Amid repeated Russian drone incursions, Baltic states are aiming to radically improve their C-UAV capabilities. Looking at one such example, Lithuania has opted to deploy a Ukrainian acoustic detection system known as Sky Fortress. Here, parabolic acoustic mirrors – a pre-radar technology – have found a second life as a low-cost distributed sensor for the age of massed drones.
The Baltic states, as part of NATO’s frontier facing onto Russia, have existential concerns about Russia’s intentions in the region. In particular, they are keen observers of the ongoing war in Ukraine.
One defining aspect of the war is the ongoing and ubiquitous use of unmanned aerial vehicles (UAVs) for numerous uses, including strike, reconnaissance, supply, and more recently, even casualty evacuation. Estimates vary, but figures of 100,000 drone sorties per month on both sides of the conflict have circulated.
The Baltic states in general and Lithuania in particular, have confronted the spectre of drone incursion. A number of incidents involving suspicious drones have been reported in Lithuania. As one example, a drone alert was issue to residents of several regions of Lithuania in October 2026.
As part of a broader increase in defence spending across the Baltic states, Estonia, Lithuania, and Latvia are increasing their ability to detect and counter hostile UAVs. It should also be noted that Ukrainian drones have allegedly occasionally turned up in Lithuanian airspace, having been confused or diverted by Russian electronic warfare (EW) efforts.
Many aspects of the air defences of the Baltic states are rightly sensitive, but observers can occasionally catch a glimpse. A 2 October 2026 press release by the Lithuanian Ministry of Defence (MoD) is a notable example. The Lithuanians are fielding a Ukrainian made system called ‘Sky Fortress’ to detect both UAVs and cruise missiles.
UAVs, particularly those low-flying and very small models, have proven to be difficult to detect by traditional air defence radar systems, particularly at longer ranges. In the case of low-flying UAVs and cruise missiles, this is due to the curvature of the Earth setting the radar horizon. In the case of small UAVs, this is due to their small surface area and materials providing insufficient energy return for detection at greater distances.
Sky Fortress, produced in Ukraine, was reportedly originally developed to detect Iranian Shahed-type one-way attack (OWA) drones, and by extension, their Russian Geran-series derivatives. Exact details of the origin of Sky Fortress are scant.
[CORRECTION 10/11/2026: An earlier version of this article mistakenly claimed that Sky Fortress was a renaming of an earlier effort called ‘Zvook’, based on an article in a US military journal. A Zvook representative reached out to clarify that the two companies are distinct Ukrainian producers of acoustic detection systems. The article has been updated to reflect this.]
The Sky Fortress system works by acoustic means. A widely dispersed network of detectors uses electronic microphones installed in parabolic acoustic mirrors to increase their fidelity and sensitivity, tied to a central processing unit (CPU) and sound card. These parabolic mirrors are, in fact, small replicas of acoustic sensors used in the pre-radar era. The networked sensors use machine learning to detect and locate possible drone threats.
Earlier systems used Android cell phones as the CPU for each sensor, but dedicated hardware has been developed. The first generation sensors units were approximate 30 cm x 30 cm x 5 cm, and were believed to be less than USD 1,000 each. The sensors are mounted on existing radio towers, telephone poles, and similar vantage points. The command and control platform is called SkyMap.
Allegedly, a network of 14,000 to 15,000 Sky Fortress sensors covers Ukraine. The system appears to be cheap enough to allow for widely proliferated sensor coverage. Since the unoccupied parts of Ukraine are approximately 7.5 times larger than Lithuania, a reasonable estimate is that 2,000 systems would be needed for the Lithuanians to have similar coverage. A particular advantage is that these small sensors are quite passive compared to radar, particularly if they communicate by wire. Even ones that use the cell phone system to communicate are likely to be indistinguishable from routine mobile phone background signals.
None of this works particularly well if there is not a library of UAV sound signatures. To the human ear, many of these systems sound more or less the same, but acoustic engineering ferrets out numerous fine differences in the acoustic signatures. The contract between Ukraine and Lithuania, according to the aforementioned Lithuanian MoD press release, specifies sound signature libraries as part of the agreement.
Two obvious issues with acoustic systems are the possibility of false positives from friendly systems, and deliberate attempts to modify hostile UAV signatures. The former can be easily managed by white-listing Lithuanian and NATO ally systems, both in terms of sound signature and time and place of use. The latter may end up in yet another arms race, with defenders having to rapidly update libraries every time something new appears.
There are some reports that indicate some aspect of the current Sky Fortress also collects radio frequency (RF) signals. Earlier versions of the system, seen in photographs by your correspondent, do not appear to support such data collection, but it is unclear how many generations of hardware Sky Fortress has progressed through.
It should be noted that reports have circulated of other users of Sky Fortress. Numerous outlets have reported that Sky Fortress was demonstrated to possible NATO clients at Ramstein Air Force Base in Germany in 2024. The news service Reuters reported that US forces have employed Sky Fortress and Sky Map to protect a US air base in Saudi Arabia during the ongoing US-Iran conflict.
Counter-UAV (C-UAV) technologies are proliferating nearly as much as drones themselves, and this Lithuanian endeavour is just the tip of an iceberg. Sky Fortress may be in use elsewhere, so perhaps the Lithuanian acquisition is just one of many.
Estonia and Latvia are well known as having expressed strong interest in C-UAV and have allocated budgetary resources. Latvia is known to be trying out an Anduril command and control (C2) platform, while Estonia is using a system from an Estonian firm Hexwatch which evidently uses both RF and acoustic sensors. Meanwhile, NATO has recently launched its C-UAV marketplace to further accelerate procurement. This is an area to watch, and Warsight will be on top of both the Baltic defence and counter-UAV topics as they evolve.
