Real-time video mosaicking of the ocean floor

An approach was developed for real-time creation of video mosaics of the ocean floor. The approach uses visual correspondence during both image acquisition and consolidation to insure that there are no gaps in the mosaic. The use of visual information for mosaic creation offers an accuracy previously unachievable in real time. A complete mosaicking approach was developed, including a technique for image registration and several strategies for image acquisition control. The approach was implemented and tested experimentally. Ocean-floor mosaics were created in real time using a remotely-operated vehicle. In addition, extensive mosaicking experiments were conducted in a test-tank environment using a semiautonomous underwater robot     

Concurrent mapping and localization using sidescan sonar

This paper describes and evaluates a concurrent mapping and localization (CML) algorithm suitable for localizing an autonomous underwater vehicle. The proposed CML algorithm uses a sidescan sonar to sense the environment. The returns from the sonar are used to detect landmarks in the vehicle's vicinity. These landmarks are used, in conjunction with a vehicle model, by the CML algorithm to concurrently build an absolute map of the environment and to localize the vehicle in absolute coordinates. As the vehicle moves forward, the areas covered by a forward-look sonar overlap, whereas little or no overlap occurs when using sidescan sonar. It has been demonstrated that numerous reobservations by a forward-look sonar of the landmarks can be used to perform CML. Multipass missions, such as sets of parallel and regularly spaced linear tracks, allow a few reobservations of each landmark with sidescan sonar. An evaluation of the CML algorithm using sidescan sonar is made on this type of trajectory. The estimated trajectory provided by the CML algorithm shows significant jerks in the positions and heading brought about by the corrections that occur when a landmark is reobserved. Thus, this trajectory is not useful to mosaic the sea bed. This paper proposes the implementation of an optimal smoother on the CML solution. A forward stochastic map is used in conjunction with a backward Rauch-Tung-Striebel filter to provide the smoothed trajectory. This paper presents simulation and real results and shows that the smoothed CML solution helps to produce a more accurate navigation solution and a smooth navigation trajectory. This paper also shows that the qualitative value of the mosaics produced using CML is far superior to those that do not use it.     

Real-time path planning and obstacle avoidance for RAIS: anautonomous underwater vehicle

This paper describes a navigation and guidance system (NGS) with real-time path planning and obstacle avoidance capabilities that has been developed for the autonomous underwater vehicle RAIS. The vehicle is designed to accomplish two missions: pre-deployment survey of sea bottom, and visual inspection of pipelines. In the first mission, the NGS must be able to track a predefined path while avoiding the unplanned occurrence of obstacles. In the second mission, the NGS must track a pipeline by locally reconstructing its location from visual information; also in this case, the unplanned occurrence of obstacles must be handled. Furthermore, the NGS must properly take into account the presence of ocean current and some drastic constraints due to sensor and actuator characteristics. Numerical and hardware-in-the-loop simulations have been developed to verify the effectiveness of the proposed NGS     

Autonomous underwater vehicles - challenging developments and technological maturity towards strategic swarm robotics systems

ABSTRACT

Reliable power supply, precise position determination and effective communication are the key requirements for strategic autonomous underwater vehicles (AUV) involved in long duration scientific missions, search operations and when operated as a swarm. The paper presents the challenging range of AUV developed for deep water, Polar and intervention applications; demanding technical requirements for strategic AUV; reliability modeling done on the lithium-ion batteries to identify the redundancy requirements for achieving near-zero failures; navigation model to estimate the achievable level of position accuracies using the state-of-the-art navigation system; limitations in underwater communication; and their importance in realizing vehicle autonomy and swarm intelligence. It is identified that a strategic grade Doppler velocity- aided inertial navigation system could provide position accuracies of about 0.5% of the distance travelled when navigated using sea bottom or ice reference, and a 38?kWh lithium-ion battery pack requires about 7% redundant battery capacity to achieve a failure probability of <?1% in a period of 1 year.     

