When connected to a Lynx, the GPS accuracy value provided by the Lynx and shown within ForeFlight Mobile is based upon the Navigation Accuracy Category for Position (NACp) standard. The NACp value is determined by the GPS accuracy of the aircraft’s horizontal GPS position information.
The NACp value specifies, with 95% probability, that the reported position information is correct. A minimum NACp value of "8" must be transmitted in order to operate in airspace defined in 14 CFR 91.225. The table below explains the acceptable NACp values 8-11 and the corresponding value that is shown within ForeFlight.
|NACp Value||GPS Accuracy Limits||GPS Accuracy Value Shown in ForeFlight|
|8||Value between 92.6m and 30m||92.6m|
|9||Value between 10m and 30m||30m|
|10||Value between 3m and 10m||10m|
|11||Value less than 3 meters||3m|
Table 1. NACp values, accuracy limits, and what is shown within ForeFlight.
Seeing GPS accuracy values of either 92.6m, 30m, 10m, or 3m within ForeFlight is normal and within acceptable NACp limits.
Of note, in L3 test flights, GPS accuracy in ForeFlight Mobile (and on the GPS status screen of the NGT-9000) was displayed as 30m (NACp = 9) as well as 10m (NACp = 10).