it will be 2000 feet out. If a small course correction needs 10 seconds to be effected at 180 kts it will take 3000 ft. It will be possible if initiated at 300 feet of height, but not at 100 feet. Consequently, only a smaller course correction can be tolerated at the lower height, and the system needs to be more accurate. This imposes a requirement for the ground-based, guidance element to conform to specific standards, as well as the airborne elements. Thus, while an aircraft may be equipped with an autoland system, it will be totally unusable without the appropriate ground environment. Similarly, it requires a crew trained in all aspects of the operation to recognize potential failures in both airborne and ground equipment, and to react appropriately, to be able to use the system in the circumstances from which it is intended. Consequently, the low visibility operations categories (Cat I, Cat II and Cat III) apply to all 3 elements in the landing – the aircraft equipment, the ground environment, and the crew. The result of all this is to create a spectrum of low visibility equipment, in which an aircraft's autoland autopilot is just one component. The development of these systems proceeded by recognizing that although the ILS would be the source of the guidance, the ILS itself contains lateral and vertical elements that have rather different characteristics. In particular, the vertical element (glideslope) originates from the projected touchdown point of the approach, i.e. typically 1000 ft from the beginning of the runway, while the lateral element (localizer) originates from beyond the far end. The transmitted glideslope therefore becomes irrelevant soon after the aircraft has reached the runway threshold, and in fact the aircraft has of course to enter its landing mode and reduce its vertical velocity quite a long time before it passes the glideslope transmitter. The inaccuracies in the basic ILS could be seen in that it was suitable for use down to 200 ft. only (Cat I), and similarly no autopilot was suitable for or approved for use below this height. The lateral guidance from the ILS localizer would however be usable right to the end of the landing roll, and hence is used to feed the rudder channel of the autopilot after touchdown. As aircraft approached the transmitter its speed is obviously reducing and rudder effectiveness diminishes, compensating to some extent for the