Precision Approach Path Indicator (PAPI) was invented in England .. the name Tony Smith is often associated with this invention .. which is probably true ..
PAPI was adopted by the International Civil Aviation Organization (ICAO) and was to replace the older Visual Approach Slope Indicator (VASI) by 1995 ... this was based on the premise that the life of VASI was 15 years, so that when it came time to replace VASI, it could be done with PAPI ... however, the reality was that some countries did not have funding arranged to do this, so VASI continues at many sites ...
PAPI is the same as VASI in the sense of the light units ... all that has been done is to relocate the units to the "theoretical touchdown point" and to vertically align the four units A, B, C, and D separately at about 20 minute increments of A=2'30", B=2'50", C=3'10" and D=3'30" ... the units B and C define the "approach corridor" with which the approach path (slope) is (B+C)/2=3'00" ... pilots fly the "approach corridor" (two whites and two reds) and not the approach path since the path itself is not seen ...
the PAPI signal is not coincident with the ILS path ... it can only be made to be approximately close to the ILS path through increasing the width of the approach corridor which is indicated by the color signals ... the PAPI is a visual aid
information on PAPI is available in the ICAO Annex 14, FAA advisory circulars and Transport Canada manual TP312 ... as well as in the pilot flight manuals ...