Precision Approach Path Indicator

The Precision Approach Path Indicator (PAPI) is a visual system next to the touch-point on a runway. The purpose of the system is to lead a plane in correct approach. The PAPI creates ray of light which gives pilot information about his actual height. 
Principle:
The rules about the parameters are written in standard ANNEX 14 ICAO. The brochure contains more specific information about intensity of ray, location where we can get device etc.