Page 55 - Electronic Navigation Cyber Book

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 displayor latitude and longitude; elevation or altitude
information may be included, based on height above the geoid .

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 travelled between two position
measurements drops below or near the random error of position
measurement. GPS units can use measurements of thedoppler shift of
the signals received to compute velocity accurately. More advanced

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