A Heuristic Approach of Radiometric Calibration for Ocean Colour Sensors: A Case Study Using ISRO’s Ocean Colour Monitor-2

Abstract

Ocean colour spectral observations play a significant contribution in mapping the earth marine resources through measurements with its inverted geo-physical/biophysical parameters. The retrieval of parameters from the basic sensor measurements highly depends on atmospheric scattering and absorption of light energy by its constituents. Hence the quantitative applications using these datasets are directly affected by the uncertainty in radiative transfer modeling towards atmospheric scattering and absorption and associated sensor degradation with time. Here authors presented an automation of radiometric calibration approach for ocean colour monitor of Oceansat-II (Jan 2017–Dec 2017) dataset through top of the atmosphere radiance simulation using a non-linear optimization technique. This algorithm also provides an alternative approach of calibrating the sensor vicariously through reduced dependency of systematic congruent in-situ measurements. Since Kavaratti in Lakshadweep, India is already a well-known site for calibrating the ocean colour sensors. The OCM cloud free images over this calibration site are utilized to perform its radiometric assessment for the year 2017 using radiative transfer model coupled with bio-optical model where the synchronous, relevant model inputs are simulated. The significant variations in the radiometric calibration coefficients were realized across the spectral bands 412 to 865 nm i.e. 5.5% to 11.8% change were recorded in the year 2017 followed by 2.65 to 5.23% change within a month of March respectively.