Thanks for that, PEI, however I am not sure the PAPI would do what I want. As I understand your description the PAPI would throw a beam divided into three zones - I will call them upper, middle and lower. I am assuming that the zones would be parallel, so that across my site I would have a beam of (say) 2 inches depth of the middle "slice" and (say) 4 inches depth of the upper and lower slices. If that is the case, then I can certainly see that it could be set up to be very accurate, but I am assuming that the beam would only be a certain width (so that, if circular, it would be the diameter of whatever size lens was used). Similar would apply with a laser, whereas with a VASI-type device it could be made so that the central light band was visible over an area of, say 90 degrees horizontal (so it could stand at the corner of the site and be seen anywhere), and the upper and lower bands were visible at quite wide vertical angles, say from the horizantal to 30 degrees up or down. What I am having difficulty in visualising is how these bands could be created. I have been assuming that there are some sort of baffles employed so that the lights can only be seen in certain directions, maybe like what are now fitted to some traffic lights to prevent drivers from seeing signals not intended for them.
Thanks again,
UFO