the world and at any time, can see a sufficient number of satellites to generate a position

solution.  Constraints are placed upon satellite visibility in terms of mask a ngle and

geometry, to minimize the possibility  of a GPS receiver generating a marginal position

solution. Coverage characteristics over any given region vary slightly over time, due

primarily to small shifts in satellite orbits.

Since GPS is a space based system, coverage is defined as a function of each

satellite s antenna beamwidth.  The GPS satellite antenna s nominal beamwidth is

approximately 28 degrees.  If a user on the Earth s surface were to view a satellite

which is just above the local horizon, the user could elevate from that location to an

altitude of approximately 200 kilometers above the Earth s surface before effectively

losing that satellite s signal.  This condition defines the maximum altitude associated

with the term "on or near the Earth."

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

that a sufficient number of satellites are transmitting a usable ranging signal within view

of any point on or near the earth, given that coverage is provided.

Just because a satellite is operational does not mean that it is currently transmitting a

usable GPS ranging signal.  Satellites will, on occasion, be removed temporarily from

service for routine maintenance.  As a result, the number of satellites actually

transmitting usable ranging signals will vary over time.  Service availability is the

measure of how GPS coverage deviates from nominal conditions due to the temporary

removal of satellites from service.  This measurement represents the percentage of

time that coverage is provided by those satellites which are transmit ting usable ranging

signals to generate a position solution.  Variations in service availability are a function

of which satellites are removed from service, the length of the service outage, and

where on the globe a user is located in relation to any resulting outage patterns.

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

the instantaneous predictable horizontal error is maintained within the normal accuracy

distribution at any point on or near the earth, given that coverage is provided and the

service is available.

GPS can be used anywhere in the world.  A failure in a system with such global

coverage may affect a large percentage of the globe.  A natural concern about using

GPS is whether or not it provides a satisfactory level of service reliability.  Service

reliability as it is used in a GPS context is somewhat more restrictive than the classical

definition, which includes times that the service is available as well as when it is

performing within specified tolerances.   GPS service reliability is viewed as a measure

only of how well GPS maintains horizontal errors within the normal predictable PPS

horizontal accuracy distribution.  100% service reliability is provided when the

horizontal error remains within the normal accuracy distribution within the conditions

specified for coverage and service availability. Periods where  the service does not

provide a sufficient number of satellites or adequate geometry to  support position

solution generation are assessed against the coverage service availability performance

standard.

6.1.2  Service Coverage Characteristics

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