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GPS/GLONASS Glossary of Terms
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GPS/GLONASS Receiver User Manual Rev 1
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GPS/GLONASS GLOSSARY OF TERMS
ASCII — A 7 bit wide serial code describing numbers, upper and lower case characters, special and non-printing
characters.
Accuracy — The degree of conformance between the estimated or measured position, time, and/or velocity of
a GPS receiver and its true time, position, and/or velocity as compared with a constant standard.
Radionavigation system accuracy is usually presented as a statistical measure of system error and is
characterized as follows:
Predictable - The accuracy of a radionavigation system’s position solution with respect to the
charted solution. Both the position solution and the chart must be based upon the same geodetic
datum.
Repeatable - The accuracy with which a user can return to a position whose coordinates have been
measured at a previous time with the same navigation system.
Relative - The accuracy with which a user can measure position relative to that of another user of
the same navigation system at the same time.
Address field — for sentences in the NMEA standard, the fixed length field following the beginning sentence
delimiter "$" (HEX 24). For NMEA approved sentences, composed of a two character talker
identifier and a three character sentence formatter. For proprietary sentences, composed of the
character "P" (HEX 50) followed by a three character manufacturer identification code.
Almanac — a set of orbit parameters that allows calculation of approximate GPS satellite positions and
velocities. The almanac is used by a GPS receiver to determine satellite visibility and as an aid
during acquisition of GPS satellite signals.
Almanac data — a set of data which is downloaded from each satellite over the course of 12.5 minutes. It
contains orbital parameter approximations for all satellites, GPS to universal time conversion
parameters, and single-frequency ionospheric model parameters.
Analog — A type of transmission characterized by variable waveforms representing information, contrasted
with digital. Modern computers are invariably digital, but when they communicate over telephone
lines, their signals must be converted to analog using a modem ( a modulator/demodulator). The
analog signal is converted back into a digital form before delivering it to a destination computer.
Antipodal Satellites — Satellites in the same orbit plane separated by 180 degrees in argument of latitude.
Arrival alarm — an alarm signal issued by a voyage tracking unit which indicates arrival at or at a pre-
determined distance from a waypoint [see arrival circle].
Arrival circle — an artificial boundary placed around the destination waypoint of the present navigation leg,
and entering of which will signal an arrival alarm.
Arrival perpendicular — crossing of the line which is perpendicular to the course line and which passes
through the destination waypoint.
Attenuation — reduction of signal strength.
Attitude — the position of an aircraft or spacecraft in relation to a given line or plane, as the horizon.
Availability — The percentage of time that the services of a navigation system can be used within a particular
coverage area. Signal availability is the percentage of time that navigational signals transmitted from
external sources are available for use. Availability is a function of both the physical characteristics
of the operational environment and the technical capabilities of the transmitter facilities.
Azimuth — the horizontal direction of a celestial point from a terrestrial point, expressed as the angular
distance from 000° (reference) clockwise through 360°. The reference point is generally True North,
but may be Magnetic North, or Relative (ship's head).
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GPS/GLONASS GLOSSARY OF TERMS