increased sensitivity of the transmitted signal. More significantly however it means the safety of the aircraft is still dependent on the ILS during rollout. Furthermore, as it taxis off the runway and down any parallel taxiway, it itself acts a reflector and can interfere with the localizer signal. This means that it can affect the safety of any following aircraft still using the localizer. As a result, such aircraft cannot be allowed to rely on that signal until the first aircraft is well clear of the runway and the "Cat. 3 protected area". The result is that when these low visibility operations are taking place, operations on the ground affect operations in the air much more than in good visibility, when pilots can see what is happening. At very busy airports, this results in restrictions in movement which can in turn severely impact the airport's capacity. In short, very low visibility operations such as autoland can only be conducted when aircraft, crews, ground equipment and air and ground traffic control all comply with more stringent requirements than normal. The first "commercial development" automatic landings (as opposed to pure experimentation) were achieved through realizing that the vertical and lateral paths had different rules. Although the localizer signal would be present throughout the landing, the glide slope had to be disregarded before touchdown in any event. It was recognized that if the aircraft had arrived at decision height (200 ft) on a correct, stable approach path – a prerequisite for a safe landing – it would have momentum along that path. Consequently, the autoland system could discard the glideslope information when it became unreliable (i.e. at 200 ft), and use of pitch information derived from the last several seconds of flight would ensure to the required degree of reliability that the descent rate (and hence adherence to the correct profile) would remain constant. This "ballistic" phase would end at the height when it became necessary to increase pitch and reduce power to enter the landing flare. The pitch change occurs over the runway in the 1000 horizontal feet between the threshold and the glide slope antenna, and so can be accurately triggered by radio altimeter. Autoland was first developed in BLEU and RAF aircraft such as the English Electric Canberra, Vickers Varsity and Avro Vulcan, and later for BEA's Trident fleet, which entered service in the early 1960s. The Trident was a