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News from the AMT cruise

Sentinel 2 ocean colour, September 2016, English Channel
On 20 September 2016 scientists from Plymouth Marine Laboratory, the University of Southampton and IFREMER collectively embarked on a 7000 mile transect from the UK to the Southern Ocean on board the RRS James Clark Ross to ensure the accuracy of the recently launched Copernicus Sentinel satellites.

During this 26th Atlantic Meridional Transect (AMT) cruise a huge variety of measurements were taken, particularly important were Ocean Colour (OC) radiometry and Sea Surface Temperature (SST) which are fundamental oceanographic parameters necessary to monitor and manage the marine ecosystem for aquaculture, fisheries, water quality, mapping and monitoring harmful algal blooms, and climate change. Using satellites to observe these parameters from space has become increasingly important as satellites can observe vast areas of the ocean that are difficult to access using traditional sampling methods and provide cost-effective global data coverage of oceanic conditions.

The array of sensors which were deployed for continuous ship measurements included Infrared Sea surface temperature Autonomous Radiometer (ISAR) which measures SSTskin, Above-water Satlantic HyperSAS radiometers which measures remote sensing reflectance at the sea surface, optical sensors for the measurement of backscattering, beam-attenuation and absorption coefficients to characterize the optical signal from the dominant substances (marine algae, detrital or inorganic sediment and dissolved organic material) in the sea surface and C-band radar which measures the surface roughness of the ocean. These sensors made measurements every minute of the day for 46 days until 4 November when the ship docked in the Falkland Islands.

In addition, the ship stopped 3 times per day to deploy instruments from cranes and 38 depth-resolved optical property measurements were made along the transect using a TRIOS Ramses radiometer, 230 discrete samples for the determination of the active photosynthetic pigment chlorophyll-a by High Performance Liquid Chromotography were collected, 157 meteorological observations from weather balloons using Vaisala RS92 radiosondes were made and 142 bucket samples were collected for further measurements of temperature.

Together these measurements will provide the range and diversity of parameters required to independently validate the suite of Sentinel sensors. The measurements are of sufficient quality and accuracy to be classified as Fiducial Reference Measurements (FRMs) as they follow satellite validation protocols and procedures, have documented SI traceability and maintain an associated uncertainty budget over the duration of the satellite mission. Following the cruise, all of the sensors used will be re-calibrated to SI standards which will be used to calculate the uncertainty in each ship-board sensor deployed. With this uncertainty budget, the scientists will be able to accurately determine the error in each of the Sentinel satellites so that we can more accurately define the uncertainty in the satellite data.

The new Sentinel satellites, developed by the European Space Agency, form the heart of the European Commission’s Copernicus programme – the largest global environmental monitoring initiative ever conceived. The Copernicus programme is unique because of the continuity of coverage over seas and oceans of global maps of phytoplankton chlorophyll, the green pigment found in plants, temperature and other sea surface properties that contribute to the air-sea flux of carbon dioxide and other climate relevant gases. The Sentinels carry a vast range of state-of-the-art instruments to deliver a stream of complementary imagery and data for monitoring our land, ice, oceans and atmosphere at unprecedented and synoptic spatial and temporal resolutions. The instruments utilised as part of the ESA AMT4SentinelFRM project include the Sentinel-2A colour scanner, the Sentinel-3A Operational Colour (OLCI) and Sea Surface Temperature (SLSTR) Instruments and surface ocean dynamic properties from Sentinel 1.

Ocean-colour sensors retrieve spectral radiance at the top of the atmosphere (TOA) from which the water-leaving radiance (Lw) is estimated after correction from atmospheric effects. Lw is then used to estimate geo-physical and biogeochemical products through the use of bio-optical algorithms. Similarly, SST sensors use infrared emission at the TOA that is then corrected for atmospheric attenuation and emission to estimate SST at the surface ocean (SSTskin).

AMT is a multidisciplinary research programme which undertakes biological, chemical and physical oceanographic research during an annual voyage of a NERC-UK research vessel to the Southern Ocean. AMT is heralded as one of NASA SeaWiFS 10 greatest highlights since it provides an ocean observing platform to measure vital calibration and validation data to support ocean colour satellite missions. The transect covers a vast range of environments from the productive coastal and eutrophic regions to the desert-like gyres in the centre of the ocean and the Southern Ocean, which are rarely accessed by research ships. Over the past 8 years the AMT programme has published 136 papers of which 66 were published by PML and 29 were on the topic of remote sensing and optics, with an average impact factor of 3.6.

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