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dB
Decibel. A notation of relative unit such as the satellite signal strength received.
Datum
A math model which depicts a part of the surface of the earth. Latitude and longitude lines on a
paper map are referenced to a specific map datum. The map datum selected on a GPS receiver
needs to match the datum listed on the corresponding paper map in order for the position readings
to match.
Elevation mask
Elevation mask is an adjustable feature of GPS receivers that specifies a satellite must be a certain
number of degrees above the horizon before its signals are used for positioning. Satellites at low
elevation angles (five degrees or less) have lower signal strengths and are more prone to loss of
lock thus causing noisy solutions.
Ellipsoid
A geometric surface which all of whose plane sections are either ellipses or circles.
Ephemeris
Ephemeris is a set of parameters used by a global navigation satellite receiver to predict the location
of a satellite and its clock behavior. Each satellite contains and transmits ephemeris data about its
own orbit and clock. Ephemeris data is more accurate than the almanac data but is applicable over a
short time frame from four to six hours. Ephemeris data is transmitted by the satellite every 30
seconds. The predictions of current satellite position are transmitted to the user in the data
message.
Geodetic coordinate
A coordinate system whose elements are latitude, longitude and geodetic height. The latitude is an
angle based on the perpendicular to the ellipsoid. Longitude is the angle measured in the XY plane.
GIS (Geographic Information System)
A computer based system that is capable of collecting, managing and analyzing geographic spatial
data. This capability includes storing and utilizing maps, displaying the results of data queries and
conducting spatial analysis.
Hot Start
Start mode of the GPS receiver when current position, clock offset, approximate GPS time and
current ephemeris data are all available.
