Analytical techniques for the calculation of leeway as a basis for search and rescue planning

Leeway, defined as the movement of the search object through water caused by the action of wind on the exposed surfaces of the object, is fundamental to search planning. Over the past several years, the U.S. Coast Guard (USCG) Research and Development Center (R&DC) and the Canadian Coast Guard (CCG) have participated in leeway studies of various drift targets such as life rafts, evacuation vessels, sailboats, and other targets of interest. The leeway coefficients computed for each drift target generated from these leeway studies will be used in the new USCG Search and Rescue (SAR) planning software, the Search and Rescue Optimal Planning System (SAROPS), to define potential search areas during SAR operations. In the fall of 2005, the R&DC conducted leeway testing of two specific drift objects on behalf of the U.S. Naval Submarine Medical Research Laboratory: the Mark-10 Submarine Escape and Immersion Equipment (SEIE) life raft, and the Submarine Emergency Position Indicating Radio Beacon (SEPIRB). These studies were performed off the coast of St. John's, Newfoundland, Canada where open ocean conditions can be obtained within several miles from shore. Multiple drift runs were completed for each type of object to evaluate their behavior in response to various wind and sea conditions, producing object drift data under a wide variety of conditions. During the course of the study, each target was tracked by an on-board GPS receiver and data logger to yield high-resolution speed and direction over ground data. Wind velocity and sea conditions were measured by meteorological and wave rider buoys deployed within the study area to characterize environmental forcing conditions. The leeway of one SEIE raft was measured directly by a 1200 kHz acoustic Doppler current profiler (ADCP) gimbal-mounted and in a down-looking orientation, as well as multiple Self Locating Datum Marker Buoys (SLDMBs) that reported their respective positions via satellite at 30-minute intervals. Leeway of the remaining objects was determined indirectly by subtracting the surface current vector of adjacent SLDMBs from the drift object vector motion recorded by its onboard GPS receiver. Because all search objects were in the vicinity of the SLDMB field, a comparison between the direct and indirect leeway motion could be made for the ADCP-equipped SEIE raft. This value was then correlated to recorded wind speed and direction, and subjected to error analysis and statistical validation. This paper focuses on the methodology employed during the field study and provides a detailed description of the post-processing routines used to derive leeway coefficients for the SEIE for U.S. Navy search planning, and for use by the USCG in its SAROPS planning software. Estimates of the surface current for each drift target to support indirect leeway calculations relied on a statistical interpolation technique and consisted of steps described in the paper below. The resulting time series constituted a data base for the calculation of downwind and crosswind leeway coefficients, derived from a least-squares linear regression between the corresponding velocity components of wind and target drift. The success of the indirect approach is evaluated by comparing the estimates with directly measured velocities. It was concluded that this statistical interpolation technique performed particularly well when the drifting target stayed within an imaginary polygon delineated by the available SLDMBs. Three of the four rafts considered in the study drifted with 12-15deg leeway angles to the right of the wind direction. Downwind leeway coefficient was 0.02 for the drogued rafts and 0.03 for the undrogued rafts. Scatter of estimated leeway velocity with respect to wind speed suggested a tighter relationship for higher wind speeds (>7 m/sec).