The Underway Conductivity–Temperature–Depth Instrument

Abstract

Abstract The development of the Underway Conductivity–Temperature–Depth (UCTD) instrument is motivated by the desire for inexpensive profiles of temperature and salinity from underway vessels, including volunteer observing ships (VOSs) and research vessels. The UCTD operates under the same principle as an expendable probe. By spooling tether line both the probe and a winch aboard ship, the velocity of the line through the water is zero, the line drag is negligible, and the probe can get arbitrarily deep. Recovery is accomplished by reeling the line back in. Recovering the UCTD has some advantages: 1) the cost per profile decreases with increasing use, 2) sensors can be calibrated postdeployment, 3) the UCTD carries a pressure sensor so depth is measured directly, and 4) no hazardous materials are left behind. The design goal for the UCTD was to obtain profiles deeper than 100 m at 20 kt (typical of a VOS). This goal has been surpassed, as it is able to profile to over 150 m at 20 kt and to over 400 m at 10 kt. The first operational use of the UCTD occurred during a May–June 2004 cruise, the purpose of which was to examine the effect of internal waves and spice on long-range acoustic propagation. Over 160 UCTD casts were completed, resulting in a hydrographic section with resolutions of 10 km horizontally and 5 m vertically.