such study gives the long term probabilities for the number of GPS satellites

operational any given time.

In most cases, a satellite vacancy from the full constellation of 24 satellites will result in

reduced service coverage.  For convenience, the lack of a four satellite positioning

solution or a condition where PDOP > 6 will be termed an "outage".  In general, the

number of outages, individual durations of outages, and areas affected by outages will

increase with each additional vacancy from the constellation.  As long as the U.S. can

maintain 21 or more satellites on orbit, and worst case situations can be avoided, the

service coverage is likely to remain between 99% and 100%.  Table 6 8 below gives

some representative values of service coverage for a 24 Satellite constellation with

"typical" deviations from the nominal orbit positions.  During the worst case three 

satellite failure condition, the worst location in the world may experience as low as 86%

average positioning availability over a 24 hour period, while the best location may still

experience 100% availability.

Table 6 7.  Probability of Operational Satellites

Number of

State

Cumulative

 Satellites

Probability

Probability

24

0.72

0.72

23

0.17

0.89

22

0.064

0.954

21

0.026

0.980

20

 0.0116

0.9916

19

 0.0064

0.9980

Table 6 8.  Service Coverage of a Typical 24 Satellite Constellation

Number of

Best Global

Average Global

Worst Global

Satellites

Service Coverage

Service Coverage

Service Coverage

23

100.00%

99.99%

99.97%

22

100.00%

99.93%

99.61%

21

 99.98%

99.69%

97.69%

20

 99.97%

99.05%

94.75%

As suggested above, there are several options the U.S. DoD may employ to minimize

the impact of reduced service coverage.  Suc h options include launches in anticipation

of satellite end of life failures, planning normal maintenance to minimize service

availability impact, deferring normal maintenance, and even minor rephasing of certain

satellites in the constellation.  In this respect, the standards quoted above for service

availability under "normal" operating conditions have some flexibility to compensate for

reduced service coverage and still maintain a high composite availability of a position

solution.

6 12