Expendable Bathythermographs (XBTs) are single-use instruments that are used to measure temperature profiles of the ocean. XBTs are launched from hand-held units, connected to the XBT itself by a thin copper wire that allows temperature data to be directly transmitted as the XBT sinks (up to 800 m). After the length of the copper wire has been expelled, the XBT is released and sinks to the bottom of the ocean.
Able to be deployed from any ship, XBTs provide a quick and inexpensive means of collecting temperature data and a vital tool of the Ships of Opportunity Facility. Using ships that undertake the same path on a regular basis allows the collection of high-density transects, capturing high-resolution and temporal information that allows for smaller-scale oceanic features, including fronts and eddies to be resolved.