CHAPTER 6:  SERVICE COVERAGE, SERVICE AVAILABILITY,

AND SERVICE RELIABILITY; SATELLITE SELECTION

CRITERIA AND FIGURE OF MERIT DESCRIPTION

6.1  SERVICE COVERAGE, SERVICE AVAILABILITY, AND SERVICE RELIABILITY

This section describes the minimum performance an authorized user can expect to

obtain from PPS receiver which is designed and operated in accordance with

"Technical Characteristics of the Navstar GPS".  Performance is specified in terms of

minimum performance standards for each performance parameter.  Each standard

includes a definition of applicable conditions and constraints.  The information provided

in this section is derived and extracted from "The Global Positioning System (GPS)

SPS Performance Specification", dated November 5, 1993, published by the U.S. DoD.

Although the GPS SPS Performance Specification is directed toward SPS users of

GPS, the specified performance of the system with respect to service coverage, service

availability, and service reliability is the same for PPS users.

The data and associated statements provided in this chapter represent conservative

performance expectations, based upon extensive observations of the system.  The

performance standards are limited to GPS Control Segment and Satellite contributions

to the PPS signal in space characteristics and their effects on the position solution.

The standards do not include enhancements or degradations to this service that might

be provided by the UE or local environment.  Examples of possible enhancements

include altitude aiding, clock aiding, differential corrections, or integrity algorithms.

Examples of possible local degradations include multipath, jamming, terrain masking,

or receiver errors.

6.1.1  Parameter Definitions

The three parameters defined below are service coverage, service availability, and

service reliability. These definitions and the relationships between them are different

from traditional definitions of similar parameters.  A dependent relationship is defined to

exist between these performance parameters.  Each successive layer of performance

definitions are conditioned on the preceding layers.  That is, coverage must be

provided before the service may be considered available and it must be available

before it can support service reliability requirements.

Service coverage is defined as the percentage of time over a specified interval that a

sufficient number of satellites are above a specified mask angle and provide an

acceptable position solution geometry at any point on or near the earth.

GPS coverage is viewed somewhat differently than coverage for existing terrestrial

positioning systems.  Traditionally coverage has been viewed as the surface area or

volume in which a system may be operated.  Since a terrestrial system s beacons are

fixed, coverage does not change as a function of time.  Since the GPS concept relies

upon the dynamics of a satellite  constellation, coverage must take into consideration a

time dependency.  GPS coverage is by definition intended to be global.  GPS coverage is

viewed alternatively as the percentage of time over a time interval that a user, anywhere in

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