Radiometric calibration of all optical sensors is a key component to achieve high accuracy data, which is traceable to international standards and repeatable through time. This study introduces a Hyperspectral Imaging (HSI) system that RAL Space has developed and presents a thorough analysis of the radiometric reflectance calibration chain required for both static and aerial applications, where the HSI system is mounted on an Unoccupied Aerial System (UAS). RAL Space HSI system comprises two commercially available Ximea snapshot mosaic imagers, one sensitive in the visible (VIS) and the other in the near infrared (NIR) spectral ranges, that are coupled with a dedicated PC and customized LabVIEW code for the capture, process and storage of the raw hyperspectral information (hypercubes). Both imagers are optically characterized for their electrical gain and offset, the vignetting and stray light effects, while the spectral response of each band has also been determined. The corrected digital numbers (DN) are then translated into reflectance values with the use of reflectance standards. For the aerial surveys, an additional step has been developed into the calibration chain that corrects for any changes in ambient illumination during flight with the use of ground based upward-looking measured spectra. Future work will include any correction needed due to temperature dependencies of the calibration chain steps.
The sea and land surface temperature radiometer (SLSTR) to be flown on the European Space Agency’s (ESA) Sentinel-3 mission is a multichannel scanning radiometer that will continue the 21 year dataset of the along-track scanning radiometer (ATSR) series. As its name implies, measurements from SLSTR will be used to retrieve global sea surface temperatures to an uncertainty of <0.3 K traced to international standards. To achieve, these low uncertainties require an end-to-end instrument calibration strategy that includes prelaunch calibration at subsystem and instrument level, on-board calibration systems, and sustained postlaunch activities. The authors describe the preparations for the prelaunch calibration activities, including the spectral response, the instrument level alignment tests, and the solar and infrared radiometric calibrations. A purpose built calibration rig has been designed and built at the Rutherford Appleton Laboratory space department (RAL Space) that will accommodate the SLSTR instrument, the infrared calibration sources, and the alignment equipment. The calibration rig has been commissioned and results of these tests will be presented. Finally, the authors will present the planning for the on-orbit monitoring and calibration activities to ensure that the calibration is maintained. These activities include vicarious calibration techniques that have been developed through previous missions and the deployment of ship-borne radiometers.
The Sea and Land Surface Temperature Radiometer (SLSTR) to be flown on ESA's Sentinel-3 mission is a multichannel
scanning radiometer that will continue the 21-year datasets of the Along Track Scanning Radiometer (ATSR)
series. As its name implies, measurements from SLSTR will be used to retrieve global sea surface temperatures to an
uncertainty of <0.3K traced to international standards. To achieve these low uncertainties requires an end to end
instrument calibration strategy that includes pre-launch calibration at subsystem and instrument level, on-board
calibration systems and sustained post launch activities.
The authors describe the preparations for the pre-launch calibration activities including the spectral response, instrument
level alignment tests, solar and infrared radiometric calibration. A purpose built calibration rig has been designed and
built at RAL space that will accommodate the SLSTR instrument, infrared calibration sources and alignment equipment.
The calibration rig has been commissioned and results of these tests will be presented.
Finally the authors will present the planning for the on-orbit monitoring and calibration activities to ensure that
calibration is maintained. These activities include vicarious calibration techniques that have been developed through
previous missions, and the deployment of ship-borne radiometers.
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