基于单水声信标距离量测的匹配定位方法

水声通信和测距能力是实现水下航行器准确定位的重要技术手段。当前基于水声定位的方法主要有利用测距和测向功能的水声定位技术以及水声测距辅助导航技术,二者的系统物理复杂度都比较高。本文提出了一种基于单水声信标距离量测的匹配定位方法,航行器在水声信标测距覆盖范围内,利用航行过程中多次测距信息构建测距圆序列形成位置约束,基于航位推算导航信息,将航行器在连续测距时间段内的相对航迹在圆序列上进行最优匹配,从而获得位置估计,通过对测距误差进行补偿可进一步提升定位精度。本方法所需物理系统结构复杂度低、可操作性强,仿真实验表明,该方法可以独立实现较高精度的定位。     

Mine avoidance techniques for underwater vehicles

It is shown that by implementing certain mine avoidance techniques, an underwater vehicle equipped with an obstacle avoidance sonar (OAS) and a navigation system can safely navigate an unknown minefield. The mine avoidance techniques take into account the physical limitations of the sonar and the navigation system, the maneuverability constraints on the underwater vehicle, and the required safe standoff distance from all mines. Extensive computer simulations have verified the mine avoidance capability in more than 50 different minefields. In all 50 simulations the vehicle reached a predetermined end point and maintained at least the specified, minimum safe standoff distance from each mine. The simulation accurately models the major difficulties associated with the sonar, the navigation system, and the vehicle dynamics. The sonar model includes surface, bottom, and volume reverberation; thermal, ambient, and flow noises; actual receiver and projector beam patterns; and false alarms and missed detections. The navigation system model contains the effects of biases, random noises, and scale factor errors. The vehicle dynamic model simulates angular velocities and accelerations associated with underwater vehicles     

Mosaic-based positioning and improved motion-estimation methods forautomatic navigation of submersible vehicles

Knowledge of the camera trajectory, which may be determined from the motions between consecutive frames of a video clip, can be used to register images for constructing image mosaics. We discuss a mosaic-based positioning framework for building photo-mosaics and concurrently utilizing them for improved positioning. In this approach, the mosaic is directly exploited in bounding the accumulation of position errors as we integrate the incremental motions of the camera. It is also shown that two earlier closed-form solutions for the estimation of motion directly from spatio-temporal image gradients, as for most gradient-based techniques based on the application of linear(ized) image motion constraint equations, are corrupted with systematic biases. These can be reduced significantly by incorporating the higher-order terms. We propose recursive methods to solve the new nonlinear constraint equations, and investigate the performance of the new solutions in a number of experiments with synthetic and real data     

Geometrical aspects of a sonar-based navigation system for underwater vehicles

A novel self-contained navigation system has been devised for underwater vehicles operating in and around offshore installations. This system matches data from a sector-scanning sonar device to a computer model of the installation. The paper begins by highlighting the existing approaches to subsea navigation before outlining the main features of the proposed system. It then concentrates on a key component of this system which is a method for calculating the position and heading of an underwater vehicle navigating in the vincinity of tubular steel structures. An iterative solution method is presented which incorporates six degree of freedom vehicle motions and this is verified in a series of laboratory experiments with various arrangements of structural members and using a commercial sonar device. The key features, applications and performance of this method are discussed. The main conclusion is that the proposed method for calculating the position and heading of an underwater vehicle contributes towards achieving an accurate and reliable subsea navigation capability.     

Large Area 3-D Reconstructions From Underwater Optical Surveys

Robotic underwater vehicles are regularly performing vast optical surveys of the ocean floor. Scientists value these surveys since optical images offer high levels of detail and are easily interpreted by humans. Unfortunately, the coverage of a single image is limited by absorption and backscatter while what is generally desired is an overall view of the survey area. Recent works on underwater mosaics assume planar scenes and are applicable only to situations without much relief. We present a complete and validated system for processing optical images acquired from an underwater robotic vehicle to form a 3-D reconstruction of the ocean floor. Our approach is designed for the most general conditions of wide-baseline imagery (low overlap and presence of significant 3-D structure) and scales to hundreds or thousands of images. We only assume a calibrated camera system and a vehicle with uncertain and possibly drifting pose information (e.g., a compass, depth sensor, and a Doppler velocity log). Our approach is based on a combination of techniques from computer vision, photogrammetry, and robotics. We use a local to global approach to structure from motion, aided by the navigation sensors on the vehicle to generate 3-D submaps. These submaps are then placed in a common reference frame that is refined by matching overlapping submaps. The final stage of processing is a bundle adjustment that provides the 3-D structure, camera poses, and uncertainty estimates in a consistent reference frame. We present results with ground truth for structure as well as results from an oceanographic survey over a coral reef.      

