Apparatus and method for detecting a digital change in data

In a DME system having a transceiver in an aircraft communicating with a ground station, a method of evaluating passage of the aircraft through predetermined distance intervals comprising the steps of:

• a. storing an encoded representation of the present distance between the aircraft and the ground station, increments in the encoded representation corresponding to said intervals;
• b. storing a representation of prior aircraft to ground station distance, said prior representation being incremented by a first control signal in a sense established by a second control signal;
• c. developing an encoded difference between said present and said prior distances;
• d. developing said second control signal in response to the sense of said encoded difference;
• and
• e. generating an output pulse comprising said first control signal in response to an inequality between said present and said prior distance, each output pulse representing one of said intervals.

In a system utilizing BCD encoding, apparatus for detecting change in a stored reference quantity comprising:

• a. means for storing an encoded representation of the least significant decade of said reference quantity;
• b. counting means for storing a prior representation of said stored least significant decade representation, said counting means being incremented by a first control signal in a sense established by a second control signal;
• c. means for developing an encoded difference between said present and said prior stored decade representations;
• d. means, responsive to the sense of said difference, for developing said second control signal;
• and
• e. means, responsive to an inequality between said present and said prior stored decade representations, for generating an output pulse comprising said first control signal, each output pulse representing a single increment of said reference quantity.