Brian M. Emery

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...

Interpretation of Coastal HF Radar–Derived Surface Currents with High-Resolution Drifter Data

Abstract Dense arrays of surface drifters are used to quantify the flow field on time and space scales over which high-frequency (HF) radar observations are measured. Up to 13 drifters were repetitively deployed off the Santa Barbara and San Diego coasts on 7 days during 18 months. Each day a regularly spaced grid overlaid on a 1-km2 (San Diego) or 4-km2 (Santa Barbara) square, located where HF radar radial data are nearly orthogonal, was seeded with drifters. As drifters moved from the square, they were retrieved and replaced to maintain a spatially uniform distribution of observations within the sampling area during the day. This sampling scheme resulted in up to 56 velocity observations distributed over the time (1 h) and space (1 and 4 km2) scales implicit in typical surface current maps from HF radar. Root-mean-square (RMS) differences between HF radar radial velocities obtained using measured antenna patterns, and average drifter velocities, are mostly 3–5 cm s−1. Smaller RMS differences compared wi...

Measuring Antenna Patterns for Ocean Surface Current HF Radars with Ships of Opportunity

AbstractHF radars measure ocean surface currents near coastlines with a spatial and temporal resolution that remains unmatched by other approaches. Most HF radars employ direction-finding techniques, which obtain the most accurate ocean surface current data when using measured, rather than idealized, antenna patterns. Simplifying and automating the antenna pattern measurement (APM) process would improve the utility of HF radar data, since idealized patterns are widely used. A method is presented for obtaining antenna pattern measurements for direction-finding HF radars from ships of opportunity. Positions obtained from the Automatic Identification System (AIS) are used to identify signals backscattered from ships in ocean current radar data. These signals and ship position data are then combined to determine the HF radar APM. Data screening methods are developed and shown to produce APMs with low error when compared with APMs obtained with shipboard transponder-based approaches. The analysis indicates tha...

Measurement of Antenna Patterns for Oceanographic Radars Using Aerial Drones

AbstractA new method is described employing small drone aircraft for antenna pattern measurements (APMs) of high-frequency (HF) oceanographic radars used for observing ocean surface currents. Previous studies have shown that accurate surface current measurements using HF radar require APMs. The APMs provide directional calibration of the receive antennas for direction-finding radars. In the absence of APMs, so-called ideal antenna patterns are assumed and these can differ substantially from measured patterns. Typically, APMs are obtained using small research vessels carrying radio signal sources or transponders in circular arcs around individual radar sites. This procedure is expensive because it requires seagoing technicians, a vessel, and other equipment necessary to support small-boat operations. Furthermore, adverse sea conditions and obstacles in the water can limit the ability of small vessels to conduct APMs. In contrast, it is shown that drone aircraft can successfully conduct APMs at much lower c...

Automatic calibrations for improved quality assurance of coastal HF radar currents

CODAR Ocean Sensors, Ltd. and the University of California, Santa Barbara are developing a method by which HF radar antenna response patterns can be calibrated automatically over time. Currently, over 130 HF radar units are providing coastal surface current maps to the public via the U.S. Integrated Ocean Observing System (USIOOS): http://www.ioos.gov/hfradar/. These real-time data are used for Coast Guard Search and Rescue, Hazardous Materials Spills Response, Water Quality Monitoring, Monitoring Harmful Algal Blooms, Fisheries Management, Modeling, Marine Navigation, Ocean Energy Production. Techniques for improved, automated quality assurance of the data provided by coastal radar stations, such as the one discussed here, will improve the efficacy of efforts in these areas. Passing vessels provide a steady supply of targets for which the echoes in the HF Doppler spectra can be used as source signals. The Automatic Identification System (AIS) transmissions from these vessels provide the position and, therefore, bearing to the vessel. By associating the known AIS positions with HF Doppler echoes, a low-cost calibration procedure can be implemented which can reduce or eliminate more labor-intensive alternatives. This method is demonstrated using data from mid-range systems in the Santa Barbara channel that are operating in the 13 MHz band. A prototype package has been deployed on a system monitoring the Gulf of Farallones and the shipping lanes approaching the San Francisco Bay. Performance and data quality metrics for this prototype will be discussed.

Antenna calibration for oceanographic radars using aerial drones

We describe using small drone aircraft for antenna pattern measurements (APMs) of high-frequency (HF) oceanographic radars for observing ocean surface currents. Prior studies show that accurate surface current measurements using HF radar require APMs. Typically APMs are obtained using small research vessels carrying radio signal sources or transponders in circular arcs around individual radar sites. This procedure is expensive because it requires sea-going technicians, a vessel, and other equipment necessary to support small boat operations. Furthermore adverse sea conditions and obstacles in the water can limit the ability of small vessels to conduct APMs. We have found that drone aircraft can conduct APMs at lower cost and in a broader range of sea states with comparable accuracy. This simplified process for obtaining APMs can lead to more frequent calibrations and improved surface current measurements.

Improved direction of arrival methods for oceanographic HF radars

Advances in oceanographic HF radar processing methods improving the accuracy and resolution of the ocean current measurements, would reveal new understanding of coastal ocean dynamics. Processing techniques presently employ MUSIC for direction finding, with a low number of data snapshots, often with low SNR. We report on the investigation of alternative signal processing methods, including the application of Maximum Likelihood Estimation (MLE) for direction finding. Simulated radar backscatter, based on the realistic flows of a high-resolution regional ocean model, is used to evaluate new processing techniques. Preliminary results suggest some improvement with the use of MLE. By addressing limitations in HF radar observations, this study addresses the important problem of observing coastal circulation at the scales that are relevant to near shore dynamics, and thus to the many practical applications employing these observations.