NavCom LAND-PAK Rev.N - Manual (Page 189)

Technical Reference Manual Rev. N
Glossary - 187
DTE Data Terminal Equipment. See DCE.
dual-frequency a type of GPS receiver that uses both L1 and L2 signals from GPS satellites. A
dual-frequency receiver can compute more precise position fixes over longer distances and
under more adverse conditions because it compensates for ionospheric delays. The SF-2050 is
a dual frequency receiver.
dynamic mode when a GPS receiver operates in dynamic mode, it assumes that it is in motion
and certain algorithms for GPS position fixing are enabled in order to calculate a tighter position
fix.
EGNOS (European Geostationary Navigation Overlay Service) a European satellite system
used to augment the two military satellite navigation systems now operating, the US GPS and
Russian GLONASS systems.
elevation distance above or below Local Vertical Datum.
elevation mask the lowest elevation, in degrees, at which a receiver can track a satellite.
Measured from the horizon to zenith, 0º to 90º.
ellipsoid a mathematical figure approximating the earth’s surface, generated by rotating an
ellipse on its minor axis. GPS positions are computed relative to the WGS-84 ellipsoid. An
ellipsoid has a smooth surface, which does not match the earth’s geoidal surface closely, so
GPS altitude measurements can contain a large vertical error component. Conventionally
surveyed positions usually reference a geoid, which has an undulating surface and
approximates the earth’s surface more closely to minimize altitude errors.
epoch literally a period of time. This period of time is defined by the length of the said period.
GAGAN (GPS Aided Geo Augmented Navigation) an Indian satellite system that provides a
set of corrections for the GPS satellites, which are valid for the Indian region. They incorporate
satellite orbit and clock corrections.
geoid the gravity-equipotential surface that best approximates mean sea level over the entire
surface of the earth. The surface of a geoid is too irregular to use for GPS readings, which are
measured relative to an ellipsoid. Conventionally surveyed positions reference a geoid. More
accurate GPS readings can be obtained by calculating the distance between the geoid and
ellipsoid at each position and subtracting this from the GPS altitude measurement.
GIS (Geographical Information Systems) a computer system capable of assembling, storing,
manipulating, updating, analyzing and displaying geographically referenced information, i.e.
data identified according to their locations. GIS technology can be used for scientific
investigations, resource management, and development planning. GIS software is used to
display, edit, query and analyze all the graphical objects and their associated information.
Global Positioning System (GPS) geometrically, there can only be one point in space, which
is the correct distance from each of four known points. GPS measures the distance from a point
to at least four satellites from a constellation of 24 NAVSTAR satellites orbiting the earth at a
very high altitude. These distances are used to calculate the point’s position.
GMT see Greenwich Mean Time.
GPS see Global Positioning System.
GPS time a measure of time. GPS time is based on UTC, but does not add periodic ‘leap
seconds’ to correct for changes in the earth’s period of rotation. As of September 2002 GPS
time is 13 seconds ahead of UTC.
Greenwich Mean Time (GMT) the local time of the 0° meridian passing through Greenwich,
England.