Geography Reference
In-Depth Information
Figure 43.1 A plot of a position measurement over time
with selective availability (SA) in action.
include multipath errors, ionospheric delay and poor
satellite geometry (Gilbert 1995). These errors are
significant and often determine the accuracy of
the data collected.
Multipath error is caused by signal
propagation where the signal transmitted to the
ground receiver is bounced or reflected during
its journey from the satellite. The error usually
occurs when the GPS unit receives signals from
very low-elevation satellites (<15°) (ibid.) . This
error may be significant when operating GPS
technology in very steep mountainous terrain or
urban environments, where many surfaces are
present which can reflect the satellite signal.
Often the only way to deal with multipath signal
error is to implement minimisation methods
such as mask angles (which reduce the angle of
the field of view in which a receiver will search
for satellites) or to avoid multipath-prone sites
(ibid) .
Ionospheric delay results from atmospheric
disruption of the GPS signal. The ionosphere
disperses the GPS signal, thus delaying the arrival
of the signal. This can result in miscalculation of
distance. One of the primary causes of delay is the
amount of water vapour in the atmosphere.
The quality of positional information is also
influenced by the geometry of the satellites in the
user's field of view at the time of observation. GPS
receivers provide the user with several quality
indicators relating to the geometry of the satellites.
The most important is the geometric dilution of
precision (GDOP). Higher GDOP values indicate
poor satellite geometry, which may give rise to
poor positional information. Figure 43.2 shows
Therefore, as SA affects the C/A code and not the
carriers L1 and L2, carrier phase observations are
immune from SA.
Finally, the GPS receiver also needs to receive
information about any atmospheric effects and
satellite geometry that may result in a degraded
signal. This gives an indication of the quality and
reliability of location information received.
OPERATIONAL ISSUES IN GPS:
SIGNAL QUALITY AND RELIABILITY
Both pseudorange and carrier phase observations
are subject to a number of potential errors. These
Figure 43.2 Good and bad
satellite geometry.
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