VASIS vs PAPI
BGS,
Check the post above your latest reply.
There is a principle difference between VASIS and PAPI and that is that the VASIS glidepath is assembled from lights at two widely separated positions, whereas PAPI light are all at the same distance from threshold.
Using VASIS, your aimpoint essentially is in between the white unit and the red unit that follows the white one. Using the "upper VASIS bars" thus moves your aimpoint further down the runway than when using the "lower VASIS bars" and so increases threshold crossing height.
With PAPI, the aimpoint is right next to the PAPI unit, so using the steeper indication will only give you a very small increase in threshold height.
ICTZ stated it correctly: take out your calculator and check.
Placing PAPI 1500 ft beyond threshold, threshold crossing height of a 3 degree glidepath is 78 ft (sine of 3 degrees times 1500 ft).
There is a 1 degree difference in slope from outer to inner PAPI light, and with four lights there are three intervals, so one third degree difference from one light to the other. The next higher slope would be 3,33 degrees, giving a TCH of 87 ft.
The distance of the PAPI from threshold will be the most important factor in threshold crossing height. The nominal glideslope of the PAPI will be determined by the terrain profile, just like an ILS glideslope: normally 3 degrees, steeper if needed because of distant obstacles in the approach path.