Ocean Surface Current Measurements Research Articles

Enhancing Arctic Maritime Domain Awareness Through an Off-Grid Multi-sensor Instrument Platform

AbstractSurface current mapping high-frequency radars were installed along the northwest corner of Alaska during the open water periods of 2011‐2013. A combination of wind and solar renewable energy inputs supplied electricity to a single radar site operating at Point Barrow, Alaska, via an off-grid remote power module (RPM). The radar at Point Barrow was able to simultaneously collect measurements of ocean surface currents, measure the position and velocity of passing vessels, and provide meteorological data in real time. This paper provides a summary of the performance of the power module from 2011 to 2013 and vessel detection results from 2013 with corresponding Automated Identification System (AIS) data. The RPM provided infrastructure to meteorological stations, high-frequency radars, and AIS in a resilient and robust manner and serves as an example of how the multi-use capability of integrated sensor modules can provide enhanced maritime domain awareness and persistent surveillance capabilities in remote Arctic environments.

Assimilation of satellite-derived ocean surface current in an Indian Ocean circulation model

Although satellites do not provide direct measurement of ocean surface currents, these currents can be indirectly inferred from satellite measurements. Such a product is regularly generated with the simultaneous use of three satellite-derived surface variables, namely sea surface height, sea surface winds and sea surface temperature. Ocean surface current has been assimilated in an Indian Ocean circulation model using the nudging technique, which involves extra surface stress acting on the ocean. This stress is the result of a difference between the satellite-derived currents and the model currents. The effect of the assimilation has been judged statistically. The impact has also been quantified in a free run following the assimilation. The assimilation has been found to exhibit a significant positive impact on the model simulation.

Inherent Limitations in High-Frequency Radar Remote Sensing Based on Bragg Scattering From the Ocean Surface

In this paper, the existence and statistical properties of the first-order (Bragg) fluctuations in high-frequency (HF) radar Doppler spectra due to ocean wave random phase are investigated. Here, a pulsed radar waveform is used and the ocean surface is assumed to first order to be a time-varying random rough surface representable as a zero-mean, Gaussian process. Theoretical analysis for the source of the Bragg fluctuations begins with the radar backscattered electric fields in the time domain. The statistical properties of the Bragg fluctuations are investigated through simulation for various radar operating frequencies and pulse widths. The magnitude and significance of the Bragg fluctuations are discussed in terms of the operating parameters. The results can be used in setting minimum error bounds for HF radar ocean surface current measurement.

HF Radar Sea-echo from Shallow Water.

HF radar systems are widely and routinely used for the measurement of ocean surface currents and waves. Analysis methods presently in use are based on the assumption of infinite water depth, and may therefore be inadequate close to shore where the radar echo is strongest. In this paper, we treat the situation when the radar echo is returned from ocean waves that interact with the ocean floor. Simulations are described which demonstrate the effect of shallow water on radar sea-echo. These are used to investigate limits on the existing theory and to define water depths at which shallow-water effects become significant. The second-order spectral energy increases relative to the first-order as the water depth decreases, resulting in spectral saturation when the waveheight exceeds a limit defined by the radar transmit frequency. This effect is particularly marked for lower radar transmit frequencies. The saturation limit on waveheight is less for shallow water. Shallow water affects second-order spectra (which gives wave information) far more than first-order (which gives information on current velocities), the latter being significantly affected only for the lowest radar transmit frequencies for extremely shallow water. We describe analysis of radar echo from shallow water measured by a Rutgers University HF radar system to give ocean wave spectral estimates. Radar-derived wave height, period and direction are compared with simultaneous shallow-water in-situ measurements.

