ashtech DG16 Reference Manual page 365

Earth Centered, Earth Fixed (ECEF)
A cartesian coordinate system centered
at the earth's center of mass. The Z-
axis is aligned with the earth's mean
spin axis. The X-axis is aligned with the
zero meridian. The Y-axis is 90
degrees west of the X-axis, forming a
right-handed coordinate system.
EDOP
Elevation Dilution of Precision
ELEV
Elevation
Elevation
Height above mean sea level. Vertical
distance above the geoid.
Elevation mask
An adjustable feature of GPS receivers
that specifies that a satellite must be at
least a specified number of degrees
above the horizon before the signals
from the satellite are to be used.
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.
Elevation mask angle
That angle below which it is not
advisable to track satellites. Typically
the elevation mask is set to 5° to avoid
interference problems caused by
buildings and trees and multipath
reflections.
Ellipsoid
In geodesy, unless otherwise specified,
a mathematical figure formed by
revolving an ellipse about its minor axis.
It is often used interchangeably with
spheroid. Two quantities define an
ellipsoid; the length of the semimajor
axis, a, and the flattening, f =- (a - b)/a,
where b is the length of the semiminor
axis. Prolate and triaxial ellipsoids are
invariably described as such.
Ellipsoid height
The measure of vertical distance above
the ellipsoid. Not the same as elevation
above sea level. GPS receiver output
position fix height in the WGS-84 datum.
Ephemeris
A set of parameters used by a GPS
receiver to predict the location of a single
GPS satellite and its clock behavior.
Each GPS satellite contains and
transmits ephemeris data for its own orbit
and clock. Ephemeris data is more
accurate than the almanac data but is
applicable over a short time frame (four
to six hours). Ephemeris data is
transmitted by the satellite every 30
seconds.
Epoch
Measurement interval or data frequency,
as in making observations every 15
seconds. Loading data using 30-second
epochs means loading every other
measurement.
FCC
Federal Communications Commission
Firmware
The coded instructions relating to
receiver function, and (sometimes) data
processing algorithms, embedded as
integral portions of the internal circuitry.
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