Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements

Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of /spl plusmn/30cm (2-/spl sigma/) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder's horizontal position was determined with an accuracy of /spl plusmn/8 cm (2-/spl sigma/) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to /spl plusmn/10 cm (2-/spl sigma/) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to /spl plusmn/1 m (2-/spl sigma/). Simulations show that /spl plusmn/30 cm (2-/spl sigma/) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicles in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.     

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.     

Centimeter-Level Positioning of Seafloor Acoustic Transponders from a Deeply-Towed Interrogator

An array of three seafloor transponders was acoustically surveyed to centimeter precision with a deeply-towed interrogator. Measurements of two-way acoustic travel time and hydrostatic pressure made as the interrogator was towed above the array were combined in a least-squares adjustment to estimate the interrogator and transponder positions in two surveys spanning two years. No transponder displacements were expected at this site in the interior of the Juan de Fuca Plate (48^?11′ N, 127^?12′ W) due to the lack of active faults. This was confirmed to a precision of ±2 cm by least-squares adjustment. Marginally detectable blunders in the observations were shown to affect the transponder position estimates by no more than 3 mm, demonstrating the geometric strength of the data set. The accumulation of many hundreds of observations resulted in a significant computational burden on the least-squares inversion procedure. The sparseness of the normal matrix was exploited to reduce by a factor of 1000 the number of calculations. The acoustic survey results suggested that the near-bottom sound speed fields during the two surveys were in better agreement than inferred from yearly single-profile conductivity, temperature, and pressure (CTD) measurements.     

Accurate GPS measurement of the location and orientation of a floating platform

Abstract

This article describes the design and initial tests of the GPS portion of a system for making seafloor geodesy measurements. In the planned system, GPS antennas on a floating platform will be used to measure the location of an acoustic transducer, attached below the platform, which interrogates an array of transponders on the seafloor. Since the GPS antennas are necessarily some distance above the transducer, a short‐baseline GPS interferometer consisting of three antennas is used to measure the platform's orientation.

A preliminary test of several crucial elements of the system was performed at the Scripps Institution of Oceanography (SIO) in December 1989. The test involved a fixed antenna on the pier and a second antenna floating on a buoy about 80 m away. GPS measurements of the vertical component of this baseline, analyzed independently by two groups using different software, agree with each other and with an independent measurement within a centimeter.

The first test of an integrated GPS/acoustic system took place in the Santa Cruz Basin off the coast of southern California in May 1990. In this test a much larger buoy, designed and built at SIO, was equipped with three GPS antennas and an acoustic transducer that interrogated a transponder on the ocean floor. Preliminary analysis indicates that the horizontal position of the transponder can be determined with a precision of about a centimeter. Further analysis will be required to investigate the magnitude of systematic errors.     

Least-Squares Estimation of Bottom Topography Using Horizontal Velocity Measurements in the Tsushima/Korea Straits

Several bathymetric data sets are compared and assessed with constraints of an ocean current model and velocity observations. The root-mean-square (rms) differences among the data sets reach 20 m in the shallow Tsushima/Korea Straits. The numerical experiments to simulate the Tsushima Warm Current are performed using four different topography data sets. The JTOPO1 data (MIRC, 2003) give the smallest rms difference to long-term horizontal velocity observations. Several least-squares combinations of the topography data sets are then sought to minimize the rms difference between the observed and modeled barotropic velocities. Most of the data sets reveal a large bias of 30–60 m at the Western Channel compared to independent sounding depths     

Precise positioning for near-bottom equipment using a relay transponder

The Marine Physical Laboratory of the Scripps Institution of Oceanography has developed an acoustic relay transponder for precise relative positioning of near-bottom instruments and geologic sampling devices. Although specifically designed to position equipment lowered on standard wire ropes without a need to maintain direct electrical contact with the surface ship, the relay transponder may be used to track free vehicles, such as deep submersibles, from the surface. The relay transponder is positioned relative to an array of bottom-anchored acoustic transponders. It is interrogated acoustically from the surface ship; it then sequentially interrogates the bottom transponders which, in turn, reply to the ship. From the measurement of the total travel time (ship to relay transponder to bottom transponder to ship) and assuming, or knowing, the sound velocity of the water, we obtain a relayed range measurement. These relayed ranges, used in conjunction with ship to bottom-transponder ranges, allow us to calculate the position of the relay transponder. A recent application of this technique is described in which several gravity core samples from the crest of the Horizon Guyot were positioned with respect to the detailed bathymetry and the geology within the area. The estimated error in positioning the samples is less than 20 m inside a navigational net extending over 100 km².Contribution of the Scripps Institution of Oceanography, new series.     