HF Radar Ocean Current Algorithm Based on MUSIC and the Validation Experiments

Wuhan University's ocean state measuring and analyzing radar (OSMAR2000), working at around 7.5 MHz in the low region of the HF band with a 120-m-long linear receiving antenna array, can measure ocean surface current at ranges of up to 200 km. An ocean surface current algorithm based on direction finding (DF) using the multiple signal classification (MUSIC) method is developed for the OSMAR2000 radar. This paper describes the OSMAR2000 ocean surface current algorithm based on MUSIC and the validation experiments in the East China Sea. The results of the ocean surface current measurements demonstrate that the OSMAR2000 ocean surface current algorithm based on MUSIC is feasible for the long range of ocean surface current mapping with a sufficient bearing resolution.

Measurements of ocean surface waves and currents using l- and c-band along-track interferometric sar

Along-track interferometric synthetic aperture radar (ATI-SAR) is an active coherent imaging system, utilizing two antennas separated along the platform flight direction. The phase information of ATI-SAR from the Doppler shift of the backscattered signal represents the line-of-sight velocity of the water scatterers. While the advent of ATI-SAR provided us with a potentially powerful technique for ocean surface current and wave mapping, the surface current has not been measured exactly from the ATI-SAR velocity because the Doppler shift is not simply proportional to the component of the mean surface current. It also includes other types of contributions associated with the phase velocity of the Bragg waves and orbital motions of all ocean waves that are longer than Bragg waves. In this paper, we review how the phase difference measured by ATI-SAR is related to the mean Doppler frequency, and we develop a new and practically useful method to extract the surface current component utilizing simultaneously measured L- and C-band ATI-SAR data. Since the measured ATI-SAR velocity shows a different value at a different radar-frequency, we investigate the influence of Bragg-resonant waves and long ocean wave motions on the ATI-SAR velocity according to the radar frequency. The Bragg-wave phase velocity component, which is a significant source of error for extracting the surface current, can be effectively eliminated by using L- and C-band ATI-SAR. The method is applied to L- and C-band ATI-SAR measurements acquired at the Ulsan coast in the southeastern part of the Korean peninsula. The resulting ocean surface current vectors are compared with in situ measurements collected by recording current meter. We furthermore extract ocean surface wave information from the ATI-SAR phase image using a quasi-linear transform.

Dual-beam interferometry for ocean surface current vector mapping

The recent use of along-track interferometry (ATI) in synthetic aperture radar (SAR) has shown promise for synoptic measurement of ocean surface currents. ATI-SARs have been used to estimate wave fields, currents, and current features. This paper describes and analyzes a dual-beam along-track interferometer to provide spatially resolved vector surface velocity estimates with a single pass of an aircraft. The design employs a pair of interferometer beams, one squinted forward and one squinted aft. Each interferometric phase is sensitive to the component of surface Doppler velocity in the direction of the beam. Therefore, a proper combination of these measurements provides a vector surface velocity estimate in one pass of the aircraft. The authors find that precise measurements dictate widely spaced beams and that the spatial resolution for the squinted SAR is essentially identical to the sidelooking case. Practical instrument design issues are discussed, and an airborne system currently in development is described. Through computer simulation, they observe the azimuthal displacement of interferometric phases by moving surfaces identical to those of conventional SAR and find that such displacement can bias the estimated surface velocity.

Simulation-based evaluations of HF radar ocean current algorithms

A computer simulation is used to analyze errors in high-frequency (HF) radar ocean surface current measurements. Two pointing algorithms used for current extraction, a direction finding approach using MUltiple SIgnal Characterization (MUSIC) developed by Schmidt (1986), and conventional beam forming, are compared in terms of the effect of variations in sea state parameters on current measurement error. The radar system parameters used in the simulation were taken from the University of Michigan's multi-frequency coastal radar (MCR), which operates on four frequencies from 4.8 to 21.8 MHz and employs an eight-element linear phased array for its receive antenna. Results show MUSIC direction finding to be applicable to phased array systems and to have a better sensitivity to sharp current features, but larger random error than traditional beam forming methods. Also, for cases where beam forming errors are dominated by beam width or low signal to noise ratio, results show MUSIC to be a viable alternative to beam forming.

