Leica Geosystems GPS Basics - Manual (Page 51)

51
GPS Basics -1.0.0en Glossary
Dilution of precision (DOP)
A description of the purely geometrical
contribution to the uncertainty in a
position fix. The DOP factor indicates the
geometrical strength of the satellite
constellation at the time of measure-
ment. Standard terms in the case of
GPS are
GDOP three position coordinates plus
clock offset
PDOP three coordinates
HDOP two horizontal coordinates
VDOP height only
TDOP clock offset only
HTDOP horizontal position and time
Doppler shift
The apparent change in frequency of a
received signal due to the rate of change
of the range between the transmitter and
receiver.
Eccentricity
The ratio of the distance from the centre
of an ellipse to its focus to the
semimajor axis.
e = (1 - b 2 /a 2 ) 1/2
where a and b are the semimajor and
semiminor axis of the ellipse, respec-
tively.
Elevation
Height above the Geoid. See
Orthometric height.
Ellipsoid
In geodesy, unless otherwise specified,
a mathematical figure formed by revolv-
ing an ellipse about its minor axis
(sometimes also referred to as sphe-
roid). Two quantities define an ellipsoid;
these are usually given as the length of
the semimajor axis a and the flattening f.
Ellipsoid height
The vertical distance of a point above the
ellipsoid.
Ephemeris
A list of positions or locations of a
celestial object as a function of time.
Ephemeris error
Difference between the actual satellite
location and the location predicted by the
satellite orbital data (ephemerides).
Epoch
A particular fixed instant of time used as
a reference point on a time scale.
Equipotential Surface
A mathematically defined surface where
the gravitational potential is the same at
any point on that surface. An example of
such a surface is the geoid.