Abstract
High-frequency (HF) radars are routinely used for remotely sensing ocean surface currents. However, the performance of the most widely used direction-finding HF radar is degraded due to the effect of the inevitable deviations of actual antenna pattern on the direction of arrival (DOA) estimation. In this paper, we quantify the DOA estimation error resulting from the deviation of the actual antenna pattern from the ideal one. Theoretical analysis and field experiment results suggest that the ratio of the deviations for the two loops dominates the DOA estimation error. Thus, eliminating the effect of the antenna pattern deviations on DOA estimation error is transformed into eliminating the effect of this ratio. From this, a calibration method based on the time-averaged local spatial coverage rate (TLSCR) is proposed to reduce the effect of the antenna pattern deviations on current extraction, which uses the ideal antenna pattern to estimate the DOA of the echoes. To validate this proposed calibration method, we assess the radar-derived radial velocities by comparing with in situ observations. The comparison results indicate that the proposed TLSCR calibration method can effectively reduce the DOA estimation error and improve the performance of the direction-finding HF radar in current observation.
Highlights
Shore-based high-frequency (HF) radars, working at a frequency between 3 MHz and 30 MHz, have been widely used to remotely sense the coastal-ocean-surface current velocities [1]
To validate the theory-deduced result that the direction of arrival (DOA) estimation error is dominated by the relative amplitude deviations for the two loops, we perform the current extraction procedure again with an extra manipulation; that is, multiplying a constant factor on the sea echoes received by the two loops after achieving the time-averaged local spatial coverage rate (TLSCR) calibration method
If the claim that the DOA estimation error is dominated by the relative amplitude deviations for the two loops is true, the radial velocities retrieved by involving the extra manipulation will agree well with the radial velocities retrieved by the normal TLSCR calibration
Summary
Shore-based high-frequency (HF) radars, working at a frequency between 3 MHz and 30 MHz, have been widely used to remotely sense the coastal-ocean-surface current velocities [1]. There are two types of current-observing HF radar systems: direction-finding (e.g., the Coastal Ocean Dynamics Application Radar (CODAR; [8]) and Ocean State Measuring and Analyzing Radar, type S (OSMAR-S; [9])) and beam-forming (e.g., Wellen Radar (WERA; [10])). These two types of HF radar systems employ two distinct methods as well as two types of different antenna arrays to resolve the direction of arrival (DOA) of the sea echoes. The preferred and more widely used HF radar system is the direction-finding HF radar because it uses a small-aperture antenna to accomplish surface current mapping, which leads to easy installation and maintenance
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