GPS integrity drops: when aircraft stop trusting their position

ADS-B reports carry two numbers about the quality of an aircraft's position: the navigation integrity category (NIC) and the navigation accuracy category for position (NACp). Both come from the aircraft's own GPS equipment. When they fall to the bottom of their scales, the aircraft is saying it can no longer vouch for the position it broadcasts.

The site looks for an airborne aircraft that reported good values, NIC 6 or more and NACp 8 or more, for at least a minute, then dropped to 1 or less for at least three reports over 20 seconds. One aircraft's drop only adds context to another story about it. When three or more aircraft lose integrity within ten minutes and about 60 nautical miles of one another, the site writes one story about the group.

An integrity drop can come from radio interference with GPS, a satellite or receiver fault, or the equipment on one aircraft, and the data cannot tell these apart. The stories call these events integrity drops and never call them jamming or spoofing, because nothing in the broadcast identifies a cause. Positions reported during the drop can still look normal on a map, and only the integrity fields show that the aircraft stopped vouching for them.