Improving the Understanding of HF Radar Antenna Patterns and Their Influence on Nearshore Surface Current Measurements

The use of oceanographic HF radars to investigate nearshore processes and circulation requires accurate calibrations to minimize the largest source of error, uncertainty, and gaps in the measurement. These calibrations, known as antenna pattern measurements (APMs), are used to locate radial current vectors (radials) on the ocean surface in bearing. While the fundamentals of obtaining the APM are well understood, the effects of different APM processing decisions, including the ultimate bearing resolution, are not well constrained. Furthermore, in long term deployments of HF radars, APMs have been observed to change in time, and the implications of these changes on the accuracy of the measurements are not well quantified. We use a simulation based approach along with APMs obtained from the field, to investigate the effects of APM distortions on radial coverage patterns. We show how these changes lead to gaps in coverage, and then develop an explanation for the coverage gaps based solely on differences in the geometrical properties of the APMs. Further investigations apply angular smoothing to the APMs, and address how that smoothing influences the direction of arrival chosen by Multiple Signal Classification (MUSIC). These suggest that the average error in the bearing determination is a function of smoothing level. This work aims to develop a set of recommendations for HF radar operators, with the goal of producing more accurate coverage from HF radars in the U.S. Integrated Ocean Observing System (IOOS).