用于水下潜器的惯性导航系统安装和初始对准技术

水下潜器通常采用捷联惯性导航和多普勒计程仪组合导航系统。为提高惯性导航系统的导航精度,保障水下潜器顺利作业,文章以高性能惯导系统PHINS为例,介绍其安装和初始对准技术。PHINS在安装时须计算其与船体的偏差以及与船体和外部传感器的力臂,尤其与多普勒计程仪组合时应执行传感器自动对准程序;PHINS的初始对准是其精确导航的前提,可在海上或静态时进行;惯导系统的安装误差和初始对准精度对于其导航精度具有决定性的影响。     

单应答器导航深海试验

布放于海底的单只应答器能够为安装有超短基线的水下潜器定位导航。介绍了单应答器导航的基本原理,简要分析了各种可能误差源对导航精度的影响。为了验证系统在深海中的工作性能,在中国南海进行了深海试验并给出了结果。     

A high-resolution pulse-Doppler underwater acoustic navigation system

A high-resolution underwater acoustic pulse-Doppler navigation system has been developed and tested at sea. The system provides continuous, highly accurate tracking of underwater and ocean-surface platforms in a fixed 50-km²navigation net. Three reference buoys, moored 20 m from the ocean bottom, provide the navigation net used by shipboard processing equipment. Each reference buoy contains an acoustic transponder, used to obtain the acoustic travel times from the transponder to the platform, and a continuous-tone beacon, used to obtain the Doppler shift due to platform motion. The system is capable of determining the position of a platform with respect to the reference net with an error of 2-3 m. The relative position of the platform on a fix-to-fix basis can be determined within several centimeters over short time intervals (approx 10min).     

The autonomous underwater glider "Spray"

A small (50-kg, 2-m long) underwater vehicle with operating speeds of 20-30 cm/s and ranges up to 6000 km has been developed and field tested. The vehicle is essentially an autonomous profiling float that uses a buoyancy engine to cycle vertically and wings to glide horizontally while moving up and down. Operational control and data relay is provided by GPS navigation and two-way communication through ORBCOMM low-Earth-orbit satellites. Missions are envisioned with profile measurements repeated at a station or spaced along a transect. The initial instrument complement of temperature, conductivity, and pressure sensors was used to observe internal waves and tides in the Monterey underwater canyon     

In Situ Alignment Calibration of Attitude and Doppler Sensors for Precision Underwater Vehicle Navigation: Theory and Experiment

This paper reports the development and experimental evaluation of two in situ least squares techniques for estimating the alignment matrix of Doppler sonars commonly used for precision navigation of oceanographic submersibles. Most previously reported methods addressed the problem of single degree-of-freedom heading alignment using bottom-lock Doppler sonar data and global positioning system (GPS) navigation data. This paper reports and evaluates two techniques for three degree-of-freedom calibration of attitude and Doppler sonar sensors using sensor data available to vehicles at full ocean depth. The first technique provides a general linear least squares estimate of the alignment matrix. The second technique results in a least squares alignment matrix estimate constrained to the group of rotation matrices. The performance of these estimates is evaluated with a laboratory remotely operated vehicle (ROV) and a field-deployed autonomous underwater vehicle (AUV). Experimental results are reported which demonstrate that Doppler navigation employing the reported alignment calibration techniques significantly improves navigation precision. The experiments show that the latter technique provides calibration estimates that improve Doppler navigation precision not only on the calibration data set itself, but also provide improved precision over a wide variety of vehicle trajectories other than the calibration data set.     

Low-bit-rate coding of underwater video using wavelet-basedcompression algorithms

Recent advances in autonomous underwater vehicle (AUV) and underwater communication technology have promoted a surge of research activity within the area of signal and information processing. A new application is proposed herein for capturing and processing underwater video onboard an untethered AUV, then transmitting it to a remote platform using acoustic telemetry. Since video communication requires a considerably larger bandwidth than that provided by an underwater acoustic channel, the data must be massively compressed prior to transmission from the AUV. Past research has shown that the low contrast and low-detailed nature of underwater imagery allows for low-bit-rate coding of the data by wavelet-based image-coding algorithms. In this work, these findings have been extended to the design of a wavelet-based hybrid video encoder which employs entropy-constrained vector quantization (ECVQ) with overlapped block-based motion compensation. The ECVQ codebooks were designed from a statistical source model which describes the distribution of high subband wavelet coefficients in both intraframe and prediction error images. Results indicate that good visual quality can be achieved for very low bit-rate coding of underwater video with our algorithm     

