Providing LAAS Flexibility and Airborne Architecture Autonomy Through Definition of Robust Signals-in-Space

Larry D. Huffman and Karl Kovach

Abstract:	The FAA CAT IIIb flight demonstrations performed by Raytheon E-Systems, Ohio University, Stanford University and Wilcox were recently completed. In summarizing the results of these demonstrations, the FAA announced that one of the next logical steps is the selection of the GPS technology (code or carrier phase tracking) to be implemented in LAAS.[ 1] This paper suggests an alternative to this selection that may reduce risk to the government and industry. Current definitions of Signals-in-Space in RTCA/DO-2 17 for SCAT-I and those being defined in RTCASC-159 (CAT II/III MOPS) provide only for code based GPS systems. The GNSS Panel Working Group D is in the early stages of ICAO SARPS development and is considering carrier phase based GPS systems. Code tracking systems are attractive since they provide reasonable navigation sensor accuracy for CAT I/II and may satisfy TSE requirements for CAT IIIb at a lower equipment cost than KCPT systems. Further, issues of a second frequency and the technical problems associated with ambiguity resolution and cycle slips are avoided. However, the NSE capabilities of code tracking technology may not adequately support integrity and availability performance. Also, the vast majority of commercial aircraft most likely to employ LAAS CAT III technology are those aircraft currently in service using ILS. For these aircraft, certification costs could be prohibitive if the GPS Landing System is not truly ‘ILS Lookalike’, including the ability to satisfy both the 0.6 meter NSE requirement at the 50 foot HAT and the autopilot voting requirements for redundant systems. These requirements are beyond current code tracking systems accuracy performance capabilities. Carrier phase tracking systems have an inherent centimeter level accuracy capability that can satisfy these requirements and improve both integrity and availability. With the rapid advance of GPS technology coupled with the current FAA schedule for LAAS implementation, the real and perceived deficiencies of both technologies may be resolved in time. However, the immediate challenge is to minimize risk while advancing the definition and development of the LAAS ground station and not restricting industry to a single airborne solution. If signals-in-space are defined such that they accommodate both code and carrier phase tracking systems, the government’s ground station development risk is minimized while airborne implementations can use the technology that best satisfies the application. These robust signals-in-space will ensure that airborne systems can comply with existing requirements that will result in an ‘ILS Lookalike’ functional and human interface. The following paragraphs provide an introduction and background information with a perspective of the current state of WAAS, LAAS, SCAT-I, regulatory activities, and AEEC characteristic development. This is followed by a discussion of accuracy performance of code and carrier phase systems. Overviews of five proposed Navigation Augmentation Broadcast Systems (or data links) are provided. Then a proposed ground system architecture is presented including discussion of the proposed signals-in- space. This paper concludes with recommended LAAS goals and a summary and conclusion.
Published in:	Proceedings of the 52nd Annual Meeting of The Institute of Navigation (1996) June 19 - 21, 1996 Royal Sonesta Hotel Cambridge, MA
Pages:	313 - 326
Cite this article:	Huffman, Larry D., Kovach, Karl, "Providing LAAS Flexibility and Airborne Architecture Autonomy Through Definition of Robust Signals-in-Space," Proceedings of the 52nd Annual Meeting of The Institute of Navigation (1996), Cambridge, MA, June 1996, pp. 313-326.
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