KEYWORDS: Image processing, Signal attenuation, Ranging, Backscatter, Scattering, Modulation, Image enhancement, Detection and tracking algorithms, Sensors, Signal to noise ratio
A processing technique for enhancing imagery and ranging data collected by an underwater modulated pulse laser system is presented. Laser-based sensors offer high-resolution and high-accuracy for imaging and ranging applications in the underwater environment. However, these capabilities can be degraded in turbid waters due to scattering. We present experimental results demonstrating a technique inspired by image processing which reduces the effects of both backscatter and forward scatter. By combining individual return waveforms of the modulated pulse laser system together and applying the processing approach as described, images can be formed in which an object in the scene can be distinguished from scatter using an edge detector. Results obtained by applying the technique to laboratory experimental data are presented and compared to a baseline approach. Useful three-dimensional imagery was generated out to 6.9 attenuation lengths, a 25% improvement over the baseline. In range finding experiments without a range-gate, a test object was detected at 11.1 attenuation lengths downrange, compared to 5.7 attenuation lengths for the baseline.
The scattering of light observed through the turbid underwater channel is often regarded as the leading challenge when designing underwater electro-optical imaging systems. There have been many approaches to address the effects of scattering such as using pulsed laser sources to reject scattered light temporally, or using intensity modulated waveforms and matched filters to remove the scattered light spectrally. In this paper, a new method is proposed which primarily uses the backscattering asymmetry property for object detection and geometric profiling. In our approach, two parallel and identical continuous wave (CW) laser beams with narrow beam widths (~2mm) are used as active illumination sources. The two beams also have controllable spacing and aiming angle, as well as initial phase difference for convenience of scanning and profiling a target. Through theory and experimental results, it will be shown that when an object leans or tilts towards one of the beam’s central trajectory, the asymmetry in the backscattered signals can be used to indicate the location or slope of the target’s surface, respectively. By varying the spacing or aiming angle of the two beams, a number of surface samples can be collected to reconstruct the object’s shape geometrically. The resolution and range limit of our approach are also measured and reported in this work. In application, our proposed method provides an economic solution to perform imaging through turbid underwater environments. Additionally, the idea can be combined with the pulsed or modulated laser signals for enhanced imaging results.
Adaptive filtering and channel estimation techniques are applied to laser based ranging systems that utilize wide-band intensity modulation to measure the range and reflectivity of underwater objects. The proposed method aims to iteratively learn the frequency dependent characteristics of the underwater environment using a frequency domain adaptive filter, which results in an estimate for the channels optical impulse response. This work presents the application of the frequency domain adaptive filter to simulated and experimental data, and shows it is possible to iteratively learn the underwater optical channel impulse response while using Hybrid Lidar/Radar techniques.
Blue-green laser systems are being developed for optical imaging and ranging in the underwater environment. The imaging application requires high range resolution to distinguish between multiple targets in the scene or between multiple target features, while the ranging application benefits from measurements with high range accuracy. The group at the Naval Air Warfare Center Aircraft Division (NAWCAD) in Patuxent River, MD has been investigating the merging of wideband radar modulation schemes with a pulsed laser system for underwater imaging and ranging applications. For the imaging application, the narrow peak produced by pulse compression at the receiver offers enhanced range resolution relative to traditional short pulse approaches. For ranging, the selection of modulation frequency bands approaching 1GHz provides backscatter and forward scatter suppression and enhanced range accuracy. Both passband and baseband digital processing have been applied to data collected in laboratory water tank experiments. The results have shown that the choice of processing scheme has a significant impact on optimizing the performance of modulated pulse laser systems for either imaging or ranging applications. These different processing schemes will be discussed, and results showing the effect of the processing schemes for imaging and ranging will be presented.
KEYWORDS: Principal component analysis, Backscatter, Signal to noise ratio, LIDAR, Modulation, Scattering, Statistical signal processing, Ranging, Absorption, Signal processing, Sensors, Environmental sensing, Photons
This work presents a new statistical signal processing approach to reduce the effects of forward scatter on range accuracy for an underwater modulated pulse lidar. Lidar sensors offer the potential for high-resolution, high-accuracy ranging in the underwater environment. For the modulated pulse lidar rangefinder, performance is limited in turbid waters primarily due to forward scatter, which causes decreased range resolution and accuracy. This work presents simulated and experimental results demonstrating the ability of statistical signal processing to reduce range error for systems operating in these turbid conditions. Experimental results demonstrated 60% reduction in range error compared to a baseline approach.
Measurements with high range resolution are needed to identify underwater threats, especially when two-dimensional contrast information is insufficient to extract object details. The challenge is that optical measurements are limited by scattering phenomena induced by the underwater channel. Back-scatter results in transmitted photons being directed back to the receiver before reaching the target of interest which induces a clutter signal for ranging and a reduction in contrast for imaging. Multiple small-angle scattering (forward-scatter) results in transmitted photons being directed to unintended regions of the target of interest (spatial spreading), while also stretching the temporal profile of a short optical pulse (temporal spreading). Spatial and temporal spreading of the optical signal combine to cause a reduction in range resolution in conventional laser imaging systems. NAVAIR has investigated ways in which wide bandwidth, modulated optical signals can be utilized to improve ranging and imaging performance in turbid water environments. Experimental efforts have been conducted to investigate channel effects on the propagated frequency content, as well as different filtering and processing techniques on the return signals to maximize range resolution. Of particular interest for the modulated pulses are coherent detection and processing techniques employed by the radar community, including methods to reduce sidelobe clutter. This paper will summarize NAVAIR’s work and show that wideband optical signals, in combination with the CLEAN algorithm, can indeed provide enhancements to range resolution and 3D imagery in turbid water environments.
