Alarm filtering in a ground proximity warning system

Claim 1

A ground proximity warning system for determining the proximity of an aircraft with respect to the ground utilizing flight positional input parameters, comprising: means for calculating the position of the aircraft with respect to a plurality of specified flight warning mode envelopes; means for providing information on the aircraft's actual position; means for comparing said calculated position with said information on the aircraft's actual position during successive sampling periods, and producing predetermined alarm condition signals if the aircraft has penetrated a respective one of said flight warning mode envelopes constituting an alarm condition; and filter means connected to said comparing means for filtering said alarm condition signals and for generating a plurality of alarm generating signals depending upon the alarm condition signal during the continuing flight of the aircraft in comparison with predetermined maximum and minimum alarm condition signal levels; including means for incrementing each flight warning mode alarm condition signal by a constant during each sampling period if said alarm condition is present; means for generating a predetermined one of said alarm signals when the predetermined maximum level of said alarm condition signal is reached; means for decrementing said alarm condition signal by a constant when its magnitude is between said maximum and minimum levels and the corresponding alarm condition ceases to exist; and means for resetting said alarm condition signal to zero instantaneously when its magnitude is below said minimum level and said alarm condition ceases to exist.