Currently, the most widely used technology to detect underwater objects is sound navigation and ranging (SONAR), because acoustic waves can penetrate the water depths to the bottom of the sea. SONAR also suffers from unwanted multipass echoes that are due to reflections from the surrounding terrain. Thus, high-resolution underwater imaging using SONAR is difficult.
Alternatively, 3-D imaging Lidars have emerged as underwater remote sensors for several applications like detection and ranging of submersible targets . They use Lasers operating in the blue-green region of the light spectrum(420 : 570nm). These wavelengths suffer minimum attenuation through water ( less than 0.1 m-1) and maximum laser reflection from estimated target (like mines or submarines) to provide a long range of detection.
The U.S. Navy has developed ALMDS (Airborne Laser Mine Detection System), designed to operate from the MH-60S helicopter, that uses a Laser Imaging Detection and Ranging blue-green laser to detect, and identify naval mines near the surface. ALMDS operates from the low flying, and smaller, helicopters. Surface mines are either moored (via a chain to the bottom) or floating (a favorite terrorist tactic), and many float just below the surface. The laser works very quickly, and enables the ALMDS equipped helicopter to quickly check out large areas for surface mines.
Researchers from Mitsubishi Electric Corporation and Japan Agency for Marine-Earth Science and Technology have developed an underwater three-dimensional (3-D) imaging sensor using a 532-nm laser.
China is developing a satellite with a powerful laser for anti-submarine warfare that researchers hope will be able to pinpoint a target as far as 500 metres below the surface. Project Guanlan, meaning “watching the big waves”, was officially launched in May 2018 at the Pilot National Laboratory for Marine Science and Technology in Qingdao, Shandong. Five hundred metres is ‘mission impossible’,” said a lidar scientist with the Shanghai Institute of Optics and Fine Mechanics at the Chinese Academy of Sciences, who is not involved in the project.
Experiments carried out by the United States and former Soviet Union achieved maximum detection depths of less than 100 metres, according to openly available information. That range has been extended in recent years by the US in research funded by Nasa and the Defence Advanced Research Projects Agency (DARPA). A device developed by DARPA, for example, was mounted on a spy plane and achieved reliable results at a depth of 200 metres, detecting targets as small as sea mines.
Underwater Laser Imaging technology
Lidars (Light Detection and Ranging) are similar to radars in that they operate by sending light pulses to the targets and calculate distances by measuring the received time. Since they use light pulses that have about 100,000 times smaller wavelength than radio waves used by radar, they have much higher resolution. Such systems perform high-resolution line scans that can be assembled into highly detailed images of sea floors to aid in searches for downed aircraft or sunken ships. The acoustic systems have limitations in the range resolution and accuracy; while, the potential benefits of a laser based underwater target detection include high directionality, high response, and high range accuracy. Range gating method is applied to underwater laser imaging system
The developing underwater laser target imaging detection technique can illustrate contour, shape of target and then detect target conveniently. Because effect of water medium and floating particles, water character of laser absorbing and scattering determines laser transmission energy and intensity and they affect traits of signal received.
In laser image target-to-background ratio is very low, background is complex and target is submerged by backscatter and formidable noise. This makes it very difficult to detect weak target. From view of fact application, on the one hand resolution limitation of imaging system and integrative effects of water transmission characteristic make difference between man-made target and natural background faint. Target becomes illegibility while it is far from the receiver.
Jaffe laboratory has been working for several decades on developing advanced laser imaging systems to improve image contrast and range in poor visibility waters typical in bays, harbors, and coastal environments. The latest development effort was tasked with reducing the size and power consumption of the system to fit on the REMUS 600 AUV. The system is currently operational and has performed several trials on the REMUS 600 with excellent results compared to conventional imaging methods. The system is currently operational and has performed several trials on the REMUS 600 with excellent results compared to conventional imaging methods.
Kraken Develops 3D Underwater Laser Imaging System for AUVs
Kraken Sonar Inc. has announced that its subsidiary, Kraken Robotik GmbH, has developed SeaVision™ systems, the world’s first RGB underwater laser imaging system that offers the resolution, range and scan rate to deliver dense full colour 3D point cloud images of subsea infrastructure with millimetre accuracy in real time. The initial system is designed for deployment on underwater robotic platforms such as Remotely Operated Vehicles (ROVs) and Autonomous Underwater Vehicles (AUVs). A hand-held diver system is planned for release later this year.
In recent years, 3D imaging sensors have increased in popularity in fields such as human-machine interaction, augmented reality, cartography and movies. These sensors provide raw 3D data that’s processed by imaging software to obtain 3D volumetric information. This workflow is known as 3D reconstruction and is a tool that to date has been primarily used in terrestrial and aerospace applications.
