Synthetic Aperture Radar Drone Gets Interferometric Imaging | Hackaday

Synthetic Aperture Radar Drone Gets Interferometric Imaging | Hackaday

Backyard Coherence: A €1,000 Drone SAR Achieves Repeat-Pass Interferometry

A hobbyist C-band FMCW radar on a 7-inch FPV quadcopter now produces sub-meter-accurate elevation maps. The enabling step is a residual-motion estimator built on generalized phase-gradient autofocus, not exotic hardware.

---

BLUF

Finnish engineer Henrik Forstén has shown repeat-pass interferometric SAR (InSAR) from a sub-kilogram FPV quadcopter carrying a self-built ~6 GHz polarimetric FMCW radar. He puts total hardware cost for radar and drone at roughly 1,000 EUR, a small fraction of comparable commercial InSAR systems. The hardware is not for sale, but the processing code is published under the MIT license in his torchbp repository. hforsten

The results are these. Two passes with a 1 m vertical baseline produced mean scene coherence rising from 0.21 without motion correction to about 0.47 with autofocus, a new residual-motion-estimation (RME) algorithm, and a reference DEM. The resulting DEM agreed with national lidar to 0.4 m RMS and 0.11 m median absolute error. Three things made this possible: a €144 RTK/PPK GNSS receiver, fixes to ArduPilot's raw-GNSS logging, and a backprojection-domain RME method that Forstén believes is novel.

The work sits alongside a growing peer-reviewed literature on drone InSAR at L-, Ku-, and K-band. It is notable mainly for cost, for being fully open, and for what it implies about the spread of dual-use SAR capability. No court filings or government releases specific to this project were found.

---

From Snow-Field Demo to Interferometer

Forstén's first airborne system appeared in February 2025. He mounted the radar on the cheapest no-name 7-inch FPV kit he could buy, set a radar budget under 500 EUR, and accepted lossy FR4 PCB material for both electronics and antennas as a result. The carrier sits near 6 GHz, which he chose as the highest band with plentiful inexpensive consumer RF parts. Cheap power amplifiers there reach about 30 dBm. Transmit and receive polarization switches let the radar collect all four of HH, HV, VH, and VV. Homemade polarimetric synthetic aperture radar drone +2

The digital back end uses a Zynq-7020 SoC. The antennas are dual-polarized, slot-fed stacked patches surrounded by a hand-cut sheet-metal horn, with a simulated peak gain of 10 dBi. Without its battery the airframe weighs 752 g. With the smaller 1,300 mAh pack the whole system comes to 948 g. The first demonstration flew a straight 500 m track at 110 m altitude and 5 m/s, transmitting VV only with a 400 µs sweep, 500 MHz of bandwidth, and 1 kHz PRF. Homemade polarimetric synthetic aperture radar drone +2

That first system relied on consumer GNSS with meter-class error, so image quality depended on autofocus. In October 2025 Forstén described a new pipeline. It combines generalized phase gradient autofocus (GPGA) with a 3-D trajectory-deviation estimator adapted from Ding et al. The image is split into sub-images, and the phase error in each is converted to a range error. Per-pulse 3-D position corrections then come from a linearized, overdetermined least-squares solution weighted by signal-to-clutter ratio. The same post added antenna-pattern normalization and a posteriori polarimetric calibration following Ainsworth, Ferro-Famil, and Lee. Synthetic aperture radar autofocus and calibration +2

Hardware Changes for Interferometry

Precision positioning. Forstén found an F9P-based RTK module on AliExpress for 144 EUR, bundled with a helical antenna. The listing gave no connector pinout, so he traced the pins himself and filed down part of the PCB to clear the antenna. The base station, also F9P-based and sold with a tripod and antenna, cost 180 EUR. He reports about 2 cm accuracy against roughly 1 m for the non-RTK receiver. hforstenhforsten

Because image formation happens offline, he also uses post-processed kinematic (PPK) solutions. He found that ArduPilot's raw-GNSS logging dropped nearly every measurement, because a fixed buffer held only 32 satellite observations and he routinely saw about 80. It also failed to record the signal-band ID needed for carrier-phase solutions. He submitted a fix upstream. In the pull request, an ArduPilot maintainer said the change should be merged while suggesting the configuration parameter be redesigned later. hforstengithub

A second problem came from the radio link. RTCM correction traffic exceeded the capacity of the ExpressLRS MAVLink link, so he wrote his own bandwidth-limited RTCM server. hforsten

Sweep linearity. The chirp comes from an LMX2491 PLL. Vendor PLL simulators model fixed-frequency operation only, so Forstén built, with LLM assistance, a simulator of a sweeping PLL with a MASH delta-sigma modulator and used it to tune the loop filter. The optimizer favored a very narrow loop, about 60 kHz bandwidth with 55° phase margin. That suppressed integer-boundary spurs and a constant-frequency spur, but at the cost of longer relock time between sweeps. hforstenhforsten

Data throughput. The 12-bit, 50 MSPS ADC produces 75 MB/s. The Zynq's SD interface tops out at 25 MB/s in theory and about 20 MB/s in practice. His fix exploits a property of FMCW data: after dechirp, near-range returns are strong and low in frequency, so successive samples change slowly. He delta-encodes adjacent samples, zigzag-maps the result, and uses a three-code nibble-aligned entropy scheme. This gives about 2.3:1 lossless compression, roughly 5.7 bits per sample on the mission data. FLAC compresses better but is impractical to run in real time on the FPGA. The compressor runs in programmable logic at about three samples per clock. With 2× decimation enabled, the stream fits comfortably within SD bandwidth. Interferometric Synthetic Aperture Radar Drone +2

Ground control. Forstén replaced Mission Planner with his own ground station, mostly written with AI coding help. It auto-generates linear, circular, and interferometric SAR missions. All flight-critical logic stays in ArduPilot on the flight controller. hforsten

The InSAR Processing Chain

The geometry is conventional repeat-pass, flown with a multicopter. Both passes follow the same track in the same direction, separated by 1 m in altitude. Flying the same direction avoids differential Doppler phase and lets common timing errors between the GNSS tag and the sweeps cancel. hforsten

The processing chain runs in five steps:

1. Each pass is autofocused independently with GPGA.
2. Each pass is backprojected onto a flat ground plane or a reference DEM.
3. The two images are resampled to a common grid.
4. RME corrects the differential trajectory error between passes.
5. The interferogram is formed, filtered, unwrapped, and converted to height.

Because backprojection onto a ground plane already accounts for the path-length difference between passes, perfectly flat ground produces zero interferometric phase. Classical slant-range processing would instead show a flat-earth phase ramp. hforsten

The RME contribution. The best-known airborne RME technique is multisquint, which has well-established backprojection variants. Examples include Cao et al. in IEEE TGRS (2018) and the BP-MSQ algorithm of Xie et al., which derives an explicit expression for residual motion error in a backprojected image. nih

Forstén took a different route. He extended his GPGA formulation so that it works directly on the per-pulse backprojection terms rather than only on finished images, and says that to his knowledge the method is new. The central problem is that real topography contaminates a naive global estimate. Each sweep illuminates the scene differently, so pixels with different topographic phase partly cancel and leak terrain into the motion estimate. He fixes this by summing over small blocks where topographic phase is roughly constant. He then estimates each block's phase and removes it before solving for the per-sweep motion phase common to all blocks. hforstenhforsten

Pixels are weighted by squared power coherence, which ignores topographic phase and suppresses decorrelated vegetation and poorly lit regions. Blocks are weighted by how consistent their phase history is across sweeps. Cross-track and vertical errors come from a least-squares fit across range bands that are evenly spaced in the sine of elevation angle. Along-track error is solved separately by differencing blocks ahead of and behind the platform. This needs a beam covering both sides, so it would fail with a narrow squinted beam. Unlike autofocus, the method needs no point targets and no iteration, and it actually works better over flat ground. Interferometric Synthetic Aperture Radar Drone +3

Measured Performance

The main scene used 17,000 sweeps imaged onto a 4,156 × 16,473-pixel grid. On an RTX 3090 Ti, autofocus took 22 seconds and fast factorized backprojection took 0.9 seconds per polarization. Ground truth was the National Land Survey of Finland 2 m lidar elevation model. hforsten

