An instrument used to monitor oceanic fronts since Rossby (1969) first introduced the concept of using variations in acoustic travel time to measure changes in the depth of the main thermocline. They were initially deployed in the MODE project and have been used extensively in many other regions. The use of an IES requires calibration of travel time measurements into other scientific quantities of interest, with calibration requiring knowledge of the variations in temperature and salinity stratification and the resulting density and sound speed profiles in the region where the IES is to be deployed.