Technical data
Tables
The figures refer to 95% position fix accuracies, expressing the value of
two standard deviations of radial error from the actual antenna
position, this position being an estimate made under specified satellite
elevation angle and PDOP conditions.
Dilution Of Precision (DOP) is a measure of the satellite geometry, and
is an indicator of the potential quality of the solutions. The lower the
numerical value, the better the potential accuracy (for example, a PDOP
below 3 indicates good satellite geometry). For 3-D positioning,
fluctuations in DOP can be harmful to the solution, especially in
Kinematic/Dynamic modes.
The following DOP terms are computed by GPS receivers:
HDOP Horizontal Dilution of Precision (Latitude, Longitude)
VDOP Vertical Dilution of Precision (Height)
TDOP Time Dilution of Precision (Timing errors)
PDOP Position Dilution of Precision (3-D positioning)
GDOP Geometric Dilution of Precision (3-D position & Time)
Estimated accuracy = DOP x measurement accuracy
While each of these terms can be individually computed, they are
formed from co-variances, and are not independent of each other. For
example, a Horizontal accuracy figure of 95% is the equivalent to 2RMS
(twice root-mean-square), or twice the standard deviation radial error.
Similarly, for vertical and time errors, a figure of 95% is the value of 2
standard-deviations of vertical or time error.
Root-mean-square (RMS) error is the value of one standard
deviation (67%) of error.
Circular Error Probability (CEP) is the value of the radius of a circle,
centred at a position containing 50% of the position estimates.
Spherical Error Probability (SEP) is the spherical equivalent of CEP,
which is centred at a position containing 50% of the position
estimates.
CEP and SEP are not affected by large errors which could make the
values an overly optimistic measurement. These probability statistics
are not suitable for use in a high accuracy positioning system. The GPS
Receiver reports all accuracies in the form of a standard deviation (RMS)
value
GPS Timing Receiver Installation and Operation Manual
D-4