Natural frequencies of a vibrating repaired panel in an ocean structure

The present work arose from the need of repairing steel plates badly damaged by corrosion in a portion of the structural element of the naval vehicle. The possibility of removing the portion and replacing it by a patch of a composite material was considered. Its dynamic behavior is altered by the introduction of the patch and the prediction of its new behavior is of great interest in many situations. This condition would appear in other real-life situations like as a localized orthotropic effect caused in the panel by a welding procedure or a metallurgical process. The first four natural frequency coefficients of the composite repaired panel with different types of boundary conditions are determined by means of a variational approach. The displacement function is approximated making use of complete sets of beam functions. The eigenvalues have been computed from (225×225) secular determinants. An independent solution is obtained using the finite element method and a reasonably good agreement with the analytical solution is encountered.     

Accuracy of tsunami travel‐time charts

Abstract

The tsunami travel‐time charts that are presently in use by the Tsunami Warning Center were constructed originally in 1948 based on the hydrographic data available in the mid 1940s. Even the revised charts of 1971 made use of essentially the same data. It is shown here that the travel times deduced by these charts could be in error by as much as two hours in some cases. Even worse, the compiled travel times as deduced from these charts are generally greater than the observed travel times, which is a dangerous situation from a tsunami warning point of view.     

Ocean tides for satellite geodesy

Abstract

Spherical harmonic tidal solutions have been obtained at the frequencies of the 32 largest luni‐solar tides using prior theory of the author. That theory was developed for turbulent, nonglobal, self‐gravitating, and loading oceans possessing realistic bathymetry and linearized bottom friction; the oceans satisfy no‐flow boundary conditions at coastlines. In this theory the eddy viscosity and bottom drag coefficients are treated as spatially uniform. Comparison of the predicted degree‐2 components of the Mf, PI, and M2 tides with those from numerical and satellite‐based tide models allows the ocean friction parameters to be estimated at long and short periods. Using the 32 tide solutions, the frequency dependence of tidal admittance is investigated, and the validity of sideband tide models used in satellite orbit analysis is examined. The implications of admittance variability for oceanic resonances are also explored. By extending the theory to include a second constraint derived from tide observations or data‐constrained tide models, it is possible to assess those models from a fluid dynamic perspective. One general conclusion from such exercises is that the large higher‐degree admittances of current short‐period tide models are dynamically incompatible with their degree‐2 admittances. Eventually it may prove possible to produce dynamically sound, observationally consistent tide models by combining the author's tide theory with satellite orbit determination.     

Selecting management methodologies for marine resources,with an illustration for southern African hake

There are many different methodologies which could be used in the management of renewable marine resources. In order to manage a particular resource appropriately, it is necessary to select those methodologies which perform most satisfactorily for that resource. In this paper, an approach is outlined which can be used to select such a "best" methodology from a set of alternatives for a particular situation. The approach involves applying alternative methodologies to a large number of data sets generated by a model of the situation and using that model to examine the consequences of each application. The approach is applied to select model-estimation and management procedures for the Cape hake resources off southern Africa. The observation error form of the Butterworth-Andrew model performs the best of the model-estimation procedures investigated, and the Maximum Allowable Catch strategy is the most satisfactory harvesting strategy of those considered. These results are relatively robust to model structure and the parameter values for the actual situation.     

