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Chapter 2 - NAVSTAR GPS Overview

ABOUT THE GPS NAVIGATION MESSAGE

The NAVigation Satellite Timing and Ranging (NAVSTAR) Global Positioning System is an all

weather, radio based, satellite navigation system that enables users to accurately determine 3-

dimensional position, velocity, and time worldwide. The overall system consists of three major

segments: the space segment, the ground control segment, and the user segment.

Space Segment

The space segment is a constellation of satellites operating in 12-hour orbits at an altitude of

20,183 km (10,898 nm). The constellation is composed of 24 satellites in six orbital planes, each

plane equally spaced about the equator and inclined at 55 degrees.

Ground Control Segment

The ground control segment consists of a master control center and a number of widely

separated monitoring stations. The ground control network tracks the satellites, precisely

determines their orbits, and periodically uploads almanac, ephemeris, and other system data to

all satellites for retransmission to the user segment.

User Segment

The user segment is the collection of all GPS user receivers (such as your Motorola Oncore GPS

Receiver) and their support equipment. The receiver determines position by a process known as

passive multi-lateration. More simply, the GPS receiver's position is determined by the geometric

intersection of several simultaneously observed ranges (satellite to receiver distances) from

satellites with known coordinates in space.

The receiver measures the transmission time required for a satellite signal to reach the receiver.

Transit time is determined using code correlation techniques. The actual measurement is a

unique time shift for which the code sequence transmitted by the satellite correlates with an

identical code generated in the tracking receiver. The receiver code is shifted until maximum

correlation between the two codes is achieved. This time shift multiplied by the speed of light is

the receiver's measure of the range to the satellite. This measurement includes various

propagation delays, as well as satellite and receiver clock errors. Since the measurement is not a

true geometric range, it is known as a pseudo-range. The receiver processes these pseudo-range

measurements along with the received ephemeris data (satellite orbit data) to determine the

user's three-dimensional position. A minimum of four pseudo-range observations are required to

mathematically solve for four unknown receiver parameters (i.e., latitude, longitude, altitude, and

clock offset). If one of these parameters is known (altitude, for example) then only three satellite

pseudo-range observations are required, and thus only three satellites need to be tracked.

Motorola GPS Products - M12+ User's Guide Revision 6.X 09FEB05

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