Surface Current Measurements Research Articles

A new Doppler frequency anomaly algorithm for surface current measurement with SAR

A new Doppler frequency anomaly algorithm for surface current measurement with SAR

Using Artificial Neural Networks for the Estimation of Subsurface Tidal Currents from High-Frequency Radar Surface Current Measurements

An extensive record of current velocities at all levels in the water column is an indispensable requirement for a tidal resource assessment and is fully necessary for accurate determination of available energy throughout the water column as well as estimating likely energy capture for any particular device. Traditional tidal prediction using the least squares method requires a large number of harmonic parameters calculated from lengthy acoustic Doppler current profiler (ADCP) measurements, while long-term in situ ADCPs have the advantage of measuring the real current but are logistically expensive. This study aims to show how these issues can be overcome with the use of a neural network to predict current velocities throughout the water column, using surface currents measured by a high-frequency radar. Various structured neural networks were trained with the aim of finding the network which could best simulate unseen subsurface current velocities, compared to ADCP data. This study shows that a recurrent neural network, trained by the Bayesian regularisation algorithm, produces current velocities highly correlated with measured values: r2 (0.98), mean absolute error (0.05 ms−1), and the Nash–Sutcliffe efficiency (0.98). The method demonstrates its high prediction ability using only 2 weeks of training data to predict subsurface currents up to 6 months in the future, whilst a constant surface current input is available. The resulting current predictions can be used to calculate flow power, with only a 0.4% mean error. The method is shown to be as accurate as harmonic analysis whilst requiring comparatively few input data and outperforms harmonics by identifying non-celestial influences; however, the model remains site specific.

Measuring Ocean Surface Current in the Kuroshio Region Using Gaofen-3 SAR Data

The Kuroshio is the strongest warm current in the western North Pacific, which plays a crucial role in climate and human activities. In terms of this, the accurate acquisition of ocean surface current velocity and direction in the Kuroshio region is of great research value. Gaofen-3 synthetic aperture radar (SAR) provides data support for the study of ocean surface current measurements in the Kuroshio region, but no relevant experimental result has been published yet. In this paper, four available stripmap mode SARs’ data acquired by Gaofen-3 in the Kuroshio region are used for measuring the ocean surface current field. In general, the Doppler centroid anomaly (DCA) estimation is a common method to infer ocean surface currents from single-antenna stripmap data, but only the radial velocity component can be retrieved. In order to measure current vectors, a novel method combining the sub-aperture processing and the least squares (LS) technology is suggested and demonstrated by applying to the Gaofen-3 SAR data processing. The experiment’s results agree well with model-derived ocean current data, indicating that the Gaofen-3 SAR has the capability to accurately retrieve the ocean surface current field in the Kuroshio region and motivate further research by providing more data.

Optimisation of Parameters in a German Bight Circulation Model by 4DVAR Assimilation of Current and Water Level Observations

Uncertain parameters in a 3D barotropic circulation model of the German Bight are estimated with a variational optimisation approach. Surface current measurements from a high frequency (HF) radar are used in combination with acoustic Doppler current profiler (ADCP) and tide gauge observations as input for a 4DVAR assimilation scheme. The required cost function gradients are estimated using an adjoint model code. The focus of the study is on systematic errors of the model with the control vector including parameters of the bathymetry, bottom roughness, open boundary forcing, meteorological forcing as well as the turbulence model. The model uses the same bathymetry, open boundary forcing, and metereological forcing as the operational model run at the Federal Maritime and Hydrographic Agency (BSH). The baroclinic BSH model is used as a reference to put the performance of the optimised model into perspective. It is shown that the optimised model has better agreement with HF radar data and tide gauge observations both within the fortnight training period and the test period 1 month later. Current profile measurements taken at two platforms indicate that both models have comparable error magnitudes at those locations. The optimised model was also compared with independent drifter data. In this case, drifter simulations based on the BSH model and the respective operational drift model including some surface wave effects were used as a reference. Again, these comparison showed very similar results overall, with some larger errors of the tuned model in very shallow areas, where no observations were used for the tuning and surface wave effects, which are only explicitly considered in the BSH model, play a more important role. The tuned model seems to be slightly more dissipative than the BSH model with more energy entering through the western boundary and less energy leaving toward the north. It also became evident that the 4DVAR cost function minimisation process is complicated by momentum advection, which leads to non-differentiable dependencies of the model with respect to the control vector. It turned out that the omission of momentum advection in the adjoint code still leads to robust estimates of descent directions.

