A CTD or Sonde is an oceanography instrument used to determine the conductivity, temperature, and depth of the ocean.
The CTD may be incorporated into an array of Niskin bottles referred to as a carousel or rosette. The sampling bottles close at predefined depths, triggered either manually or by a computer, and the water samples may subsequently be analyzed further for biological and chemical parameters.
The CTD may also be used for the calibration of sensors.
A sound velocity probe is a device that is used for measuring the speed of sound, specifically in the water column, for oceanographic and hydrographic research purposes.