Complementing GNSS for resilient performance based navigation
Creators: name: Osechas, Okuary
name: McGraw, Gary
Publisher: name: ZHAW Zürcher Hochschule für Angewandte Wissenschaften
Year: 2024
Subjects: GNSS resilience
Performance based navigation
629: Luftfahrt- und Fahrzeugtechnik

description: GNSS positioning excels in providing Performance Based Navigation (PBN) services, both Area Navigation (RNAV) and Required Navigation Performance (RNP), to aviation users on a global scale, delivering better performance than most enroute and terminal operations require. However, it has become apparent that radio frequency interference (RFI) makes sole reliance on GNSS untenable. As depicted in Figure 1 and in the spirit of the “Protect, Toughen and Augment” (PTA) paradigm promoted by the National Space-Based PNT Advisory Board (GPS.Gov: PNT Advisory Board), ensuring aviation PBN capabilities will require action on multiple fronts. 
GNSS receiver technologies exist that would make GNSS more resilient to RFI, i.e., “Toughen”, but there are limits to how much GNSS can be toughened and other threats to GNSS that must be considered, so this can only be a piece of the solution. A key aspect of making PBN resilient to RFI and other GNSS threats is modernization of Complementary Positioning, Navigation and Timing (CPNT) systems, i.e., the “Augment” part of PTA. But as illustrated in Figure 1, “Toughen” and “Augment” are only a part of aviation’s problem: other aspects, like improving how PNT information is integrated with other aircraft systems and how better RFI situational awareness will be provided to operators and flight crews are needed to fully exploit the capabilities provided by toughened GNSS and CPNT augmentation. Note that while RFI situational awareness is an important aspect of “Protect” in PTA, such situational awareness relies on having interference detection and dissemination capabilities that are not currently standard.
This report focuses on the “Augment” aspect of PTA. While it considers a broad range of potential CPNT systems, it lays no claim to being an exhaustive survey. Instead, its focus is on CPNT systems that have the greatest potential to meet the needs of aviation, with an emphasis on the evolution of systems that are either currently approved for civil aviation, have previously been approved, or already in an approval process. While it must be acknowledged that no single terrestrial navigation system can possibly match GNSS accuracy and coverage performance, there are solutions that offer viable alternatives, especially to over-land enroute and terminal area users, preserving the PBN capabilities that currently only GNSS can provide.  A set of performance criteria are proposed to evaluate the candidate CPNT sources. Applying these evaluation criteria to the list of proposed CPNT technologies yields our recommendations for future-proofing resilient PNT for aviation, with consideration given to other applications as well.  
The analysis indicates that systems not developed by the aviation community, e.g., 5G cellular, Low Earth Orbit (LEO) systems, signals of opportunity, or commercial beacons, and pseudolites, lack some of the basic functions and provider commitments at the core of safety-critical applications requiring high integrity. Because of the high costs and likely lengthy timelines for adapting these technologies for aviation CPNT, the focus should be on modernizing terrestrial aviation current RADIONAV. Accordingly, our recommendations are:
•	Accelerate development of Enhanced DME (eDME), as eDME can help to address intra-system interference issues with current DME, as well as offer performance improvements;
•	Support the continued development of Enhanced LORAN (eLORAN) for aviation, as this system can provide coverage over larger areas with less infrastructure, as well as timing for aviation and other users;
•	Continue to develop a navigation capability in the L-band Digital Aviation Communications System (LDACS-NAV), which is capable of providing higher performance ranging than either eDME or eLORAN. 
The analysis here also suggests that there are synergies between these different systems. For example, eDME and LDACS-NAV measurements could be integrated together in airborne solutions, reducing the required number of ground installations needed in a region for each system. Furthermore, eLORAN can also serve as a timing aid to eDME and LDACS ground stations. In addition, LDACS could disseminate differential eLORAN corrections with very low latency.