Surveying, GPS, Digitization - A Primer of GIS: Fundamental Geographic and Cartographic Concepts - page 144

The only GPS satellite on public display.
From http://commons.wikimedia.org/wiki/Image:Global_Positioning_System_satellite.jpg
NAVSTAR GPS.) Over 50 satellites have been launched, each about the size
of a school bus. The system costs around $400 million yearly to maintain, but
is freely available all around the world. The accuracy and availability can be
limited by the Department of Defense if they see a need through selective
availability, which degrades the signals received by commercial GPS units.
With a GPS receiver, which can be a computer chip close to the size of a
postage stamp attached to an antenna, a device can receive and process the
GPS satellite signals and determine location and elevation. How this works is
quite complex, but the general idea is rather straightforward. Instead of
using a measurement of distance, as in surveying, GPS uses the time it takes
radio signals to travel. With much simplification we can say that the GPS
receiver calculates the difference between its own clock's time and the time
communicated in signals from GPS satellites, then uses this difference to cal-
culate the distance between the receiver and the satellites. The time differ-
ence is detected in the difference between the signal sequence (a binary sig-
nal called “pseudorandom”) received by the GPS receiver and the signal
sequence it has. Each satellite broadcasts a signal that contains data about
the satellite and the time on its clock. The GPS receiver's time calculations
also should take a variety of interferences into account, especially interfer-
ence in the atmosphere of the earth that can slow down the transmission of a
signal from a satellite. If the GPS receiver is traveling, the corresponding
movement must also be taken into account.
Obviously some limits to the accuracy of the GPS measurements arise,
related to the number of satellites available. The most complex part of GPS
positioning is the determination of location. With the signal from one GPS
satellite, a GPS receiver can only determine how far it is from that satellite,
but not where. It could be anywhere on an imaginary sphere drawn at that
distance from the satellite. To determine the position of the GPS receiver,