ഹ്‌ Satellite clock errors cause additional errors to the GPS measurements.
ഹ These errors are common to all users observing the same satellite and can be removed through
differencing between the receivers.
ഹ്‌ Applying the satellite clock correction in the navigation message can also correct the satellite
clock errors.
GPS receivers use inexpensive crystal clocks, which are much less accurate than the satellite
clocks. As such, the receiver clock error is much larger than that of the GPS satellite clock. It can,
however, be removed through differencing between the satellites or it can be treated as an
additional unknown parameter in the estimation process. Precise external clocks (usually cesium
or rubidium) are used in some applications instead of the internal receiver clock.

4. Multipath Error

YA signal that bounces of a smooth object and hits the receiver antenna.

ഹ്‌ Increases the length of time for a signal to reach the receiver.

vA big position error results. ٠ Gravel roads * Open water * Snow fields ٠ Rock walls °
Buidlings

ഹ്‌ multipath error occurs when the GPS signal arrives at the receiver antenna through
different paths.

ഫ്‌ These paths can be the direct line of sight signal and reflected signals from objects
surrounding the receiver antenna.

ഹ്‌ Multipath distorts the original signal through interference with the reflected signals at the
GPS antenna.

ഹ്‌ Multipath-mitigation techniques, multipath error is reduced to several meters, even in a
highly reflective environment

ഹ്‌ There are several options to reduce the effect of multipath. The straightforward option is
to select an observation site with no reflecting objects in the vicinity of the receiver
antenna.

ഹ്‌ Another option to reduce the effect of multipath is to use a chock ring antenna (a chock

ring device is a ground plane that has several concentric metal hoops, which reduce the
reflected signals).

5. Antenna-phase-center Variation

ഹ്‌ GPS antenna receives the incoming satellite signal and then converts its energy into
an electric current, which can be handled by the GPS receiver.

ഹ്‌ The point at which the GPS signal is received is called the antenna phase center.

ഹ്‌ Generally, the antenna phase center does not coincide with the physical (geometrical)
center of the antenna.

ഹ്‌ It varies depending on the elevation and the azimuth of the GPS satellite as well as the
intensity of the observed signal.

ഹ്‌ As aresult, additional range error can be expected.