Appendix B: Glossary of Terms
Antenna: The interface between a free-space electromagnetic wave and a
guided wave. It can also be described as the input and output inter-
face of a spacecraft.
Antenna illumination: The distribution of radio energy across an antenna
or reector surface. Illumination affects antenna gain. Distribution
of the radio energy depends on the feed horn.
Aperture: The surface area of an antenna that is exposed to a radiofre-
quency (RF) signal.
Apogee: The highest point along an elliptical orbit where the orbit is far-
thest from earth.
Attenuation: The power loss between one transmitting source and the
receiver due to path losses.
Bandwidth: The range of frequencies required to represent the information
contained in a signal.
Bearer service: A type of telecommunication service that provides the
capacity for the transmission of signals between the user and net-
work interface.
Bit error rate (BER): An empirical record of a system’s actual bit error
Cell: A short block of information, usually of xed length, that is identied
with a label.
Channel: A specic portion of the information carrying capacity of a net-
work interface, and specied by a specic transmission rate.
Channel structure: The structure that denes the maximum digital carry-
ing capacity in terms of bit rates across a network interface.
Circuit switching: The method of transferring information in which switch-
ing and transmission functions are achieved by permanently allo-
cating a number of channels or bandwidth between the connections.
Codec: A contraction of code and decode.
Code division multiple access: A multiple-access scheme where stations
use spread-spectrum modulations and orthogonal codes to avoid
interfering with one another.
Common channel signaling network: The network that provides physical
and transmission capacity for the transfer of connection control sig-
nals between components of interexchange network.
Communication satellite: An electronic relay station orbiting in space that
picks up messages transmitted from the ground and retransmits
them to a distant location.
Constellation: A collection of similar satellites designed to provide multiple
coverage or multiple redundancy.
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264 Satellite Communication Engineering, Second Edition
Cryptographic key management: The generation, distribution, recognition,
and reception of the cryptographic keys (passwords).
Cryptography: A science that comprises encryption, decryption, and a
cryptographic key management unit.
Decoding: The process required to reconstruct the data bit sequence
encoded onto the carrier.
Decryption: The unlocking of the locked (decrypted) message; the reverse
of encryption.
Demodulation: A process by which the original signal is recovered from a
modulated carrier; the reverse of modulation.
Delay: Time a signal takes to go from the sending station through the satel-
lite to the receiving station.
Downlink: The space-to-earth telecommunication pathway.
Drag: The friction between a moving vehicle through the earths atmo-
sphere opposing the vehicle’s forward motion.
Earth station: A terrestrial terminal that provides a means of transmitting
the modulated radiofrequency (RF) carrier to the satellite within the
uplink frequency spectrum and receiving the RF carrier from the
satellite with the downlink frequency spectrum. It is the vital link
between satellite and communication users.
Encryption (enciphering): The process of converting messages, informa-
tion, or data into a form unreadable by anyone except the intended
Feed: The radiofrequency (RF) input/output device.
Footprint: An area on earth illuminated by a satellite antenna. Its shape and
size depend on a number of factors, including the antenna design
and the satellites angle of elevation.
Forward error correction: A transmission error detection and correction
technique that, in addition to detecting errors, also corrects errors at
the receive end of the link.
Frequency division multiple access: A technique by which the transponder
bandwidth is shared in separate frequency slots.
Frequency reuse: Using the same frequency band several times in such a
way as to increase the overall capacity of a network without increas-
ing the allocated bandwidth.
Functional group: A set of functions required for carrying out tasks in
cooperation with another functional element or group.
Gain: A measure of amplier power expressed in watts (W) or decibels (dB).
Gateway: A special switch used to link part of a network.
Geostationary satellite (GEO): A satellite placed in the geosynchronous
orbit in the equatorial plane. Although it appears not to be moving
when seen from the earth, its velocity in space equates to 3.076 km/s
(11,071.9 km/h).
Gigahertz (GHz): One billion (10
) cycles per second.
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