High-Frequency Radars: Beamforming Calibrations Using Ships as Reflectors*

Linear array antennas and beamforming techniques offer some advantages compared to direction finding using squared arrays. The azimuthal resolution depends on the number of antenna elements and their spacing. Assuming an ideal beam pattern and no amplitude taper across the aperture, 16 antennas in a linear array spaced at half the electromagnetic wavelength theoretically provide a beam resolution of 3.5° normal to the array, and up to twice that when the beam is steered within an azimuthal range of 60° from the direction normal to the array. However, miscalibrated phases among antenna elements, cables, and receivers (e.g., caused by service activities without recalibration) can cause errors in the beam-steering direction and distortions of the beam pattern, resulting in unreliable ocean surface current and wave estimations. The present work uses opportunistic ship echoes randomly received by oceanographic high-frequency radars to correct an unusual case of severe phase differences between receiver channels, leading to a dramatic improvement of the surface current patterns. The method proposed allows for simplified calibrations of phases to account for hardware-related changes without the need to conduct the regular calibration procedure and can be applied during postprocessing of datasets acquired with insufficient calibration.