Precise Positioning of Ocean Bottom Seismometer by Using Acoustic Transponder and CTD

We have obtained precise estimates of the position of Ocean Bottom Seismometers (OBS) on the sea bottom. Such estimates are usually uncertain due to their free falling deployment. This uncertainty is small enough, or is correctable, with OBS spacing of more than 10 km usually employed in crustal studies. But, for example, if the spacing is only 200 m for OBS reflection studies, estimates of the position with an accuracy of the order of 10 m or more is required.The determination was carried out with the slant range data, ship position data and a 1D acoustic velocity structure calculated from Conductivity–Temperature–Depth (CTD) data, if they are available. The slant range data were obtained by an acoustic transponder system designed for the sinker releasing of the OBS or travel time data of direct water wave arrivals by airgun shooting. The ship position data was obtained by a single GPS or DGPS. The method of calculation was similar to those used for earthquake hypocenter determination.The results indicate that the accuracy of determined OBS positions is enough for present OBS experiments, which becomes order of 1 m by using the DGPS and of less than 10 m by using the single GPS, if we measure the distance from several positions at the sea surface by using a transponder system which is not designed for the precise ranging. The geometry of calling positions is most important to determine the OBS position, even if we use the data with larger error, such as the direct water wave arrival data. The 1D acoustic velocity structure should be required for the correct depth of the OBS. Although it is rare that we use a CTD, even an empirical velocity structure works well.     

Sensor position estimation and source ranging in a shallow water environment

Underwater acoustic transient signals are generated mechanically at known positions along a wharf. These signals are received by a wide aperture planar array of four underwater acoustic sensors, whose positions relative to the wharf are unknown. A method is described that enables the positions of the sensors to be estimated from accurate differential time-of-arrival measurements (with 0.1 /spl mu/s precision) as the signal wavefronts traverse the array. A comparison of the estimated positions with the nominal positions of the first three sensors, which form a 20-m-wide aperture horizontal line array, reveals a 2-cm displacement of the middle sensor from the line array axis. This slight bowing of the line array results in overranging (bias error of 3%) when the wavefront curvature method is used with the nominal collinear sensor positions to locate a static source of active sonar transmissions at a range of 59.2 m. The use of the spherical intersection method coupled with the estimated sensor positions of the line array provides an order of magnitude improvement in the range estimate (within 0.3% of the actual value). However, systematic ranging errors are observed when the sound propagation medium becomes nonstationary. Next, the differences in the arrival times of the direct path and boundary-reflected path signals at the middle sensor of the wide aperture line array are estimated using the differential phase residue of the analytic signal at the sensor output. These multipath delays are used to estimate the range and depth of the source. Although the average value of the multipath range estimates is within 0.5% of the actual value, the variance of the range estimates is 50 times larger when compared with the results of the spherical intersection and wavefront curvature methods. The multipath delay data are also processed to provide a reliable estimate of the temporal variation in the water depth enabling the tidal variation to be observed.     

基于LoRa的海洋多功能信标组网及特性研究

针对目前海洋观测网络通信距离、功耗、复杂度、抗干扰能力、成本之间的矛盾,提出了一种基于LoRa(long range)的海洋多功能信标组网观测系统。通过运用新型的LoRa调制技术,采用星型链路网络对多功能信标进行组网设计,并结合北斗通信,构建了一套覆盖范围广、功耗低,组网简单,抗干扰性强的海洋监测网络系统,能够有效实现近海和远海的全范围覆盖,同时降低了组网的成本。通过对多功能信标的低功耗设计,可有效降低信标功耗,延长了工作周期。通过室内和室外的模拟实验,验证了组网通信的有效性,以及远距离传输数据特性,为信标的升级设计和海洋组网观测打下了基础。     

适用于中国近海观测研究浮标的北斗/GPS双模信标系统的研制与应用

为了提高海洋观测研究浮标在位运行的安全性,自主设计了一种北斗/GPS双模定位信标。该信标采用传统的GPS技术进行定位结合“北斗”卫星特有的报文通信和定位功能,通过解决传统GPS信号盲区问题,为中国近海海洋观测研究浮标安全运行提供了更加有力的保障,同时提高了数据保密能力。该系统采用外部供电和内部自主供电相结合的模式,即使外部供电中断,也可保证正常运行不低于1 a,而且基本上不需要对信标进行专门维护,大大降低了运行和维护成本。与该信标配套的岸站管理系统具备发送和短信和邮件报警的功能,可实现浮标运行状态实时监控。     

