Page 59 - Electronic Navigation Cyber Book

THE FOLLOWING ERRORS AFFECT THE ACCURACY OF GPS
POSITION FIXES

Satellite clock error

It has already been stated that a satellite clock oscillator is a precision
instrument, but it is still necessary to re-adjust it from the ground support
network. Error introduced by SV clock error is unlikely to exceed 1 m and
regular uplinking of clock data reduces it to a minimum. Block IIA and
Block IIR satellites, the latest SVs, carry better clock oscillators and will
consequently provide higher accuracy fixes.

Ionospheric delay error

As the two transmitted carriers must pass through the ionosphere, a
speed reduction caused by refraction of the radio wave occurs. The
extent of the delay, and consequently the error introduced into the
pseudo-range measurement calculation, is dependent upon the electron
density the radio wave encounters along the signal path. Electron density
is itself dependent upon three main factors:

• the time of day
• the SV elevation
• the latitude of the receiver.

© 2018 Digital Galaxy Index 59