ranging codes and navigation data messages.  The Space, Control, and User Segments are

described in more detail in paragraph 1.2.

The ranging codes broadcast by the satellites enable a GPS receiver to measure the transit time of

the signals and thereby determine the range between each satellite and the receiver.  The

navigation data message enables a receiver to calculate the position of each satellite at the time

the signals were transmitted.  The receiver then uses this information to determine its own

position, performing calculations similar to those performed by other distance measuring

navigation equipment.  Conceptually, each range measurement defines a sphere centered on a

satellite.  The common intersection point of the spheres on or near the earth s surface defines the

receiver position.

For GPS positioning, a minimum of four satellites are normally required to be simultaneously "in

view" of the receiver, thus providing four range measurements.  This enables the receiver to

calculate the three unknown parameters representing its (3 D) position, as well as a fourth

parameter representing the user clock error.  Treating the user clock error as an unknown enables

most receivers to be built with an inexpensive crystal oscillator rather than an expensive precision

oscillator or atomic clock.  Precise time estimates are required for precise positioning, since a time

error of 3 nanoseconds is roughly equivalent to a range error of 1 metre.  Less than four satellites

can be used by a receiver if time or altitude is precisely known or if these parameters are available

from an external source.  A more detailed explanation of the GPS theory of operation is provided

in paragraph 1.4.

1.2  SYSTEM OVERVIEW

1.2.1  Space Segment

The GPS Space Segment consists of 24 Navstar satellites in semi synchronous (approximately 12 

hour) orbits.  The satellites are arranged in six orbital planes with four satellites in each plane. 

The orbital planes have an inclination angle of 55 degrees relative to the earth s equator.  The

satellites have an average orbit altitude of 20200 kilometres (10900 nautical miles) above the

surface of the earth.  Figure 1 2 illustrates the GPS satellite constellation.

The satellites complete one orbit in approximately 11 hours and 58 minutes.  Since the earth is

rotating under the satellites, the satellites trace a track over the earths surface which repeats every

23 hours and 56 minutes.  A user at a fixed location on the ground will observe the same satellite

each day passing through the same track in the sky, but the satellite will rise and set four minutes

earlier each day, due to the 4 minute difference between the rotational period of the earth and two

orbital periods of a satellite.  The satellites are positioned in the orbital planes so that four or more

satellites, with a good geometric relationship for positioning, will normally be observable at every

location on earth.  The effect of geometric relationships on GPS positioning accuracy is explained

in further detail in Chapter 3

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