Viavi 87339, Operation Manual

Page: 133 / 198

Principals of Operation
6 -16
Distance to three points is known, position is known in 3D space.
Fig. 6-22 Triangulation using three known points in 3D space
6.1.4.G Time Delay Errors
Sources of Time Delay Error are:
Ephemeris data :
The data concerning ephemeris errors may not exactly model the true satellite motion.
The disparity in ephemeris data can introduce 1 to 5 meters of positional error.
Ephemeris data is valid for a period of about 4 hours
Satellite clocks:
The data concerning the satellite’s four atomic clocks may not reflect the exact rate of
clock drift. Distortion of the signal by measurement noise can further increase positional
error. Clock drift disparity can introduce 0 to 2.5 meters of positional error and
measurement noise can introduce 0 to 10 meters of positional error.
Receiver Clock Inaccuracies and Rounding Errors :
Despite the synchronization of the receiver clock with the satellite time during the
position determination (compensation for clock bias), the remaining inaccuracy of the
time still leads to an error of about 2 m in the position determination. Rounding and
calculation errors of the receiver sum up approximately to 1 m.
Multi-path
:
GPS signals can also be affected by multi-path issues where the radio signals reflect off
of surrounding terrain such as buildings, canyon walls, and hard ground. These delayed
signals can result in periodic signal cancellation but typically they change dynamically
with location and may just cause short term inaccuracy.
Fig. 6-23 Multi-path Delays