However, the ability to generate accurate 3D reconstruction of underwater infrastructure is an important requirement for commercial, military and ocean research applications. While sonar is the technology of choice for covering large areas, 3D laser systems such as Kraken’s SeaVision™ provide significantly higher resolution and accuracy at inspection ranges of under 10 metres.
SeaVision™ uses a full colour laser scanning process that’s repeated thousands of times per second to generate coordinate values of millions of points on a reflected surface. The coordinates and intensity associated with each reflected laser pulse are processed in real time to generate an ultra high resolution point cloud. SeaVision produces over 300,000 colored points per second and can reconstruct a 3D object in real-time with typical spatial accuracy of less than 2 millimetres. These datasets can be used to create highly detailed models for 3D visualization, asset management, artificial intelligence and predictive analytics.
Unlike other underwater laser scanning systems, SeaVision™ does not have any externally moving parts. It is integrated in a compact twin pod configuration with flexible mounting options and localized auto-calibration. This enables the system to be mounted at-sea without the need for a specialist or technical support.
Using Structure from Motion photogrammetric range imaging and correlation techniques similar to Kraken’s Synthetic Aperture Sonar technology, SeaVision’s highly sensitive colour cameras are used for motion compensation and micro-navigation. Advanced signal processing algorithms correct vehicle motion during laser scanning without the need for an expensive inertial navigation system. The laser scans are co-registered to the camera images to provide both optical data and 3D point clouds for quantitative measurements.
Another unique feature is the application of six laser lines in Red, Green and Blue (RGB) colours to reproduce full colour information. All data is processed on-board in real-time and can be directly streamed and viewed topside or stored on the system’s multi-terabyte solid state drive. SeaVision™ can also be used in profiling mode, where the lasers automatically maintain optimal scan angles and acquire colour 3D data as the ROV or AUV platform moves along the target.
Dr. Jakob Schwendner, Managing Director of Kraken Robotik GmbH said, “3D laser scanning unlocks the potential of underwater surveys for subsea asset assessment. SeaVision™ enables the existing conditions of underwater assets to be captured as millions of data points, which can then be imported into 3D modeling software for creating realistic, to-scale images of the asset. The data available in 3D models can help improve decisions. During meetings and evaluations, 3D models will benefit both technical and non-technical people because they can easily interpret the model. The level of detail provides more useful information that helps in easy visualization and advanced analysis.”
Northrop Grumman’s ALMDS (Airborne Laser Mine Detection System) for mine warfare
The US Navy has attained initial operating capability (IOC) for its AN/AES-1 Airborne Laser Mine Detection System (ALMDS).
Designed and built by Northrop Grumman,The ALMDS will be integrated into the MH-60S helicopter to provide rapid wide-area reconnaissance and assessment of mine threats in littoral zones, confined straits, choke points, and amphibious objective areas. The ALMDS will be embarked as part of the mine countermeasures (MCM) mission package on the Littoral Combat Ship (LCS). ALMDS provides rapid wide-area reconnaissance and assessment of mine threats in sea lanes, littoral zones, confined straits, choke points and amphibious areas of operations. This agile system’s detection speed and accuracy will significantly improve the U.S. Navy’s mine detection capabilities and help ensure the safety of service members around the world.
The ALMDS system features several capabilities that make it the first of its kind. It leverages a sensor pod to rapidly sweep the water using laser technology. The sensor pod can also be rapidly installed on a medium-lift helicopter and quickly removed after mission completion.
“Using forward motion of the aircraft, ALMDS’ pulsed laser light generates 3-D images of the near-surface volume to detect, classify and localize near-surface moored sea mines,” said Mark Skinner, vice president, directed energy, Northrop Grumman. “Highly accurate in day or night operations, the untethered ALMDS sensor conducts rapid wide-area searches with high accuracy.” The target data generated by ALMDS is displayed on a console and stored for post-mission analysis.
Northrop Grumman successfully integrated and demonstrated ALMDS on a UH-60M Blackhawk helicopter. The first international sale of ALMDS occurred in 2012 to the Japan Maritime Self Defense Force (JMSDF), and the JMSDF has completed flight qualification testing of ALMDS on an MCH-101 helicopter.
ALMDS uses Light Detection and Ranging (LIDAR) technologies to detect, classify and localize naval mines near-surface moored sea mines. The untethered sensor is capable of day or night operations and can attain high area search rates with accurate localization to support follow-on mine neutralization. The ALMDS uses the forward motion of the aircraft to generate image data, which simplifies the scanning process and helps ensure high system reliability.
