Assessment of CTD and optical sensor calibration differences between standalone sensors and those mounted within an undersea glider

摘要

The NATO Undersea Research Centre upgraded its oceanographic calibration facility in Italy in 2010 and built a new optic calibration capability for providing sensor calibration within the glider of both CTD and optical sensors. The new facility was described during the latest OCEANS 2011 conference in Santander (Spain). The aim is for NURC to improve accuracy and efficiency in calibrating glider sensors by eliminating the need to dismount the sensors from the gliders, ship the sensor to the manufacturer, reassemble the hull and pressure test the system. NURC considers the calibration of sensors within the glider extremely important and is committed, as with its standard oceanographic instrumentation, to calibrate before and after all deployment campaigns, thus ensuring scientists get accountable measurements essential for their research. To meet these stringent requirements an in-house facility capable of producing high quality calibrations on sensors mounted within the glider is essential. For that purpose, NURC needed to understand how the glider hull may affect the calibration measurements. Very few results were given on the topic in the Santander paper where initial findings showed some discrepancies between calibrations of sensors mounted within the whole glider and the science bay only. These were mainly seen on the conductivity, where differences of 0.035 mS/cm were observed. However, the results shown were made from data acquired on a single glider and hence did not allow quantifiable conclusions to be derived. To understand the uncertainties when comparing established procedures to those being newly developed a more extensive data set was required. The purpose of this paper is to present the calibration results that have been performed on different sensors, being both CTD and optical, in order to assess how the glider itself is affecting them. The aim was to evaluate if and by how much the data obtained from the in situ glider measurements could differ from- - the manufacturer calibration. Procedures were repeated for the different vehicles of the NURC's glider fleet and data analyzed in order to understand the repeatability of calibrations and the consistency of the sensors and platforms. Results compared will be presented from multiple gliders where data were collected from a) the sensors mounted within a whole glider, b) the sensor mounted within the glider science bay and c) as a standalone sensor. The preliminary results, that will however have to be verified on more glider calibrations, show the necessity of performing the calibration of the sensor within the glider for the conductivity measurements. This is not as critical for the temperature measurements and the optic measurements where it is shown that the calibration do not change significantly if the calibration is made with the sensor within the science bay or the sensor within the whole glider.