Satellite Observations of Ocean Color
Ocean Color is the water hue due to the presence of tiny plants containing the pigment chlorophyll within phytoplankton and other oceanic particles such as sediments, and colored dissolved organic material (DOM). Phytoplankton consists of small, single-celled ocean plants which constitute the base of the oceanic food web and produce organic carbon through photosynthesis. The rate at which photosynthesis proceeds is known as primary productivity. Different types of phytoplankton that contains different concentrations of chlorophyll have the effect of changing the color of water to green hues from the deep blue of its pure state.
Satellite sensing of ocean color provides important information of chlorophyll and other oceanic particle concentrations. Ocean color observed by satellite sensors (e.g., MODIS/Aqua and VIIRS/NPP) is the radiance emanating from the ocean due to scattering by phytoplankton (e.g., chlorophyll), sediments, and DOM. The observation of ocean color reveals the variability in the distribution and concentration of phytoplankton and the extent of primary productivity, and therefore permits a quantum leap in our understanding of oceanographic processes from regional to global scales. Moreover, satellite observations provide the rapid, global coverage required for studies of ocean productivity worldwide and play an important role in the remote sensing of ocean color.
Moderate-resolution Imaging Spectroradiometer (MODIS)
The Moderate-resolution Imaging Spectroradiometer (MODIS) is a instrument launched by NASA in 1999 on board the Terra (10:30 a.m.) satellite, and in 2002 on board the Aqua (1:30 p.m.) satellite. The instruments have 36 spectral bands ranging in wavelength from 0.4 to 14.4 µm and at varying spatial resolutions. It has 20 reflective solar bands from 410 nm to 2.1 µm and has 16 thermal emissive bands from 3.7 to 14.4 µm (see the tables).
The Visible/Infrared Imager/Radiometer Suite (VIIRS) is one of five instruments onboard National Polar-orbiting Partnership (NPP) and crosses the equator at about 1:30 p.m. local time (npp.gsfc.nasa.gov/viirs.html). The planned lifespan for the VIIRS is five years although its potential life expectancy could be seven years. The VIIRS builds on MODIS and AVHRR heritage and provides high radiometric accuracy and spatial resolution multispectral imagery. The instrument has 22 bands, i.e., 16 "Moderate" resolution (750 m) bands (M bands), 5 "Imagery" resolution (375 m) bands (I bands), and the low-light Day-Night-Band (750 m). Generally, the VIIRS can provide information about cloud, aerosols, ocean color, lake color, forest cover and productivity, sea ice coverage, etc.