Just a point of interest, which I hadn't realised until a while ago:
PAPIs go quite far out of true, quite quickly. This is hardly surprising considering that they are precision optical devices placed very close to a bit of tarmac which people keep throwing heavy bits of metal onto at high speed.
PAPIs are regularly re-aligned by a simple process using a sighting device a short distance in front of the installation.
This is one reason why the GS is sometimes not completely co-incident with the PAPIs, and a mention of this to the tower will enable them to send the man out with the kit to put the situation right.