No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
A Model for the Simulation of a Pulsed Laser Line Scan System
Abstract
Several underwater electro-optical (EO) imaging systems are currently available for both civilian and military use. One type of EO sensor, the laser line scan (LLS) sensor, uses a narrow collimated beam to scan the ocean floor, sweeping out lines as the sensor platform moves through the water. Current LLS systems use a continuous laser beam, and mitigate the laser volume backscatter by offsetting the laser source from the collector. An LLS system that uses a pulsed laser beam is currently under development. The pulsed system can also reduce the collection of laser backscatter by time gating the collector aperture so as to block light during specified time intervals. We present a detailed numerical model for the return signal for the pulsed LLS system and make some rudimentary comparisons between the projected performance for the pulsed and continuous wave variants of the LLS system. We find that the pulsed LLS system has the potential to significantly improve the signal-to-noise ratio relative to the continuous LLS system
... This method also offers a reduced form factor for payload operation on small UUVs. Computer model developers have been concentrating on synoptic model development for LLS and PLLS systems to avert the computational burden of Monte-Carlo ray trace methods and to allow system designers to consider the trade-off of system parameters [21]. Recent studies indicate that the PLLS approach allows temporal separation of the volume scatter and target signals improving the image contrast and the operational range of the system [22, 23]. ...
... This in turn makes it possible to reject the lower frequency components of backscatter and ambient light, further increasing the range capability of the system. This type of system has previously been investigated by various research laboratories [25, 34] and has been the subject of recent simulations using Metron's radiative transfer solver [21] and other radiative transfer codes developed specifically for pulsed underwater laser imaging systems [35]. Sub-systems hardware development (lasers and detection means) and verification of simulation codes continue to remain a primary activity for these systems. ...
Obtaining satisfactory visibility of undersea objects has been historically difficult due to the absorptive and scattering properties of seawater. Mitigating these effects has been a long term research focus, but recent advancements in hardware, software, and algorithmic methods have led to noticeable improvement in system operational range. This paper is intended to provide a summary of recently reported research in the area of Underwater Optics and Vision and briefly covers advances in the following areas: 1) Image formation and image processing methods; 2) Extended range imaging techniques; 3) Imaging using spatial coherency (e.g. holography); and 4) Multipledimensional image acquisition and image processing.
... Though TracePro™ uses a Monte Carlo ray trace technique with an importance sampling feature, the time consumed in modeling scattering from realistic water conditions becomes excessive when a large number of rays is desired. Consequently, we have used simulation software developed by Metron (Reston, VA) for both performance prediction of continuous wave excitation [1] and for pulsed excitation [19] where time history plots are also required. The ongoing testing of the prototype imagers will also serve to validate these codes. ...
... However, the performance of the scanner with a pulsed laser and gated receiver is expected to further improve viewable range by reducing background noise from near field back scatter as well as ambient light. This has been investigated through simulation [18] [19]. ...
Synchronous scan imagers have demonstrated improved ability to see underwater allowing operation at up to 6 optical attenuation lengths. Recent developments in laser and scanning systems show promise for further miniaturization of these systems to allow operation from small AUVs. This paper describes an approach that exhibits several advantages compared to larger systems having a similar receive aperture and Field of View (FOV)
... Previous work on EO data has been focused on Streak Tube Imaging Lidar (STIL) system [1]- [4], and laser line scan (LLS) [5]- [7] based systems. STIL sensor produces highresolution 3-D images of underwater objects by scanning (line by line), on the target field [1]. ...
The problem of detecting underwater targets from electro-optical (EO) images is considered in this paper. A block-based log-likelihood ratio test has been developed for detection and segmentation of underwater mine-like objects in the EO images captured with a CCD-based image sensor. The main focus of this research is to develop a robust detection algorithm that can be used to detect low contrast and partial underwater objects from the EO imagery with low false alarm rate. The detection method involves identifying frames of interest (FOI) containing the potential targets. Once the FOI have been identified, regions of interest (ROI) within the FOI are segmented from the background. Performance of the detection method is tested in terms of probability of detection, false alarm rate, and receiver operating characteristic (ROC) curves for FOI in the selected data runs. The algorithm shows promising results in target detection and generation of good silhouettes for subsequent classification.
