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I GPS/GLONASS Glossary of Terms
Global Positioning System (GPS) — full name NAVSTAR Global Positioning System, a space-based radio
positioning system which provides suitably equipped users with accurate position, velocity and time
data. When fully operational, GPS will provide this data free of direct user charge worldwide,
continuously, and under all weather conditions. The GPS constellation will consist of 24 orbiting
satellites, four equally spaced around each of six different orbiter planes. The system is being
developed by the Department of Defence under U.S. Air Force management.
GLONASS — The Soviet Ministry of Defense Global Navigation Satellite System: A constellation of 24
antipodal satellites orbiting the earth at a very high altitude. GLONASS satellites transmit signals
that allow one to determine, with great accuracy, the locations of GLONASS receivers. The receivers
can be fixed on the Earth, in moving vehicles, aircraft, or in low-Earth orbiting satellites. GLONASS
is used in air, land and sea navigation, mapping, surveying and other applications where precise
positioning is necessary.
GPS — The U.S. Department of Defense Global Positioning System: A constellation of 24 satellites orbiting
the earth at a very high altitude. GPS satellites transmit signals that allow one to determine, with
great accuracy, the locations of GPS receivers. The receivers can be fixed on the Earth, in moving
vehicles, aircraft, or in low-Earth orbiting satellites. GPS is used in air, land and sea navigation,
mapping, surveying and other applications where precise positioning is necessary.
Great circle — the shortest distance between any two points along the surface of a sphere or ellipsoid, and
therefore the shortest navigation distance between any two points on the Earth. Also called Geodesic
Line.
HDOP — Horizontal Dilution of Precision - A numerical value expressing the confidence factor of the
horizontal position solution based on current satellite geometry. Makes no constraint assumptions
about time, and about height only if the FIX HEIGHT command has been invoked. The lower the
HDOP value, the greater the confidence in the solution.
HTDOP — Horizontal position and Time Dilution of Precision - A numerical value expressing the confidence
factor of the position solution based on current satellite geometry. Assumes height is known if the
FIX HEIGHT command has been invoked. If not, it will give the normalized precision of the
horizontal and time parameters given that nothing has been constrained. The lower the HTDOP
value, the greater the confidence factor.
Heading — the direction in which a vessel points or heads at any instant, expressed in degrees 000° clockwise
through 360° and may be referenced to True North, Magnetic North, or Grid North. The heading of
a vessel is also called the ship's head. Heading is a constantly changing value as the vessel oscillates
or yaws across the course due to the effects of the air or sea, cross currents, and steering errors.
Integer Ambiguity Estimates — carrier phase ambiguity estimates which are only allowed to take on integer
values.
Iono-free Carrier Phase Observation — a linear combination of L1 and L2 carrier phase measurements
which provides an estimate of the carrier phase observation on one frequency with the effects of the
ionosphere removed.
measurement on that frequency.
Ionosphere — The band of charged particles 80 to 120 miles above the earth's surface, which represent a
nonhomogeneous and dispersive medium for radio signals.
Ionospheric Delay — A wave propagating through the ionosphere experiences delay. Phase delay depends on
electron content and affects carrier signals. Group delay depends on dispersion in the ionosphere as
well and affects signal modulation (codes). The phase and group delay are of the same magnitude
but opposite sign.
Ionospheric Refraction — The change in the propagation speed of a signal as it passes through the ionosphere.
Kinematic — Your GPS antenna is moving. In GPS, this term is typically used with precise carrier phase
positioning, and the term dynamic is used with pseudorange positioning.
L-band — The group of radio frequencies extending from 390 MHz to 1550 MHz. The GPS carrier frequencies
(1227.6 MHz and 1575.42 MHz) are in the L-band.
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It provides a different ambiguity value (non-integer) than a simple
GPS/GLONASS Receiver User Manual Rev 1
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