SA consists of
two different components - Dither and Epsilon
(Figures
9.2 and 9.3)
Dither:
An intentionalmanipulation of
the satellite clock frequency resulting in the generation
of the carrier waves and the codes with varying wavelengths.
It results in variable distance between each C/A code
chip, rather than the fixed 293 m aklthough the replica
code generated within the receiver will still assume the
chip length to be 293 m and consequently will cause pseudorange
errors.
Typical pseudorange errors for satellites with SA are ±100
m.
Figure
9.2 Dither (GPS Tutor, 1998)
Figure
9.3 GPS pseudorange errors - an example time
series (GPS Tutor, 1998)
Epsilon: It
refers to errors imposed within the description of the satellite
orbit in the ephemeris data sent in the broadcast message.
For positioning purposes, the coordinates of the satellites
are derived using this incorrect information, and errors in
these coordinates propagate into the position of the receiver.
