The FAA released a policy statement (PDF) last week assuring operators that “the FAA will not consider aircraft non-compliant with the ADS-B Out rule during periods of GPS performance degradation that are outside the operator’s control. The circumstances are identified in the policy and are valid provided the operator has exercised appropriate due diligence prior to conducting an operation.”
As long as operators exercise “due diligence” in determining GPS and, therefore, ADS-B Out coverage prior to launching, the FAA says it will not dock pilots who experience unexpected GPS glitches that affect the ADS-B system from providing a position solution. Similarly, the FAA says that pilots who are given a routing other than the one filed that results in ADS-B “failure” will not be violated.
The bulk of the FAA’s concern has to do with so-called SA-On or SA-Aware GPS units providing a position solution to the ADS-B network. The FAA notes that WAAS-enabled, complying GPS position sources, such as an IFR-approved GPS, are equivalent to radar in terms of position accuracy.