CC BY 4.0Ji, JingxianJingxianJiMukherjee, ShamboShamboMukherjeeKuhl, AlexanderAlexanderKuhlKoke, SebastianSebastianKokeLeipe, MarkusMarkusLeipeRothe, MarkusMarkusRotheSteinlechner, Fabian OliverFabian OliverSteinlechnerKronjäger, JochenJochenKronjäger2025-10-012025-10-012025https://publica.fraunhofer.de/handle/publica/496530https://doi.org/10.24406/publica-558210.1364/OE.55770810.24406/publica-55822-s2.0-105015588654Optical clock networks connected by phase-coherent links offer significant potential for advancing fundamental research and diverse scientific applications. Free-space optical frequency transfer extends fiber-based connectivity to remote areas and holds the potential for global coverage via satellite links. Here we present a compact and robust portable, rack-integrated two-way free-space link characterization system. Equipped with plug-and-play capabilities, the system enables straightforward interfacing with various optical systems and facilitates quick deployment for field experiments. In this work, we achieve a fractional frequency instability of 2.0 × 10-19 for an averaging time of 10 s over a 3.4 km horizontal fully folded intra-city free-space link. Moreover, the system maintains an uptime of 94% over 15 hours, illustrating its reliability and effectiveness for high-precision optical frequency comparisons over free-space.entruejournal article