The coherence progression shows what each correction stage buys:

• No autofocus or RME: mean coherence 0.210 hforsten
• Autofocus only: 0.297, still very poor at near range hforsten
• Autofocus plus RME: 0.453 including decorrelated forest and poorly lit edges, with near-range ground above 0.9 hforsten
• Autofocus, RME, and reference DEM: 0.468 hforsten

The solved RME was mostly vertical, which is both the axis where RTK GNSS is weakest and the axis left out of autofocus. For radar engineers, the practical point is this: the amplitude images before and after RME look almost the same, yet the interferograms differ dramatically. In other words, a well-focused SAR image says little about whether it will support interferometry. hforstenhforsten

After unwrapping with SNAPHU, the SAR DEM differed from lidar by 0.4 m RMS and 0.11 m median absolute error, counting areas where the scene had genuinely changed. A constant baseline error of (9.2, 24.4, 2.3) mm was estimated by fitting to the reference DEM and removed. Uncorrected, it would have produced several meters of height error at the image edges with a 1 m baseline. hforstenhforsten

A second mission, flown minutes later on a track rotated 90°, reached 0.480 mean coherence, with a smaller baseline error. hforsten

The limitations are standard physics, and Forstén states them plainly:

• Forest decorrelates between passes, shadowed areas return nothing, and smooth roads give little backscatter at these look angles. hforsten
• At a bridge, the deck and the ground beneath share the same range. That breaks the single-surface assumption the interferometry relies on. hforsten
• The 1 m baseline works at close range, but farther ranges would benefit from a longer baseline to increase height sensitivity. hforsten
• Without special permits, flight altitude is capped at 120 m, which forces very shallow grazing angles and long shadows at range. hforsten

Context: The Drone InSAR Research Landscape

Forstén's result follows several years of institutional work across multiple bands.

L-band. Frey and Werner of Gamma Remote Sensing reported repeat-pass interferograms and a first tomographic profile at EUSAR 2021. Their compact FMCW L-band SAR flew on Aeroscout's Scout B1-100 VTOL UAV, with temporal baselines up to 43 days. Gamma now fields the system on a Freefly Alta X quadcopter, specifying 1.2–1.4 GHz operation and 0.75 m range resolution. vde-verlaggamma-rs

Ku-band. Ruiz-Carregal et al. demonstrated repeat-pass interferometry with a dual-channel Ku-band FMCW drone SAR that also has single-pass cross-track capability. They used the single-pass DEM to coregister the repeat-pass stack. Their 2025 IEEE TGRS follow-up addresses the fact that GNSS errors are comparable to wavelengths at X-band and above. They introduced a complex-domain enhancement of multisquint RME that is more robust to large errors and decorrelation. The same group has since published drone-based multi-temporal DInSAR for large 3-D displacement retrieval with daily revisits in IEEE JSTARS (2026). Remote Sensing (Oct 2024) +2

K-band. In IEEE Transactions on Radar Systems (March 2026), Li, Bekar, Martorella, and Antoniou of the University of Birmingham quantified motion-error tolerances for high-frequency UAV InSAR. They showed that autofocus can restore height-estimation performance and validated this with a 24 GHz UAV demonstrator, which they describe as the first UAV InSAR demonstration in that band. birmingham

The Birmingham group's earlier low-cost drone SAR work was, by Forstén's account, part of what motivated his own effort. He noted that the published system was quoted at £15,000, beyond his personal budget. hforsten

Ultra-wideband. Multi-baseline UWB UAV interferometry has been demonstrated experimentally by Mustieles-Perez et al. at EUSAR 2024.

A shared theme runs through all of this work. Whether the method is Gamma's DEM-aided time-domain backprojection, the Ku-band team's multisquint refinements, the Birmingham autofocus analysis, or Forstén's block-wise GPGA estimator, the limiting factor is residual trajectory error at the millimeter level, not radar hardware. Forstén's RME has not been peer reviewed or benchmarked against multisquint on common data. That comparison would be a natural next step.

Dual-Use and Supply-Chain Notes

Two points deserve attention from AES readers.

First, component access. Mouser accepted Forstén's order for an RF component and then declined to ship it because the supplier bars sales to individuals, apparently to keep parts out of defense applications. He substituted an obsolete pin-compatible part rated only to 4 GHz. hforsten

Second, export classification. SAR capability is a named parameter in the multilateral dual-use lists. Wassenaar-derived entry 6A008.d covers radar able to operate in SAR, ISAR, or side-looking airborne modes. In U.S. regulations the corresponding text is ECCN 6A008 in 15 CFR Part 774. Separately, publicly available software is treated differently under the General Software and Technology Notes. Whether and how these controls apply to a non-commercial, unsold hobby build with MIT-licensed processing code is a question for export counsel. This article draws no conclusion on it. thetradehub

The broader policy point stands regardless. Autofocus, RME, and GPU backprojection are now open source. Sub-€200 carrier-phase GNSS is widely available. Together these lower the barrier to coherent change detection and terrain mapping from small UAS. That is a boon for geoscience, infrastructure monitoring, and disaster response, and a factor for counter-UAS and operational-security planners.

---

Sources

Sources [1]–[3], [5]–[16], and [18]–[20] were accessed or confirmed during research for this article. Source [4] is the article supplied by the requester. Source [17] is cited in [1] and was not independently opened.

[1] H. Forstén, "Interferometric synthetic aperture radar drone," Henrik's Blog, Sep. 14, 2026. https://hforsten.com/interferometric-synthetic-aperture-radar-drone.html

[2] H. Forstén, "Homemade polarimetric synthetic aperture radar drone," Henrik's Blog, Feb. 11, 2025. https://hforsten.com/homemade-polarimetric-synthetic-aperture-radar-drone.html

[3] H. Forstén, "Synthetic aperture radar autofocus and calibration," Henrik's Blog, Oct. 7, 2025. https://hforsten.com/synthetic-aperture-radar-autofocus-and-calibration.html

[4] "Synthetic aperture radar drone gets interferometric imaging," Hackaday, Sep. 28, 2026. https://hackaday.com/2026/09/28/synthetic-aperture-radar-drone-gets-interferometric-imaging/

[5] "Budget-minded synthetic aperture radar takes to the skies," Hackaday, Feb. 13, 2025. https://hackaday.com/2025/02/13/budget-minded-synthetic-aperture-radar-takes-to-the-skies/

[6] Ttl (H. Forstén), "AP_GPS: GPS_RAW_DATA fixes for PPK processing," ArduPilot pull request #32395, GitHub. https://github.com/ArduPilot/ardupilot/pull/32395

[7] H. Forstén, torchbp: differentiable GPU SAR image formation and autofocus (MIT license), GitHub. https://github.com/Ttl/torchbp

[8] Y. Li, A. Bekar, M. Martorella, and M. Antoniou, "Unmanned aerial vehicle (UAV)-based, K-band interferometric synthetic aperture radar (SAR)," IEEE Trans. Radar Syst., vol. 4, pp. 535–548, Mar. 2026, doi: 10.1109/TRS.2026.3661483. https://research.birmingham.ac.uk/en/publications/unmanned-aerial-vehicle-uav-based-k-band-interferometric-syntheti/

[9] G. Ruiz-Carregal et al., "Ku-band SAR-Drone system and methodology for repeat-pass interferometry," Remote Sens., vol. 16, no. 21, Art. no. 4069, Oct. 2024, doi: 10.3390/rs16214069. https://www.mdpi.com/2072-4292/16/21/4069

[10] G. Ruiz-Carregal et al., "Accurate residual motion error estimation for high-frequency drone-borne SAR interferometry," IEEE Trans. Geosci. Remote Sens., vol. 63, pp. 1–20, 2025. https://ieeexplore.ieee.org/document/11104077/

[11] G. Ruiz-Carregal et al., "Drone-based MT-DInSAR for high-magnitude 3-D displacement retrieval with daily revisits," IEEE J. Sel. Topics Appl. Earth Observ. Remote Sens., vol. 19, pp. 850–874, 2026. Bibliographic record: https://dblp.dagstuhl.de/pid/386/1752.html

