Design considerations for achieving MLS Category III requirements

The authors examine microwave landing system (MLS) ground equipment design considerations for achieving the requirements for Category III landing operations. Specifically addressed are accuracy, continuity of service, and integrity requirements. Accuracy near the touchdown region is achieved by providing adequate transmitter power, sufficiently small antenna beamwidth, low effective sidelobes (e.g. by introducing a degree of randomness in the dynamic sidelobes and by using a directive element pattern), and, in the azimuth antenna, by shaping the vertical pattern. High continuity of service is achieved by using a high-reliability design, through redundancy of critical elements, by providing an uninterruptible power supply, and by incorporating prealarm warnings. High integrity is achieved by providing an appropriate configuration of redundant monitors, a capability to verify monitor performance periodically, a high-reliability control circuit, and sufficiently independent implementation of transmitter, monitor, and control functions