Leica Geosystems GPS Basics - Manual (Page 16)

16
GPS Basics -1.0.0en How GPS works
4 2. Satellite and Receiver clock errors
Even though the clocks in the satellite
are very accurate (to about 3 nanosec-
onds), they do sometimes drift slightly
and cause small errors, affecting the
accuracy of the position. The US Depart-
ment of Defense monitors the satellite
clocks using the Control Segment (see
section 2.2) and can correct any drift that
is found.
3. Multipath Errors
Multipath occurs when the receiver
antenna is positioned close to a
large reflecting surface such as a
lake or building. The satellite signal
does not travel directly to the antenna
but hits the nearby object first and is
reflected into the antenna creating a
false measurement.
Multipath can be reduced by use of
special GPS antennas that incorpo-
rate a ground plane (a circular,
metallic disk about 50cm (2 feet) in
diameter) that prevent low elevation
signals reaching the antenna.
For highest accuracy, the preferred solution is
use of a choke ring antenna. A choke ring
antenna has 4 or 5 concentric rings around
the antenna that trap any indirect signals.
Multipath only affects high accuracy, survey-
type measurements. Simple handheld
navigation receivers do not employ such
techniques.
Choke-Ring Antenna