K. Laws

Bio: K. Laws is an academic researcher from University of California, Santa Cruz. The author has contributed to research in topics: Radar & Buoy. The author has an hindex of 7, co-authored 29 publications receiving 216 citations.

Simulation-based evaluations of HF radar ocean current algorithms

Abstract: 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.

Estimation and Assessment of Errors Related to Antenna Pattern Distortion in CODAR SeaSonde High-Frequency Radar Ocean Current Measurements

Abstract: A simulation-based investigation of errors in HF radar‐derived, near-surface ocean current measurements is presented. The simulation model is specific to Coastal Ocean Dynamics Application Radar (CODAR) SeaSonde radar systems that employ a compact, collocated antenna geometry. In this study, radial current retrievals are obtained by processing simulated data using unmodified CODAR data processing software. To avoid limiting the results to specific ocean current and wind wave scenarios, the analyses employ large ensembles of randomly varying simulated environmental conditions. The effect of antenna pattern distortion on the accuracy of retrievals is investigated using 40 different antenna sensitivity patterns of varying levels of distortion. A single parameter is derived to describe the level of the antenna pattern distortion. This parameter is found to be highly correlated with the rms error of the simulated radial currents (r 5 0.94) and therefore can be used as a basis for evaluating the severity of site-specific antenna pattern distortions. Ensemble averages of the subperiod simulated current retrieval standard deviations are found to be highly correlated with the antenna pattern distortion parameter (r 5 0.92). Simulations without distortions of the antenna pattern indicate that an rms radial current error of 2.9 cm s 21 is a minimum bound on the error of a SeaSonde ocean radar system, given a typical set of operating parameters and a generalized ensemble of ocean conditions.

Using HF surface wave radar and the ship Automatic Identification System (AIS) to monitor coastal vessels

Abstract: We compare the ship detection capabilities of the Automatic Identification System AIS (installed on some ships) and coastal, surface wave HF radars, showing how to use both systems together to enhance ship detection performance in coastal regions. Practical reasons to want better real-time awareness of the location, velocity and type of vessels along coasts include vessel safety, protection of the coastal environment and national security. Our model for the HF radar aspect uses an example radar with significant power and aperture, similar to the Pisces radar. The AIS model is for the high power (12.5 W) AIS unit and a significantly elevated receiver (~ 250 ft asl). The HF system show good capability to ranges of ~ 150 km for small ships to 250 km for large ships. The AIS system shows excellent capability out to a typical horizon of ~ 50 km with irregular coverage beyond using ducted propagation to several hundred km and more. Use of both systems allows monitoring of both AIS and non-AIS equipped ships and enhances probability of detection for situations where both systems are functional.

Prototype autonomous mini-buoy for use in a wireless networked, ocean surface sensor array

Abstract: We report the design, prototype construction and initial testing of a small minibuoy that is aimed at use in a coordinated, wireless networked array of buoys for near-surface ocean sensing. This vehicle is designed to fill the gap between larger ocean surface vessels and/or moored buoys and subsurface gliders. The size and cost is low enough that these versatile sensor platforms can be deployed easily and in quantity. Since these minibuoys are mobile, they can keep station in currents as large as 25 cm/s or move as an adaptive, coordinated sensor array for high resolution in both time and space. The buoy is about 74 cm (29 in) long, 41 cm (16 in) wide (max) and weighs about 14.5 kg (32 lbs); hence, it can be deployed easily from small craft. Deployment times are about 1 to 2 days or more - longer with solar power. The buoy structure is fiberglass and PVC with two 2 W DC motors. Control is done with GPS and magnetic heading sensors and a PID scheme to maintain course. Communication is via a 900 MHz system with a range of 1 to 2 km and plans for a longer range HF/VHF or satellite system. The initial sensor system is designed for ocean hyperspectral observations as surface truth for airborne system calibration and validation and other ocean color applications. Acoustic, wave, air & water temperature sensors as well as GPS are included. The Mark I prototype has been successfully tested in a pool with manual control.

Identifying ship echoes in CODAR HF radar data: A Kalman filtering approach

Abstract: Coastal nations have an interest in maritime domain awareness for applications in national security, coastal conservancy, fishery and stewardship of the exclusive economic zones (EEZs) along their coastlines. Maritime situational awareness involves knowing the location, speed and bearing of ships and boats in the EEZ. HF radar is a useful tool in providing ship information in real time. It is especially effective when combined with ship-borne AIS beacons. Our previously developed HF radar and AIS ship detection models estimate signal to noise ratio (SNR) as a function of range, including ducted propagation for the AIS radio signals. However, ship detection is hampered by the high variability of HF echoes from ships. This is due in part to the aspect dependence of ship radar cross-section and to the presence of clutter bands at known Doppler shifts from both the ground and ocean waves. Tracking ships using their HF radar echoes becomes the means for effectively monitoring the presence of ships in the coastal ocean. We explore the application of Kalman filtering to the ship tracking problem, following the techniques described by J. V. Candy. This approach is described and demonstrated with a simple example.

