G/S vs papi or vasis
hello squak ident,
normally a papi gives correct glidepath information till 100ft AGL, where as a vasis can be followed till 200ft AGL.
when there is a small difference between electronic glide slope & papi or vasis,use common sense, have a good look at the rwy & if your aiming point is where it is supposed to be & remain steady in your windscreen & the rwy shape is ok(having taken into account the rwy geography, uphill, downhill, small, large widths etc. ...)your are on the right angle. keep in mind that differences that might exist between the electronic G/S & papi/vasis stems from the different limitations of both systems.
vasis/papis are correct somewhere from 4nm out(not a lot), daylight, haze, brume & pollution can discolor the lights, twr selected brightness has an influence, system can be inop or just not switched on, long grass can obstruct clear vision & finally the system can indeed be misaligned for a number of reasons.
electronic glide slopes are mostly certified from about 10nm, provided you are on the localizer(ok from 25nm out) & on the right altitude/height AGL, about 3000ftAGL/9-10nm out on a 3° glide path:3x3=9.
if the distance is limited, it should be mentioned on your approach chart, eg. catania,sicily, where glide slope is ok from about 5nm out if my memory is correct.
finally, i would say that if the difference is small, you can use either. if the difference is significant & in your view unacceptable, report it to the twr & use the one mark eyeball to determine which system is correct & make a safe landing.
hope this will be of any help.