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The capacity of current aeronautical datalinks is reaching its limits and becomes a hindrance for growth of worldwide civil aviation. To modernize Air Traffic Management (ATM) and digitize aeronautical communications, successors for current technologies are being researched and deployed. The envisioned successor for the VHF Datalink mode 2 (VDLm2) for European air traffic is the L-band Digital Aeronautical Communications System (LDACS). Similar to VDLm2, LDACS is a terrestrial, cellular Air-Ground (A/G) communications system. Contrary to VDLm2, LDACS shall also provide an Air-Air (A/A) communication mode in the future, called LDACS A/A, which operates in a radius of 200 Nautical Miles (NM) for aircraft above altitude of 3000m. Long-distance multi-hop A/A communications could be used to extend the range of LDACS ground stations into oceanic and remote areas, increasing the utility of the terrestrial infrastructure. While LDACS A/G offers sound cybersecurity measures, the development of such for an LDACS A/A extension is currently in its infancy and needs to be investigated thoroughly. One particular design constraint for cybersecurity for aeronautical multi-hop A/A networks is the topology of the underlying mesh network. The objectives of this paper are to investigate (1) the number of concurrent aircraft that are within communication range to each other and (2) the number of hops necessary to cover given distances and (3) to propose possible cybersecurity approaches for LDACS A/A in particular. With actual flight traces data from the OpenSky database for European air traffic, we identify high fluctuations of results based on the time of day and region. The following results were obtained: (1) concurrent aircraft are ranging from 0 to 258, (2) on an exemplary route from Istanbul to Dublin, ranging roughly 3000km, 9 hops were necessary on average with stable routes lasting 1m 21s on average and (3) up to 19% of the total stable connection time is used for establishing a secure Peer-to-Peer (P2P) tunnel via mutual authentication between all hops.

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