Offshore Dispersal and Predation of Sea Turtle Hatchlings I: A Study of Hawksbill Turtles at Chagar Hutang Turtle Sanctuary, Malaysia

Sea turtle hatchlings emerge from underground nests at night, rapidly crawling seaward to swim off shore. Once in the water, hatchlings might experience high predation rates while in shallow water before reaching deeper water where encounters with predators, and consequently mortality rates, likely decline. Behavioral studies have described different swimming strategies used by hatchlings to counter nearshore predation. Coastal and oceanographic conditions are also likely to influence dispersal away from near shore to the open ocean. This study assessed predation rates of Hawksbill Turtle (Eretmochelys imbricata) hatchlings as they dispersed from shore at Chagar Hutang Bay, Malaysia and the role surface currents play in the transport of hatchlings off shore in the nearshore environment. An acoustic doppler current profiler was used to measure surface currents, and direct observations of hatchlings swimming off shore were made from a kayak using GPS loggers to track hatchling swimming paths. Six of the 31 hatchlings tracked (19.4%) were predated, most within 50 m of shore, indicating that predators are more abundant in shallower areas of the bay where a coralline-rocky bottom predominates. Survival tended to be greater under dark conditions when moonlight was absent or minimal. We quantified the relative importance of the tidal current in a hatchling's offshore swim, and found that in most cases, tidal surface currents assisted the offshore movement of Hawksbill hatchlings as they dispersed from the beach. These findings provide a better understanding of how sea turtle hatchling dispersal is affected by predation, moonlight, and physical oceanographic conditions at Chagar Hutang Turtle Sanctuary.

Ocean surface current multiscale observation mission (OSCOM): Simultaneous measurement of ocean surface current, vector wind, and temperature

Ocean current is one of the major drivers of water mass, energy, and biogeochemical cycles in the global ocean–atmosphere boundary layer and also a key variable in the formation of extreme climate events (e.g., El Niño). Direct measurement of the global ocean surface current is of great scientific interest and application value for understanding multiscale ocean dynamics, air-sea interaction, ocean mass and energy balance, and ocean carbon budget, as well as their variabilities under climate change. This paper reviews the state-of-the-art developments of ocean dynamics and technology on the observation of multiscale ocean circulation and related scientific frontiers. Presently, measurements of global ocean surface currents, which are mainly geostrophically derived from satellite altimeter data, are only available to resolve quasi-geostrophic current at large- to meso-scale in the off-equatorial open ocean. This becomes a bottleneck impeding the application and the development of ocean circulation dynamics. With the ambition of breaking the bottleneck, Ocean Surface Current multiscale Observation Mission (OSCOM) will launch a satellite equipped with a Doppler Scatterometer to directly measure ocean surface currents for the first time with a very high horizontal resolution of 5–10 km and a 3-day global coverage. Through carrying a Surface Temperature Infrared Radiometer and a Surface Temperature Microwave Radiometer, the OSCOM satellite is supposed to have the capability of observing ocean surface current, ocean surface vector wind, and sea surface temperature simultaneously. With a cutting-edge design, OSCOM will provide an in-depth picture of non-equilibrium ocean state and air-sea interaction from mesoscale to submesoscale, and helps to construct the fine structure of deep ocean current through a combination with Array for Real-time Geostrophic Oceanography profiling. Those direct measurements and derived dynamic parameters will further facilitate analyses of ocean carbon budget and ocean biogeochemical cycle, and provide a novel and improved pathway to data assimilation, coupling of General Circulation Models, and the Earth System Modelling for ocean prediction and climate change.

