Precision Approach Path Indicator - Used in avionics to guide planes to the runway using the approach path (Glide Path).
A series of 3 or 4 lights that change colour from
white to red in accordance to the height of the aircraft.
Red over Red - Too low
Red over
White - On the Glide path
White over White - Too
highThe PAPI lights can be seen up to 5 miles in daylight and upto 20 miles at night.