
Created by Charles E. Ellison
Last Updated: 3 September 1997