The final metric for the WAAS system is availability which emphasizesthe operational economy of the navigation system. It is computedas the fraction of time the WAAS system is providing positionfixes to the specified level of accuracy, integrity and continuity. The Minimum Operational Performance Standards (MOPS)for WAAS specify the computation of the Vertical Protection Level(VPL) and Horizontal Protection Level(HPL) of the differentially correctednavigation solution which must be met at a probability of 99.99999%. Thus the true error must not exceed the protection level more thanonce in 107 seconds. If the computed protection level exceeds thecorresponding alert limit then the alarm is dropped and theoperation cannot proceed. If the operation has already begun thiscondition is a continuity breach and a missed approach must beconducted. Otherwise the system is declared unavailable for thatepoch.
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