Abstract. The estimation of chlorophyll concentration in marine waters is fundamental for a number of scientific and practical purposes. Standard ocean color algorithms applicable to moderate resolution imaging spectroradiometer (MODIS) imagery, such as OC3M and MedOC3, are known to overestimate chlorophyll concentration ([CHL]) in Mediterranean oligotrophic waters. The performances of these algorithms are currently evaluated together with two relatively new algorithms, OC5 and SAM_LT, which make use of more of the spectral information of MODIS data. This evaluation exercise has been carried out using in situ data collected in the North Tyrrhenian and Ligurian Seas during three recent oceanographic campaigns. The four algorithms perform differently in Case 1 and Case 2 waters defined following global and local classification criteria. In particular, the mentioned [CHL] overestimation of OC3M and MedOC3 is not evident for typical Case 1 waters; this overestimation is instead significant in intermediate and Case 2 waters. OC5 and SAM_LT are less sensitive to this problem, and are generally more accurate in Case 2 waters. These results are finally interpreted and discussed in light of a possible operational utilization of the [CHL] estimation methods.

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 ...

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