Interference-Resistant Vertically-Guided Approaches for Military Aircraft in Civil Airspace

D.A. Stratton

Abstract:	Military aircraft require means to operate in civil airspace, and avionics solutions that would enable continuously-guided vertical descent to low-altitude minima are particularly desireable. While the Defense Department has authority to self-certify its aircraft, these certifications need to address requirements for operational approvals in domestic and foreign civil airspace. Such operations normally require the use of civil augmentation services to provide the requisite integrity. Civil augmentation services rely exclusively on civil signals such as GPS Standard Positioning Service (SPS), FAA Wide-Area Augmentation System (WAAS), as well as counterpart GNSS systems in Europe and Asia. On the other hand, operation in degraded and challenged environments, particularly Radio-Frequency Interference (RFI), represents a growing concern for all GNSS users. The U.S. military requires the GPS Precise Positioning Service (PPS) which uses Selective Availability and Anti-Spoofing Modules (SAASM). SAASM receivers and the PPS service are highly resistent to jamming and spoofing, and offer other benefits to the military. Solutions are needed to leverage the capabilities of PPS service while obtaining the benefits of operation in civil airspace. The Military Standard Orders (MSOs), including MSO-C145, have been introduced as a means to obtain approval for avionics that could utilize PPS in civil airspace. Alternatives for MSO-C145 compliance include separate civil and military receivers, SPS/PPS switchable military receivers and integrated SPS/PPS receivers. The integrated SPS/PPS receiver performs SPS reception and PNT calculation in parallel and indepedent of PPS functions. The integrated receiver offers several unique benefits including the ability to monitor PPS PNT with civil approved signals, enabling use of PPS in civil airspace with unquestioned integrity. Parallel solution paths decrease the likelihood of common-mode fault, and provide an economical means to maintain stringent levels of DO-178B qualification. The substantial advancements in commercially-available ASIC, FPGA and memory technology have reduced space, weight and power requirements to well within the most challenging application requirements including manned and unmanned aerial vehicles. The MSO-C145 approval is directed at the use of PPS for civil RNP operations, where the GNSS is used as a source for lateral navigation only with barometric altimetry used to descend through a series of constant-altitude segments to a missed approach point. Fault Detection and Exclusion (FDE) algorithms are used to define a Horizontal Protection Level, which is guaranteed to bound the horizontal position error with high confidence. Since the release of the MSO C145, the FAA has certified over 5,000 Lateral Approach with Vertical Guidance or LPV approaches, in which the civil GPS/WAAS receiver provides vertical as well as lateral guidance on a continuous descent directly to the runway. Continuously-guided vertical descent is highly beneficial from operational and safety standpoints. In order to bound the vertical position error, the WAAS receiver uses the civil WAAS signal to construct a much tighter Vertical Protection Level than what would be obtained from the use of FDE alone. This paper expands the capabilities of integrated SPS-PPS receivers to capitalize on augmentation services to provide LPV approach procedures for military aircraft. A key element of the approach is a hybrid SPS-PPS integrity algorithm in which ionosphere-corrected observables from augmented SPS are compared with inertially-complemented PPS observables to bound potential errors due to spoofing of the civil signals. The resulting validation of the civil solution mitigates civil spoofers, while the PPS solution is simultaneously bounded with superior confidence compared to conventional PPS FDE. The parallel PNT solutions of the integrated receiver are both available to the aircraft, enabling flexibility for mission systems to utilize military signals while navigating with full civil compliance and having access to the latest civil procedures. An extension of the method can be used to construct virtual GLONASS and/or Galileo measurements from the PPS in a future multi-constellation SPS-PPS receiver. This paper will address the design approach, simulation results, and address the civil threats and their mitigation using an integrated SPS-PPS receiver.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2569 - 2576
Cite this article:	Stratton, D.A., "Interference-Resistant Vertically-Guided Approaches for Military Aircraft in Civil Airspace," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2569-2576.
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