Search

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.

ARGOS drifters deployed in the Northern Gulf of California in September 1995 showed the presence of a cyclonic gyre, while a second deployment in March 1996 revealed an anticyclonic gyre. A circulation pattern consisting of a seasonally reversing gyre had been proposed before on the basis of satellite images, geostrophic calculations, and numerical models, but so far no direct observations have been made to test its existence. In September the gyre was cyclonic, baroclinic, very well defined, stable, and strong; its mean speed and rotation time were 0.3 m s−1 and ∼7 days. In March the gyre had the same mean speed, but it was anticyclonic and displaced to the northwest of the summer position. The March gyre has barotopic and baroclinic characteristics, but the observed speeds are stronger than in numerical simulations. These data and a data bank analysis suggest that the summer gyre is a persistent summer feature, but the winter–spring situation remains ill-defined and requires further research.