In message , Bruce
wrote:
It hasn't changed, except that the VASIs (visual approach slope
indicators) on both runways have been upgraded to PAPIs (precision
approach path indicators). Both are purely visual aids working on
similar optical principles using polarised light, and are sensitive to
cloud conditions.

They aren't polarized; they simply involve lights shining above or below
an aligned plate.

VASIs give an up/down indication of whether the
aircraft is on the correct glide path. PAPIs give up/down and also
left/right indications.

I don't believe PAPIs give sideways indications; they just give a better
indication of the actual slope.

As I understand it, VASIs consist of two sets of lights which show red
below the glide path angle and white above it. So if you're on the
correct path, you see red over white. PAPIs, on the other hand, consist
of one set of four lights but each light is set at a different angle. So
the number of white lights indicates what angle you're at - the correct
angle is shown by two reds and two whites.

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