Maritime surveillance relies on advanced technologies to ensure the safety and security of national and international waters, particularly in monitoring vessel activities. Distributed Acoustic Sensing (DAS) has emerged as a powerful technology for detecting and analyzing underwater acoustic signatures along fiber-optic cables. However, the lack of annotated DAS datasets in maritime contexts, combined with the high dimensionality and unstructured nature of recorded data streams, hinders the deployment of automated solutions that rely on labeled data for vessel detection. This work introduces DASBoot, a novel annotation toolkit designed to enhance maritime surveillance by aligning vessel signatures from DAS data with Automatic Identification System (AIS) messages. Our approach integrates data processing, fusion, and visualization within a cohesive workflow that significantly reduces the cognitive load on analysts while improving the accuracy of vessel identification. The experimental results demonstrate the effectiveness of our method for dataset annotation and pave the way for future advancements in DAS-based automated maritime surveillance.
In the maritime domain, fog represents a significant challenge, e.g. for the detection of castaways in search and rescue missions or obstacle avoidance/warning systems, particularly at night. In scattering environments, conventional active imaging techniques often struggle due to backscattering resulting in low contrasts or completely saturated pixels from backscattering at fog particles (water droplets). The effective suppression of atmospheric backscatter in foggy environments by using Gated-Viewing technology has been demonstrated in previous studies. However, another alternative of blocking backscattered light is using polarization if the scattering event preserves polarization, which is the case for water droplets (maritime fog). In the present study, we modified an existing, unpolarized light emitting Gated-Viewing instrument and added the option to emit and receive linear polarized light through installation of polarization filters. All images taken with this modification are acquired in Continuous Exposure (CE) mode, i.e. without gating. The objective was to investigate whether polarization can serve as an alternative to Gated-Viewing in the maritime domain where backscatter is expected to largely preserve polarization. In initial field tests the efficiency of the modified (polarized) instrument in suppressing retroreflections under clear weather conditions was evaluated. Further tests with artificial fog showed that the modified instrument could effectively suppress backscatter, achieving a contrast loss comparable to that observed in clear weather conditions.
Two compact and portable SWIR active imaging instrument configurations aiming at vision enhancement in indoor applications are tested and compared, working at 1300 nm and 1550 nm, respectively. Both configurations are in-house developments, but based on a limited number of standard and commercially available components (cameras, LEDs). The instruments provide images (640×512, resp. 1280×1024 pixels) at a rate of ca. 17 Hz (live stream) that can be displayed either directly on an integrated display or send via (wireless) network. Key specifications (optical power, field of view, heat development) have been characterized in laboratory tests. The performance of the two system configurations in terms of vision enhancement is compared both practically (field tests) and theoretically (Mie scattering theory). The 1300 nm illuminator has almost double power compared to the 1550 nm illuminator. However, Mie calculations predict more backscatter and less transmission through fog and smoke, which is highly depending on the particle size. Field tests using artificial fog and an in-house developed transmissiometer have been performed to validate the findings from modeling and found a vision enhancement in the order of one magnitude due to use of SWIR (instead of the Visible) for use in typical environments for which the instruments are designed for. A substantial additional improvement in terms of vision enhancement could be achieved by using polarized light and polarization optics to reduce the backscatter signal. In contrast to other research studies, this vision enhancement is not based on polarization difference imaging but on reducing the backscatter component only, enabling a robust and simple system design.
A portable short-wave infrared (SWIR) sensor system was developed aiming at vision enhancement through fog and smoke for support of emergency forces such as fire fighters or the police. In these environments, wavelengths in the SWIR regime have superior transmission and less backscatter in comparison to the visible spectral range received by the human eye or RGB cameras. On the emitter side, the active SWIR sensor system features a light-emitting diode (LED) array consisting of 55 SWIR-LEDs with a total optical power output of 280 mW emitting at wavelengths around λ = 1568 nm with a Full Width at Half Maximum (FWHM) of 137 nm, which are more eye-safe compared to the visible range. The receiver consists of an InGaAs camera equipped with a lens with a field of view slightly exceeding the angle of radiation of the LED array. For convenient use as a portable device, a display for live video from the SWIR camera is embedded within the system. The dimensions of the system are 270 x 190 x 110 mm and the overall weight is 3470 g. The superior potential of SWIR in contrast to visible wavelengths in scattering environments is first theoretically estimated using the Mie scattering theory, followed by an introduction of the SWIR sensor system including a detailed description of its assembly and a characterisation of the illuminator regarding optical power, spatial emission profile, heat dissipation, and spectral emission. The performance of the system is then estimated by design calculations based on the lidar equation. First field experiments using a fog machine show an improved performance compared to a camera in the visible range (VIS), as a result of less backscattering from illumination, lower extinction and thus producing a clearer image.
