Geoscience Reference
In-Depth Information
Satellite 3
(9000 km)
Satellite 2
(8000 km)
Satellite 1
(7000 km)
Satellite 4
(check)
GPS receiver
in car
Figure 2.30 GPS triangulation.
By determining the distance from three different satellites, the GPS receiver nar-
rows its location to the region where three different spheres intersect. The distance to a fourth satellite can be deter-
mined as a check. Locations can be determined to within one meter with expensive receivers.
So how does this system work (Figure 2.30)? Imagine
your car or cell phone contains a GPS receiver and receives
a signal from a single satellite indicating that it is 7000 km
(4350 mi) away from your location. This lone signal indicates
that you are anywhere on the surface of a sphere that is 7000
km (4350 mi) from the satellite. If the signal from a second
satellite is added, for example, one that is 8000 km (4971 mi)
away from the receiver, your location can be narrowed to the
region contained within the space where both spheres inter-
sect. Unfortunately, the size of this overlapping region is still
considerable and might tell you that your location, for ex-
ample, is someplace within the eastern United States. How-
ever, by adding the distance to a third satellite, say, one that is
9000 km (5592 mi) away, the location is further narrowed to
a very small area where all three spheres intersect. To further
increase the confidence of the measured location, the distance
to a fourth satellite can be measured as a check. In this way, the
system is more immune to fluctuations in the speed of the PRC
signal as it travels through the atmosphere and inconsistencies
that may exist between the time indicated by the incredibly
accurate atomic clocks on satellites and less reliable clocks in
receivers.
Through this process of triangulation, it is possible
to pinpoint geographic location within about 20 m (about
90 ft) with inexpensive (less than $100) systems and to less
than 1 m (3.3 ft) with more expensive systems (more than
$10,000). This degree of accuracy is especially useful in
the military because missiles and smart bombs can be pre-
cisely delivered, as you may have seen in television footage
from recent conflicts. Within the geographic community,
high-resolution GPS has been used in a wide variety of cir-
cumstances, including coastal navigation, determining the
precise location of study sites, and the monitoring of plate
movement along faults in earthquake zones (Figure 2.31).
It is now used at an everyday level by millions of ordinary
people to find their way around as they travel. You probably
have a GPS yourself.
Although GPS is an exciting tool that has great potential for
geographic analysis, it has several potential sources of error with
which you should be familiar. One potential source of error is
that the altitude of individual satellite orbits may vary from time
to time. This variation is a problem because the receiver bases
its distance calculations on a predetermined satellite altitude.
Another source of error is the effect of the atmosphere on the
satellite signal as it beams toward Earth. This effect is especially
a problem when dust, water vapor, and ionized particles in the at-
mosphere interfere with the speed of the GPS signal, which leads
to errors in distance calculations. A final transmission problem is
