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76 SUPERSTAR II User Manual Rev 3
Appendix H Glossary of Terms
ASCII - A 7 bit wide serial code describing numbers, upper and lower case alpha characters, special and non-
printing characters.
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.
Attenuation - reduction of signal strength.
Carrier - the steady transmitted RF signal whose amplitude, frequency, or phase may be modulated to carry
information.
Checksum - by NMEA standard, a validity check performed on the data contained in the sentences, calculated
by the talker, appended to the message, then recalculated by the listener for comparison to determine if the
message was received correctly. Required for some sentences, optional for all others.
Circular Error Probable (CEP) - the radius of a circle, centered at the user’s true location, that contains 50
percent of the individual position measurements made using a particular navigation system.
Coarse Acquisition (C/A) Code - a spread spectrum direct sequence code that is used primarily by
commercial GPS receivers to determine the range to the transmitting GPS satellite. Uses a chip rate of 1.023
MHz.
Control segment - the Master Control Station and the globally dispersed Monitor Stations used to manage the
GPS satellites, determine their precise orbital parameters, and synchronize their clocks.
Differential GPS (DGPS) - a technique to improve GPS accuracy that uses pseudorange errors measured at a
known location to improve the measurements made by other GPS receivers within the general geographic area.
Dilution of Precision (DOP) - A numerical value expressing the confidence factor of the position solution
based on current satellite geometry. The lower the value, the greater the confidence in the solution. DOP can be
expressed in the following forms:
GDOP all parameters are uncertain (latitude, longitude,
height, clock offset)
PDOP 3D parameters are uncertain (latitude, longitude, height)
HTDOP 2D parameters and time are uncertain (latitude, longitude, time)
HDOP 2D parameters are uncertain (latitude, longitude)
VDOP height is uncertain
TDOP clock offset is uncertain
Earth-Centered-Earth-Fixed (ECEF) -a right-hand Cartesian coordinate system with its origin located at the
center of the Earth used by GPS to describe three-dimensional location. ECEF coordinates are centered on the
WGS-84 reference ellipsoid, have the “Z” axis aligned with the Earth’s spin axis, the “X” axis through the
intersection of the Prime Meridian and the Equator and the “Y” axis is rotated 90 degrees East of the “X” axis
about the “Z” axis.
Ephemeris - a set of satellite orbit parameters that is used by a GPS receiver to calculate precise GPS satellite
positions and velocities. The ephemeris is used in the determination of the navigation solution and is updated
periodically by the satellite to maintain the accuracy of GPS receivers.
Field - a character or string of characters immediately preceded by a field delimiter.