TCAS
BLUE empty diamond (image1) with +07 indicates that an aircraft is flying 700 feet above is descending towards us. When the proximity increases, the blue diamond will fill in (image 2; 400ft above) and gives an aural warning “traffic”. As it gets closer, the blue diamond changes into YELLOW circle (image 3; 300ft above). Once the aircraft approaches the danger zone, the yellow circle changes into RED square (image 4; 200ft above) and resolution advisory warning “descend” is given to the pilot. Then the pilot descends the aircraft and avoids collision.