Reduced averaging times in the long range seasonde

Abstract

High frequency (HF) radar systems are useful for measuring the velocity of sea surface currents. Besides the ecological and economic importance of being able to accurately measure currents, the US Coast Guard uses HF radar to make decisions involved in search and rescue operations. Currently, 5 MHz or long-range HF radar systems output radial vector files that are calculated using a 180-minute averaging interval. Reducing the averaging interval to 60 minutes could improve accuracy in areas where the current changes significantly over this 3 hour averaging interval. To test this, the track of a real drifter was compared to the track of two virtual drifters over a 48-hour period. One virtual drifter was tracked using HF radar total surface currents derived from radial measurements averaged over the default 180-minute interval and the other using HF radar data averaged over the 60-minute interval. Distance between real drifter and virtual was determined for both cases. For the first 24-hour period, both HR radar intervals had similar differences between the real drifter and the virtual. The largest difference was approximately 5 km, and the smallest was less than 2 km. The next 24 hours were less similar with the HF radar for the 60-minute interval being more accurate. For example, the 180 minute interval, the distances between the virtual and real drifter reached a maximum of approximately 12 km while the maximum difference for the HF radar averaged over 60 minute intervals was only 7km. The improvement of accuracy when switching the HF radar over to 60 minute intervals has implications in the broad sense for studies looking at biological and ecological species as well as for transportation at sea but where this difference maybe most important is in the ability to narrow a search area during US Coast Guard Search and Rescue missions.