Measurement of Ocean Surface Currents by the CRL HF Ocean Surface Radar of FMCW Type. Part 2. Current Vector

The Communications Research Laboratory (CRL) has been developing high-frequency ocean surface radars (HFOSRs). The CRL dual-site HFOSR system can clarify the distribution of surface currents with a nominal range of 50 km. This paper presents a theoretical and experimental analysis of the measurement error of the current vector obtained by the CRL HFOSR system, using a comparison of instantaneous current vectors acquired by the HFOSR system and current meters moored at a depth of 2 m, taking account of the vertical current shear. The theoretical analysis shows that the probability distribution of the measurement error of the current vector forms concentric ellipses at a spatial scale that depends on the RMS measurement error of radial current velocity and with an aspect ratio that depends only on the azimuthal difference of the radar beams. When the azimuthal difference is a right angle, the measurement error of the current vector is at a minimum. A comparison between instantaneous current vectors measured by the CRL HFOSR system and moored current meters shows that the distribution of the difference vector between the radar current and the meter current agrees well with the theoretical measurement error of the current vector and that the RMS of difference vector length is about 10 cm s−1 while the azimuthal difference between two radar beams is between 45 and 135 degrees. The accuracy of current measurement by the dual-site HFOSR system is therefore considered to be less than 10 cm s−1 in this range of azimuthal difference. The theoretical analysis will be applicable for a wider range of the azimuthal difference of the radar beams.

Current Structure Variations Detected by High-Frequency Radar and Vector-Measuring Current Meters

Abstract Ocean surface current measurements from high-frequency (HF) radar are assessed by comparing these data to near-surface current observations from 1 to 30 October 1994 at two moored subsurface current meter arrays (20 and 25 m) instrumented with vector-measuring current meters (VMCMs) and Seacat sensors during the Duck94 experiment. A dual-station ocean surface current radar (OSCR) mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 25 km × 44 km domain using the HF (25.4 MHz) mode and directly overlooked these moorings. In response to wind, tidal, and buoyancy forcing over 29 days, surface current observations were acquired 95% of the time in the core of the OSCR domain, decreasing to levels of about 50% in the offshore direction. Regression analyses between surface and subsurface measurements at 4 and 6 m indicated biases of 2–6 cm s−1, slopes of O(1), and rms differences of 7–9 cm s−1. Episodic freshwater intrusions of about 30 practical salinity units (psu)...

Measurements of Ocean Surface Currents from a Moving Ship Using VHF Radar

Abstract Shore-based, high-frequency (HF) and very high frequency (VHF) radar systems for measuring ocean surface currents have been well detailed in the literature. The use of these systems has been limited to coastal ocean applications since both of the required radar sites must be positioned ashore. The geometric constraints imposed by two shore sites can be relieved by using at least one sea-based system, thus allowing coverage of more coastal areas. A reduction in the number of required sites to one can be achieved by placing the site on a platform that transits between transmit–receive cycles to generate an overlapping field of coverage. Reducing the number of required sites to one increases the ocean area that can be measured. The feasibility of using a single VHF radar system based on a moving ship for both nearshore and deep-water area coverage is the subject of this paper. In December 1993 and July 1994 a nearshore, ship-based dataset was taken off the east coast of Key Biscayne, Florida, with o...

Mapping Wind Directions with HF Radar

BESIDES THE MEASUREMENT of ocean surface currents, high-frequency (HF) radar has also been demonstrated to be effective at measuring the wind direction at scales on the order of one to several kilometers and over areas of millions of square kilometers in the case of sky-wave HF radars (Georges et al. , 1993) and many hundreds of square kilometers in the case of groundwave HF radars. The capability to estimate the wind direction at high resolution over large areas makes HF radar unique in that it is ground based, yet is capable of collecting remotely sensed measurements of the oceanic wind field that could prove beneficial to mariners, weather forecasters, ocean and atmospheric modelers, offshore operators, and recreational users. Not only are such measurements useful to a variety of people, but they would be expensive and difficult, if not impossible, to obtain through other means, such as through the use of buoys at this resolution.