[12] O. Frey and C. L. Werner, "UAV-borne repeat-pass SAR interferometry and SAR tomography with a compact L-band SAR system," in Proc. EUSAR 2021, 2021, pp. 1–4. https://vde-verlag.de/proceedings-en/455457040.html

[13] O. Frey, C. L. Werner, and R. Coscione, "Car-borne and UAV-borne mobile mapping of surface displacements with a compact repeat-pass interferometric SAR system at L-band," in Proc. IEEE IGARSS, 2019, pp. 274–277. https://www.research-collection.ethz.ch/entities/publication/4fdfccd7-499c-4e71-af51-cee08f08be21

[14] Gamma Remote Sensing, "GAMMA SAR systems information," v1.8, Oct. 14, 2025. https://www.gamma-rs.ch/files/instruments/SAR/GAMMA_SAR_Systems_information.pdf

[15] A. Bekar, M. Antoniou, and C. J. Baker, "Low-cost, high-resolution, drone-borne SAR imaging," IEEE Trans. Geosci. Remote Sens. https://pure-oai.bham.ac.uk/ws/portalfiles/portal/136457382/Final_Version_TGRS.pdf

[16] P. Xie, M. Zhang, L. Zhang, and G. Wang, "Residual motion error correction with backprojection multisquint algorithm for airborne synthetic aperture radar interferometry," Sensors, vol. 19, no. 10, Art. no. 2342, May 2019, doi: 10.3390/s19102342. https://pmc.ncbi.nlm.nih.gov/articles/PMC6567132

[17] Z. Ding et al., "An autofocus approach for UAV-based ultrawideband ultrawidebeam SAR data with frequency-dependent and 2-D space-variant motion errors," IEEE Trans. Geosci. Remote Sens. https://ieeexplore.ieee.org/document/9380507

[18] N. Cao et al., "Estimation of residual motion errors in airborne SAR interferometry based on time-domain backprojection and multisquint techniques," IEEE Trans. Geosci. Remote Sens., vol. 56, no. 4, pp. 2397–2407, Apr. 2018. https://smu.edu/lyle/departments/cee/faculty/-/media/EF97FA1312EA4C7597D128EE31F15C51.ashx

[19] V. Mustieles-Perez et al., "Experimental demonstration of UAV-based ultra-wideband multi-baseline SAR interferometry," in Proc. EUSAR 2024, pp. 1156–1161. Cited in the reference list of: https://arxiv.org/pdf/2507.20792

[20] EU Dual-Use Annex I / Wassenaar Arrangement, entry 6A008 (radar systems), secondary summary: https://www.thetradehub.eu/de/customs/export-control/annex-i/6a008. Authoritative U.S. text: 15 CFR Part 774, Supp. 1, ECCN 6A008.

 

Why the US Navy Is Fighting Iran With a Fleet That Carries No One - YouTube

Why the US Navy Is Fighting Iran With a Fleet That Carries No One - YouTube

The Fleet With No One Aboard: What Hormuz Is Teaching the Navy About Uncrewed Warfare

Sep 26, 2026 · @Stephen L Pendergast

BLUF

• Uncrewed systems crossed from experiment to combat in the 2026 Iran war. Confirmed firsts include combat use of the $35,000 LUCAS one-way attack drone on 28 February, the first U.S. rescue by an unmanned surface vessel (USV) in June, and the first U.S. sea-drone strike, three Saronic Corsairs against Bandar Abbas naval base, on 12 July.
• Below the surface, unmanned vehicles did the finding. U.S. officials say Navy underwater drones located more than 100 suspected mines in the Strait of Hormuz Traffic Separation Scheme; divers, SEALs, and contractors did much of the neutralizing.
• The losses are real and instructive. The U.S. military has lost at least 45 MQ-9 Reapers (roughly a quarter of the fleet), a $238 million MQ-4C Triton on 9 April, and an Anduril Dive-LD captured by Iran on 8 September. Iran tried and failed to seize a Saildrone on 14 September.
• The deeper lesson is adaptation, not cheapness. Sensors, shooters, and rescuers are becoming interchangeable nodes: a wind-powered Saildrone Surveyor fired two JAGMs at RIMPAC 2026 on carrier-group targeting data, and a Corsair built to strike also pulled aviators from the water.
• Much is unproven. "Attritable" USVs still cost about $1.1 million each, evidence for USV strike at scale is thin, mine countermeasures remain fragile, and the law of the sea has not settled whether a crewless hull is a warship.
• Recommendation: Measure the fleet by its adaptation rate, not its hull count. Fund the loop that converts combat lessons into fielded changes in weeks: open architectures, rapid sustainment, delegated autonomy doctrine, and allied integration through CENTCOM's new Task Force Falcon Strike.

The Fight at the Gate

Seven months into the war with Iran, the most consequential change in U.S. sea power may be the fleet that carries no one. A U.S. official told Military Times that American forces now use unmanned systems "on, above, and below" the sea, and that they have been effective chiefly in reducing risk to personnel. None of these machines would register on a traditional order of battle, which counts hulls, guns, and crews.

The war began on 28 February 2026, when U.S. Central Command (CENTCOM) and Israel opened coordinated strikes on Iran: Operation Epic Fury for the United States, Roaring Lion for Israel. Iran closed the Strait of Hormuz within days using warnings, mine-laying, ship seizures, and Islamic Revolutionary Guard Corps Navy (IRGCN) swarm-boat activity. A ceasefire began 8 April, a U.S. blockade followed on 13 April, and the truce collapsed in early July after renewed Iranian attacks on shipping.

Iran fought the way a weaker navy should. U.S. forces destroyed or disabled more than 120 Iranian naval vessels within the first month, so Tehran leaned on the tools of denial: fast attack craft, coastal anti-ship missiles, naval mines, one-way attack drones, and small explosive boats. The strategy rested on one assumption. Whatever the United States sent into the strait would have people aboard who could be killed, captured, or made to hesitate.

Striking where no crew should go. On 12 July, three Saronic Corsair USVs struck a submarine and ship-maintenance facility at Bandar Abbas naval base, which CENTCOM called the first time American forces had employed sea drones in combat. The Corsair is a 24-foot craft with a range beyond 1,000 nautical miles, a 1,000-pound payload, and a 35-knot sprint. Task Force 59 had fielded Corsairs in theater only since late March.

The lesson of the MQ-9. The Reaper was the war's workhorse hunter-killer, but three U.S. officials told The Washington Post in August that at least 45 had been lost, about a quarter of the fleet, at $30 million to $50 million apiece. Most belonged to the Air Force, and a defense official said some were lost to data-link failures rather than Iranian fire. When holding an eye over the strait costs a quarter of the aircraft, seeing has come apart from hitting and needs a solution of its own.

A Century-Old Idea, Finally at Scale

The crewless naval weapon is as old as the aircraft carrier. On 6 March 1918, a Navy-funded Curtiss-Sperry Flying Bomb left its launch track on Long Island, flew straight under gyroscopic control, and cut its own throttle at the preset 1,000 yards. Admiral Delmar Fahrney later called it the first successful flight of an automatic missile in the United States, and possibly the world.

Each later war moved the idea a notch. The Navy flew the QH-50 DASH drone helicopter from Cold War destroyers to deliver antisubmarine torpedoes, jet-powered Ryan reconnaissance drones penetrated North Vietnamese air defenses, and Pioneer drones spotted 16-inch gunfire for Iowa-class battleships in 1991. What changed in 2026 was economics and autonomy: drones became cheap and capable enough to buy by the hundred rather than the handful.

The institutional groundwork was already laid. Fifth Fleet stood up Task Force 59 in 2021 to integrate unmanned systems and artificial intelligence, and in January 2024 added Unmanned Task Group 59.1 to operate them alongside specialized crews. When Hormuz caught fire, the unmanned fleet was not born there. It was put on trial there.

The Strike Layer: Cheap Mass and New Roads

The strike arm faced two pressures at once: an arithmetic of cheap threats met by million-dollar interceptors, and targets where a crew was too costly or too exposed to send. It answered by spreading in two directions, cheaper on one side and into new forms on the other.

