|
A B
C D
E F
G H
I J
K L
M N
O P
Q R
S T
U V
W X
Y Z
|
| Earth centred Earth fixed (ECEF) |
Cartesian coordinate system where the origin is nominally
the centre of the earth. It is actually the centre of a datum,
which is thought to best fit the size and shape of the earth.
For GPS calculations it is defined and named WGS 84. The X direction
is the intersection of the prime meridian (Greenwich) with the
equator. The vectors rotate with the earth. The Z direction
is from the centre of the earth to the north pole. |
| Eccentric anomaly E |
The regularizing variable in the two body problem. E is related
to the mean anomaly M by Keeper's equation: M = E - e.sinE (e
stands for eccentricity). |
| Eccentricity |
The ratio of the distance from the centre of an ellipse to
its focus to the semi-major axis. e = (1 - b2/a2)-_ where a
and b are the semi-major and semi-minor axes of the ellipse. |
| ECEF |
See earth
centred earth fixed |
| Ecliptic |
The earth sun orbital plane. North is the direction of the
angular momentum of the system. Also called the ecliptic pole. |
| Elevation |
Height above mean sea level. Vertical distance above the
Geoid. |
| Elevation mask angle |
That angle below satellites should not be tracked. Normally
set to 15 degrees to avoid interference problems caused by buildings
and trees and multipath errors. |
| Ellipsoid |
A mathematical figure formed by revolving an ellipse about
its minor axis. It is often used interchangeably with spheroid.
Two quantities define an ellipsoid; these are usually given
as the length of the semi-major axis a , and the flattening,
f = (a - b) / a, where b is the length of the semi-minor axis.
When the values are defined, related to the geoid and used in
geodesy, the ellipsoid surface constitutes a datum. Prolate
and triaxial ellipsoids are always described as such. |
| Ellipsoid Height |
The height (vertical distance) above an ellipsoid. Not the
same as elevation above sea level. GPS receivers output positions
and heights above the WGS84 datum. |
| Encryption |
A method of making a code to be secret. In GPS, the military
P code is encrypted with a key so that civilian users cannot
make use of it and so that it has "anti spoofing"
qualities. This means that it is difficult for an unfriendly
force to interfere with the signal or transmit a false signal.
When encrypted, it is called the Y code. The P code has been
permanently encrypted since March 1994. |
| Ephemeris |
A list of (accurate) positions or locations of celestial
object as a function of time. Available as "broadcast ephemeris"
or as postprocessed "precise ephemeris". |
| Ephemeris Error |
An innaccurate prediction of the satellite's orbit will cause
an error in your position calculation, and is refered to as
Ephermeris Error |
| Epoch |
An instant in time. In GPS, an epoch is the moment an measurement
is made by a receiver. The epoch rate is the measurement interval,
observation interval or recording rate used by a receiver when
recording data. e.g. making observations every 15 seconds. |
| Event Market/Icon |
A graphic symbol that can be placed on the plotter screen
(and saved in memory) to represent some special event or area
of interest to the GPS user. |
|
A B
C D
E F
G H
I J
K L
M N
O P
Q R
S T
U V
W X
Y Z
|