The ALMDS uses pulsed laser light and streak tube receivers housed in an external equipment pod on the MH-60 helicopter. These lasers are designed to search the water column from the surface to about 40 feet in depth — the area where mines are the biggest threats and coincidentally where mine-hunting sonar systems are least effective. The system takes an image of the entire near-surface water column potentially containing mines.
The ALMDS projects a pulsed wide 538-nanometer blue-green laser beam into the water and samples at rates greater than 100 per second. According to a Defense Science Journal publication, the wavelength of the blue green laser has the unique ability to maintain about 50 percent of its radiation intensity when penetrating ocean water. Blue-green lasers emit wavelengths of about 450 to 550nm, according to a report by the Australian Defense Force, with wavelengths closer to 550 nm used for more opaque water, and shorter wavelengths used to penetrate clearer water.
Will China’s new laser satellite become the ‘Death Star’ for submarines?
Project Guanlan, meaning “watching the big waves”, was officially launched in May at the Pilot National Laboratory for Marine Science and Technology in Qingdao, Shandong. It aims to strengthen China’s surveillance activities in the world’s oceans, according to the laboratory’s website.
Scientists are working on the satellite’s design at the laboratory, but its key components are being developed by more than 20 research institutes and universities across the country. Song Xiaoquan, a researcher involved in the project, said if the team can develop the satellite as planned, it will make the upper layer of the sea “more or less transparent”. “It will change almost everything,” Song said. While light dims 1,000 times faster in water than in the air, and the sun can penetrate no more than 200 metres below the ocean surface, a powerful artificial laser beam can be 1 billion times brighter than the sun.
Song said the team aimed to use every available sensing method to achieve the maximum possible depth of detection. “Sometimes there may not be enough light to reach 500 metres and back, but we can still try to work out what’s down there by taking an indirect measurement at a shallower depth,” he said.
The device is designed to generate high-power laser beam pulses in different colours, or frequencies, that allow sensitive receivers to pick up more information from various depths. Those laser beams can scan an area as wide as 100km, or concentrate on one spot just 1km wide. It will be used in conjunction with a microwave radar, also mounted on the satellite, to better identify targets. Although the radar cannot penetrate water, it can measure the surface movement with extremely high accuracy – so when a moving submarine creates small disturbances on the surface, for example, the radar will tell the satellite where to throw the laser beam.
Zhang Tinglu, another researcher involved in the project, said the main target for the satellite was the thermocline – a thin layer of water where the temperature changes abruptly. He declined to elaborate on the role of the satellite in anti-sub warfare, but the thermocline is known to be important for submarine captains because it can reflect active sonar and other acoustic signals. That means a vessel could potentially avoid detection in the thermocline, but not by a laser beam.
Small LIDARs on UAV
Bathymetric lidar is used to determine water depth by measuring the time delay between the transmission of a pulse and its return signal.
A team at the Georgia Tech Research Institute (GTRI) has developed bathymetric lidars that are much smaller and more efficient than the current full-size systems. The new technology, developed under the Active Electro-Optical Intelligence, Surveillance and Reconnaissance (AEO-ISR) project, would let modest-sized unmanned aerial vehicles (UAVs) carry bathymetric lidars, lowering costs substantially.
And, unlike currently available systems, AEO-ISR technology is designed to gather and transmit data in real time, allowing it to produce high-resolution 3-D undersea imagery with greater speed, accuracy, and usability.These advanced capabilities could support a range of military uses such as anti-mine and anti-submarine intelligence and nautical charting, as well as civilian mapping tasks.
Underwater three-dimensional imaging laser sensor with 120-deg wide-scanning angle using the combination of a dome lens and coaxial optic
Researchers from Mitsubishi Electric Corporation and Japan Agency for Marine-Earth Science and Technology have developed an underwater three-dimensional (3-D) imaging sensor using a 532-nm laser. The key feature of this sensor is the wide-scanning angle combined with pressure resistance and compactness. This feature is realized by the combination of the dome lens, the coaxial optics, and an optical design that avoids internal light reflections.
The sensor system realize a wide-scanning angle of 120 deg (horizontal)×30 deg (vertical)120 deg (horizontal)×30 deg (vertical) while having a compact size of 25-cm diameter and 60-cm length. The received signals are compensated using a sensitivity time control (STC) circuit in which the reception gain is increased according to the TOF in order to compensate the attenuation effect of the signal intensity in the water.
A detector sensitivity time control circuit and a time-to-digital converter are used to detect a small signal and suppress the unwanted backscattered signals due to marine snow. 3-D imaging of the seafloor with 20-m width and 60-m length was demonstrated in the sea around Ishigaki Island, Japan. “The results of the field experiments indicate that our sensor exhibits significant potential for underwater exploration using AUVs and ROVs,” write the authors.
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