Coastal Zone Color Scanner
Encyclopedia
The Coastal Zone Color Scanner (or CZCS) was a multi-channel scanning radiometer
aboard the Nimbus 7
satellite. Nimbus 7 was launched 24 October 1978, and CZCS became operational on 2 November 1978. It was only designed to operate for one year (as a proof-of-concept), but in fact remained in service until 22 June 1986. Its operation on board the Nimbus 7 was limited to alternate days as it shared its power with the passive microwave Scanning Multichannel Microwave Radiometer
.
CZCS measured reflected solar energy in six channel
s, at a resolution
of 800 meters. These measurements were used to map chlorophyll
concentration in water, sediment
distribution, salinity
, and the temperature of coastal waters and ocean current
s.
CZCS lay the foundations for subsequent satellite ocean color sensors, and formed a cornerstone for international efforts to understand the ocean's role in the carbon cycle
.
imagery. The "color" of the ocean in CZCS images comes from substances in the water, particularly phytoplankton
(microscopic, free-floating photosynthetic organisms), as well as inorganic particulates.
Because ocean color data is related to the presence of phytoplankton and particulates, it can be used to calculate the concentrations of material in surface waters and the level of biological activity; as phytoplankton concentration increases, ocean color shifts from blue to green (note that most CZCS images are false colored, so that high levels of phytoplankton appear as red or orange). Satellite-based ocean color observations provide a global picture of life in the world's oceans, because phytoplankton is the basis for the vast majority of oceanic food chain
s. By recording images over a period of years, scientists also gained a better understanding of how the phytoplankton biomass changed over time; for instance, red tide
bloom
s could be observed when they grew. Ocean color measurements are also of interest because phytoplankton removes carbon dioxide from the sea water during photosynthesis, and so forms an important part of the global carbon cycle.
Raw data from the scanner were transmitted, at an average bit rate
of 800 kbit/s, to the ground station, where they were saved on magnetic tape. The tapes were then sent to the Image Processing Division at Goddard Space Flight Center
. The processed data were archived at Goddard, and available to scientists worldwide. The data were originally stored on 38,000 nine track magnetic tapes, and later migrated to optical disc.
The archive was one of the first instances of a system that provided a visual preview ("browse" ) of images, which assisted in ordering data. It became a model to be followed later by the Earth Observing System
's Distributed Active Archive Centers.
The Sea-viewing Wide Field-of-view Sensor (SeaWiFS
) was a follow-on to CZCS, launched in 1997. The MODIS
/Aqua instrument currently provides ocean color data.
Reflected solar energy was measured in six channels to sense color caused by absorption due to chlorophyll, sediments, and gelbstoffe in coastal waters. The CZCS used a rotating plane mirror at a 45 degree angle to the optic axis of a Cassegrain telescope. The mirror scanned 360 degrees but only the 80 degrees of data centered on nadir
were collected for ocean color measurements. The instrument viewed deep space and calibration
sources during the remainder of the scan. The incoming radiation was collected by the telescope and divided into two streams by a dichroic
beam splitter
. One stream was transmitted to a field stop that was also the entrance aperture
of a small polychromator
. The radiance that entered the polychromator was separated and re-imaged in five wavelengths on five silicon detectors in the focal plane of the polychromator. The other stream was directed to a cooled mercury cadmium telluride detector in the thermal region (10.5-12.5 micrometer). A radiative cooler was used to cool the thermal detector
. To avoid sun glint, the scanner mirror was tilted about the sensor pitch axis on command so that the line of sight of the sensor was moved in 2-degree increments up to 20 degrees with respect to the nadir. Spectral bands at 0.443 and 0.670 micrometers centered on the most intense absorption bands of chlorophyll, while the band at 0.550 micrometers centered on the "hinge point," the wavelength of minimum absorption. Ratios of measured energies in these channels were shown to closely parallel surface chlorophyll concentrations. Data from the scanning radiometer were processed, with algorithms developed from the field experiment data, to produce maps of chlorophyll absorption. The temperatures
of coastal waters and ocean currents were measured in a spectral band centered at 11.5 micrometers. Observations were made also in two other spectral bands, 0.520 micrometers for chlorophyll correlation and 0.750 micrometers for surface vegetation. The scan width was 1556 km centered on nadir and the ground resolution was 0.825 km at nadir.
