ION GNSS 2012
Session B4: Preserving the Availability & Integrity of GNSS in Harsh Environments

Title: Performance of Current Distance Measuring Equipment and Implications for Alternative Position Navigation and Timing for Aviation
Author(s): T. Murphy, M. Harris, K. Zhu, Boeing Commercial Airplanes; R.W. Lilley, Aviation Management Associates, Inc.; R.H. Erikson, Federal Aviation Administration
Date/Time: Thursday, September 20, 2012, 4:46 p.m.
Room: 205 (NCC)

This paper discusses some opportunities and challenges related to use of DME as an alternate source of positioning, navigation and timing (PNT), usable in the absence of GNSS services for aviation applications.
Future air traffic management systems such as the US FAA´s Next-Generation Air Transportation System (NextGen) and Europe´s Single European Sky ATM Research (SESAR) as currently envisioned will depend heavily on three- and four- dimensional position determination derived onboard airplanes. GNSS will certainly play a central role in providing the necessary Position Navigation and Timing (PNT) services that will support area navigation (RNAV) operations as well as surveillance services through ADS-B; however, vulnerability to RF interference is a cause for concern that GNSS alone will be unable to fully meet the requirements for PNT services to support future ATM. Consequently, the need for some backup capability has become a subject of robust debate.

In the near to medium term, Distance Measuring Equipment (DME) is the obvious solution to the need for backup to GNSS for aviation applications for the following reasons: DME systems already exist, are standardized and airborne equipage is nearly ubiquitous on air transport class airplanes. Furthermore, existing wide deployment of DME ground facilities and a long history of successful service globally both point to potential benefits of DME improvement and continued use. However, the current specified performance of DME systems limits their usefulness as a backup to GNSS. A general investigation into the performance of DME systems is warranted, to include the real performance of currently fielded DME equipment (both ground and airborne) and the modifications that could be made in the equipment and/or standards to allow more advantageous use of DME systems as an alternative to GNSS.

This paper describes distance measurement data that was collected using current- generation DME avionics in FAA flight inspection, in the laboratory, and during many hours of flight testing using large air transport airplanes and a high precision truth reference system. Performance of the current DME system is compared with the existing standards, and opportunities are considered for improvement of the standards so that the current performance can be exploited without replacing existing airborne equipment. Similarly, potential improvements to ground-based DME transponders are considered. Several potentially viable alternatives for improving DME utility are identified and discussed. Finally, the paper includes some general discussion of potential future generations of DME equipment and standards.

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