“A very detailed answer indeed,” but not necessarily correct ??
Re “Most runways have an aimpoint marking associated with the 1,000 ft point (US FAA) or 1,312 ft/400m ICAO, regardless of whether it is an IFR or VFR runway. It seems this marker is not just an aimpoint, but also an excellent reminder of how far the jet has progressed down the runway.”
I understanding that the ‘aim point’ discussed is more likely a fixed distance marking from the threshold, which normally coincides with (or is close to) the glide-slope origin. The markings are coded to define the distance from the threshold and are often used as ‘an acceptable landing area’, but this is aircraft dependant and is rarely mandated except in theory for Cat 2/3 operations.
The old style VASI was not a projection system – the red / white lamps interfered and gave an ambiguous pink value at low altitude; however if we assume that the definition used is an abbreviation for the PAPI (VASI), then PAPI is a very accurate projected system. Then as stated, the PAPI visual glide path is harmonized with the electronic glideslope to main the correlation between pilot’s eye position and glidepath aerial – the electronic glidepath.
Again as I understand a two PAPI system (long body aircraft option), the visual glide-slopes are identical and at the same angle as the electronic glidepath (parallel beams); the second set of PAPI lights are positioning further down the runway to enable a long body aircraft to follow a similar flight path to that of standard aircraft although the pilots eye position is much higher. This enables the aircraft to track the same electronic glideslope whilst giving the pilot a ‘normal’ visual cue; it also maintains the wheel height over threshold as explained (plus/minus all the calculations as given).
Re "Visual Aim Point”; I know of very few runway markings or indeed 'techniques' to ensure main landing gear touchdown at the desired point on the runway. Some limiting runways have a yellow landing box marked on the runway (Faeroe Islands?) where the aircraft must either touch down or fly a baulked landing. London City has a non standard fixed distance marking that has been recommended as the last point of touchdown for jet aircraft, but this is not directly associated with the steep approach.
I do not recall that a steep approach specifically requires different runway markings. At LCY the geometry change in the visual glidepath (PAPI) and the ILS is countered by repositioning the PAPI and ILS closer to the threshold whilst maintaining their correlation. The other geometric change accounted for is the 35 ft threshold crossing height, but again all glide-paths are aligned; as yet, there are no complexities of long body aircraft at LCY!