Claiming the cheap side of the ledger. The Low-cost Uncrewed Combat Attack System (LUCAS) is SpektreWorks' reverse-engineered copy of Iran's Shahed-136: roughly 10 feet long with an 8-foot wingspan, a small piston engine, several hundred miles of range, and a CENTCOM-stated price near $35,000. Task Force Scorpion Strike, a one-way attack squadron led by Special Operations Command Central and stood up in December 2025, flew LUCAS in combat for the first time on 28 February against IRGC command posts, air defenses, launch sites, and airfields. Those combat launches were from land; the maritime proof came earlier, on 16 December 2025, when Task Force 59 launched a LUCAS from the flight deck of USS Santa Barbara (LCS-32) in the Arabian Gulf.

Striking where no crew should go. On 12 July, three Saronic Corsair USVs struck a submarine and ship-maintenance facility at Bandar Abbas naval base, which CENTCOM called the first time American forces had employed sea drones in combat. The Corsair is a 24-foot craft with a range beyond 1,000 nautical miles, a 1,000-pound payload, and a 35-knot sprint. Task Force 59 had fielded Corsairs in theater only since late March.

The lesson of the MQ-9. The Reaper was the war's workhorse hunter-killer, but three U.S. officials told The Washington Post in August that at least 45 had been lost, about a quarter of the fleet, at $30 million to $50 million apiece. Most belonged to the Air Force, and a defense official said some were lost to data-link failures rather than Iranian fire. When holding an eye over the strait costs a quarter of the aircraft, seeing has come apart from hitting and needs a solution of its own.

 

The Sensing Layer: Climb Out of Reach, or Become Too Cheap to Matter

Information cuts both ways: the picture that shows where mines lie and boats gather is the same picture that cues the fleet's weapons. The Reaper's losses forced the fleet's eyes to evolve in two opposite directions at once.

Up: the MQ-4C Triton. The Navy's roughly 20 Tritons fly a 130-foot wingspan, wider than a Boeing 737's, for more than 24 hours at altitudes above 50,000 feet. Altitude is the whole argument. For a surface target under standard 4/3-earth refraction, radar horizon in nautical miles is approximately:

d_{\text{nmi}} \approx 1.23\sqrt{h_{\text{ft}}}

A 100-foot masthead radar sees about 12 nmi to a sea-level target; a Triton at 52,000 feet sees about 280 nmi. One aircraft therefore covers more ocean per orbit than a line of destroyers. The cost of that reach is concentration: on 9 April, a Triton operating from NAS Sigonella squawked a general emergency after a Hormuz patrol and went down, a Class A mishap on a platform the Navy prices near $238 million. The cause has not been disclosed.

Down: the Saildrone. At the other end of the scale sits the Saildrone Explorer, a roughly 25-foot wind- and solar-powered USV that holds station for months without fuel and has monitored Gulf traffic for CENTCOM for years. Iran has twice treated these as prizes, seizing Explorers in 2022 before returning them under U.S. pressure. On 14 September 2026, two IRGC small boats tried again near Larak Island; CENTCOM said they failed after it "forcefully responded," and Axios reported a U.S. drone destroyed both boats with two missiles. The episode proves the design premise: a sensor that costs little to keep at sea yields little to capture.

The Road Below: Mines and the Seabed

The strait does not open until the road beneath it is clear, and that road has always been the Navy's weakest domain. A mine costing a few thousand dollars can cripple a billion-dollar warship; the tanker Bridgeton struck an Iranian mine on the first Earnest Will convoy in 1987. Writing in Proceedings in April, analysts warned that the Knifefish UUV, built to hunt Iranian influence mines on or buried in the seabed, was still experiencing significant growing pains.

Hunting by machine. CENTCOM announced in April that underwater drones would join a new clearance effort. The Independence-class littoral combat ship's Mine Countermeasures Mission Package pairs an MH-60S with unmanned surface vehicles towing AN/AQS-20 sonar or influence sweeps, plus Knifefish for buried and high-clutter mines. On 25 August, U.S. officials said Navy underwater drones had systematically scanned the Traffic Separation Scheme over several months and located more than 100 suspected mines.

Humans still closed the loop. Contractors, Navy divers, and SEALs operating from inflatable boats neutralized many of those contacts. The distinction matters: unmanned systems moved the fleet's sailors out of the search, not yet out of the kill. Clearance also is not permanence. On 30 August U.S. forces struck launchers on Larak Island that were preparing to fire mines into the strait, and a late-August maritime security advisory kept the strait's threat level at "severe."

The Dive-LD and the price of presence. Anduril's Dive-LD is a roughly 19-foot, three-ton large UUV rated to 6,000 meters with about 10 days of endurance, at a reported unit cost near $2.5 million. On 8 September, the IRGC Navy recovered one largely intact near the entrance to the strait. CENTCOM said the "older model" had malfunctioned and sat dead in the water for more than a day, and carried no sensitive data or classified sonar. Anduril called it an attritable system whose loss was an expected possibility; the harder question is how much an adversary learns from each loss.

Roles That Will Not Stay Put

The clearest sign of change is not on any spec sheet; it is in how these machines take over each other's jobs. An old fleet is a collection of hulls welded to single missions. The emerging fleet is a pool of nodes whose roles shift as the moment demands.

The watcher becomes the shooter. During RIMPAC 2026 off Hawaii, under the Navy's Fleet Experimentation program, a 65-foot Saildrone Surveyor fitted with a Lockheed Martin JAGM Dual Launcher fired two missiles at a surrogate high-speed surface craft. Targeting data came from the USS Theodore Roosevelt (CVN-71) carrier strike group through the Navy's Adjunct Remote Engagement System (ARES), and the same vessel demonstrated passive electronic-warfare detection with an MH-60S. Lockheed published no accuracy or range results, and nothing released says the USV selected a target or authorized fire on its own: the machine carried the weapon, not the authority.

The striker becomes the lifeguard. In early June, when an Army AH-64 Apache went down near the coast of Oman, a Task Force 59 Corsair helped recover both crew members. A month later, the same class of hull struck Bandar Abbas.

Doctrine is catching up. The CNO's February 2026 Fighting Instructions assign medium USVs to "scouting, screening, and striking," task UUVs with water-space denial, survey, and mine warfare, and call for attritable, easily replenished USVs in the force design. Adm. Daryl Caudle introduced "Delegated Autonomy Levels" to define what any unit, manned or unmanned, may do without waiting for higher headquarters. In August, CENTCOM announced Task Force Falcon Strike, a SOCCENT-led, multinational attack-drone force spanning air, surface, and subsurface, to scale Scorpion Strike's model with regional partners.

What Hormuz Has Not Yet Proven

The temptation is to declare a revolution; the evidence supports a transition. CNA's Joshua Tallis cautions that the war's public examples are "an iterative learning moment, not a fully formed vision of the future fight." Four problems remain open.

1. Attritable is not the same as cheap. Open-source estimates put a Corsair near $1.1 million and a Dive-LD near $2.5 million. Tallis notes such systems are cheaper than crewed platforms but "not, per se, cheap," and the Navy has no agreed rule for when a system has delivered enough value to justify its loss.
2. Strike at sea is still thinly evidenced. One three-boat raid and one two-round exercise shot do not establish USV strike as a repeatable capability under jamming, weather, and damaged links. Surveillance remains the unmanned fleet's proven strength.
3. Every loss is also a transfer. Reapers lost to link failures, a Triton lost to an undisclosed cause, and an intact Dive-LD in IRGC hands show that spoofing, jamming, and capture are part of the price. Anti-tamper design, data minimization, and recoverability deserve the same weight as endurance.
4. The law has not decided what these hulls are. UNCLOS Article 29 defines a warship as commanded by a commissioned officer and "manned by a crew" under armed forces discipline. Analysts at CIMSEC and the Naval War College argue that remote human command can satisfy the test, but the status of a crewless vessel, especially one with humans out of the loop, remains unsettled. That affects boarding rights, sovereign immunity, and whether seizing one is an act of war.

