DME/N Error Budget Allocation and DME-Next Proof-of-Concept Flight Test and Performance Evaluation

摘要

Since its inception in 1952, the Distance Measuring Equipment (DME) has formed an essential part of worldwide aviation navigation. An extensive DME ground infrastructure is present globally, most aircraft are outfitted with DME interrogator equipment, the system has decades of proven robustness, and has dissimilar failure modes from satellite-based navigation systems. These characteristics reaffirm DME's potential in current and future aviation Position, Navigation, and Timing (PNT). The transition to Performance Based Navigation (PBN) [1] increases the performance demands on the navigation systems, which warrants modernization of the existing DME/N system to improve its accuracy, availability, continuity and integrity, while maintaining backwards compatibility with existing legacy interrogators. DME/N is a terrestrial pulsed two-way ranging system. The interrogating aircraft sends a pulse pair; a ground-based DME transponder receives the pulse pair, inserts a known delay, and sends a reply pulse-pair back at a different frequency. The slant-range between interrogator and transponder is calculated from the measured roundtrip delay. The ranging accuracy of DME/N is currently specified as 0.17 nmi (or 0.25% of the range, whichever is greater) (TSO-C66c) [2], or 0.2 nmi maximum total system error according to FAA-E-2996 [3]. These specifications are conservative; modern equipment performs significantly better. However, without further analysis and subsequent tightening of the specifications no credit can be taken for this improved performance. The DME/N error budget can be divided into 3 parts: errors induced by the airborne equipment (interrogator), by the ground equipment (transponder), and propagation-related errors. The equipment-related errors can be mostly reduced by evolutionary performance improvements associated with modern technology whereas the propagation-related errors will likely require more revolutionary enhancements of the system. This paper presents a flight-test methodology to characterize the ranging error associated with each DME sub-system. A precisely time-tagged RF data collection system, previously developed by Ohio University [4-7], has been expanded from one-way (ground-to-air) to two-way (air-to-ground and ground-to-air) via synchronized RF recordings of transmission and reception at both interrogator and transponder. The calibration of this system is performed in three steps: pre-flight precise signal strength and delay calibration, pre- and post-flight instrumentation bias calibration, and in-flight phase coherency and amplitude calibration. Post-processing of the data recordings, combined with high-fidelity position, attitude, time, and frequency truth, enables a detailed breakdown of the DME/N error budget into the aform-mentioned ground, air, and propagation categories. Flight-test performance measurements are presented using a low-power DME/N transponder operated at Ohio University's UNI airport and a transport-grade DME/N interrogator installed in one of Ohio University Avionics Engineering Center's flight test aircraft. Evolutionary DME/N performance improvement are likely not sufficient to meet the more stringent future navigation requirements such as RNP 0.3 (0.3 nmi Total System Error with 0.6 nmi 10-7 containment) and surveillance requirements such as NACP-8 (92.6 m 95%) and NIC-7 (10-7 containment of 370.4 m). Revolutionary DME performance enhancements are introduced in previous work: [5] introduces the usage of DME carrier phase and [7] introduces Pulse Noise Multipath (PNMP). These enhancements dramatically improve DME's accuracy and integrity. The proposed DME-Next system brings DME carrier phase and PNMP together and combines one-way ranging with occasional two-way ranging to also minimize DME/N's capacity challenges without the need for transponder time synchronization [6].