VIIRS reflective solar bands calibration changes and potential impacts on ocean color applications

Abstract

The VIIRS (Visible-Infrared Imaging Radiometer Suite) instrument onboard the Suomi NPP (National Polar-orbiting Partnership) spacecraft started acquiring Earth observations in November 2011. Since then, radiometric calibration applied to the VIIRS RSB (Reflective Solar Band) measurements for the SDR (Sensor Data Record) production has been improved several times. In this paper, timeline of the main upgrades to the calibration software and parameters is compared with the changes of the radiometric coefficients applied in the operational production of the VIIRS SDR. Initially, radiometric calibration coefficients were updated once per week to correct for the responsivity degradation that occurs for some of the sensor’s spectral bands due to contamination of the VIIRS telescope’s mirrors. Despite the frequent updates, discontinuities in the radiometric calibration could still affect ocean color time series. In August 2012, magnitude of the radiometric coefficient changes was greatly reduced by implementing a procedure that predicts (about a week ahead) values of the calibration coefficients for each Earth scan until a subsequent update. The updates have been continued with the weekly frequency, and the coefficient prediction errors were monitored by comparisons with the initial invariant coefficients from the following week. The predicted coefficients were also compared with the coefficients derived once per orbit from the onboard solar diffuser measurements by an automated procedure implemented in the VIIRS data operational processing software. The paper evaluates the changes in the VIIRS RSB coefficient updates for bands M1 to M7 and potential impacts of these changes on ocean color applications.