hi
I just read this somewhere recently...the reason vasi was abandoned and papi was started.
seems to me that 50' was when papi gives out for practical use
try looking up the US Aeronautical Information Manual online and then looking up airport lighting.
you could also look up PAPI in google
I offer this only from memory, as someone who flys should actually take the time to look it up himself.
some papis have limits placed upon them for specific airports due to terrain near by. I"ve seen one as four miles.
good luck, and learn to find your spot
oh, ok, read about it here and why VASI was done away with
Precision approach path indicator - Wikipedia, the free encyclopedia