Optical Remote Sensing

Coastal Zone Color Scanner (CZCS)

The Coastal Zone Color Scanner was a scanning radiometer that viewed the ocean in six spectral bands: five (443, 520, 550, 670, 750 nm) in the visible and IR, and one in the thermal IR (10,500-12,500 nm). The CZCS, created to provide estimates of the near-surface pigment concentration with an active life of seven and a half years (1979-1986), provided an unparalleled synoptic view of the color (see example) of the world's oceans. In a sun-synchronous orbit at a nominal height of 955 km, a viewing swath width of 1,600 km and a ground resolution of 825 m was achieved. The CZCS coverage, however, was inhomogeneous with space and time, so research of oceanic processes was limited to mesoscale processes.

Several empirical facts were utilized in correcting the CZCS images for atmospheric effects and in retrieving the pigment concentration. Gordon and Clark (1981) found the water leaving radiance in the 520 and 550 nm bands could be determined a priori in low chlorophyll waters ([C]>0.25 mg/m³). Due to the strong effect of phytoplankton pigments in the blue regions, it could not be so easily determined in the 443 nm band. These two values were used to determine the type of aerosol present between the ocean and the sensor for any clear-water portions of a scene. While the type of aerosol was assumed the same for the entire scene, the amount of aerosol could vary; this was determined at 670 nm, where the water-leaving radiance was usually less than one digital number (water was assumed to be black). If no clear water was found in the scence, the aerosol was assumed to be "white" (equal aerosol reflectance for all spectral bands). With the aerosol identified, and Rayleigh scattering known using computer code, the total radiance received at the sensor in the blue 443 nm waveband was used to determine the water-leaving radiance at this wavelength. Empirical observations by Gordon et al. (1983) of the relationship between the water-leaving radiances at 550 and 443 nm and pigment concentration were then used to determine the pigment (i.e., chlorophyll) concentration at the site in question.

Theoretically, CZCS images should allow recovery of the blue water-leaving radiance to within 10-15% and the pigment concentration to within 30-40%, under optimal conditions. Surface winds, multiple scattering, and mixing of aerosol types all work to confound pigment retrieval. Much current research is focused on quantifying these effects for the next generation of ocean-viewing sensors.

Advanced Very High Resolution Radiometer (AVHRR)

The Advanced Very High Resolution Radiometer (AVHRR) is a part of an environmental satellite system named the TIROS-N series (launched October 1978) that was later continued as the NOAA-N series. At present, NOAA-12 and NOAA-14 are operational. The system is a result of a cooperative effort between the United States, United Kingdom, and France to provide daily environmental monitoring and to promote global change research.

Many instruments operate onboard each NOAA satellite. Of particular interest to oceanic optical remote sensing is the AVHRR. A 5-channel scanning radiometer (580-680; 725-1,100; 3,550-3,930; 10,350-11,300; 11,500-12,500 nm), it observes in the red, near infrared, and infrared parts of the spectrum at a nominal resolution of 1 km (see example). Such resolution, however, provides a data stream too voluminous for daily capture; global area coverage is provided at 4-km resolution after the data is subsampled and averaged onboard the instrument. The NOAA-N series satellites possess two modes of operation: an afternoon ascending node (northward Equator crossing) and a descending (southward Equator crossing) node in the morning. The orbit is near-polar and sun synchronous, with a period of approximately 102 minutes (14/day); each cross-track scan corresponds to a swath width of 2,700 km. Further technical details may be found in Kidwell (1991).

The daily coverage provided by the AVHRR is a vast improvement over the sparse (weekly or worse) imagery previously obtained by sensors such as SPOT and Landsat. Such temporal frequency allows for observations and analyses of rapidly changing physical processes such as those found during floods and hurricanes, or in estuarine and near-coastal regions. The saturation radiance of the AVHRR's sensors is much greater than that of the CZCS, allowing for evaluation of quite turbid water where the total reflectance is much greater than for clear water. More information on the remote sensing of turbid estuaries and other areas may be found in Geological Processes.

For more information on AVHRR go to http://edc.usgs.gov/products/satellite/avhrr.html

AVIRIS (Airborne Visible-Infrared Imaging Spectrometer AVIRIS is a test-bed for future spacecraft imaging spectrometers such as MODIS, HIRIS, and SeaWiFS. With 224 spectral channels between 400 and 2,400 nm and 20-m square pixels at 65,000 feet altitude, a signal-to-noise ratio of 10-20% of that of CZCS is obtained (Carder et al. 1993). Recent improvements (see example) have significantly increased responsivity of AVIRIS at the blue end (R. Greene, Jet Propulsion Laboratory; personal communication). Spatial averaging can increase the signal-to-noise ratio, at the cost of spatial resolution. Vicarious calibration must be performed on each flight by modeling the upwelling light field using computer code, such as Lowtran 7 multiple-scattering code, and accounting for the atmospheric effects. The calibrated sensor can then provide spectral values of water-leaving radiance, providing the signal-to-noise ratio is large enough.

A large advantage to this type of spectrometer is the greatly increased spatial resolution. Even a 14-fold increase in pixel signal-to-noise ratio resulted in a 280-m spatial resolution for the backscattering coefficient at 671 nm, and a 50-fold increase at 415 nm still resulted in a 1,000-m resolution for the absorption coefficient in the analysis of Carder et al.

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