IMOS - Satellite Remote Sensing - MODIS - 01 day - Diffuse attenuation coefficient (k490)

Updated: 2026-07-28

The Aqua satellite platform carries a MODIS sensor that observes sunlight reflected from within the ocean surface layer at multiple wavelengths. These multi-spectral measurements are used to infer the diffuse attenuation coefficient (Kd) at 490nm wavelength which provides information on how light is attenuated in the water column. It is defined as the scaling length of the exponential decrease of the downwelling irradiance and has units (m^-1). The MODIS K490 product estimates Kd at 490nm wavelength, using a semi-empirical model based on the ratio of water leaving radiances at 490nm and 555nm. The algorithm is described at https://oceancolor.gsfc.nasa.gov/atbd/kd/. Production of contemporary MODIS data ceased at 2024/12/26. Users requiring similar Ocean Colour Radiometry data beyond 2024 should consider the VIIRS instruments on the SNPP and NOAA20+ series of satellites, also provided by IMOS.