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Global Navigation Satellite Systems (GNSS) are becoming the primary means of navigation for civil aviation. Nevertheless, concerns about GNSS outages remain, driving the need for Alternative Positioning, Navigation and Timing (APNT) systems to provide availability and continuity for performancebased navigation services. Although the existing Distance Measuring Equipment (DME) infrastructure is able to provide Required Navigation Performance (RNP) 1.0 accuracy, it is not robust for individual station outages. Additionally, we show that in the European airspace DME is reaching the capacity limit. To address these two problems, we propose a methodology based on modular APNT. In the presented approach, the complementary ranging sources are optimally placed to obtain robustness. It is assumed that the L-Band Digital Aeronautical Communications System (LDACS) can provide this capability. As shown in the results, the modular APNT system is able to provide robust RNP 1.0 coverage for Germany using 17 new LDACS ground stations to complement the network of 73 existing DME installations in Germany.

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It is a well-known and documented problem within the aviation community that outages in satellite navigation services make performance-based navigation (PBN) operations, as defined by ICAO (2008), using low required navigation performance (RNP) values impossible in various parts of the world (Berz, 2016). In these areas, Global Navigation Satellite System (GNSS) receivers do not always function as desired. Radio-frequency interference (RFI) has long been touted as the main reason for such outages in GNSS-based services, not only for aviation users but also for maritime applications (Pérez Marcos, et al., 2018).Here, we summarize the findings of a test flight conducted specifically to assess the impact of the RFI on operational GNSS avionics' multimode receivers (MMR), such that the findings could be representative of a broader class of mainline passenger aircraft. The test flight included equipment to study the issue at different levels of abstraction: directly in the cockpit as observed by pilots in the operational environment, at the level of GNSS observables, and directly in the radio spectrum. EXPERIMENT DESIGNOne location that has reported difficulties with GNSS outages is the Nicosia Flight Information Region. The Department of Civil Aviation of the Republic 1 German Aerospace Center (DLR)