On the Surface Current Measurement Capabilities of Spaceborne Doppler Scatterometry

AbstractWide‐swath spaceborne Doppler scatterometry is a promising technique for the simultaneous measurement of global ocean surface winds and currents. The technique has been proven from airborne platforms, and here we use the lessons learned to examine a range of implications for a spaceborne system. We use a Doppler scatterometer simulator and a state‐of‐the‐art global circulation model to generate surface current measurements and their random errors. We find that a feasible instrument could measure 5 km gridded surface currents with typical random errors between 10 and 25 cm/s. For higher wind speeds, the random error in surface current decreases logarithmically. This level of accuracy allows for the computation of surface current relative vorticity and horizontal divergence with typical wavelength resolutions of 15–30 and 25–60 km, respectively. Unlike previous studies, we find that these measurements do not require multiday averaging, opening up new avenues for monitoring global ocean circulation.

Improved Method to Suppress Azimuth Ambiguity for Current Velocity Measurement in Coastal Waters Based on ATI-SAR Systems

Measurements of ocean surface currents in coastal waters are crucial for improving our understanding of tidal atlases, as well as for ecosystem and water pollution monitoring. This paper proposes an improved method for estimating the baseline-to-platform speed ratio (BPSR) for improving the current line-of-sight (LOS) velocity measurement accuracy in coastal waters with along-track interferometric synthetic aperture radar (ATI-SAR) based on eigenvalue spectrum entropy (EVSE) analysis. The estimation of BPSR utilizes the spaceborne along-track interferometry and considers the effects of a satellite orbit and an inaccurate baseline responsible for azimuth ambiguity in coastal waters. Unlike the existing methods, which often assume idealized rather than actual operating environments, the proposed approach considers the accuracy of BPSR, which is its key advantage applicable to many, even poorly designed, ATI-SAR systems. This is achieved through an alternate algorithm for the suppression of azimuth ambiguity and BPSR estimation based on an improved analysis of the eigenvalue spectrum entropy, which is an important parameter representing the mixability of unambiguous and ambiguous signals. The improvements include the consideration of a measurement of the heterogeneity of the scene, the corrections of coherence-inferred phase fluctuation (CPF), and the interferogram-derived phase variability (IPV); the last two variables are closely related to the determination of the EVSE threshold. Besides, the BPSR estimation also represents an improvement that has not been achieved in previous work of EVSE analysis. When the improved method is used on the simulated ocean-surface current LOS velocity data obtained from a coastal area, the root-mean-square error is less than 0.05 m/s. The other strengths of the proposed algorithm are adaptability, robustness, and a limited user input requirement. Most importantly, the method can be adopted for practical applications.

Comparison of Measured Surface Currents from High Frequency (HF) and X-Band Radar in a Marine Protected Coastal Area of the Ligurian Sea: Toward an Integrated Monitoring System

Two different ground-based remote sensing instruments can be used for the near-real-time monitoring of surface waves and currents, namely the high frequency HF radar and the microwave X-band radar. The HF system reaches larger offshore distances at lower spatial resolutions and provides a poorer measurement of the wave-induced currents in very shallow waters. On the other hand, the X-band system achieves significantly higher spatial resolutions with a smaller offshore coverage. This study provides a preliminary comparison of the measured surface currents, obtained by the two different tools where they overlap. The comparison showed a good agreement between the measures with some discrepancies ascribable to the difference in the characteristics of the two radar technologies.

