GUIDANCE ACCURACY CONSIDERATIONS FOR THE MICROWAVE LANDING SYSTEM PRECISION DME

R. J. Kelly and E. F. C. LaBerge

Peer Reviewed

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.). Because there are user airborne equipment space and cost restrictions, the PDME must be interoperable with conventional (e.g., VORTAC) DME ground transponders. The PDME must, therefore, assume channel as- signments at L-band. Key to the PDME achieving its operational objectives is a complete accuracy specification and its rationale, including a measurement methodology. Not only must the error budget allocations between equipment instrumentation errors and multipath-induced errors be specified, but also the degree of airborne in- terrogator output data smoothing. These PDME filter time constants must be compatible with the flight control system characteristics of the aircraft to be served. This paper presents a consistent accuracy specification suitable for MLS operational requirements and a PDME implementation common for CTOL, STOL, and VTOL. Reference 22 reviews the U.S. MLS program.
Published in: NAVIGATION, Journal of the Institute of Navigation, Volume 27, Number 1
Pages: 1 - 22
Cite this article: Kelly, R. J., LaBerge, E. F. C., "GUIDANCE ACCURACY CONSIDERATIONS FOR THE MICROWAVE LANDING SYSTEM PRECISION DME", NAVIGATION, Journal of The Institute of Navigation, Vol. 27, No. 1, Spring 1980, pp. 1-22.
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