Recommendations. The Navy should measure unmanned progress by cycle time, not inventory: how many days from a combat lesson to a fielded fix. That argues for open, modular architectures like the containerized launchers now riding Saildrones; forward sustainment and replenishment pools, since a search-and-rescue role demands drones pre-positioned across a theater; a mine countermeasures force with enough Knifefish-class depth to survive attrition; and early work with allies on the legal status of armed USVs, starting inside Falcon Strike.

The decade ahead will not belong to the navy that owns the most hulls. It will belong to the one that can change fastest when the water changes around it. Hormuz did not create that force. It showed the Navy has begun to build one.

Sidebar: The Black Sea Parallel

Hormuz is teaching the U.S. Navy what Ukraine and Russia have been teaching each other since 2022: the side that adapts fastest, not the side with more hulls, controls the water. The two wars differ in who holds the uncrewed edge, but converge on the same four lessons.

Dimension	Black Sea: Ukraine vs. Russia	Strait of Hormuz: U.S. vs. Iran
Who uses drones, and why	Ukraine, with almost no navy left, uses USVs for sea denial against a larger fleet	The dominant navy adds uncrewed systems to spare crewed ships from cheap threats
Surface strike record	Magura V5 and Sea Baby sank or damaged roughly a dozen warships and pushed the Black Sea Fleet from Sevastopol to Novorossiysk; in 2026 Ukraine struck frigates inside Novorossiysk harbor	One confirmed raid: three Corsairs against Bandar Abbas, 12 July 2026
Cheap air mass	Russia launched 6,000+ missiles and drones in January 2026 alone; Ukrainian interceptor drones at about $3,000 each claim up to 90% of Shaheds	LUCAS, a $35,000 Shahed copy, gives the U.S. cheap strike; U.S. and Gulf states have sought Ukrainian interceptors
Roles converging	Magura V7s carry air-to-air missiles against aircraft; Sea Babies launch FPV drones; the Magura MV11 is a drone-carrying mothership	Saildrone Surveyor fires JAGMs on carrier-group data; a strike Corsair doubles as a rescue craft
Communications as the weak point	Russia's 2026 USV campaign reportedly stalled after it lost Starlink access; Ukraine builds in autonomy for jammed conditions	Some Reaper losses came from data-link failures; a Dive-LD sat dead in the water before capture
The adversary adapts	Russia added booms, nets, helicopter patrols, and its own USVs; Ukraine destroyed three Russian USVs on 23 June 2026	IRGC tried to seize a Saildrone and captured a Dive-LD
Industrial model	Competing private firms; 40,000 interceptors delivered in one month; Magura V7 licensed for U.S. production	CENTCOM's Scorpion and Falcon Strike task forces; a multiyear Pentagon drone buy run as competitive "gauntlets"

Where the lessons converge. Both wars show that sea denial no longer requires fleet parity, only attritable mass and a working kill chain. Both show that the cost exchange decides sustainability: a $3,000 interceptor against a $50,000 Shahed in Ukraine, a $35,000 LUCAS against targets that once demanded a Tomahawk over Iran. And both show that the link, not the airframe or hull, is the component most often lost.

Where they diverge. Ukraine adopted uncrewed systems because it had nothing else; its drones are its navy. The U.S. Navy is grafting them onto a fleet whose crewed carriers and destroyers still carry most of the combat power. That makes the U.S. risk subtler: institutions built around exquisite hulls may absorb the new tools slowly, while a force with no legacy to protect iterates in weeks.

The warning. Russia's Black Sea Fleet did not lose because it was small. It lost because it adapted more slowly than its opponent. Iran, which supplies the Shahed design at the center of both wars, is watching the same lessons.

 

Sources

Official releases and primary documents

1. U.S. Central Command. "CENTCOM Launches First-Ever Multinational Attack Drone Task Force." Public release, 13 August 2026. Mirrored at GlobalSecurity.org. https://www.globalsecurity.org/military/library/news/2026/08/mil-260813-centcom01.htm
2. U.S. Department of Defense. "Operation Epic Fury Fact Sheet." 6 April 2026. https://media.defense.gov/2026/Apr/06/2003907108/-1/-1/1/OPERATION-EPIC-FURY-FACT-SHEET-APRIL-6-2026.PDF
3. Caudle, Adm. Daryl, USN. U.S. Navy Fighting Instructions. Office of the Chief of Naval Operations, February 2026. https://www.navy.mil/Leadership/Chief-of-Naval-Operations/display-cnotes/Article/4400384/us-navy-fighting-instructions/
4. Lockheed Martin. "Lockheed Martin and Saildrone Demonstrate Launch from USV Platform." Press release, Bethesda, MD, 20 August 2026. https://news.lockheedmartin.com/2026-08-20-Lockheed-Martin-and-Saildrone-Demonstrate-Launch-from-USV-Platform

News reporting

5. Oliverio, Natalie. "CENTCOM Has Become a Battle Lab for Unmanned Systems." Army Times / Military Times, 22 September 2026. https://www.armytimes.com/news/your-military/2026/09/22/centcom-has-become-a-battle-lab-for-unmanned-systems/
6. Barrett, Claire. "Sea Drones Strike Iranian Port in Combat First for US." Military Times, 13 July 2026. https://www.militarytimes.com/news/your-military/2026/07/13/sea-drones-strike-iranian-port-in-combat-first-for-us-navy/
7. "U.S. Military Has Lost Roughly 25% of Its Reaper Drones as Iran War Depletes Arsenal." The Washington Post, 13 August 2026. https://www.washingtonpost.com/national-security/2026/08/13/us-military-has-lost-roughly-25-its-reaper-drones-iran-war-depletes-arsenal/
8. "US Confirms First Combat Use of LUCAS One-Way Attack Drone in Iran Strikes." Military Times, 28 February 2026. https://www.militarytimes.com/news/your-military/2026/02/28/us-confirms-first-combat-use-of-lucas-one-way-attack-drone-in-iran-strikes/
9. "Launch of LUCAS One-Way Attack Drone from Navy Ship at Sea Called 'Significant Milestone.'" Stars and Stripes, 18 December 2025. https://www.stripes.com/branches/navy/2025-12-18/lucas-live-launch-20134156.html
10. "Navy Acknowledges Triton Crash as Drone Losses in Iran Mission Mount." Stars and Stripes, 17 April 2026. https://www.stripes.com/theaters/middle_east/2026-04-17/drone-persian-gulf-navy-triton-sigonella-21404402.html
11. "Navy MQ-4C Triton Surveillance Drone Crash in the Middle East Finally Confirmed." The War Zone, 14 April 2026. https://www.twz.com/air/navy-mq-4c-triton-surveillance-drone-crash-in-the-middle-east-finally-confirmed
12. "U.S. Strikes Iranian Boats Trying to Seize Navy Drone in Strait of Hormuz." Axios, 15 September 2026. https://www.axios.com/2026/09/15/us-iran-boats-navy-drone-strait-hormuz
13. "US 'Forcefully' Deters 2 Iranian Boats from Taking Navy Drone in Middle East." Stars and Stripes, 15 September 2026. https://www.stripes.com/theaters/middle_east/2026-09-15/iran-boats-destroyed-middle-east-drone-22861851.html
14. "U.S. Military's Anduril Dive-LD Drone Was 'Dead in the Water' Before Iran Nabbed It." DefenseScoop, 8 September 2026. https://defensescoop.com/2026/09/08/us-militarys-anduril-dive-ld-drone-captured-by-iran/
15. "Underwater Drone Captured by Iran Matches American Anduril Model." Naval News, September 2026. https://www.navalnews.com/naval-news/2026/09/underwater-drone-captured-by-iran-matches-american-anduril-model/
16. "US Admits Underwater Drone Loss as Iran Claims Seizure in Strait of Hormuz." Naval Technology, September 2026. https://www.naval-technology.com/news/us-underwater-drone-iran/
17. "Navy to Use Underwater Drones to Help Clear Iranian Mines from Strait of Hormuz." DefenseScoop, 11 April 2026. https://defensescoop.com/2026/04/11/strait-of-hormuz-mine-clearance-navy-centcom-underwater-drones/
18. "Trump Announces All Mines Have Been Removed from the Strait of Hormuz." Axios, 25 August 2026. https://www.axios.com/2026/08/25/hormuz-mines-remove-oil-iran-trump
19. "How US Used Navy Divers, Underwater Drones to Clear Strait of Hormuz Mines." Gulf News, September 2026. https://gulfnews.com/world/mena/how-us-used-navy-divers-underwater-drones-to-clear-strait-of-hormuz-mines-1.500666159
20. "CENTCOM Details How US Cleared Iranian Mines in Strait of Hormuz." Fox News, August 2026. https://www.foxnews.com/politics/inside-dangerous-us-mission-cleared-irans-mines-hormuz-shipping-lanes
21. "Centcom Creates First 'Multinational' Attack Drone Task Force." DefenseScoop, 13 August 2026. https://defensescoop.com/2026/08/13/centcom-creates-multinational-attack-drone-task-force/
22. "US Navy Launches Missiles from Its First Drone Sailboat." Tom's Hardware, August 2026. https://www.tomshardware.com/tech-industry/drones/us-navy-launches-missiles-from-its-first-drone-sailboat-saildrone-surveyor-launches-dual-jagm-missiles-carrier-strike-group-tests-armed-usv-and-electronic-warfare
23. Frantzman, Seth J. "Heavy MQ-9 Losses in Iran War Signal Push for Mass-Producible Replacement Drones." The Jerusalem Post, September 2026. https://www.jpost.com/defense-and-tech/article-908788