The EODES suite of electro-optical system models is designed to provide a high-fidelity simulation capability and accurate performance prediction tools. EODES simulations are being used to analyze the performance of existing electro-optical identification (EOID) systems and to inform new system design concepts. Performance prediction tools are being incorporated into tactical decision aids with near real-time performance assessments for available EOID assets to assist mine countermeasures (MCM) commanders and mission planners in formulating effective tactics. The long-term goal is to develop a complete set of validated models for underwater and airborne electro-optical systems and extend the simulation and performance prediction capabilities to all relevant Navy systems. OBJECTIVES EODES electro-optical sensor models are based on high-fidelity physical models of radiative transfer in turbid media under the assumption of small-angle scattering and Fourier optics models for various scanning systems [1]. A key objective of this program is to develop flexible and efficient numerical solution techniques for these physical models to provide high-fidelity, computationally efficient simulation and performance analysis. The physical and systems models already incorporated within EODES have undergone a rigorous validation process [7,8] and continuous validation remains a key objective. The aim is to provide modeling and simulation capabilities that are relevant to the Navy, whether in the engineering design phase of next-generation EOID systems or for tactical performance prediction. System models, as they are developed, will be submitted to the Oceanographic and Atmospheric Master Library (OAML) Software Review Board (SRB) for certification as Navy standard models. Performance prediction models are expected to be transitioned to Naval Oceanographic Office (NAVOCEANO) upon receiving OAML certification.
Electro-optical identification (EOID) systems are playing an increasingly important role in mine countermeasures (MCM) operations. EOID systems, and other MCM assets, can be used more effectively when tactics and mission planning account for the state of the battlespace environment. The long-term goal of several ONR programs in the Ocean, Atmosphere & Space Division is to develop and transition systems and software to survey the battlespace environment and provide performance estimates for EOID systems operating in that environment. The environmental characterizations and EOID performance predictions are provided to MCM commanders and mission planners anywhere around the world in near real-time to aid in asset allocation and the development of an appropriate and effective course of action. Performance prediction software will be certified for inclusion in the Oceanographic and Atmospheric Master Library (OAML) and placed under configuration management. OBJECTIVES The objectives of this project were to demonstrate a complete tactical performance prediction capability for electro-optical identification (EOID) systems during the RIMPAC-08 Naval exercise and to prepare EODES performance prediction code for submission to the Oceanographic and Atmospheric Master Library (OAML) Software Review Board (SRB) for certification as a Navy standard model. The RIMPAC-08 exercise is described in last year's annual report for this same contract, so we will limit the present review to the submission of EODES software to the OAML SRB.
A new technique has been found that uses in-phase and quadrature phase (I/Q) demodulation to optimize the images produced with an amplitude-modulated laser imaging system. An I/Q demodulator was used to collect the I/Q components of the received modulation envelope. It was discovered that by adjusting the local oscillator phase and the modulation frequency, the backscatter and target signals can be analyzed separately via the I/Q components. This new approach enhances image contrast beyond what was achieved with a previous design that processed only the composite magnitude information.
The effects of ocean waves on lidar imaging of submerged objects are investigated. Two significant consequences of wave focusing or defocusing are quantified: (a) intensification of near-surface backscatter in which the mean return is increased relative to that for a flat interface, and (b) spatial-temporal modulations of the backscattered return. For the former, mean returns can be as much as 50% larger than flat surface returns at shallow depth. For the latter, the strong modulations induced by wave motion present a dominant clutter field that significantly affects the imaging of shallow objects. Both effects are compensated at greater depths by beam spreading caused by multiple scattering, which diminishes the intensity of the wave focusing.
The radiance N, resulting from an initially narrow collimated beam, propagating in the ocean, is investigated with the aid of the small-angle-scattering theory of electron scattering. The resulting expression for N is calculated approximately and compared both with experiments and numerical calculations. Various properties of the approximate expression for N are explored; an observer, located off the axis of the beam at an axial distance z from the source, who attempts to determine the direction to the source by seeking the maximum of N with respect to ray direction, would sight a point on the beam axis at a distance z/3 from the source.
It is shown that the radiative transfer theorems are the consequences of
a simple identity. A uniqueness theorem essential to later arguments is derived.
Various results which have been called ''Reciprocity Principles'' are obtained as
special cases. In plane problems, Chandrasekhar's equations for the reflection
function for a haif-space are derived. These equations are used to obtain the
Green's function for one velocity neutron diffusion in the same geometry. A
specific reciprocity result is used to obtain the Green's function for two
adjacent half-spaces. Chamdrasekhar's equations for a slab are obtained as
another specialization of the general reciprocity identity together with certain
invariance considerations. (M.H. R.)