深海机器人视景仿真系统研究

以载人深潜器的各种水动力参数和实际尺寸为基础,根据几何空间坐标方程建立了其运动学模型,采用MultiGen公司的Creator建模工具和Vega视景环境完成了在深海虚拟环境下的系统仿真。该系统可以实现深海机器人的可视化,更加直观、生动和实时的反映其位姿状态和水面、水下巡航过程。该系统实际应用在中国科学技术馆深海机器人展馆项目上,一方面展示载人深潜器的水下工作过程,同时也使得观众有机会亲身体验潜水器的操纵与驾驶。实际运行结果表明,该系统逼真地演示了载人深潜器水面备航、无动力下潜以及近海底巡航等仿真过程,能够满足系统仿真的实时性要求。该系统还可以应用到深海环境模拟研究、水下机器人运动仿真、控制系统调试以及操纵驾驶训练等中。     

Image registration for the underwater inspection using the maximum a posteriori technique

Describes an image registration method for underwater inspection tasks. A remotely operated vehicle equipped with a video camera and a scanning sonar is used as the testbed vehicle. Each image of the underwater scene is saved along with the video camera's position and orientation. The images are then combined to create a large composite picture of the underwater structure being inspected. This method is based upon a maximum a posteriori estimation technique and provides smooth and robust estimates of image shifts. Our results demonstrate the feasibility of this highly promising underwater inspection procedure.     

Sediment mobility in the Pomeranian Bight (Baltic Sea): a case study based on sidescan-sonar images and hydrodynamic modelling

Sidescan-sonar surveys were performed on a 2×4 km area of seafloor in the southern Baltic Sea (Pomeranian Bight) in 1996 and 1998. Overlapping sub-areas of the individual surveys showing characteristic details were processed into geographically referenced mosaics. Sediment types were identified from echo characteristics and by comparison with granulometric data. The sea bottom covered by the mosaics consists predominantly of sand, with subordinate lag sediments with stones and small ripple fields consisting of coarse sand to fine gravel. A comparison of the two mosaics did not reveal any significant changes of the sea bottom over the 2-year period. Characteristic sedimentary features remained almost unchanged over this period on detailed sonar images of smaller sub-areas. Substantial transport of sandy sediments can thus be excluded in the course of the observation period. Grid files of advective velocity components and orbital velocity of wave motion of a three-dimensional hydrodynamical model for the period from September 1996 to October 1997 were used to estimate the current regime in the study area for the interval between the two sidescan surveys. Comparing critical velocities for the dominant sediment types with the results of the numerical bottom current simulations and the observations from sidescan images, it is apparent that strong current events during the modelled time interval were still too weak to resuspend and transport sand of any grain size, even though maximum current velocities of 30 cm/s at the seafloor were modelled. Only a few patches of newly accumulated (acoustically soft) material (mud, fluff and/or soft plant remnants), with a horizontal extension of about 10 m at a terrain step feature, were recognised in the 1998 mosaic. Our results imply that sand deposits in the southern Baltic Sea can remain stationary over time periods of several years, and that the transport of organic material, nutrients and associated pollutants to depositional areas in deeper water is predominantly accomplished by the movement of material finer than sand. A significant portion of this fine material is evidently transported in the bottom boundary layer under conditions of moderate hydrodynamical forcing.     

潜水器控制系统层次故障诊断模型设计

在潜水器控制系统的基础上,按照结构、功能和组件的关系,为潜水器控制系统进行故障诊断建模。依据基于模型的层次故障诊断技术,建立了适用于潜水器控制系统故障诊断的具体诊断和推理策略,开发了故障诊断软件系统,描述了故障诊断具体过程,并利用数字仿真验证系统设计的有效性。     

基于可重置联邦滤波器的水下运载体导航定位方法

单一导航系统无法满足水下运载体高精度、高可靠性的导航需求,文中根据常用水下导航系统的特点,提出基于可重置联邦滤波器的水下运载体导航定位方案,设计了以惯导系统(INS)为主参考系统,声学定位系统/深度计、多普勒计程仪(DVL)、罗经为子参考系统的联邦滤波器,并进行了仿真研究。结果表明文中设计的滤波器能有效融合各个传感器的导航信息,实现水下运载体高精度导航定位,并具有一定容错性。