Non-augmented GNSS simply use the satellites in sight and positioning. Timing errors or the loss of a satellite signal are not notified to the flight deck nor is the error in the position a solution.

Thus, it is important before flight to check the availability of the relevant satellites and any potential outages for an accurate position at all stages of the flight, but especially for the destination airfield and alternate at the estimated times of arrival.

This GNSS achievability can be verified by checking NOTAMs and the Notice Advisory to Navstar Users (NANU) messages, which are issued jointly by the United States Coast Guard and the GPS Operations Centre in Colorado and provides updates on the GPS constellation.

NANUs are issued 72 hours (3 days) before any change of a GPS satellite.