Calibration of HF radar systems with ships of opportunity

This paper addresses the application of ships of opportunity to determine the proper phase corrections for HF ground-wave phased array radar systems. Such HF radar systems are being used more and more extensively for measurements of coastal ocean surface currents. In order to correctly determine the spatial bearing of the currents that such systems measure, antennas must be appropriately calibrated for amplitude and phase variations within the array. Phase and amplitude corrections are often accomplished through ship-based transponder runs. Such calibration runs, however, require the use of a ship and thus are expensive and may not be done as frequently as is needed, such as if there are changes in the elements of the array or the local terrain. A technique is described below that can potentially make use of ships of opportunity with unknown bearings to determine the required phase corrections for the array elements and to determine the bearing of the unknown ship. Examples run thus far from transponder data collected with an 8-element loop receiving antenna array indicate that with this technique, for cases where the SNR exceeds 20 dB, the antenna phase corrections are generally within 0.2-0.3 radians of the values obtained from a direct transponder run where the ship´s bearing is known. When the ship´s bearing is assumed unknown and is uniquely determined from different radar frequencies, for cases where the SNR exceeds 20 dB, the results for the ship bearing are consistent to within 2-3°.