Figure 1 4.  Monitor Station and Ground Antenna

The PCS primarily operates under control of the MCS to support prelaunch compatibility testing

of GPS satellites via a cable interface.  The PCS also includes an RF transmit/receive capability

that can serve as a Control Segment ground antenna, if necessary.  The U.S. Air Force Satellite

Control Network (AFSCN) consists of a multipurpose worldwide network of ground  and space 

based satellite control facilities.  Various AFSCN resources are available to support GPS but are

not dedicated exclusively to GPS.

1.2.3  User Segment

The User Segment consists of receivers specifically designed to receive, decode, and process the

GPS satellite signals.  Receivers can be stand alone, integrated with or embedded into other

systems.   GPS receivers can vary significantly in design and function, depending on their

application for navigation, accurate positioning, time transfer, surveying and attitude reference. 

Chapter 2 provides a general description of GPS receiver types and intended applications.

1.3  GPS SERVICES

Two levels of service are provided by the GPS, the Precise Positioning Service (PPS) and the

Standard Positioning Service (SPS). 

1.3.1  Precise Positioning Service

The PPS is an accurate positioning velocity and timing service which is available only to

authorized users.  The PPS is primarily intended for military purposes.  Authorization to use the

PPS is determined by the U.S. Department of Defense (DoD), based on internal U.S. defense

requirements or international defense commitments.  Authorized users of the PPS include U.S.

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