Abstract
An efficient system to produce in situ high quality radiometric measurements is compulsory to rigorously perform the vicarious calibration of satellite sensors dedicated to Ocean Color Radiometry (OCR) and to validate their derived products. This requirement is especially needed during the early stages of an OCR satellite activity or for remote areas poorly covered by oceanographic cruises with possible bio-optical anomalies. Taking advantage of Argo’s profiling float, we present a new autonomous profiling float dedicated to in situ radiometric measurements. The float is based on the Provor CTS5 (manufacturer NKE) with an added novel two protruding arm design allowing for sensor redundancies, shading mitigation and near-surface data. Equipped with two identical radiometers on each arm that measure downward irradiance and upwelling radiance at seven wavelengths, the ProVal float generates both redundant radiometric profiles as well as an estimate of Remote Sensing Reflectance. Results from 449 profiles sampled obtained in the NW Mediterranean Sea and in the Indian sector of the Southern Ocean are presented to illustrate the ProVal float technical maturity. Analysis of the behavior of the profiling float, including tilting and ascent speeds, is presented. The vertical stability of the ProVal exhibits 85% of surface data of the Mediterranean Sea with a tilt smaller than 10 degrees. This percentage is 45% in the Southern Ocean due to rougher seas. Redundant sensors provide a characterization of the relative drift between sensors over the deployment which is found to be less than 0.15% per month over a year. Post-cruise calibration of a recovered float revealed no significant drift. As an example of the utility of ProVal floats, a match-up of Remote Sensing Reflectance measured with the European Space Agency Ocean and Land Color Imager (OLCI onboard Sentinel-3A & 3B) is shown. It follows that profiling floats, such as ProVal, could provide a significant contribution to an upcoming global System Vicarious Calibration of space-based radiometers.
Highlights
Since the first satellite sensor dedicated to Ocean Color Radiometry (OCR) was launched in 1978, the “Coastal Zone Color Scanner” (CZCS, NASA) (Hovis et al, 1980), the number and types of satellites observing the color of the world’s ocean has greatly increased
The ProVal float was designed to respond to the need of a “validation float” as defined by the (IOCCG, 2011)
The 14 months of a ProVal float mission deployed in the Southern Ocean have demonstrated this capability
Summary
Since the first satellite sensor dedicated to Ocean Color Radiometry (OCR) was launched in 1978, the “Coastal Zone Color Scanner” (CZCS, NASA) (Hovis et al, 1980), the number and types of satellites observing the color of the world’s ocean has greatly increased. SVC consists of adjusting the sensor’s spectral gains (g-factors) by comparing reflectances derived from the satellite observations to reflectances derived from highly accurate in situ radiometric measurements over the mission lifetime (Gordon, 1987, 1998). The scarcity of in situ data still remains an issue for the realization of the full potential and capabilities of OCR satellite missions. To perform the first SVC, an operational mission such as Ocean Land Color Instrument (OLCI) on board the Sentinel-3A satellite has relied on climatological averages derived from other satellite missions, in addition to in situ measurements (Sentinel3A Product Notice–OLCI Level-2 Ocean Colour, 2018). Proper calibration, validation and interpretation of satellite OCR data on a global basis could benefit greatly from an extensive observational record of real time in situ radiometric measurements early in the mission
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