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     

GIS-Supported Airfield Selection near Zhongshan Station,East Antarctica,based on Multi-Mission Remote Sensing Data

Abstract

As Antarctica attracts increasing attention in global climate change studies, the demand for field expeditions has increased in recent decades. Aircraft has become the most efficient mode of transportation because of the advantages of short travel times over long distances, access to unreachable locations and capability to carry different types of sensors to obtain large areal coverages. However, few studies have been published regarding Antarctic airfield site selection for heavy-wheeled aircraft. In this paper, we present methods and results of blue-ice airfield preliminary selection near Zhongshan station, East Antarctica. The geographic information system (GIS)-based method is supported by multi-mission high-resolution images from the ZY-3, WorldView-2 and Landsat-8 satellites along with existing remote sensing products. Ground truth observations were integrated with satellite panchromatic and spectral information to identify runway candidate areas. The information inferred by remote sensing data, including firn type, ice movement, surface slope and ice fracture, is used for evaluation of the airfield selection rules. Finally, the rankings and recommendations of runway candidates were performed in a GIS analysis environment. The site selection approach developed in this paper can be applied in preconstruction studies of other similar cryosphere environments to provide appropriate candidates for a final stage field investigation.     

Seasat data applications by commercial users

Abstract

The Seasat Program was initiated as a proof‐of‐concept mission to evaluate the effectiveness of remotely sensed oceanographic phenomena from a satellite platform. From inception, this program has been user‐oriented. These users, within the academic, government, and commercial ocean communities, have served as the architects of the program and are continuing to be involved in the validation and application of the satellite‐derived data. While an early failure of the satellite deprived the user communities of a real‐time multiseasonal data set, the program did yield a rare and valuable archive of data now being analyzed and validated by a variety of users.

A unique program of evaluation exists to assess the impact of satellite‐derived oceanographic observations in several segments of the commercial sector. Commercial users from such segments as marine transportation, ocean mining, offshore oil and gas exploration and operations, marine Fisheries, and marine safety are analyzing Seasat data to understand the operational and economic impact of such data on their various business activities.

Early results from these studies show that satellite observations alone can provide commercial users with historical data for new and remote areas of operation which, to obtain by in situ measurements, would require enormous expenditures of funds for oceanographic instrumentation. More significantly, these studies are showing that timely oceanographic and weather forecasts, improved by the use of satellite‐derived observations of winds, waves, and surface temperatures, can result in more efficient and safe operations, yielding industry savings of millions of dollars annually.     

Erratum

Abstract

Several physical aspects of continuous subbottom profiling are presented, and their relationship to operating characteristics of profiling instruments is discussed. Interpretation of data produced by shallow‐penetration continuous profilers, such as the SONIA system, is a process of transformation of the continuous measurement of the travel times of reflected sound pulses into geologic phenomena. Essentially, the continuous profiler is a geologic tool; interpretation of the data is based on geologic observation and reasoning, irrespective of the further use of the information obtained in applied science. However, interpreters must have sufficient knowledge of the geophysical system and of the equipment design to be in a position to evaluate the significance of their interpretation.     

Estimating hypocentral uncertainties for marine microearthquake surveys: A comparison of the generalized inverse and grid search methods

For microearthquake surveys conducted with small networks in regions where the seismic velocity structure has large vertical gradients, the formal errors accompanying hypocentral solutions obtained by a generalized inverse method may be misleading since they do not incorporate the effects of nonlinearity in travel times. An alternative method for estimating uncertainties involves calculating travel time residuals over a regular grid and using the F statistic to contour confidence volumes. We present a statistical expression for the latter confidence limits that is applicable when an independent estimate of arrival time errors is available from observations accumulated for a number of earthquakes. Synthetic experiments comparing the results of the grid search and generalized inverse methods show that in cases where solutions are obtained either without S wave information or for epicenters which lie well outside the network, the effects of nonlinearity on the shape of the confidence regions may be significant. However, for the well-observed events both methods yield comparable confidence volumes in good agreement with the distribution of hypocenters obtained from repeated locations incorporating random errors. The generalized inverse method has the advantage that it requires fewer calculations, so the examination of systematic errors in hypocentral parameters produced by uncertainties in the seismic velocity structure can be studied in a more computationally efficient manner. Except in the cases of poorly resolved earthquakes, the effects of nonlinearity on uncertainties in hypocentral parameters can be observed by the application of the F statistic to the variation of the generalized inverse travel time residuals with focal depth.     

