Correcting for distorted antenna patterns in CODAR ocean surface measurements

Abstract

CODAR systems employ compact antenna elements such as electrically small loops and monopoles to extract bearing information in ocean surface observations. Past analysis methods have assumed that these element patterns are perfect, i.e., cosine and omnidirectional. Operations from metallic offshore platforms usually distort these patterns because of unavoidable objects in their near field. When such distortions are ignored, previous methods are shown to produce <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim35\deg</tex> rms bearing errors. Therefore least-squares methods are presented and demonstrated that deal with differential element pattern distortions. It is shown how the required relative patterns are easily measured by a boat circling the antenna, and these patterns are then stored as look-up tables in the least-squares inversion methods. Relative patterns (i.e., one element pattern divided by the other), rather than absolute, are all that are required for extraction of surface current, wave-height directional spectra, wind direction, and drifting transponder information with CODAR.