On the accuracy of HF radar surface current measurements: Intercomparisons with ship‐based sensors

High-frequency (HF) radar systems can provide periodic, two-dimensional, vector current estimates over an area approaching 1000 km . As the use of these HF systems has gained wider acceptance, a number of attempts have been made to estimate the accuracy of such systems. However, comparisons of HF radar current estimates with in situ sensors are difficult to interpret since HF systems measure currents averaged over an area of-1 km 2 and to a depth of only -50 cm while in situ sensors measure currents at a point and somewhat greater depths (-1 to 10 m). Previous studies of the accuracy of HF radar technology have thus attributed the differences observed between HF radar and in situ sensors to an unknown combination of vertical shear, horizontal inhomogeneity, in situ instrument errors, and HF radar system errors. This study examines the accuracy of HF radar current measurements using data from the 1993 High Resolution Remote Sensing Experiment, conducted off Cape Hatteras, North Carolina. Data from four shipborne in situ current meters are compared with data from an Ocean Surface Current Radar (OSCR), a commercial current-measuring radar. We attempt to discern the predominant sources of error in these data by using multiple simultaneous measurements from different sensors and by examining the variation of observed current differences as a function of location. The results suggest an upper bound on the accuracy of the OSCR-derived radial currents of 7 to 8 cm/s. the measurement of near-surface currents in the coastal environ- ment is an important problem.