Air traffic control

Claim 1

THE METHOD OF INDICATING ON A SINGLE BOARD THE DIS TANCES OF A PLURALITY OF AIRCRAFT IN AN AIR CORRIDOR FROM A CENTRAL STATION, INCLUDING THE STEPS OF TRANSMITTING TO THE AIRCRAFT IN THE CORRIDOR A SIGNAL MODULATED BY A LOW FRE QUENCY AND FURTHER MODULATED BY A SELECTIVE AIRCRAFT CALL FREQUENCY FOR ENABLING THE VOICE COMMUNICATION CHANNEL AND THE TRANSMITTER IN THE AIRCRAFT CORRESPONDING WITH THE PARTICULAR SELECTIVE CALL FREQUENCY TRANSMITTED; AUTOMATICALLY REBROADCASTING THE LOW-MODULATED FREQUENCY FROM THE SELECTED AIRCRAFT ON A CARRIER; RECEIVING THE LOWFREQUENCY MODULATED CARRIER AT THE CENTRAL STATION; DETERMINING THE PHASE-DELAY BETWEEN THE TRANSMITTED LOW FREQUENCY AND THE LOW FREQUENCY RECEIVED AT THE STATION FROM THE AIRCRAFT; AND DISPLAYING MARKINGS ON THE BOARD IN ADJACENT COLUMNS EACH REPRESENTING AN AIRCRAFT AND INDICATING THE PHASE DELAYS MEASURED AND HENCE THE DISTANCE FROM THE STATION TO THE SELECTED AIRCRAFT.