Analysis, history, and law

24. "The Crisis in Mine Countermeasures." U.S. Naval Institute Proceedings 152, no. 4 (April 2026). https://www.usni.org/magazines/proceedings/2026/april/crisis-mine-countermeasures
25. "UDT 2026: Unmanned MCM Systems Could Help Clear Mines in the Strait of Hormuz in Weeks, Given Permissive Environment." Janes, April 2026. https://www.janes.com/defence-intelligence-insights/defence-news/sea/udt-2026-unmanned-mcm-systems-could-help-clear-mines-in-the-strait-of-hormuz-in-weeks-given-permissive-environment
26. "Operation Epic Fury and the Collapse of Iran's Layered Naval Defense." Gulf International Forum, 29 March 2026. https://gulfif.org/operation-epic-fury-and-the-collapse-of-irans-layered-naval-defense/
27. "US-Iran War 2026: Operation Epic Fury and the Hormuz Crisis." Quwa, 26 May 2026. https://quwa.org/iran/military-news-iran/us-iran-war-2026-operation-epic-fury-the-strait-of-hormuz-crisis-and-the-limits-of-american-air-power/
28. "The Navy's Secret Weapon, Circa 1918." U.S. Naval Institute Proceedings, June 1967. https://www.usni.org/magazines/proceedings/1967/june/navys-secret-weapon-circa-1918
29. Defense Technical Information Center. Technical report ADA111764 (history of U.S. guided missiles, including the 6 March 1918 Flying Bomb flight). https://apps.dtic.mil/sti/tr/pdf/ADA111764.pdf
30. "Unmanned Maritime Systems and Warships: Interpretations Under the Law of the Sea." Center for International Maritime Security (CIMSEC), 31 July 2024. https://cimsec.org/unmanned-maritime-systems-and-warships-interpretations-under-the-law-of-the-sea/
31. Materna, Malgorzata. "Adjusting the Aperture: The International Law Case for Qualifying Unmanned Vessels as Warships." International Law Studies 100 (U.S. Naval War College). https://digital-commons.usnwc.edu/cgi/viewcontent.cgi?article=3051&context=ils

Note on sourcing: Items 5, 6, and 1 were read in full; others were verified against publisher search excerpts. Unit costs for Corsair and Dive-LD are open-source estimates, not official figures. The DASH, Ryan reconnaissance drone, and Pioneer references are standard naval history and are not separately cited.

 

 

World's first Acoustic Picket Line at REPMUS26 - Naval News

World's first Acoustic Picket Line at REPMUS26 - Naval News

Listening by the Hundreds: Oshen's Acoustic Picket Line and the New Arithmetic of Undersea Surveillance

A British start-up's four-foot sailboats were tested against live targets at NATO's largest uncrewed-systems exercise. The concept is sound, but the burden of proof has barely started.

---

BLUF

At REPMUS 2026 off Portugal, Plymouth-based Oshen deployed six wind- and solar-powered C-Star micro-USVs, each carrying one hydrophone, as a distributed "acoustic picket line." This was the first test of the concept against live targets. The concept replaces a few expensive towed arrays with many cheap, expendable listening nodes. Those nodes would localize contacts by using several sensors at once and then cue high-end ASW assets such as Type 26 frigates and P-8As. The timing fits the Royal Navy's Atlantic Bastion program and NATO's push to connect hundreds of heterogeneous uncrewed systems to a common data backbone. The C-Star's endurance is well documented: it gathered data inside a Category 5 hurricane and made the first fully autonomous Atlantic crossing. Its value as an ASW sensor is not yet proven. No REPMUS detection results have been released. Physical limits still apply: a single near-surface omnidirectional hydrophone on a small hull, detecting quiet modern submarines, in the sea states of the GIUK Gap. The next data point comes next month, when eight C-Stars join the AUKUS Maritime Big Play experimentation in San Diego.

---

The Demonstration

REPMUS (Robotic Experimentation and Prototyping using Maritime Unmanned Systems) is the Portuguese Navy's annual proving ground for uncrewed systems. The 2026 edition ended on 25 September at Tróia, on the Portuguese coast, and is dedicated to testing and integrating uncrewed systems with conventional forces. NATO Allied Command Transformation reports that REPMUS 2026 drew around 1,500 participants from 36 navies, alongside industry and academic partners operating hundreds of uncrewed craft. One exercise guide puts the total at more than 300 uncrewed surface, underwater, and air systems working a box of more than 400 square nautical miles. (Some secondary reporting gives different participation figures. The ACT numbers are used here.) Task Force X-Arctic Takes its Next Step at REPMUS 2026 - NATO's ACT +2

Within that crowd, Oshen deployed six of its 1.2-meter robotic sailboats, each fitted with a hydrophone, to create a distributed surveillance network tested against live targets for the first time. The hydrophones come from Systems Engineering and Assessment (SEA), and the constellation can hold position to create an acoustic barrier across a given stretch of water. The company says the barrier's shape and density can be adjusted to the threat picture. It also says multiple sensing points can yield a target's location rather than just its bearing, allowing other assets, including towed arrays, to be directed toward a contact. World's first Acoustic Picket Line at REPMUS26 - Naval News +2

The processing architecture matters as much as the hulls. CEO Anahita Laverack has described the C-Stars as processing audio onboard at the edge and relaying only the most crucial information back for constellation-level analysis. Oshen's engineers spent the months before the exercise working with Navy counterparts so that the company's command software would integrate with NATO's, and C-Star readings were fed into a central database shared among Alliance partners. LinkedInNaval News

Oshen's defense and oceanography lead, Gordon Jones, summarized the argument: a network of many hydrophone platforms is "wider, more resilient and harder for an adversary to characterise" than one built on a handful of towed arrays. Laverack framed the goal as taking a capability that depends on scarce assets and making it something that can be "deployed at scale, cheaply and continuously." Naval NewsDefence Blog

As of this writing, neither Oshen nor NATO has published detection, classification, or localization results from the REPMUS serials.

