Navigation mode selection apparatus

An aircraft navigation system supporting a plurality of navigation modes. An error estimator (l7) for each navigation mode (l0-l6) provides an estimated navmode error signal representative of an estimate of the error in the position signal generated by the associated navmode. A navigation mode selector (l9) enables the navmode having the smallest estimated navmode error signal that is consistent with procedure specified navaid criteria. The navmode computed position signal is processed through a low pass position filter (26). The estimated navmode error signal associated with the selected navmode is processed through a low pass position error filter (28) having the same time constant as the low pass position filter (26) to provide an estimated position error. The estimated position error is utilised to correct INS and DEAD RECKONING navigation as well as to set a threshold for navigation sensor data validation.