Absolute vicarious calibration of OCM2 and AWiFS sensors using a reflectance-based method over land sites in the Rann of Kutch, Gujarat

A study was carried out to estimate vicarious calibration coefficients for the OCM2 (Ocean Color Monitor) sensor onboard Oceansat-2 and also the AWiFS (Advanced Wide Field Sensor) sensor onboard Resourcesat-1 using reflectance measurements over three land sites – Dhrangadhra, Desalpar, and Bhachau – in the Rann of Kutch, Gujarat, India, on four dates (17 October 2010, 25 and 29 April 2011, and 1 May 2011). Hyperspectral field reflectance measurements of the study sites (of extent ˜2 km × 2 km) in the wavelength range 325–2500 nm, along with measurements of atmospheric parameters (aerosol optical depth (AOD), water vapour, ozone) and sensor spectral response functions, were input to the 6S atmosphere correction code to compute top-of-atmosphere (TOA) at-satellite radiance in the eight visible and near infrared (NIR) bands of OCM2 and the four visible, NIR, and shortwave infrared (SWIR) bands of the AWiFS sensor. The uncertainty in vicarious calibration coefficients due to measured spatial variability of field reflectance, aerosol optical thickness (AOT), water vapour, and ozone, was also computed for the OCM2 sensor for three dates (25 and 29 April 2011, 1 May 2011). The effect of surface anisotropy on TOA radiance was studied using a 15 day Moderate Resolution Imaging Spectroradiometer (MODIS) Bidirectional Reflectance Distribution Function (BRDF) product covering the study sites. The results show that there is an indication of change in calibration coefficients in OCM2, for band 2 (25 April Desalpar data), bands 2 and 5 (29 April Desalpar data), and bands 2–5 and 7 in Bhachau (1 May data), all at the 1σ level. For these bands, in the inverse mode, the 6S corrected surface reflectance was closer to field surface reflectance when estimated at-sensor radiances were used as input to the code. For AWiFS, there was no evidence of change in calibration coefficients of all four bands at the 1σ level. It was found that site spatial variability was a critical factor in estimating change in sensor calibration coefficients and influencing uncertainty in TOA radiance for all three sites.

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