Why the GIUK Gap, Again

The picket line is aimed at a familiar piece of water. Since the Cold War, the Greenland–Iceland–UK Gap has been the chokepoint for tracking submarines moving into the open Atlantic, when the U.S. Navy's SOSUS seabed hydrophone network monitored the same waters. The geography is large. The gap comprises roughly 200 miles between Greenland and Iceland and 500 miles between Iceland and Scotland, and Russia has commissioned more than 30 submarines since the mid-2000s. Defence BlogForces News

The demand signal is rising while British supply is falling. The Royal Navy reports that its activations to monitor Russian activity rose 25 percent over the first seven months of 2026 compared with 2025. In April, Defence Secretary John Healey disclosed a month-long operation in which UK and allied forces tracked an Akula-class submarine and two GUGI deep-sea research submarines operating in the UK's exclusive economic zone. The Royal Navy rotates its five remaining Type 23 ASW frigates through Operation Ceto, the standing mission to detect submarines trying to shadow British SSBNs. Royal Navy steps up monitoring of increased Russian activity in UK waters +2

London's answer is Atlantic Bastion. The program is meant to connect ships, submarines, aircraft, and uncrewed vessels through AI-enabled acoustic detection feeding a digital targeting web, with capabilities due in the water in 2026. Type 26 frigates, with mission bays that can host uncrewed systems, are expected to be the centerpiece of the network. At UDT 2026, Captain James Lovell described the first of the program's pillars as expanding ASW sensing coverage with fixed systems, autonomous platforms, and existing assets, including the "introduction of massive sensors" to complement crewed platforms. The spring 2026 industry demonstrations covered seabed acoustic detection, USVs with acoustic and towed arrays, drones carrying sonobuoys, and gliders and acoustic floats. Planners anticipate an eventual government-owned, government-operated force of uncrewed platforms for persistent North Atlantic ASW. New Royal Navy undersea warfare technology unveiled to counter threat from Russia +4

NATO is building the connective layer in parallel. At REPMUS, ACT's Task Force X-Arctic connected nearly 300 capabilities to its digital backbone, and its mission-engineering layer tasked and re-tasked systems automatically and continuously. The effort supports Arctic Sentry, the enhanced vigilance activity led by Joint Force Command Norfolk and launched in February 2026, and aims for a fully digitized multi-domain situational awareness demonstration in the North Atlantic and High North by summer 2027. A picket line of cheap sensors becomes useful only when a C2 fabric like this can absorb its output. NATO ACTNATO ACT

The Platform: Endurance Is Proven

The C-Star's seakeeping record is strong for a vessel this small. The U.S. Navy's Naval Meteorology and Oceanography Command, which has tested the boats off Mississippi, puts their weight at about 100 pounds, and one person can launch one from the back of a boat. AutoNotion

Its most cited credential comes from NOAA. During Hurricane Humberto in September 2025, one C-Star measured a minimum pressure of 955 millibars and gusts above 150 mph in the Category 5 eyewall. Together with a rise in measured sunlight, the pressure drop confirmed it had passed through the eyewall to the edge of the eye. The National Hurricane Center referred to the C-Star data in an official forecast discussion. The NHC's final Tropical Cyclone Report states that C-Star surface observations helped construct Humberto's best track. Ocean robot first to collect data in Category 5 storm - NOAA/AOML +2

On 13 September 2026, C-Star PC13 finished the Microtransat Challenge east of Barbados after 127 days and 2,926 nautical miles from Gran Canaria, with no intervention from the Oshen team. The fully autonomous prize had gone unclaimed for 16 years despite more than 30 attempts, including by the U.S. Naval Academy. One caution for precision: a Norwegian vessel, SB Met, completed a Microtransat crossing in 2018, so PC13's distinction is the fully autonomous, no-intervention prize rather than the first crossing of any kind. The voyage was not flawless either: around the halfway point PC13 developed erratic steering, which Oshen attributes to rudder friction. Oshen autonomous vessel completes first unaided Atlantic crossing - Smart Maritime Network +2

On acoustics specifically, the public record is thinner. The company's own website reports that in Royal Navy-funded trials off Iceland, a C-Star hydrophone detected maritime assets up to 10 nautical miles away in storm conditions. Neither the target type nor the target's radiated noise level has been published. Oshendata

The Industrial Case

Oshen is trying to win on the production line. It took three years to build its first 15 C-Stars. The Plymouth factory now produces 15 every six weeks, another 100 are on order, and headcount has grown from seven in September 2025 to almost 30. A roughly $5 million round led by Lunar Ventures, with AlbionVC, Twin Track, and Concept Ventures, will fund manufacturing expansion and work in passive acoustics, subsea infrastructure protection, and ASW. Under a UK Defence and Security Accelerator project, the company is also testing a distributed passive acoustic network with ZeroUSV and MarineAI. In August, ZeroUSV launched two C-Stars from its 12-meter uncrewed Oceanus12, with software running the sequence. That demonstration points toward mothership-deployed pickets that do not need a crewed vessel for laydown. Oshen raises $5M to scale autonomous ocean robots for defence missions – Resilience Media +3

The company is small compared with its U.S. peers. Saildrone has raised more than $100 million, and Sofar Ocean nearly $70 million. Even sympathetic coverage notes it is unclear whether Oshen's hurricane data and production record will translate into naval purchasing decisions. Tech Funding NewsTech Funding News

Engineering Reality Check

The following assessment is the author's analysis, not company or Navy data.

Localization, not just detection. A single omnidirectional hydrophone has no array gain and no bearing. The claim that a constellation yields position rather than bearing depends on combining data across nodes, through time-difference-of-arrival, amplitude comparison, or cross-fixing of detections. That needs tight time synchronization (GNSS provides it, and GNSS is contested in the High North), accurate node positions on hulls that drift, and a good sound-speed profile. Node geometry determines localization error in the same way it does in multistatic radar. A thin line of sensors will produce poor fixes along its own axis.

The near-surface problem. A sensor near the surface is exposed to breaking waves, rain, wind, and hull and flow noise, all of which rise with sea state. That penalty is heaviest in the weather the GIUK Gap produces most of the year. More fundamentally, a hydrophone hanging near the surface may sit above the sonic layer while a submarine operates below it, in a shadow zone. This is the problem that pushed Cold War ASW toward deep seabed arrays and variable-depth towed bodies. The public record does not say how deep the SEA hydrophone deploys or whether a deeper, longer cable is planned.

Target strength. A 10-nautical-mile detection of unspecified maritime assets in a storm is encouraging if those assets were surface ships. It says little about a quiet SSN or a GUGI special-mission boat. Against those targets, a single-element sensor's effective range could be a small fraction of that figure, unless onboard processing can exploit narrowband tonals over long integration times.

Barrier arithmetic. Suppose, for illustration, a 2-nautical-mile reliable detection radius against a quiet submarine. The roughly 600 nautical miles of combined gap width then needs on the order of 150 nodes for a single contiguous line. Doubling for depth-in-barrier, and adding spares for attrition and rotation, pushes the total into the high hundreds. That is roughly consistent with Oshen's own suggestion that picket lines of 1,000 C-Stars could support the Royal Navy's new submarine-hunting frigates between Iceland, Greenland, and Britain. At 15 hulls every six weeks, current production would need several years to field such a force. Tripling output changes that calculation materially. Oshendata

Data links and C2. Satellite links from a four-foot hull carry little bandwidth, so edge detection and classification are required, not optional. False-alarm management across hundreds of nodes, much of it caused by fishing traffic and marine mammals, will decide whether operators trust the network. It will also decide whether an automated tasking layer like Task Force X-Arctic's can use the output without saturating watch floors.

Survivability versus exploitation. Low observability and expendability are real advantages. The company notes that an adversary cannot easily tell which vessels are active or what they carry. Unattended hulls can also be recovered by the other side, fouled by fishing gear, or spoofed. Hardware anti-tamper measures and GNSS-resilient timing deserve attention before any operational deployment.

None of these points defeats the concept. They define what the next round of trials must measure: probability of detection against realistic quiet targets across a range of sea states, localization error as a function of node geometry, false-alarm rates, and the time from cue to prosecution by a crewed asset.

From Tróia to San Diego

Oshen plans to bring eight C-Stars to the Maritime Big Play exercise in San Diego next month to test the same picket-line concept in a U.S. setting. Maritime Big Play is an AUKUS Pillar II program. Under it, Australia, the UK, and the United States rapidly test and integrate autonomous systems, with 2026 work focused on ASW, maritime strike, and seabed warfare. Its priorities include shared command-and-control software, a common autonomy baseline across the three nations, and a joint test and reference environment. Oshen already has a U.S. foothold: a Cooperative Research and Development Agreement with the U.S. Navy covering single-beam sonar applications. In a two-week trial with CNMOC off Cat Island, Mississippi, C-Stars fitted with single-beam echo sounders mapped the seabed to depths of about 80 meters. British robot sailboats test submarine picket line +4

For the U.S. Navy, the test matters beyond the GIUK Gap. The same model could apply to Pacific chokepoints and to the protection of undersea cables and pipelines, where persistent, low-cost coverage is scarce.