Position Correction Using Echoes From a Navigation Fix for Synthetic Aperture Sonar Imaging

In synthetic aperture sonar (SAS), the platform position must be known sufficiently accurately for signals to be added coherently along the synthetic aperture. Often, the onboard navigation system is insufficiently accurate by itself, so corrections are needed. A well-known method is the displaced phase center antenna (DPCA) procedure for correcting platform position using seabed echoes. DPCA methods have the advantage of insensitivity to changing interference patterns, moving specular reflection, and changing occlusion, with aspect. However, when seabed echoes are unusable, either because they are too weak, or because they are corrupted by multipath, the seabed DCPA method may fail. Therefore, we present an alternative DPCA method using sonar echoes from a suitable navigation fix, based on an object detected after standard beamforming. In our proposed system, look angle is obtained by tracking the centroid of the rectified image of the fix object. When the standard DPCA correction equations are modified for a fixed reflector, it turns out that they provide incremental range and look-angle errors, precisely the values required when the target itself is used as the navigation fix. Moreover, the values obtained are then self-compensating for errors in estimating seabed depth or forward motion of the platform. The navigation fix is selected by bracketing in range, and beamforming overlapping subsets of the receiver array. In this paper, we present experimental results at transmitter frequencies of 25 and 100 kHz where our method enabled well-focused SAS images to be generated with little recourse to other navigation information. Hence, SAS can be carried out, even when a sophisticated inertial navigation system (INS) is not available.      

Northwest Atlantic REX inverted echo sounder location: Acoustic range data

The Northwest Atlantic Regional Energetics Experiment (REX) will study the dynamics and energetics of the Gulf Stream and associated rings. Bottom‐moored inverted echo sounders with pressure gauges (IES/PGs) will collect in situ data which both complement and augment the GEOSAT altimeter measurements in the Northwest Atlantic. The primary objective of these data will be the intercomparison of satellite and IES/PG measured fluctuations in sea surface topography and mean ther‐mocline depth. However, more detailed and absolute intercom‐parisons of these data would be available if the IES/PG could be positioned in an inertial reference frame while the measurements were taken. This paper presents an error model for computing the accuracy of positioning with a bottom beacon of the IES/PG type. Equations for studying the use of acoustic range data for locating an IES/PG with respect to a ship's track are developed and presented. The ship's track is assumed to be located in inertial space to the 1‐ to 2‐m level by application of the GPS (Mohan, 1983, 1984). Effects of depth‐dependent acoustic speed are included. Although IES/PGs as presently deployed would have to be modified to function as the type of ranging device described in this paper, the representative parameter values that have been calculated for the Northwest Atlantic REX show that sub‐1‐meter accuracy in location determination can be expected. A crucial factor is the measurement pattern, and recommendations for optimal patterns are presented.     

Ray Theory Application in Long Baseline System

The long baseline (LBL) system is widely used to locate and track autonomous underwater vehicles (AUV) through acoustic communication.Three important issues are presented here in LBL system application with AUV.Those issues which regard the normal acoustic communication between LBL system and AUV are the depth of towed array,the length of beacon cable,and the effective area of the AUV.The first issue is the key of the LBL system,which ensures the normal communication between towed array and beacons.The second issue which impacts the normal communication from the AUV to beacons in available range should be considered after the first one has been settled.Then the last issue determines the safe work area of the AUV.The ordinary differential equations (ODE) algorithm of ray is deduced from Snell′s law.The ODE algorithm is applied to obtain sound rays from sound source to receiver.These problems are solved by the judgment that whether rays pinging from a sound source arrives at a receiver.The sea trial shows that these methods have much validity and practicality.     

Temporal domain processing for a synthetic aperture array

A method for the synthesis of an aperture with improved bearing resolution and signal gain is described. The proposed method temporally synthesizes data from an overlap correlator, which is obtained by aperture domain averaging of phase differences. Previous studies, such as extended towed array measurements (ETAM), had a restriction in that the overlapped hydrophones between successive measurements of a towed array were required to have identical positions in space. In this paper, however, it is shown that the phase correction factors can be estimated without restriction on the positions of the overlapped hydrophones. This implies that the proposed method is able to utilize more snapshots to extend the towed array. Simulation results showed that the proposed method resulted in higher estimation accuracy than ETAM. In addition, the effects of coherency and other systematic errors on the proposed method were examined     

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。     

Analysis of optical method of determining wave slopes from photographs of glitter zone

We analyze the optical method of determining wave slopes on the sea surface from measurements of the characteristics of the glitter zone on photographs. Two approaches to the implementing this are explored. Their merits and drawbacks are noted. Low-altitude (H = 5–20 m) photographs of the sea surface are used to acquire new diverse information on the fine structure of the glitter zone and its relation to the sea roughness. We present a number of new experimental data on the glitter features and structure that were omitted in the Cox-Munk method. The applicability range for this optical method is estimated.     

