Surface vessel wake detection
Inventors
Estes, Lee E • Doyle, Stephen B
Assignees
US Department of Navy • Government of the United States of America
Publication Number
US-9702819-B1
Publication Date
2017-07-11
Expiration Date
2036-06-22
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Abstract
A system is provided for detection of surface glints on a water surface. A laser of the system produces a nearly horizontal or vertical linearly polarized laser light pulse. Lens of the system form an afocal imaging system with lateral magnification and longitudinal magnification that projects an image of the laser light at a pulse location onto the water surface. A portion of the laser light is focused onto a high speed detector. The output of the high speed detector is connected to a digitizer to provide system synchronization and to monitor the laser light. A glint image of the pulse location is detected by a broad band detector. The electrical output of the detector is input to the digitizer where the output is digitized. The output of the digitizer is then sent to a computer where the output is stored and analyzed.
Core Innovation
The invention provides a system for detection of surface glints on a water surface by using a laser that produces nearly horizontal or vertical linearly polarized laser light pulses. It employs an afocal imaging system designed with lateral and longitudinal magnification to project a focused laser pulse location onto the water surface. The system includes a high speed detector and a broadband detector to detect glint images of the pulse location. The detected signals are digitized and analyzed by a computer for storage and further analysis.
The problem solved by the invention is the difficulty in detecting surface vessel wakes from underwater due to the challenges presented by water absorption and light scattering that complicate underwater detection with solar illumination and existing laser backscatter methods. Existing underwater LIDAR systems detect wakes by detecting backscattered laser light from wake bubbles, but the returns are weaker and suffer from resolution limitations. The invention aims to detect and characterize wakes made by surface vessels from underwater by exploiting the strong surface glints which are more intense and provide more detailed information than bubble backscatter.
The system achieves this by focusing a laser beam to a small optical spot on the water surface; when this spot is oriented normally to the surface, strong glints are reflected back to the detector. Scanning the laser pulses across the water surface generates a two-dimensional image of glint occurrences, which can be correlated with wake models to determine vessel characteristics such as size, speed, direction, and wake age. The invention also addresses interference from ambient light by employing temporal, spatial, spectral, and directional filtering, enabling operation in daylight and high background environments.
Claims Coverage
The patent includes two independent claims covering a system and a method for detection of glints on a water surface. The inventive features focus on the system's optical components and the method's procedural steps for glint detection and analysis.
System configuration for glint detection using afocal imaging
The system includes a laser emitting pulses with controlled diameter onto a water surface, a half wave plate to produce linearly polarized light, an afocal imaging system combining unit image magnification and power telescopes with lateral and longitudinal magnification to project a pulse location image. A polarizing beam splitter deflects a portion of the laser pulse to a high speed detector for monitoring, and reflected glints are converted from circularly to linearly polarized light to be reflected back through the beam splitter for detection. The system filters the detected light spectrally and spatially using apertures and an interference filter before broadband detection. Outputs from detectors are digitized and analyzed via computer.
Method for detecting glints on water surface using polarized pulsed laser and imaging
The method comprises emitting circularly polarized laser light pulses onto a water surface at a defined diameter, directing a portion of the laser pulse to a high speed detector for monitoring, detecting glint signals by imaging the reflected glints, filtering the image spectrally through an interference filter to pass only the laser wavelength, and detecting the filtered image with a detector for digitization and analysis.
The claims cover a system utilizing precise polarization control, afocal imaging telescopes, and advanced filtering to detect and analyze laser-induced glints from a water surface, and a method involving emitting, monitoring, filtering, imaging, and detecting glints for characterization of surface wakes.
Stated Advantages
Use of strong surface glint returns provides more intense signals compared to bubble LIDAR systems, improving detection sensitivity.
Capability to determine vessel characteristics including size, speed, direction, and wake age by analyzing glint signal geometry and amplitude.
Temporal, spatial, spectral, and directional filtering greatly reduces background light interference, enabling operation in daylight and high background environments.
Afocal optics allow operation at different water clarity conditions or stand-off distances by adjusting focal range via translation of telescope lenses.
The system serves as a sensor for general sea state measurement and detection of unwanted surface pollutants like oil slicks.
Documented Applications
Underwater detection and characterization of wakes made by surface vessels, including determination of vessel size, speed, direction, and wake age.
Operation from underwater vehicles to construct two-dimensional images of glint signal occurrences for wake analysis.
Use as a sea state sensor and sensor for surface pollutants such as oil slicks.
Operation from above the water surface for glint detection.
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