KEYWORDS: Laser systems engineering, Signal to noise ratio, Chemical species, Sensing systems, LIDAR, Ranging, Analog electronics, Modulation, Computer simulations, Detection and tracking algorithms
Empirical and experimental results are provided, proving the successful application of sub-Nyquist sampling to wide-bandwidth linear frequency modulated waveforms commonly used in ranging applications. The wide bandwidth nature of these waveforms traditionally dictate the need for high speed digitizers and matched filtering in order to determine the range to an object. We propose that a sub-Nyquist rate digitizer, along with the notions of sparse recovery, can be used to significantly reduce the rate and number of samples necessary to detect the range to an object, thereby reducing overall system cost. Specifically, the application of these concepts to a laser based ranging system is explored, and properties of the sensing matrix are investigated as well as possible sparse recovery performance.
A modulated pulse laser imaging system has been developed which utilizes coded/chirped RF modulation to mitigate the adverse effects of optical scattering in degraded visual underwater environments. Current laser imaging techniques employ either short pulses or single frequency modulated pulses to obtain both intensity and range images. Systems using short pulses have high range resolution but are susceptible to scattering due to the wide bandwidth nature of the pulse. Range gating can be used to limit the effects of backscatter, but this can lead to blind spots in the range image. Modulated pulse systems can help suppress the contribution from scattered light in generated imagery without gating the receiver. However, the use of narrowband, single tone modulation results in limited range resolution where small targets are camouflaged within the background. This drives the need for systems which have high range resolution while still suppressing the effects of scattering caused by the environment. Coded/chirped modulated pulses enable the use of radar pulse compression techniques to substantially increase range resolution while also providing a way to discriminate the object of interest from the light scattered from the environment. Linearly frequency chirped waveforms and phase shift keyed barker codes were experimentally investigated to determine the effects that pulse compression would have on intensity/range data. The effect of modulation frequency on the data produced with both wideband and narrowband modulation was also investigated. The results from laboratory experiments will be presented and compared to model predictions.
KEYWORDS: Backscatter, Modulation, Ranging, Signal attenuation, Ocean optics, Signal detection, Scattering, Digital signal processing, Receivers, Signal processing
The performance of a frequency-modulated continuous-wave (FMCW) hybrid lidar-radar system will be presented in the context of an underwater optical ranging application. In adapting this technique from the radar community, a laser is intensity-modulated with a linear frequency ramp. A custom wideband laser source modulated by a new wideband digital synthesizer board is used to transmit an 800 MHz wide chirp into the underwater channel. The transmitted signal is mixed with a reference copy to obtain a “beat” signal representing the distance to the desired object. The expected form of the return signal is derived for turbid waters, a highly scattering environment, indicating that FMCW can detect both the desired object and the volumetric center of the backscatter “clutter” signal. This result is verified using both laboratory experiments and a realistic simulation model of the underwater optical channel. Ranging performance is explored as a function of both object position and water turbidity. Experimental and simulated results are in good agreement and performance out to ten attenuation lengths is reported, equivalent to 100 meters in open ocean or 5 meters in a turbid harbor condition.
KEYWORDS: Modulation, Signal attenuation, Analog electronics, Receivers, Phase shift keying, Backscatter, Signal processing, Digital filtering, Linear filtering, Radar
Techniques have been developed to mitigate many of the issues associated with underwater imaging in turbid environments. However, as targets get smaller and better camouflaged, new techniques are needed to enhance system sensitivity. Researchers at NAVAIR have been developing several techniques that use RF modulation to suppress background clutter and enhance target detection. One approach in particular uses modulation to encode a pulse in a synchronous line scan configuration. Previous results have shown this technique to be effective at both forward and backscatter suppression. Nearly a perfect analog to modulated pulse radar, this technique can leverage additional signal processing and pulse encoding schemes to further suppress background clutter, pull signals out of noise, and improve image resolution. Additionally, using a software controlled transmitter, we can exploit this flexibility without the need to change out expensive hardware. Various types of encoding schemes were tested and compared. We report on their comparative effectiveness relative to a more conventional non-coded pulse scheme to suppress background clutter and improved target detection.
In this paper simulation and experimental results are presented for two hybrid lidar-radar modulation techniques for underwater laser ranging. Both approaches use a combination of multi-frequency and single frequency modulation with the goal of simultaneously providing good range accuracy, unambiguous range, and backscatter suppression. The first approach uses a combination of dual and single frequency modulation. The performance is explored as a function of increasing average frequency while keeping the difference frequency of the dual tones constant. The second approach uses a combination of a stepped multi-tone modulation called frequency domain reflectometry (FDR) and single frequency modulation. The FDR technique is shown to allow simultaneous detection of the range of both the volumetric center of the backscattered “clutter” signal and the desired object. Experimental and simulated results are in good agreement for both techniques and performance out to ten attenuations lengths is reported.
A hybrid approach is described that enhances the performance of an underwater optical ranging system. This approach uses high-frequency modulation and a spatial delay line filter to suppress unwanted backscatter. A dual frequency approach is also implemented to reduce the effects of forward scatter and remove the ambiguity associated with using the phase of the single, high-frequency modulation envelope to measure range. Controlled laboratory experiments were conducted to evaluate the effectiveness of the hybrid technique to reject multiple scattered light and improve range precision. The experimental results were compared with data generated from a theoretical model developed to predict the performance of the technique as a function of system and environmental variables. Model and experimental results are shown that reveal the ability of the approach to provide accurate ranging to an underwater object in a variety of water environments. Model predictions also indicate that advancements in transmitter and receiver technology will extend the range and improve the accuracy of the technique beyond what has been achieved thus far.
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