Mesoscale Ocean Surface Current Structure Detected by High-Frequency Radar

Abstract The quality and vertical correlation scales of high-frequency (HF) radar-derived ocean surface current measurements from an ocean surface current radar (OSCR) are assessed by comparing surface to subsurface current observations from 11 June to 8 July 1993 at directional discus buoys DW and DE, each instrumented with a three-axis ultrasonic current meter at the 13.8- and 9.5-m depths, respectively. A dual-station OSCR mapped the current fields at 20-min intervals at a horizontal resolution of 1.2 km over a 30 km × 44 km domain inshore of the Gulf Stream using the HF (25.4 MHz) mode. Over a 27-day experimental period, surface current observations were acquired 97% of the time extending to the maximum theoretical range of 44 km. Linear regression analyses indicated a bias of 2–4 cm s−1 and slopes of O(1). While there were periods when the daily averaged complex correlation coefficients were highly correlated (>0.8), periods of low correlation (<0.3) are explained in terms of vertical phase differenc...

Measurement of ocean surface currents using a long-range, high-frequency ground wave radar

High-frequency (HF) ground wave radar (GWR) is emerging as a significant tool for monitoring ocean surface conditions at ranges well beyond the line-of-sight horizon that limits conventional systems. An experimental GWR system at Cape Race, Newfoundland, Canada that has been operational since 1991, has the ability to performing routine surveillance of oceanic surface parameters and surface target detection. Operating in the frequency range between 5 and 8 MHz, the frequency modulated interrupted continuous wave (FMICW) radar has a nominal range capability of 200 km. An experiment was performed during the period of October 20-November 21, 1992 to test the surface current measuring capability of the Cape Race system. Here, near real-time radial surface current information is extracted from the Doppler spectra of the radar time series data and a comparison is performed to the Lagrangian velocities derived from the position-time tracks of Accurate Surface Tracker (AST) drifters. A wide range of oceanic conditions were experienced during the experimental period, and favorable results were obtained from the comparison regardless of the sea state conditions. The analysis shows the standard deviation in the radar radial velocity component to be approximately 5 cm/s.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Estimates of currents in the nearshore ocean region using interferometric Synthetic Aperture Radar

Microwave remote sensing of the ocean surface using Synthetic Aperture Radar (SAR) is of great interest due to its high resolution and the potential of nearly instantaneous coverage of large areas. The experience accumulated in ocean imaging by SAR indicates that there are substantial difficulties in interpretation of the obtained images of ocean wave systems. Ocean surface currents are almost impossible to obtain from SAR imagery. The present paper reports on measurements of ocean surface currents based on imaging of the nearshore regions of the Monterey Bay using an interferometric SAR. The method is a modification of the conventional SAR which employs two spatially separated antennas. This modification allows one to construct an interferogram from the two complex images obtained by each antenna, which provides direct mapping of the observed surface velocities. The present results demonstrate the advantages of this novel technique in studying nearshore ocean surface dynamics. In particular, spatial distribution of the longshore current velocity is presented.

