airborne expendable bathythermograph

(Abbreviated AXBT.) An expendable instrument that is dropped from an aircraft and used to measure the profile of temperature in the water column.

The probe consists of a thermistor in a weighted, streamlined case. It falls freely at a fixed, known rate so that the elapsed time can be converted to depth. It is connected by a thin, freely unwinding wire to a small buoy with a radio transmitter through which the data are transmitted to the aircraft, which continues its flight.