Finite Element Model Based Determination of Local Membrane Conductivity of Polymer Electrolyte Fuel Cells

A new non-invasive method to measure PEFC in-plane membrane conductivity distribution is presented. It is based on electrical impedance tomography that relies on surface current injection and voltage measurement at the outer cell surface. The working principle is detailed and a numerical feasibility study shows the potential of the technique.

Validation and Evaluation of a Ship Echo-Based Array Phase Manifold Calibration Method for HF Surface Wave Radar DOA Estimation and Current Measurement

Shore-based phased-array HF radars have been widely used for remotely sensing ocean surface current, wave, and wind around the world. However, phase uncertainties, especially phase distortions, in receiving elements significantly degrade the performance of beam forming and direction-of-arrival (DOA) estimation for phased-array HF radar. To address this problem, the conventional array signal model is modified by adding a direction-based phase error matrix. Subsequently, an array phase manifold calibration method using antenna responses of incoming ship echoes is proposed. Later, an assessment on the proposed array calibration method is made based on the DOA estimations and current measurements that are obtained from the datasets that were collected with a multi-frequency HF (MHF) radar. MHF radar-estimated DOAs using three calibration strategies are compared with the ship directions that are provided by an Automatic Identification System (AIS). Additionally, comparisons between the MHF radar-derived currents while using three calibration strategies and Acoustic Doppler Current Profilers (ADCP)-measured currents are made. The results indicate that the proposed array calibration method is effective in DOA estimation and current measurement for phased-array HF radars, especially in the phase distortion situation.

Ka-Band Doppler Scatterometry over a Loop Current Eddy

Doppler scatterometry is a promising new technique for the simultaneous measurement of ocean surface currents and winds. These measurements have been recommended by the recent US NRC Decadal Review for NASA as being priority variables for the coming decade of Earth observations. In addition, currents and winds are useful for many applications, including assessing the operating conditions for oil platforms or tracking the dispersal of plastic or oil by surface currents and winds. While promising, Doppler scatterometry is relatively new and understanding the measurement characteristics is an important area of research. To this end, Chevron sponsored the deployment of DopplerScatt, a NASA/JPL Ka-band Doppler scatterometer, over instrumented sites located at the edge of a Gulf of Mexico Loop Current Eddy (LCE). In addition to in situ measurements, coincident synoptic maps of surface currents were collected by the Areté ROCIS instrument, an optical current measurement system. Here we report on the results of this experiment for both surface currents and winds. Surface current comparisons show that the Ka-band Current Geophysical Model Function (CGMF) needs to include wind drift currents, which could not be estimated with prior data sets. Once the CGMF is updated, ROCIS and DopplerScatt show good agreement for surface current speeds, but, at times, direction differences on the order of 10° can occur. Remote sensing optical and radar data agree better among themselves than with ADCP currents measured at 5 m depth, showing that remote sensing is sensitive to the the currents in top 1 m of the ocean. The LCE data provided a unique opportunity to study the effects of surface currents and stability conditions on scatterometer winds. We show that, like Ku-band, Ka-band estimates of winds are related to neutral winds (and wind stress) and are referenced relative to the moving frame provided by the current. This is useful for the study of air-sea interactions, but must be accounted for when using scatterometer winds for weather prediction.

Advances in Observing and Understanding Small-Scale Open Ocean Circulation During the Gulf of Mexico Research Initiative Era