Role of the oceans in global climate

Abstract The oceans play a major role in determining the Earth's climate and its natural variations, and also the magnitude and timing of changes that may result from human activities such as the emission of carbon dioxide and other ‘greenhouse’ gases. They do so by absorbing much of the solar radiation and of the downward infrared radiation emitted by the greenhouse gases in the atmosphere, by distributing this heat by ocean currents and eddies induced by wind stress, and by density gradients set up by variations in temperature and salinity. Descriptions of the large-scale heat transfer within the oceans and the exchanges of heat, moisture, momentum and fresh water between the atmosphere and the oceans are based partly on observation, partly on theory and partly on model stimulations. The oceans will moderate any climatic effects resulting from human emissions of carbon dioxide because they absorb about half of this carbon dioxide and also delay any additional warming because of their great thermal inertia. This is demonstrated by the results from climate models in which the global atmosphere is coupled to the deep global oceans and the atmospheric carbon dioxide is allowed to increase at 1% p.a. compound to double after 70 years. The resulting rise in globally averaged temperature predicted by three different models ranges from 2·3 to 1·3 K compared with predictions of 4–5 K from models having only a shallow ocean that does not interact fully with the atmosphere. Reliable long-term predictions of climate change will not be possible unless and until there is an adequate supply of oceanic observations to feed and validate the models. Measurements of ocean surface temperatures, winds, waves and currents are now being made by ingenious satellite-borne instruments but observations from the ocean interior are very sparse and irregular. The latter cannot be obtained in sufficient quantity by ships but could, in principle, be provided by small unmanned, remotely controlled submarines located and interrogated by satellite. An international global ocean-observing system, estimated to cost about US$2 × 109 year−1, is now under discussion, but the difficult question of funding has yet to be addressed.

Comment on ‘Single station ocean current vector measurement: Application of the spaced antenna (SA) technique’

A “spaced antenna” (SA) technique for HF ocean‐surface current measurement was introduced by May et al. [1989]. It was stated that the SA technique can measure transverse surface current speed from a single HF radar station, in addition to the radial speed that is conventionally obtained; it is claimed, therefore, that a single station can thereby measure the total current vector, eliminating the need for two radars. This sequel presents the derivation showing that the SA technique actually measures the radial speed's angular variation with respect to bearing, rather than the transverse current speed. The former is equal to the transverse speed only when there is no curvature in the current streamlines. Using reported voracities, we show that neglect of the resulting curvature in attempting to deduce the transverse speed produces errors too large to allow accurate single‐site current mapping based on SA measurements for normal coastal operation. We consider applications where the SA should prove advantageous, and give limits on radial variation for required transverse speed accuracy.

Direct Measurements of Current Shear in the Tropical Pacific Ocean and Its Effect on Drift Buoy Performance

Measurements of ocean surface currents derived from drift buoy trajectories are subject to errors caused by slippage of the buoy relative to the surrounding water. This slippage error is caused by a number of forces acting on the buoy and drogue element, one of which is the current shear in the water. Idealized model calculations are used to exemplify some effects of vertical current shear on drogues, and on the performance of drogued buoy systems in current shear. It is shown that shear enhances the performance of drogues, and that long drogues should perform better than short drogues in shear, but shear also can induce slippage by adding drag force to the buoy hull. To establish environmental design parameters, average and rms current shear values between 9.7 and 22.5 meters depth were computed from Doppler acoustic current profiler measurements from the tropical Pacific Ocean. Largest values of shear (∼0.025 s−1 rms) were found near the equator in the eastern Pacific as expected. Elsewhere the shear was generally less than 0.02 s−1, mostly less than 0.01 s−1. Average values of shear were generally less than 0.007 s−1.

Interferometric radar measurement of ocean surface currents

We describe a new method of measuring surface currents using an interferometric synthetic aperture radar. An airborne implementation has been tested over the San Francisco Bay near the time of maximum tidal flow, resulting in a map of the east-west component of the current. Only the line-of-sight component of velocity is measured by this technique. Where the signal-to-noise ratio was strongest, statistical fluctuations of less than 4 cm s−1 were observed for ocean patches of 60×60 m.

Measurement of Ocean Surface Currents from Space with Multifrequency Microwave Radars-A System Analysis

The use of multifrequency microwave radars to measure ocean surface currents from satellites is considered in this paper. We discuss the effects that satellite motion background backscatter from the ocean surface, and receiver noise have on the ability of such systems to obtain current maps. We present an example which shows that radars in geostationary orbit have the potential to map ocean currents over large areas of the ocean surface.