Alignment and testing of instrument landing systems

The standard method of aligning instrument landing systems is a visual one using a theodolite to observe an aircraft flying on the localizer and glide-path beams. This method is not altogether satisfactory because it is too much dependent upon good weather and results in a great deal of flying. An improved method has been developed which is based upon aerial photography. Since photographs are taken at about 1000ft above ground level, the weather requirement is reduced to conditions in which the cloud base exceeds this figure. Moreover, the flying time is only about one-fifth of that required by the visual method. The errors inherent in the photographic method of glide-path alignment are numerically the same as those for the visual method, but for localizer alignment an error-reducing process can be applied which reduces the region of uncertainty to about half that of the visual method. Special ground equipment has been developed for measuring essential parameters of the beams before flying begins. This reduces the probability that the equipment will fail to meet its alignment specification during a flight check.