Comparative analysis of ocean color measurements of IRS-P4 OCM and SeaWiFs in the Arabian Sea

Abstract

The Indian Remote Sensing P4 (IRS-P4) satellite has carried an Ocean Color Monitor (OCM) sensor for investigations on ocean color parameters in open and coastal seas. The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) sensor of the National Aeronautics and Space Administration is also an ocean color instrument, which is in concurrent operation with IRS-P4 OCM. Results of an intercomparison analysis are described for ocean color data from OCM and SeaWiFS, using a consistent methodology for atmospheric correction and bio-optical algorithms. Near synchronous data of OCM and SeaWiFS sensors was obtained on March 22, 2000 over parts of the Arabian Sea. The OCM and SeaWiFS image data were processed to estimate normalized water-leaving radiance (nL_w) in 443-, 490-, 510-, and 555-nm spectral bands. A comparison was made for 10×10 colocated pixels of OCM and SeaWiFS images. Differences were observed in the estimated values of nL_w [443] and nL_w [490], respectively, obtained by OCM, when compared to SeaWiFS. However, OCM estimated nL_w [510] and nL_w [555] were comparable to that of SeaWiFS. An intercalibration approach for OCM was adopted using SeaWiFS calibration as "standard". An intercomparison of the top-of-the-atmosphere radiance measured by OCM and SeaWiFS sensor was performed for colocated pixels having similar viewing geometry. Individual OCM bands were vicariously recalibrated, and a gain coefficient for each OCM band was derived through this procedure. The OCM data for March 22, 2000 were reprocessed using derived gain coefficients, and new gain coefficients were further tested on an independent dataset of March 18, 2000. Results obtained for the recalibrated OCM data showed a better match up with the ocean color estimation obtained by SeaWiFS.