Detection of ocean waves using satellite altimetry: Application to equatorial Kelvin waves

A new method for wave motion detection from satellite altimetric measurements of sea surface height is presented. The essence of the approach is to construct a two‐dimensional traveling‐wave Fourier series representation of the amplitude field within a prespecified oceanic region. The method employs an iterative, nonlinear least‐squares technique based on the Marquardt‐Levenberg algorithm to solve for model parameters describing characteristic features of the evolving wave system. The Marquardt‐Levenberg Fourier series (MLFS) algorithm was applied to Kelvin waves active during the 1986–1987 El Nino event in the equatorial Pacific ocean using GEOSAT Exact Repeat Mission altimetry data. Characteristics of the wave system were found to be in essential agreement with earlier field measurements and the observations of Cheney and Miller (1987) obtained using time series developed from GEOSAT data. The advantage of the present detection scheme lies in its speed and ability to determine a wave system's dispersion relation over a finite range of wavenumbers, and hence the group velocity of that system.     

High-precision array element localization for vertical line arraysin the Arctic Ocean

Array element localization (AEL) surveys are often required to accurately localize acoustic instruments (transponders or sensors) in the ocean. These are typically based on transmitting or recording acoustic signals from or at a set of well-known positions. A significant limiting factor in many AEL surveys is the uncertainty inherent in these “known” positions. In this paper, an inversion algorithm is developed which properly treats both transponder and sensor positions as unknowns, subject to available a priori information in the form of position estimates and uncertainties. The algorithm essentially consists of an iterative linearized inversion of the raytracing equations employing the method of regularization. The approach is applied to independently localize transponders and vertical line array (VLA) sensors that form part of a three-dimensional sensor array in the Arctic Ocean. Confidence limits estimated via Monte Carlo simulation indicate that transponders and sensors are localized to less than 1 m in three dimensions. The VLA sensor motion, monitored over a seven-week period, appears to be predominately driven by tidal currents and is consistent with historical current measurements for the region     

Integrated System for Robust 6-DOF Positioning Utilizing New Closed-Form Visual Motion Estimation Methods in Planar Terrains

Estimating the relative positions and (or) trajectory of a camera from video images is a fundamental problem in motion vision. Of special relevance is the closed-form solution for planar scenes, for processing fly-over imagery from airborne and underwater robotics platforms, automated airplane landing utilizing runway landmarks, photomosaicing, etc. However, the method's robustness can break down in certain scenarios, e.g., due to inherent translation-rotation ambiguity of visual motion with short baselines and narrow field of view. The robustness can be improved by devising methods that compute a smaller set of motion parameters, utilizing other sensors to measure the remaining components. This paper addressed key issues in six degrees of freedom positioning from fly-over imagery by integrating vision with rotational angle sensors. First, we propose and utilize robust closed-form solutions for estimating the motion and orientation of a planar surface from the image flow variations up to first order, given measurements of pitch and roll motions. We also describe a calibration technique to enable the integration of angle sensor and visual measurements. Next, an error analysis enables us to evaluate the impact of inaccurate pitch and roll measurements on the estimates from the new closed-form solutions. Finally, the performance of our new methods and the integrated positioning system are evaluated in various experiments with synthetic and real data     

A Study of a Short-Baseline Acoustic Positioning System for Offshore Vessels

The underwater position of the beacon determined using the conventional mode of the short-baseline acoustic positioning system (SBL system) is usually incorrect due to the inaccuracy of underwater-measured sound speed, which is affected by the environment. This article presents a corrected mode of SBL system to offer a precise solution for the underwater beacon position. The corrected mode using the formulation of Euclidean geometry converges quickly and efficiently, obtaining the precise position of the beacon. Finally, a simulation of coring missions of a vessel is presented to verify the accuracy of the corrected mode of the SBL system.     

