Originally Posted by
Tomaski
ILS technology goes back to the 1930's. I'm not sure when the 200' basic minimum was adopted, but it's been at least 40 years, and I presume it is a function of the position tolerances of the equipment (ground and aircraft). Assuming there are no issues with terrain, obstacles or antenna interference, lower minimums are possible with the additional equipment and crew training, but all of those cost money to provide. Airports and operators have to decide if the need for the lower minimums justify the investment.
Before I got my instrument rating, which was 1958. You may recall it was 200 feet only with a middle marker. If the middle marker was inop or not received, it was then 250. Visibility without ALS was 1 mile, which later became 3/4 mile. With ALS it became 1/2 mile or RVR 2,600, which later became 1/2 mile or RVR 2,400, and RVR 1,800, with TDZ and CL lighting.