Prediction of the distribution of buoyant oil, plastics, flotsam and marine organisms near the ocean surface is a long-standing fundamental problem of practical importance. Here, progress in this area during the time of the Gulf of Mexico Research Initiative (GoMRI) (2012-2019) is synthesized, emphasizing the well-known accumulation of floating material into highly concentrated streaks on horizontal scales of meters to 10’s of kilometers. In the pre-GoMRI period, two new paradigms were developed: the importance of submesoscale frontal dynamics on the larger scales and of surface-wave-driven Langmuir turbulence on the smaller scales, with a broad transition occurring near 100 m. Rapid progress resulted from the combination of high resolution numerical modeling tools, mostly developed before GoMRI, and new observational techniques developed during GoMRI. New designs for inexpensive and biodegradable satellite tracked surface drifters and new techniques for aerial tracking of surface drift cards enabled the deployment of thousands of such drifters to measure surface currents and to act as calibrated surrogates for oil. Progress in using radar and optical remote sensing of surface waves enabled mapping of surface currents from ships and aircraft. These new tools, combined with traditional oceanographic tools, enabled a set of coordinated measurement programs which supported and expanded the new paradigms. Floating material was found to both accumulate at submesoscale fronts and to disperse by following these fronts as they moved, leading to both higher concentrations and increased dispersion. Analyses confirmed the distinct submesoscale dynamics of this process and complexity of the resulting fields. Existing tools could be developed into predictive models of submesoscale statistics; prediction of individual submesoscale features is limited by data. Away from fronts, measured rates of accumulation of material in and beneath surface windrows was found to be consistent with Langmuir turbulence, but highly dependent on the rise rate of the material and thus, for oil, on the droplet size. Models of this process were developed and tested and could be further developed into predictive tools. Both the submesoscale and Langmuir processes are sensitive to coupling with surface waves and air-sea flux processes; this is a promising area for future studies.

Best Practices on High Frequency Radar Deployment and Operation for Ocean Current Measurement

High Frequency Radar (HFR) technology refers to land based remote sensing instruments capable of measuring surface currents and ocean waves at ranges up to 200 km or more. HFR technology is widely acknowledged as a cost-efficient tool to monitor coastal regions and has potential use to monitor coastal regions all over the world. Globally, the number of HFR stations is steadily increasing. Regional networks provide real-time data in support of operational activities such as search and rescue operations, fast response in case of maritime accidents and spill of pollutants, and resource management. Each operator needs a general understanding of the working principles in order to ensure that instruments are managed properly. A set of harmonized quality assurance and quality control procedures is recommended, along with an effective approach to HFR data discovery and dissemination, to provide high quality measurements to the end users. Different documents providing best practices for operation and maintenance have emerged in the past years. They are oriented either to Direction Finding (DF) or Beam Forming (BF) systems, or to specific manufacturer’s radar systems. The main objective of this paper is to offer a comprehensive “Best Practices” document in an effort of ensuring consistency between different deployments and harmonized operations of HFR systems. This, regardless of system manufacturer, antenna design and setup. A homogeneous approach is given when possible, general concepts and definitions are introduced in order to provide a framework for both data processing and management steps. Examples are also given from the European HFR users with focus on Near Real Time data flow suitable for operational services.

Measuring Winds and Currents with Ka-Band Doppler Scatterometry: An Airborne Implementation and Progress towards a Spaceborne Mission

Ocean surface winds and currents are tightly coupled, essential climate variables, synoptic measurements of which require a remote sensing approach. Global measurements of ocean vector winds have been provided by scatterometers for decades, but a synoptic approach to measuring total vector surface currents has remained elusive. Doppler scatterometry is a coherent burst-scatterometry technique that builds on the long heritage of spinning pencil beam scatterometers to enable the wide-swath, simultaneous measurement of ocean surface vector winds and currents. To prove the measurement concept, NASA funded the DopplerScatt airborne Doppler scatterometer through the Instrument Incubator Program (IIP) and Airborne Instrument Technology Transition (AITT) program. DopplerScatt has successfully shown that pencil beam Doppler scatterometry can be used to form wide swath measurements of ocean winds and currents, and has increased the technology readiness level of key instrument components, including: Ka-band pulsed radar hardware, optimized scatterometer burst-mode operation, calibration techniques, geophysical model functions, and processing algorithms. With the promise and progress shown by DopplerScatt, and the importance of air-sea interactions in mind, the National Academy’s Decadal Survey has targeted simultaneous measurements of winds and currents from a Doppler scatterometer for an Earth Explorer class spaceborne mission. Besides DopplerScatt’s place as a technology stepping stone towards a satellite mission, DopplerScatt provides scientifically important measurements of ocean currents and winds (400 m resolution) and their derivatives (1 km resolution) over a 25 km swath. These measurements are enabling studies of the submesoscales and air-sea interactions that were previously impossible, and are central to the upcoming NASA Earth Ventures Suborbital-3 Submesoscale Ocean Dynamics Experiment (S-MODE). This paper summarizes the development of DopplerScatt hardware, systems, calibration, and operations, and how advances in each relate to progress towards a spaceborne Doppler scatterometer mission.

