Satellite Constellation - Fundamentals of Global Positioning System Receivers: A Software Approach - page 31

3.2 CONTROL SEGMENT OF THE GPS SYSTEM ( 1-3 )

This section will provide a very brief idea of the GPS system. The GPS system
may be considered as comprising three segments: the control segment, the space
segment, and the user segment. The space segment contains all the satellites,
which will be discussed in Chapters 3, 4, and 5. The user segment can be con-
sidered the base of receivers and their processing, which is the focus of this text.
The control segment will be discussed in this section.
The control segment consists of five control stations, including a master control
station. These control stations are widely separated in longitude around the earth.
The master control station is located at Falcon Air Force Base, Colorado Springs,
CO. Operations are maintained at all times year round. The main purpose of the
control stations is to monitor the performance of the GPS satellites. The data
collected from the satellites by the control stations will be sent to the master
control station for processing. The master control station is responsible for all
aspects of constellation control and command. A few of the operation objectives
are presented here: (1) Monitor GPS performance in support of all performance
standards. (2) Generate and upload the navigation data to the satellites to sustain
performance standards. (3) Promptly detect and respond to satellite failure to
minimize the impact. Detailed information on the control segment can be found
in reference 3.

3.3 SATELLITE CONSTELLATION ( 3-9 )

There are a total of 24 GPS satellites divided into six orbits and each orbit has four
satellites. Each orbit makes a 55-degree angle with the equator, which is referred
to as the inclination angle. The orbits are separated by 60 degrees to cover the
complete 360 degrees. The radius of the satellite orbit is 26,560 km and it rotates
around the earth twice in a sidereal day. Table 3.1 lists all these parameters.
The central body of the Block IIR satellite is a cube of approximately 6 ft on
each side. ( 8 ) The span of the solar panel is about 30 ft. The lift-off weight of the
spacecraft is 4,480 pounds and the on-orbit weight is 2,370 pounds.
The four satellites in an orbit are not equally spaced. Two satellites are sepa-
rated by 30.0 - 32.1 degrees. The other two make three angles with the first two

TABLE 3.1 Characteristics of GPS Satellites

Constellation

Number of satellites 24

Number of orbital planes 6

Number of satellites per orbit 4

55 ◦ Orbital inclination

Orbital radius ( 7 )

26560 km

Period ( 4 ) 11 hrs 57 min 57.26 sec

Ground track repeat sidereal day