Page 56 - Electronic Navigation Cyber Book

navigation systems use additional sensors like a compass or an inertial
navigation system to complement GPS.
In typical GPS operation, four or more satellites must be visible to obtain
an accurate result. Four sphere surfaces typically do not
[a]
intersect. Because of this, it can be said with confidence that when the
navigation equations are solved to find an intersection, this solution gives
the position of the receiver along with the difference between the time
kept by the receiver's on-board clock and the true time-of-day, thereby
eliminating the need for a very large, expensive, and power hungry clock.
The very accurately computed time is used only for display or not at all in
many GPS applications, which use only the location. A number of
applications for GPS do make use of this cheap and highly accurate
timing. These include time transfer, traffic signal timing,
and synchronization of cell phone base stations.

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

Structure
The current GPS consists of three major segments. These are the space
segment (SS), a control segment (CS), and a user segment (US). The
U.S. Air Force develops, maintains, and operates the space and control
segments. GPS satellites broadcast signals from space, and each GPS
receiver uses these signals to calculate its three-dimensional location
(latitude, longitude, and altitude) and the current time. [57]

The space segment is composed of 24 to 32 satellites in medium Earth
orbit and also includes the payload adapters to the boosters required to
launch them into orbit. The control segment is composed of a master
control station, an alternate master control station, and a host of
dedicated and shared ground antennas and monitor stations. The user
segment is composed of hundreds of thousands of U.S. and allied military
users of the secure GPS Precise Positioning Service, and tens of millions
of civil, commercial, and scientific users of the Standard Positioning
Service (see GPS navigation devices).

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