51 ARE THERE APPROACHING AIRCRAFT?
52 CALCULATE THE DISTANCE BETWEEN AIRCRAFT
53 COMPARE D WITH THRESHOLD VALUE T
54 D<T?
55 PREPARATION OF SURVEILLANCE INSTRUCTION MARK
56 WHICH ONE OF STAGES 1, 2 AND 3 D IS APPLIED TO
57 SPECIFY DISPLAY PROPERTY OF SURVEILLANCE INSTRUCTION MARK ACCORDING TO APPLIED STAGE
58 OUTPUT TO GRAPHIC ENGINE SECTION