Table 6 2.  Service Availability Standards

Service Availability

Standard

Conditions and Constraints

 99.85% global average

  Conditioned on coverage standard

  Standard based on a typical 24 hour interval, averaged over the

globe

  Typical 24 hour interval defined using averaging period of 30

days

 99.16% single point

  Conditioned on coverage standard

average

  Standard based on a typical 24 hour interval, for the worst case

point on the globe

  Typical 24 hour interval defined using averaging period of 30

days

 95.87% global average

  Conditioned on coverage standard

on worst case day

  Standard represents a worst case 24 hour interval, averaged over

the globe

 83.92% at worst case

  Conditioned on coverage standard

point on worst case day

  Standard based on a worst case 24 hour interval, for the worst case

point on the globe

6.1.3.3  Expected Service Availability Characteristics

Table 6 3 defines what service availability characteristics will be like for a given

satellite outage condition.  Service availability projections over time may be generated

by applying the information in Table 6 3 to expected satellite control operations

scenarios.  A satellite control operations scenario is based upon a conservative

estimate of satellite maintenance activity frequency and duration.  Satellite

maintenance actions requiring service downtime include periodic cesium frequency

standard maintenance, station keeping maneuvers to maintain orbits within tolerances,

and responses to component failures.  Given current routine maintenance requirements

and component failure expectations, generally three, and no more than four satellites

should be removed from service over any 30 day period.  Once a satellite is removed

from service, it is assumed that it will be down for no more than 24 hours.

The first service availability scenario to be defined represents a worst case 30 day

period. A summary of this scenario is provided in Table 6 4.  The scenario is

considered to be worst case from two perspectives:  it includes a day with three

satellites removed from service, and it includes a total of four satellite down days.  The

three satellite down scenario is based upon the simultaneous removal of two satellites

for routine maintenance, accompanied with a component failure on a third satellite.

Worst case global service availability  on a day with three satellites removed from

service is 95.87%; the associated worst case regional service availability is 83.92%.

The resulting 30 day service availability values  range from 99.85% to 99.99%,

depending on which satellites make up the four which

6 6