Applications of Wireless Sensor Networks in Marine Environment Monitoring: A Survey

Abstract: With the rapid development of society and the economy, an increasing number of human activities have gradually destroyed the marine environment. Marine environment monitoring is a vital problem and has increasingly attracted a great deal of research and development attention. During the past decade, various marine environment monitoring systems have been developed. The traditional marine environment monitoring system using an oceanographic research vessel is expensive and time-consuming and has a low resolution both in time and space. Wireless Sensor Networks (WSNs) have recently been considered as potentially promising alternatives for monitoring marine environments since they have a number of advantages such as unmanned operation, easy deployment, real-time monitoring, and relatively low cost. This paper provides a comprehensive review of the state-of-the-art technologies in the field of marine environment monitoring using wireless sensor networks. It first describes application areas, a common architecture of WSN-based oceanographic monitoring systems, a general architecture of an oceanographic sensor node, sensing parameters and sensors, and wireless communication technologies. Then, it presents a detailed review of some related projects, systems, techniques, approaches and algorithms. It also discusses challenges and opportunities in the research, development, and deployment of wireless sensor networks for marine environment monitoring.

Evaluating Radial Current Measurements from CODAR High-Frequency Radars with Moored Current Meters

Abstract: The performance of a network of five CODAR (Coastal Ocean Dynamics Application Radar) SeaSonde high-frequency (HF) radars, broadcasting near 13 MHz and using the Multiple Signal Classification (MUSIC) algorithm for direction finding, is described based on comparisons with an array of nine moorings in the Santa Barbara Channel and Santa Maria basin deployed between June 1997 and November 1999. Eight of the moorings carried vector-measuring current meters (VMCMs), the ninth had an upward-looking ADCP. Coverage areas of the HF radars and moorings included diverse flow and sea-state regimes. Measurement depths were ∼1 m for the HF radars, 5 m for the VMCMs, and 3.2 m for the ADCP bin nearest to the surface. Comparison of radial current components from 18 HF radar–mooring pairs yielded rms speed differences of 7–19 cm s−1 and correlation coefficients squared (r2) in the range of 0.39–0.77. Spectral analysis showed significant coherence for frequencies below 0.1 cph (periods longer than 10 h). At highe...

The correlation between surface drifters and coherent structures based on high-frequency radar data in Monterey Bay

Abstract: This paper investigates the transport structure of surface currents around the Monterey Bay, CA region. Currents measured by radar stations around Monterey Bay indicate the presence of strong, spatial and temporal, nonlinear patterns. To understand the geometry of the flow in the bay, Lagrangian coherent structures (LCS) are computed. These structures are mobile separatrices that divide the flow into regions of qualitatively different dynamics. They provide direct information about the flow structure but are geometrically simpler than particle trajectories themselves. The LCS patterns were used to reveal the mesoscale flow conditions observed during the 2003 Autonomous Ocean Sampling Network (AOSN-II) experiment. Drifter paths from the AOSN experiment were compared to the patterns induced by the LCS computed from high-frequency radar data. We verify that the fate of the drifters can be better characterized based on the LCS than direct interpretation of the current data. This property can be exploited to optimize drifter deployment.

Calibration and Validation of Direction-Finding High-Frequency Radar Ocean Surface Current Observations

Abstract: This paper focuses on the validation of remotely sensed ocean surface currents from SeaSonde-type high-frequency (HF) radar systems. Hourly observations during the period July 22, 2003 through September 9, 2003 are used from four separate radar sites deployed around the shores of Monterey Bay, CA. Calibration of direction-finding techniques is addressed through the comparisons of results obtained using measured and ideal (i.e., perfect) antenna patterns. Radial currents are compared with observations from a moored current meter and from 16 surface drifter trajectories. In addition, four overwater baselines are used for radar-to-radar comparisons. Use of measured antenna patterns improves system performance in almost all cases. Antenna-pattern measurements repeated one year later at three of the four radar locations exhibit only minor changes indicating that pattern distortions are stable. Calibrated results show root-mean-square (rms) radial velocity differences in the range of 9.8-13.0 cm/s, which suggest radar observation error levels in the range of 6.9-9.2 cm/s. In most cases, clear evidence of bearing errors can be seen, which range up to 30deg for uncalibrated radar-derived radial currents and up to 15deg for currents obtained using measured antenna patterns. Bearing errors are not, however, constant with angle. The results recommend use of measured antenna patterns in all SeaSonde-type applications. They also recommend an expanded simulation effort to better describe the effects of antenna-pattern distortions on bearing determination under a variety of ocean conditions