Pseudo long base line navigation algorithm for underwater vehicles with inertial sensors and two acoustic range measurements

This paper presents an integrated navigational algorithm for unmanned underwater vehicles (UUV) using two acoustic range transducers and strap-down inertial measurement unit (SD-IMU). A range measurement model is derived for a UUV having one acoustic transducer and cruising around two reference transponders at sea floor or surface. The proposed algorithm, called pseudo long base line (PLBL), estimates the position of the vehicle integrating the SD-IMU signals corrected with the two range measurements. Extended Kalman filter was applied to propagate error covariance, to update measurement errors and to correct state equation whenever the external measurements are available. Simulations were conducted to illustrate the effectiveness of the PLBL using the 6-d.o.f. nonlinear numerical model of a UUV at current flow, excluding bottom-fixed DVL. This paper also shows the error convergence of the vehicle's initial position by the additional range measurements without velocity information.     

Steric height trends at Ocean Station Papa in the Northeast Pacific Ocean

Abstract

We estimate secular changes in steric sea level in the northeast Pacific Ocean using the 27‐year time series of monthly hydrographic observations for Station PAPA (50°N, 145°W). Linear trends based on the entire data record suggest that steric heights relative to 1000 db are increasing at a rate of 0.93 mm/yr and that 67% of this increase is due to thermosteric changes at depths below 100 m; the smaller halosteric contribution to the steric trend appears to be confined to the upper 100 m. A trend of 0(1 mm/yr) is consistent with estimates of sea level rise based on coastal tide gauge records. However, a critical examination of the results indicates that sea level changes of such small magnitude would be masked by the large (1–10 cm) interannual variability of open ocean steric height. This is verified by recalculation of trends using abridged versions of the data set. We conclude that our trend estimates are still open to question and that the present 27‐year time series is too short to permit accurate resolution of possible climate‐induced changes in global sea level.     

Measurement of ocean wave spectra using a ship-mounted HF radar

The work describes an inversion algorithm for HF radar measurement of nondirectional wave spectra using an omnidirectional receive/transmit antenna. Such a radar would be suitable for deployment on a stationary ship or drill rig. In this approach, wave information is extracted from the radar observations by numerically inverting the integral equation representing the backscatter return from the ocean. Test results of this technique applied to data collected using a 25.4-MHz radar installed on a ship have been very positive. For the two measurements collected, there is a high degree of correlation between the radar wave estimates and those of a WAVE-TRACK buoy     

Computer aided piloting of a deeply towed vehicle

An instrumented vehicle is towed at the end of a 5 km long cable, gathering data about the deep sea floor and near-bottom water column. Although bottom-moored acoustic transponders are used to determine the vehicle and ship positions precisely and in real time, predicting the ship manoeuvers required to bring the vehicle over an area of interest on the sea floor is far from trivial for the ship driver. Computer software has been developed which recommends courses for the ship to steer so that the vehicle will pass near a desired target. In trials at sea, the computer steered the vehicle 80, 40 and 85 m from pre-selected targets. Analysis of the causes of the misses suggests future developments which may reduce the miss distance, provide information on current structure of the water column, and reduce the level of skill and attention required of the vehicle pilot.     

Marine Geodesy (1975–1979)— to the International Association of Geodesy,Section 1

Abstract

Geographic Information Systems (GIS) have been developed during the last decade. Their land‐based applications range from land information management, urban planning, and environmental research to engineering design and management in the utility and oil industries, geological subsurface analysis, and others. However, applications of GIS in the marine environment are still in the initial stages. This may be due to, among other things, the large marine data sets, the demands of 3D data processing, and the difficulty of ocean data acquisition.

This article presents the result of the development of an integrated Marine Geographic Information System (MGIS) for the exploration and development of the Exclusive Economic Zone (EEZ) in the U.S. Pacific Islands region by the Pacific Mapping Center, University of Hawaii. Features such as spatial marine data processing, integration of GIS and mapping systems, 3D data structures, and simulation and animation of marine operations are developed and applied in this system. Since MGIS applications in EEZ are related to many disciplines in marine sciences and engineering, an operational MGIS should be an integrated system in which both basic GIS and marine application systems are combined into a single geo‐referenced system, In addition to other applications, this MGIS has been used: (1) to select a potential deep‐water research site off the island of Hawaii for the State of Hawaii Department of Business, Economic, and Development (DBED); (2) to generate a three‐dimensional database and use it for the navigation and simulation of underwater operations of an underwater research vehicle; and (3) to produce two 2° × 2° mosaic sheets of sonar images, which meet the USGS standard for a sonar image atlas.