Guidance Accuracy Considerations for the Microwave Landing System Precision DME

Abstract

The microwave landing system (mls), developed by the FAA under a joint DOT/FAA, DOD, and NASA program is designed for extended requirements in volumetric coverage, guidance accuracy, and integrity to meet the increasing needs of aviation. It is to be a common civil-military system and provide a level of operations and equipments suitable for all classes of users. Integral to the Microwave Landing System concept is the Distance Measuring Equipment (DME) which measures range to touchdown. It must satisfy to the maximum extent possible, approach and landing operational requirements for all user aircraft (CTOL, STOL, and VTOL). These requirements dictate aircraft range and range rate measurements with an accuracy at least an order of magnitude more precise than those needed for conventional terminal DME application. This precision DME system (called the PDME) must: (1) satisfy the CTOL Category II decision height and Category III flare maneuver accuracy requirements (100 feet, 2 sigma) when combined with the appropriate elevation data, and (2) for VTOL applications provide accurate range (±40 feet, 2 sigma) and range rate information (±2 knots 2 sigma) to permit IFR decelerated approaches to within 500-foot range of hover. These specifications must be satisfied in the presence of large signal attenuation caused by ground multipath interference and signal time-of-arrival errors induced by lateral multipath (hangars, etc.).