Ocean surface current retrieval from space: The Sentinel-2 multispectral capabilities

The Sentinel-2 MultiSpectral Instrument (MSI) collects multiple spectral band images, corresponding to specific sensing wavelengths and spatial resolutions, i.e. 10 m, 20 m and 60 m, respectively. Images are collected one at the time with a given time-lag between observations. Under favorable conditions, spatio-temporal characteristics of propagating ocean surface waves can thus uniquely be retrieved. A method for surface current vector field reconstruction is then developed. Demonstrated over different deep ocean regions, the retrieved surface current fields well compare with medium-resolution ocean circulation model or derived-velocities from altimeter measurements. At finer scales, the surface wave-conservation law is recovered, with the associated relationship between current vorticity and wave-ray curvature. Over shallow water regions, the wave propagation properties well follow sea depth variations, consistent with ETOPO1 data. Finally, time-lag between detector bands can also be exploited to estimate speed and direction properties of detected surface wave breaking whitecaps. An analysis of velocity reconstruction errors further reveals that Sentinel-2 MSI inter-channel co-registration is realized with an accuracy better than 0.1 pixel. Overall, these results confirm very promising capabilities of optical imagery to provide direct surface current velocity measurements from space, over relatively large areas, O(100 km), with a spatial resolution down to O(1 km).

Wave-Height Mapping From Second-Order Harmonic Peaks of Wide-Beam HF Radar Backscatter Spectra

Compact high-frequency surface wave radar has been widely applied to the measurement of sea surface current, but its accuracy and direction resolution of wave parameter estimation are always limited due to the wide beam of the antenna. In this article, a novel wave-height mapping method based on the second-order harmonic peak (SHP) of radar Doppler spectra is proposed to address this concern. The characteristic of the SHP at the Doppler frequency of $\sqrt {2}$ times the Bragg frequency is studied through the theoretical derivation and numerical simulation. A relationship between the ratio ( $R$ ) of the SHP power to the Bragg peak power and significant wave height ( $H_{s}$ ) is derived. Furthermore, the $R$ – $H_{s}$ model is improved by incorporating influences, such as background noise and antenna beamwidth. With this improved model, a wave-height mapping algorithm based on the direction finding technique is presented. This approach enables the significant wave-height map extraction using a broad-beam radar. Finally, wave-height maps obtained at different sea states are depicted and analyzed, and the wave heights appearing on the maps are compared with buoy data over a one-month experiment to verify the validity and robustness of the algorithm. During this period, the significant wave height varies from about 0.5 to 4.5 m, and the radar measured wave heights at different range/distance bins show an overall root-mean-square error (RMSE) of 0.33–0.77 m and a correlation coefficient (CC) of 0.78–0.94, with respect to the buoy measurements.