Implications

The acoustic picket line is best understood as a tripwire, not a replacement for ASW. It is a cheap, persistent, attritable layer meant to shrink the search area that scarce frigates, P-8s, and SSNs must cover. The UK is short of hulls and submarines at the moment Russian undersea activity is rising, so that layer has obvious appeal. The C-Star has proven it can survive at sea. What it now has to prove is acoustic: detection and localization of quiet submarines, from the surface, in bad weather, with false-alarm rates operators can live with. REPMUS 2026 and Maritime Big Play should produce the first data. Navies should require that data to be made available for independent evaluation before judging how much of the promised efficiency is real.

---

Verified Sources

Naval News Staff. "World's First Acoustic Picket Line at REPMUS26." Naval News, September 2026. https://www.navalnews.com/naval-news/2026/09/worlds-first-acoustic-picket-line-at-repmus26

"British Robot Sailboats Test Submarine Picket Line." UK Defence Journal, September 2026. https://ukdefencejournal.org.uk/british-robot-sailboats-test-submarine-picket-line/

"UK Firm Tests Robot Sailboats to Track Hidden Submarines." Defence Blog, September 2026. https://defence-blog.com/uk-firm-tests-robot-sailboats-to-track-hidden-submarines/

"Oshen Tests Distributed Acoustic Surveillance Network." Smart Maritime Network, September 21, 2026. https://smartmaritimenetwork.com/2026/09/21/oshen-tests-distributed-acoustic-surveillance-network/

"UK Tests Anti-Sub Technology." Australian Naval Institute, September 2026. https://navalinstitute.com.au/uk-tests-anti-sub-technology/

Giordano, Paolo. "Task Force X-Arctic Takes Its Next Step at REPMUS 2026." NATO Allied Command Transformation, September 24, 2026. https://www.act.nato.int/article/tfx-arctic-repmus/

"NATO Exercises 2026: The Complete Guide to Allied Readiness." Grosswald, 2026. https://www.grosswald.org/nato-exercises-2026/

"A MUSt DO: REPMUS Is Primary Means for NATO to Develop Maritime Uncrewed Capability." Seapower, March 23, 2026. https://seapowermagazine.org/a-must-do-repmus-is-primary-means-for-nato-to-develop-maritime-uncrewed-capability/

Royal Navy. "AUKUS Nations to Build on Last Year's Successes as They Look Ahead to 2026." February 6, 2026. https://www.royalnavy.mod.uk/news/2026/february/06/20260206-aukus-nations-look-ahead-to-2026

"AUKUS Navies Step Up Autonomous Warfare Push in 2026." UK Defence Journal, February 9, 2026. https://ukdefencejournal.org.uk/aukus-navies-step-up-autonomous-warfare-push-in-2026/

Australian Department of Defence. "Defence Tests Cutting-Edge Autonomous Capabilities during AUKUS Maritime Big Play." February 19, 2026. https://www.defence.gov.au/news-events/releases/2026-02-19/defence-tests-cutting-edge-autonomous-capabilities-during-aukus-maritime-big-play

Royal Navy. "New Royal Navy Undersea Warfare Technology Unveiled to Counter Threat from Russia." December 8, 2025. https://www.royalnavy.mod.uk/news/2025/december/08/20241208-atlantic-bastion

Royal Navy. "Royal Navy Works with Businesses on Enhancing Anti-Submarine Warfare Using Autonomous Tech." April 22, 2026. https://www.royalnavy.mod.uk/news/2026/april/22/20260422-underwater-battlespace-demos

"More Detail Emerges on Royal Navy Atlantic Bastion." UK Defence Journal, April 17, 2026. https://ukdefencejournal.org.uk/more-detail-emerges-on-royal-navy-atlantic-bastion/

"Underwater Surveillance Tech to the Fore as Royal Navy Forges Ahead with Atlantic Bastion." Navy Leaders, April 30, 2026. https://navyleaders.com/news/underwater-surveillance-tech-to-the-fore-as-royal-navy-forges-ahead-with-atlantic-bastion/

"Atlantic Bastion: How the UK Is Addressing the Undersea Threat." DSEI Gateway, April 2, 2026. https://dsei-gateway.com/en/insights/explainers/atlantic-bastion-how-the-uk-is-addressing-the-undersea-threat/

Royal Navy. "Royal Navy Steps Up Monitoring of Increased Russian Activity in UK Waters." August 8, 2026. https://www.royalnavy.mod.uk/news/2026/august/08/20260807-uk-armed-forces-step-up-monitoring-of-increased-russian-activity-in-uk-waters

"Royal Navy Reports 25 Percent Increase in Russian Activity in U.K., North Atlantic Waters." USNI News, August 11, 2026. https://news.usni.org/2026/08/11/royal-navy-reports-25-percent-increase-in-russian-activity-in-u-k-north-atlantic-waters

Cranny-Evans, Sam. "UK Operation to Counter Russian Submarine Activity." Calibre Defence, April 9, 2026. https://www.calibredefence.co.uk/uk-details-operation-to-counter-russian-submarine-activity-in-north-atlantic/

"Bolster Our Capability or Risk Seeing Russia Dominate GIUK Gap, Warns Former Submariner." Forces News, March 10, 2026. https://www.forcesnews.com/services/navy/bolster-our-capability-or-risk-seeing-russia-dominate-giuk-gap-warns-former

NOAA Atlantic Oceanographic and Meteorological Laboratory. "Ocean Robot First to Collect Data in Category 5 Storm." September 30, 2025. https://www.aoml.noaa.gov/mini-ocean-robot-collects-data-in-category-5-hurricane/

National Hurricane Center. Tropical Cyclone Report: Hurricane Humberto (AL082025). NOAA. https://www.nhc.noaa.gov/data/tcr/AL082025_Humberto.pdf

"Oshen Autonomous Vessel Completes First Unaided Atlantic Crossing." Smart Maritime Network, September 16, 2026. https://smartmaritimenetwork.com/2026/09/16/oshen-autonomous-vessel-completes-first-unaided-atlantic-crossing/

"Tiny Autonomous Boat Completes Historic Atlantic Crossing." Interesting Engineering, September 2026. https://interestingengineering.com/transportation/tiny-autonomous-boat-crosses-atlantic-microtransat

"The Microtransat Challenge." Wikipedia. https://en.wikipedia.org/wiki/The_Microtransat_Challenge

"Oshen Raises $5M to Scale Autonomous Ocean Robots for Defence Missions." Resilience Media, August 24, 2026. https://resiliencemedia.co/oshen-raises-5m-to-scale-autonomous-ocean-robots-for-defence-missions/

"Imperial College Grads' Oshen Raises $5M to Mass-Produce Hurricane-Tested Ocean Robots." Tech Funding News, August 21, 2026. https://techfundingnews.com/oshen-raises-5m-hurricane-tested-ocean-robots/

"Oshen's Sail-Powered Robots Prove Seabed-Mapping Capability in U.S. Navy Trial." Ocean News & Technology, August 5, 2026. https://oceannews.com/news/science-technology/oshen-c-star-robots-prove-seabed-mapping-capability-in-us-navy-trial

"Uncrewed Boat Launches Uncrewed Boats." Autonocion, August 2026. https://www.autonocion.com/us/uncrewed-boat-launches-uncrewed-boats/

Oshen Ltd. "Persistent, Wide-Area Ocean Intelligence" (corporate website). Accessed September 25, 2026. https://www.oshendata.com/

A few notes on sourcing. The Iceland 10-nautical-mile detection and the 1,000-hull picket figure come from Oshen's own website (which attributes them to Janes and The Telegraph), and I could not independently retrieve those originals. Reported REPMUS participation figures conflict across outlets, so I used NATO ACT's. The Engineering Reality Check section is analysis, not reported fact.