Detailed three-dimensional (3-D) imaging of the surface of the seafloor is essential for underwater exploration. Currently, the most widely used technology for exploration is sound navigation and ranging (SONAR), because acoustic waves can propagate through a long distance in water. However, SONAR 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 for several applications in the area of detection and ranging of submersible targets has been proposed using 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.
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.
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.
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 U.S. Navy’s AN/AES-1 Airborne Laser Mine Detection System, designed and manufactured by Northrop Grumman Corporation (NYSE: NOC), has achieved Initial Operational Capability. 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.
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. 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.
“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. The Navy’s ALMDS installation aboard the MH-60S Seahawk helicopter is mounted on a Bomb Rack Unit 14, which is installed on the Carriage, Stream, Tow, and Recovery System. Northrop Grumman’s self-contained design allows the system to be installed on other aircraft types.
Earlier this year, 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.
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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