Analysis of Measured Surface Nearshore Currents Using Multiple Drifters on Anmok Beach, South Korea

Lim, H.S.; Kim, M.J.; Kim, D.H., and Lee, H.J., 2019. Analysis of measured surface nearshore currents using multiple drifters on Anmok beach, South Korea. In: Lee, J.L.; Yoon, J.-S.; Cho, W.C.; Muin, M., and Lee, J. (eds.), The 3rd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 91, pp. 46-50. Coconut Creek (Florida), ISSN 0749-0208.To understand the mechanism of sediment transport and beach erosion processes in the surf zone near the coast of the Korean Peninsula, the development of coastal erosion control technology, an R&D project supported by the Korean government, has been conducted since 2013. Anmok Beach, located on the eastern coast of the Korean Peninsula, has been studied by measuring currents and sediment transport using bottom-mounted acoustic sensors and offshore long-term wave observation since February 2015. Moreover, for measurement of surface currents, multiple Lagrangian drifters were developed and successfully deployed nine times near the surf zone and submerged breakwaters close to Kangneung northern breakwaters. The variation of the measured bottom currents was induced directly by seasonal wave height and direction, depending on the incoming waves from the NNE in winter, ENE in summer, and NE, normal to the shoreline, in the case of high waves. However, the measured surface currents were affected not only by the incoming waves in the presence of high-energy waves, but also by wind speed and direction in the presence of low-energy waves. In this study, the observed surface nearshore currents measured by small Lagrangian GPS drifters near the surf zone and submerged breakwaters at Anmok were analyzed to investigate the variability of the wave-induced currents and nearshore surface currents generated by different wave and wind environments. The drifter experiments showed that offshore surface currents in the surf zone were predominantly developed, especially by strong seaward winds that opposed the direction of the weak waves. However, onshore surface currents were developed by wave-induced currents generated by the interaction of waves from offshore accompanied by sea winds. This measurement is now used to calibrate and validate the coastal sediment modeling system for the eastern coast of the Korean Peninsula.

Integrated Observations of Global Surface Winds, Currents, and Waves: Requirements and Challenges for the Next Decade

Ocean surface winds, currents, and waves play a crucial role in exchanges of momentum, energy, heat, freshwater, gases, and other tracers between the ocean, atmosphere, and ice. Despite surface waves being strongly coupled to the upper ocean circulation and the overlying atmosphere, efforts to improve ocean, atmospheric, and wave observations and models have evolved somewhat independently. From an observational point of view, community efforts to bridge this gap have led to proposals for satellite Doppler oceanography mission concepts, which could provide unprecedented measurements of absolute surface velocity and directional wave spectrum at global scales. This paper reviews the present state of observations of surface winds, currents, and waves, and it outlines observational gaps that limit our current understanding of coupled processes that happen at the air-sea-ice interface. A significant challenge for the coming decade of wind, current, and wave observations will come in combining and interpreting measurements from (a) wave-buoys and high-frequency radars in coastal regions, (b) surface drifters and wave-enabled drifters in the open-ocean, marginal ice zones, and wave-current interaction ``hot-spots'', and (c) simultaneous measurements of absolute surface currents, ocean surface wind vector, and directional wave spectrum from Doppler satellite sensors.

A New Azimuth Ambiguity Suppression Algorithm for Surface Current Measurement in Coastal Waters and Rivers With Along-track InSAR

We present a new algorithm for suppressing azimuth ambiguities when measuring surface currents with an along-track interferometric (ATI) synthetic aperture radar in heterogeneous scenes, such as coastal waters or rivers. The key of the proposed algorithm involves a careful analysis of a parameter called the eigenvalue spectrum entropy (EVSE), which is defined as the entropy of the eigenvalue spectrum of the ATI covariance matrix computed in the Doppler domain. The physical meaning of EVSE is that it serves as a descriptor for the degree to which an unambiguous signal component and an ambiguous one are mixed. With the help of EVSE, azimuth ambiguities can be suppressed. Simulation results demonstrate that as compared with the conventional ATI method without azimuth ambiguity suppression, the proposed algorithm allows for a pronounced improvement in the current measuring accuracy. Other advantages of the proposed algorithm lie in the fact that it is not only adaptive, due to its ability to automatically capture the useful Doppler band whose width and position may both vary for different radar and scene parameters, but it also needs a minimal number of user inputs, making it a quite attractive algorithm for routine implementation.