Experimental Analysis of a Bidirectional Air-to-Ground Optical Communication Link

Under the German QuNET initiative, Key Experiment 3 investigates the integration of dynamic mobile nodes into future quantum networks using airborne optical links as an intermediate step to satellite-based scenarios. The experiment aims to demonstrate the applicability of different quantum key distribution (QKD) technologies over long-distance mobile links. The German Aerospace Center (DLR) contributes with the Free-Space Experimental Laser Terminal 2 (FELT-II) and a bidirectional classical optical link, which is essential for both data transmission and tracking operations. This paper presents a preliminary experimental analysis of the classical communication channel during the flight campaign conducted in Erlangen in September and October of 2025. Performance metrics at both the physical layer (received signal strength indicator (RSSI), signal-to-noise ratio (SNR)) and data layer (raw and coded bit error rate (BER), round trip times (RTT)) are evaluated under varying day-time and night-time conditions. The results are in line with the expected performance for scientific data transmission and these are subject to limitations arising from the pointing, turbulence and ambient light. These factors particularly affect the stability required for reliable bidirectional communication in air-to-ground link scenarios.