Target localization and tracking from Doppler-shift measurements

It is possible to localize a nonmaneuvering moving source radiating a constant frequency tone from measurements of the Doppler-shifted frequency at several sensors. Due to the nonlinear nature of the problem, it is necessary to find the solution by grid searches. However, if measurements of the rates of frequency changes are available, the search is only in three dimensions instead of the normal five in source frequency, its x-y positions, and speeds. The validity of combining frequency and frequency-rate measurements is confirmed with simulation studies. The overall system includes a least-squares track-sort algorithm to differentiate the true track from the extraneous track, and a Kalman tracker for the prediction of future source positions, thereby reducing the grid search size. An error analysis relating localization accuracy to uncertainties in frequency measurements and sensor positions is also given     

User expectations for multibeam echo sounders backscatter strength data-looking back into the future

With the ability of multibeam echo sounders (MBES) to measure backscatter strength (BS) as a function of true angle of insonification across the seafloor, came a new recognition of the potential of backscatter measurements to remotely characterize the properties of the seafloor. Advances in transducer design, digital electronics, signal processing capabilities, navigation, and graphic display devices, have improved the resolution and particularly the dynamic range available to sonar and processing software manufacturers. Alongside these improvements the expectations of what the data can deliver has also grown. In this paper, we identify these user-expectations and explore how MBES backscatter is utilized by different communities involved in marine seabed research at present, and the aspirations that these communities have for the data in the future. The results presented here are based on a user survey conducted by the GeoHab (Marine Geological and Biological Habitat Mapping) association. This paper summarises the different processing procedures employed to extract useful information from MBES backscatter data and the various intentions for which the user community collect the data. We show how a range of backscatter output products are generated from the different processing procedures, and how these results are taken up by different scientific disciplines, and also identify common constraints in handling MBES BS data. Finally, we outline our expectations for the future of this unique and important data source for seafloor mapping and characterisation.     

一种海洋潜标适用的新型GPS信标机

本研究设计了一种潜标适用的新型信标机,它集成了GPS模块、数传模块、微处理器和压力开关等部件,可有效提高潜标回收过程中的目标定位能力。该信标机安装于潜标主浮体,通过压力开关控制电源。当主浮体位于海面时,信标机处于工作状态,它可以不断地将GPS位置数据通过无线传输方式向外发送,配套的接收端可以将接收的位置信息直观地显示在...     

Accuracy assessment of GPS/Acoustic positioning using a Seafloor Acoustic Transponder System

The accuracy of GPS/Acoustic positioning is crucial for monitoring seafloor crustal deformation. However, the slant range residual is currently the only indicator used to evaluate the precision of positioning seafloor transponders. This study employs a unique Seafloor Acoustic Transponder System (SATS) to evaluate the accuracy of GPS/Acoustic seafloor positioning. The SATS has three transponders and an attitude sensor in a single unit, which provides true lengths of transponder baselines and true attitude of the SATS to ensure assessment reliability and validity. The proposed approach was tested through a GPS/Acoustic experiment, in which an off-the-shelf acoustic system was used to collect range measurements. Using GPS/Acoustic geodetic observations, the positions of three transponders on the SATS were estimated by an optimization technique combined with ray-tracing calculations. The accuracy of the GPS/Acoustic seafloor positioning is assessed by comparing the true baselines and attitude with the results derived from the position estimates of the three transponders. A sensitivity analysis is conducted to investigate the robustness of the GPS/Acoustic positioning results to changes of sound speed. Experimental results demonstrate that the use of the SATS can help to assess the validity of the GPS and acoustic travel time measurements in the GPS/Acoustic seafloor positioning.     

HF radar observations of Arctic pack-ice breakup

This paper describes the first reported high-resolution remote measurements of sea-ice velocities during the summer Arctic pack-ice breakup, made with a high-frequency (HF) radar system (CODAR, for Coastal Ocean Dynamics Applications Radar) located on Cross Island, Alaska. Each 36-min observation also gives the positions of the ice edge, the moving ice, and the open water, with an azimuthal and distance resolution of5degand 1.2 km, respectively, to a range of 15 km. The statistical uncertainties in speed are typically 2-4 cm/s. The ice breakup was observed over a two-day period starting with low ice velocity and no open water and ending with ice and current velocities of approximately 40 cm/s. The position of the ice edge is verified by a simultaneous synthetic aperture radar (SAR) image. To compare the ice, current, and wind velocities, a uniform velocity model was fitted to the measurements of radial velocity. The speed of both ice and current under free drift conditions was found to lie between 2 and 5 percent of the wind speed and the direction within20degof the wind direction.