Design
The SLDMB is based on the Davis-style drifter design, which attempts to minimize the effects of wind and surface waves. avis, R. E. (1985). Drifter Observations of Coastal Surface Currents During CODE: The Method and Descriptive View. Journal of Geophysical Research, 90(C3), 14. This is accomplished by reducing the area above the ocean surface to small floats and an antenna. Below the surface is a series of drogue vanes to 70 cm. (less than the 1 m Davis-style buoy) in depth, which catch the ocean current, along with electronic equipment that deploys the vanes and antenna, receives the GPS signal and transmits the location to the USCG. SLDMB construction varies by manufacturer, but those used by the USCG consists of four orthogonal drag vanes 0.5m wide and 0.7m high of nylon fabric. These are supported by PVC arms at top and bottom, which extend from a cylindrical hull that contains the electronic equipment. Small floats are attached to the end of each upper arm in order to maintain buoyancy, and a small antenna projects above the SLDMB.[Bang, I., Mooers, C. N. K., Haus, B., Turner, C., Lewandowski, M. (2007). Technical Report: Surface Drifter Advection and Dispersion in the Florida Current Between Key West and Jacksonville, Florida. Technical Report.]. Deployment of the SLDMB may be accomplished by aircraft (both fixed-wing and rotary) or by ship. SLDMB deployed by aircraft are encased in a tube and attached to a parachute which decreases the impact produced upon hitting the water but without so much drag that the buoy can drift off-course according to USCG SAR guidelines. Upon reaching the ocean surface, the outer casing and parachute break away from the SLDMB, and the spring-loaded antenna deploys. Electronics consist of a GPS receiver, electronic transmitter and sufficient batteries to provide continuous data collection for a period of two weeks to one month.Specifications
ThSearch-and-rescue application
Because it has a small above-water surface and high underwater surface area, the effect of surface winds and waves has a negligible effect, instead moving with the flow of the upper 1m of the water column. The USCG has found that this instrument behaves as a 'zero-leeway' object, moving with the top meter of the water column, with no additional motion due to the direct effect of the wind on the SLDMB's exposed areas. The USCG maintains several hundred SLDMBs for deployment and responds to more than 5,000 SAR cases each year. In the year 2006, more than 400 SLDMBs were deployed in SAR applications, with an average lifetime of 22 days. The USCG may release SLDMBs at their discretion to aid in search efforts. In remote areas, SLDMBs are deployed via C-130 aircraft or helicopters. The GPS unit on each SLDMB calculates its position every 30 minutes, and transmits the data via the ARGOS data collection system to USCG Operational Support Center (OSC). During high traffic periods, the USCG may pre-deploy units in order to have existing data in areas where SAR operations are more likely, reducing the time required to collect ocean current data during the SAR process. SLDMB may be released as single units or as a group, depending on the situation required. In cases where the last known position is known and the time lag to SLDMB deployment are minimal, only a single unit may be necessary. However, if a sufficient time lag exists, or the last known position is not available, multiple SLDMBs should be used. An example of this second case would be a downed fishing vessel, in which only the approximate area of the vessel is known.[U.S. COAST GUARD ADDENDUM TO THE UNITED STATES NATIONAL SEARCH AND RESCUE SUPPLEMENT (NSS) TO THE INTERNATIONAL AERONAUTICAL AND MARITIME SEARCH AND RESCUE MANUAL (IAMSAR) (2004). Retrieved from http://www.uscg.mil/directives/listing_cim.asp?id=3000-3999.]See also
* GPS buoyReferences
{{Reflist Buoyage