Robert W. Lee

Hybrid technique for enhanced optical ranging in turbid water environments

Abstract. 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.

FMCW optical ranging technique in turbid waters

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.

Waveform design considerations for modulated pulse lidar

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.

Optical ranging techniques in turbid waters

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.

Tailoring of RF coded optical pulses for underwater 3D imaging

Laser imaging of 3D targets in highly scattering environments has traditionally been done using narrow 1-2ns optical pulses in conjunction with a range-gated detector. While this approach offers range resolution determined by the width of the optical pulse and is very effective in eliminating the effects of backscatter from the turbid environment, it lacks resistance to the effects of forward scatter. The Modulated Pulse Lidar is an alternative approach developed by researchers at NAVAIR that uses longer pulses that are intensity modulated with RF coded waveforms. Radar pulse compression techniques involving the use of RF coded waveforms with large bandwidths provide range resolution that meets or exceeds that of the short pulse approach, while matched-filtering techniques are employed to suppress backscatter. More importantly, the modulated pulse approach provides resistance to forward scatter effects that the short pulse approach does not. This work focuses on the comparison of imagery generated employing both pulse schemes in the same line-scan underwater 3D imaging system over a range of water turbidities. Experimental imagery and temporal data will be presented that shows that the modulated pulse approach does in fact offer an advantage over the short pulse approach due to its resistance to forward scatter effects.

Time of flight measurements for optically illuminated underwater targets using Compressive Sampling and Sparse reconstruction

Compressive Sampling and Sparse reconstruction theory is applied to a linearly frequency modulated continuous wave hybrid lidar/radar system. The goal is to show that high resolution time of flight measurements to underwater targets can be obtained utilizing far fewer samples than dictated by Nyquist sampling theorems. Traditional mixing/down-conversion and matched filter signal processing methods are reviewed and compared to the Compressive Sampling and Sparse Reconstruction methods. Simulated evidence is provided to show the possible sampling rate reductions, and experiments are used to observe the effects that turbid underwater environments have on recovery. Results show that by using compressive sensing theory and sparse reconstruction, it is possible to achieve significant sample rate reduction while maintaining centimeter range resolution.

Statistical signal processing technique to reduce effects of forward scatter on underwater modulated pulse lidar

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.

Optimizing the performance of modulated pulse laser systems for imaging and ranging applications

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.

Adaptive underwater channel estimation for hybrid lidar/radar

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.

Wide bandwidth optical signals for high range resolution measurements in water

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.