Design and capabilities
Sky Fortress is a Ukrainian passive-acoustic detection system that identifies air threats by the sound they make rather than by radar return. Early nodes were built from a microphone and a smartphone mounted on a pole; later, purpose-built units added a dedicated processor, sound card, directional microphones, and parabolic collectors or arrays of MEMS microphones. Each node captures ambient sound, filters out background noise such as wind, traffic, and birdsong, and runs a machine-learning classifier to recognise an engine's or airframe's acoustic signature. Rather than streaming raw audio, the node sends a short message: sensor ID, timestamp, bearing to the source, and a confidence level. That acoustic data is fused with radar tracks and passed to mobile fire teams equipped with tablets and anti-aircraft guns. A single unit costs roughly 400 to 1,000 dollars, cheap enough to deploy in dense, overlapping numbers, and because each node only listens it emits nothing for an adversary to detect or jam.
Operational history
Sky Fortress emerged in 2022, started by two Ukrainian engineers, i3 Engineering CEO Pavlo Tsiupka and Respeecher co-founder Dmytro Believtsov, in response to low-flying cruise missiles that radar struggles to pick up in ground-clutter blind zones. The network has grown to roughly 14,000 sensors across Ukraine, with NATO-backed funding committed to add a further 15,000 third-generation units. The system has been demonstrated at Ramstein Air Base in Germany, and Romania has begun testing it. Lithuania is the first Baltic state to commit: General Raimundas Vaikšnoras, Commander-in-Chief of the Lithuanian Armed Forces, told LRT that funding is secured, with testing in late 2025 and deployment from 2026 to complement the country's radar-based air defence. Sky Fortress is frequently discussed alongside Zvook, a separate Ukrainian acoustic-detection company founded in Lviv in 2023; the two are distinct programmes, and Sky Fortress carries the larger sensor count and the Lithuanian deployment specifically.
AirVeto context
Acoustic detection is the direct counter-drone answer to the incursions AirVeto reconstructs: the Shahed, Geran, and FPV types that cross the eastern border are exactly what these sensors are tuned to hear. Lithuania fielding Sky Fortress from 2026 places acoustic detection over the same airspace AirVeto's incident record covers, alongside the counter-drone systems Lithuanian forces are already drilling.