NURC within glider sensors calibration, validation and monitoring facilities

Abstract

Because of the NATO Undersea Research Centre's long history of oceanographic instrument calibration, the growing application of underwater gliders in the oceanographic community and the difficulties in sensor characterization, the Centre is developing a complete set of facilities and procedures that allow within glider sensors calibration, validation and monitoring. Gliders typically carry a variety of sensors, including conductivity, temperature and depth (CTD), dissolved oxygen, and optics packages, such as fluorescence, backscattering, irradiance, and radiance To date, single sensors mounted on gliders are calibrated by the manufacturer, requires dismounting the sensor from the hull, shipping the sensor, reassembling the hull, and pressure testing the system. These steps are time consuming, and there is no precise knowledge about how a sensor calibrated outside of the glider will perform inside the vehicle. To address these issues, the NATO Undersea Research Centre (NURC) recently built dedicated unique oceanographic and optic calibration facilities and is currently developing high-accuracy in situ techniques that allow complete calibration and characterization of most of the sensors that are typically installed on a glider. Some of these procedures are conducted in proper calibration facility; some are just basic monitoring of the instruments drift, while some others are in situ validation of the sensors through the use of higher resolution and accuracy instruments that perform measurements. Through the use of dedicated new oceanographic and optic calibration facilities, basic monitoring of sensor drift, and in situ validation of sensors using high resolution and accurate instruments, NURC is developing procedures for a complete and unique high-quality characterization of the water mass properties measured during each mission. The oceanographic calibration facility is developed from the existing internationally recognized WOCE standard NURC CTD calibration facility. The facility is thermally and humidity controlled and is equipped with the highest standards for measuring of sea water temperature, conductivity-salinity and pressure parameters in CTD systems. The laboratory includes three NURC designed salt water calibration baths, two of 385 liters for CTD calibrations and a newly constructed 1000 liters bath to permit Glider CTD's to be calibrated whilst integrated and connected to the glider. All baths have high thermal stability, regulated by modular computer controlled heat exchanging units and are shown to meet the WOCE standard for temperature and salinity. The Optic Instruments Calibration OIC Laboratory is a new facility which allows calibration of different kinds of radiometers. This NIST standard facility, with high resolution components and controlled environment, enhances to produce low uncertainties in absolute radiometric calibration. A combination of a clean room/darkroom is utilized to isolate and protect optical instruments from airborne particulate and ambient light. The lamp system is the core of the calibration facility. Based on the international 1000W FEL irradiance standard lamp, in conjunction with the reflectance system, this provides the high accuracy radiance source. The high repeatability is maintained using a monitored precision power supply. The alignment of the instruments is then verified with a 2.5mW HeNe (Class IIIa, EC 3R) laser. This paper presents a detailed description of the two calibration facilities as well as the procedures developed and the preliminary results achieved for calibrating both oceanographic sensors and optic sensors within gliders.