Indian Journal of Geo-Marine Sciences Vol. 40(6), December 2011, pp. 762-769 Backscattering of light by coastal waters using hyperspectral in-situ measurements: A case study off Veraval, Gujarat, India Arvind Sahay, Prakash Chauhan, P. V. Nagamani, Nivedita Sanwalani and R. M. Dwivedi Marine and Coastal Environment Division, Marine and Earth Sciences Group 1 Space Applications Centre, Ahmedabad 380015, India [E-mail: sahayarvind@gmail.com] Received 6 May 2010; revised 24 December 2010 Backscattering of coastal waters of Arabian Sea off Veraval, Gujarat, India has been reported using Quasi Analytical Algorithm (QAA) inversion method. The same is compared with the in-situ measurements. Present study compares the backscattering coefficient retrieved from QAA with the measured values of backscattering at 470 and 700 nm. Retrieved backscattering coefficient using QAA lies between 0.0005-0.0047 m -1 at 470 nm and 0.00006-0.0040 m -1 at 700 nm. This is compared with the measured backscattering coefficients. Root mean square error (RMSE) has been computed, taking logarithm (to base 10) of the measured and modeled values. RMSE for 470 nm is 0.21 and for 700 nm is 0.23. Mean percentage error has also been computed which is 19.19% at 470 nm and 43.90% at 700 nm. Field measurements show that the QAA approach overestimates the backscattering coefficient at 470 nm and 700 nm (19.19% and 43.90% respectively) in the coastal waters of Gujarat. This is possibly due to the wavelength model used in the computation of backscattering at various wavelengths. [Keywords: Inherent Optical Properties, algorithm, backscattering coefficient, quasi analytical, ocean colour] Introduction Inherent Optical Properties (IOP) are those optical properties, which are independent of variations in the angular distribution of the incident light field, and are solely determined by the type and concentration of substances present in the medium 1 . Scattering and absorption are two basic inherent optical properties of the seawater. Different methods of retrieval of inherent optical properties were suggested by different researchers. These methods include analytical, semi analytical, quasi analytical, linear matrix inversion and artificial neural network etc. Loisel and Stramski 2 developed an analytical inverse algorithm to retrieve the total absorption, scattering, and backscattering coefficients from the irradiance reflectance just beneath the surface and the mean vertical diffuse attenuation coefficient over the first optical depth based on Monte Carlo and Hydrolight simulations. Quasi Analytical Algorithm (QAA) was developed by Lee et. al. 3 to derive inherent optical properties of ocean waters. The Linear Matrix Inversion (LMI) algorithm was developed by Hoge and Lyon 4 . This algorithm uses remote sensing reflectance at three wavelengths to simultaneously derive three major unknowns. In the past decade, this algorithm has been applied to data collected from many regions around the world (Hoge and Lyon 4-5 , Hoge et. al. 6 ). The approach of Artficial Neural Network (ANN) inversion is studied by several researchers (Doerffer et. al. 7 , Doerffer and Schiller 8 , Schiller and Doerffer 9 ). ANN algorithm is having capability to invert directional water leaving radiance directly into absorption and scattering coefficients or concentrations of different constituents present in coastal waters. GSM (Garver Siegel Maritorena) semianalytical ocean colour model was initially developed by Garver and Siegel 10 and later updated by Maritorena et. al 11 . The GSM model is based on the quadratic relationship between the remote sensing reflectance (Rrs) and the absorption and backscattering co-effcients from Gordon et. al. 12 . In the present study backscattering coefficient at 470 nm and 700 nm has been retrieved using QAA approach by Lee et. al. 3 (2002). The aim of the present study is (i) to retrieve the backscattering coefficient (m -1 ) of the coastal waters of Veraval (Gujarat, India) using hyperspectral in-situ data by Quasi Analytical Algorithm i.e QAA approach (ii) to validate it with the field measured backscattering coefficient. In the field backscattering is measured by the scattering meter 13 at 470 and 700 nm. The root mean square error (RMSE) and the mean % error have been computed to validate the results.