A
GPS receiver calculates its position by precisely timing the signals sent by
GPS satellites high above the Earth. Each
satellite continually transmits messages that include:
- the time the message was transmitted and,
- satellite position at time of message transmission.
The
receiver uses the messages it receives to determine the transit time of each
message and computes the distance to each satellite using the speed of light.
Each of these distances and satellites' locations defines a sphere. The
receiver is on the surface of each of these spheres when the distances and the
satellites' locations are correct. These distances and satellites' locations
are used to compute the location of the receiver using the navigation equations.
This location is then displayed, perhaps with a moving
map display or latitude
and longitude.
Basic GPS measurements yield only a position,
and neither speed nor direction. However, most GPS units can automatically
derive velocity and direction of movement from two or more position
measurements. The disadvantage of this principle is that changes in speed or
direction can only be computed with a delay, and that derived direction becomes
inaccurate when the distance traveled between two position measurements drops
below or near the random error of
position measurement. GPS units can use measurements of the doppler
shift of the signals received to compute velocity
accurately.
Although four satellites are
required for normal operation, fewer apply in special cases. If one variable is
already known, a receiver can determine its position using only three
satellites. For example, a ship or aircraft may have known elevation. Some GPS
receivers may use additional clues or assumptions such as reusing the last
known altitude,
dead reckoning, inertial
navigation, or including information from the
vehicle computer, to give a (possibly degraded) position when fewer than four
satellites are visible.
