Physical oceanographic data from Seaglider trials in stratified coastal waters using a new pumped payload CTD

The Seaglider, developed by the University of Washington with ONR (Office of Naval Research) funding and licensed to iRobot in 2008, is an autonomous underwater vehicle used for a wide variety of occeanographic research. Science payloads installed on gliders typically include temperature (T) and conductivity (C) sensors, or a CTD (Conductivity-Temperature-with-Depth profiler), in which the T and C measurements are used to derive salinity, density and other important physical parameters. Free-flushed CTDs by Sea-Bird Electronics, referred to here as the CT Sail, were the first science payload installed in the Seaglider. While these are still in use on many Seagliders, they are being phased out in favor of a modular, low-power pumped CTD, referred to as the GPCTD (Glider Payload CTD), also by Sea-Bird Electronics. Data gathered during field trials of the Seaglider integrated with the new GPCTD alongside Seagliders with the free-flushed CT Sails offer an opportunity to evaluate and compare the data quality between the two CTD types. Data provided by iRobot come from June 2011 mission trials conducted in the stratified waters along the coast off Massachusetts. Comparisons of dive profiles made simultaneously by pairs of Seagliders indicate the raw pumped GPCTD data show improved data quality with less salinity spiking and conductivity cell thermal mass errors compared to the free-flushed CT Sail data. Applying consistent corrections to the GPCTD data for sensor measurement alignment and time-dependent conductivity cell thermal mass errors further improves accuracy. Corrections to GPCTD data are simple in comparison to the unpumped CT Sail data, because the GPCTD pumped flow produces a steady T-C sensor response, and the GPCTD data acquisition system provides a constant sample-rate time series necessary for these time-dependent corrections.