Two Gated-Viewing instruments of different design, but similar mean optical power, were compared during a field test: The TRAGVIS sensor is an experimental, scientific development which was designed for particular needs of maritime search and rescue operations. The instrument uses pulsed VCSELs in the NIR, and a CMOS camera in multi-integration mode. As designed for distances < 400 m, a fixed focal length (wide angular FOV of ≈ 9° ) is used, and the repetition rate is high, while the pulse energy is low. The MODAR is a commercial multi-sensor platform comprising a Gated-Viewing instrument designed for security operations (e.g. police) both on sea and on land. Aiming at distances up to several kilometers, both camera and laser illumination are equipped with zoom optics, and the repetition rate is small, while the pulse energy is high. In contrast to TRAGVIS, an image intensifier is used. TRAGVIS and MODAR were compared in terms of signal-to-noise ratio (SNR) and image contrast using Lambertian reflectors at different distances. TRAGVIS was found to perform better than MODAR at distances < 350 m, but its performance decreases with distance while MODAR’s performance stays constant as a result of the laser and camera zoom. When used in ungated (continuous exposure) mode, TRAGVIS shows > 5 times larger SNR than in gated mode, and almost one order of magnitude larger SNR than MODAR due to the lack of an image intensifier. This demonstrates the instrument’s ability to be used for both, Gated-Viewing as well as simple active illumination mode. However, for the same reason (image intensifier) MODAR’s shutter suppression, which is crucial for reducing the back-scatter signal and therefore vision enhancement, was found to be at least 5-6 times better than that of TRAGVIS.
KEYWORDS: Signal to noise ratio, Backscatter, Visibility, Sensors, Reflectivity, Visibility through fog, Signal attenuation, Gated imaging, Maritime surveillance
Maritime search and rescue operations (SAR) are highly affected by harsh environmental conditions and darkness (night time operation). Especially at low visibility and high humidity scenarios like fog, mist or sea spray, gated-viewing offers an active-imaging solution to effectively suppress atmospheric back-scatter and enhance target contrast. The presented TRAGVIS gated-viewing system is designed to fill the needs in SAR operations: at least 185 m detection range at a minimum FOV of 7°x6° and operates in the NIR at 804 nm emission wavelength, combining a high repetition rate VCSEL illuminator with an accumulation mode CMOS camera. The performance of the demonstrator in a wide range of different visibility fog events and different sets of system parameters has been evaluated by analysing the target signal, contrast and signal to noise ratio SNR as a function of the optical depth OD, which was measured by an atmospheric visibility sensor. As the back-scattered signal (suppressed by the camera shutter) overcomes the target signal of a 41% reflectivity target at OD > 4, it was found, together with a low target signal, to be the major reason for the drop of contrast after a vision enhancement up to OD ≈ 3. A limitation of the system to approximately OD = 5.3 is estimated, as the image shows a decent contrast of 10%, but at an SNR of only ∼ 2.2. The highest potential for improvements was found in an optimised placement of the illuminator with respect to the receiver and scene geometry.
In order to support maritime search and rescue activities, an affordable gated-viewing instrument has been developed within the TRAGVIS project. The instrument, which got the name TRAGVIS after the project’s name, has the purpose of vision enhancement during night-time missions under bad visibility conditions. TRAGVIS consists of a compact, eye-safe NIR (near-infrared) laser light source and a monochromatic 1.3 Mpixel camera and has a field of view similar to the one of common field glasses. The camera sensor was recently upgraded from the Onyx to the Bora rev.A sensor from Teledyne–e2v, and a thorough comparison will be shown between them. Several field tests were conducted on an out of service airport and in a maritime environment. The measured gray values of the instrument were calibrated to the reflectivity of the targets at different distances. Furthermore, the performance of the instrument has been studied under different visibility conditions. Therefore, several images of the same target were taken with the gated-viewing mode enabled and disabled. These measurements showed that even during the presence of a light fog with an extinction coefficient of 3.8 km−1, the measured contrast decreased by more than a factor of 3 when gated-viewing was disabled. Contrary to this, no significant decrease in the contrast could be identified using the gated-viewing feature of our instrument. In the maritime environment, field tests at a close harbor were performed for the identification of different maritime objects such as sailing boats, rubber boats and drones. Using several TRAGVIS images taken at monotone increasing gate distances, a simple method was applied to build three dimensional images of this maritime scenery.
A novel gated-viewing instrument is presented for vision enhancement in maritime search and rescue applications predominantly under limited visibility conditions at night. The compact device consists of a camera and an eye-safe NIR (near-infrared) illuminator and has a field of view of ≈7° x 6°, which is similar to field glasses. The detection range is 250 m for Lambertian reflectors, but is much larger if clothes with retro-reflectors are worn. A key challenge is the cost effectiveness of the instrument as potential users in the field of maritime search and rescue applications usually suffer from financial limitations. As a result, no image intensifier, but an off-the-shelf CMOS camera in accumulation mode with a reasonable quantum efficiency in the NIR region is used. The active illumination is based on a self-developed illuminator consisting of 7 pulsed vertical-cavity surface-emitting laser (VCSEL) arrays. The mean optical power is 7 W, the center wavelength is λ≈804 nm, and the light pulse width is ≈100 ns at a repetition rate of 345 kHz. Detailed simulations leading to the system design are presented together with respective characterization measurements of the camera and illuminator as well as first test measurements of the complete system.
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