Radiometer
A radiometer is a device for measuring the radiant flux of electromagnetic radiation. Generally, the term radiometer denotes an infrared radiation detector, yet it also includes detectors operating on any electromagnetic wavelength....
aboard the Nimbus 7
Nimbus program
The Nimbus satellites were second-generation U.S. robotic spacecraft used for meteorological research and development. The spacecraft were designed to serve as stabilized, Earth-oriented platforms for the testing of advanced systems to sense and collect atmospheric science data...
satellite. Nimbus 7 was launched 24 October 1978, and CZCS became operational on 2 November 1978. It was only designed to operate for one year (as a proof-of-concept), but in fact remained in service until 22 June 1986. Its operation on board the Nimbus 7 was limited to alternate days as it shared its power with the passive microwave Scanning Multichannel Microwave Radiometer
Scanning Multichannel Microwave Radiometer
The scanning multichannel microwave radiometer [pronounced simmer] was a five-frequency microwave radiometer flown on the Seasat and Nimbus 7 satellites. Both were launched in 1978, with the Seasat mission lasting less than six months until failure of the primary bus. The Nimbus 7 SMMR lasted from...
.
CZCS measured reflected solar energy in six channel
Channel (digital image)
Color digital images are made of pixels, and pixels are made of combinations of primary colors. A channel in this context is the grayscale image of the same size as a color image, made of just one of these primary colors. For instance, an image from a standard digital camera will have a red, green...
s, at a resolution
Optical resolution
Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.An imaging system may have many individual components including a lens and recording and display components...
of 800 meters. These measurements were used to map chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
concentration in water, sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
distribution, salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
, and the temperature of coastal waters and ocean current
Ocean current
An ocean current is a continuous, directed movement of ocean water generated by the forces acting upon this mean flow, such as breaking waves, wind, Coriolis effect, cabbeling, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
s.
CZCS lay the foundations for subsequent satellite ocean color sensors, and formed a cornerstone for international efforts to understand the ocean's role in the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
.
Ocean color
The most significant product of the CZCS was its collection of so-called ocean colorOcean color
The "color" of the ocean is determined by the interactions of incident light with substances or particles present in the water.For pure ocean water, such like the open ocean water, it appears as a very dark navy blue. The reason the ocean is blue is due to the absorption and scattering of light...
imagery. The "color" of the ocean in CZCS images comes from substances in the water, particularly phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...
(microscopic, free-floating photosynthetic organisms), as well as inorganic particulates.
Because ocean color data is related to the presence of phytoplankton and particulates, it can be used to calculate the concentrations of material in surface waters and the level of biological activity; as phytoplankton concentration increases, ocean color shifts from blue to green (note that most CZCS images are false colored, so that high levels of phytoplankton appear as red or orange). Satellite-based ocean color observations provide a global picture of life in the world's oceans, because phytoplankton is the basis for the vast majority of oceanic food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
s. By recording images over a period of years, scientists also gained a better understanding of how the phytoplankton biomass changed over time; for instance, red tide
Red tide
Red tide is a common name for a phenomenon also known as an algal bloom , an event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column and results in discoloration of the surface water. It is usually found in coastal areas...
bloom
Algal bloom
An algal bloom is a rapid increase or accumulation in the population of algae in an aquatic system. Algal blooms may occur in freshwater as well as marine environments. Typically, only one or a small number of phytoplankton species are involved, and some blooms may be recognized by discoloration...
s could be observed when they grew. Ocean color measurements are also of interest because phytoplankton removes carbon dioxide from the sea water during photosynthesis, and so forms an important part of the global carbon cycle.
Raw data from the scanner were transmitted, at an average bit rate
Bitrate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
of 800 kbit/s, to the ground station, where they were saved on magnetic tape. The tapes were then sent to the Image Processing Division at Goddard Space Flight Center
Goddard Space Flight Center
The Goddard Space Flight Center is a major NASA space research laboratory established on May 1, 1959 as NASA's first space flight center. GSFC employs approximately 10,000 civil servants and contractors, and is located approximately northeast of Washington, D.C. in Greenbelt, Maryland, USA. GSFC,...
. The processed data were archived at Goddard, and available to scientists worldwide. The data were originally stored on 38,000 nine track magnetic tapes, and later migrated to optical disc.
The archive was one of the first instances of a system that provided a visual preview ("browse" ) of images, which assisted in ordering data. It became a model to be followed later by the Earth Observing System
Earth Observing System
The Earth Observing System is a program of NASA comprising a series of artificial satellite missions and scientific instruments in Earth orbit designed for long-term global observations of the land surface, biosphere, atmosphere, and oceans of the Earth. The satellite component of the program was...
's Distributed Active Archive Centers.
The Sea-viewing Wide Field-of-view Sensor (SeaWiFS
SeaWiFS
SeaWiFS stands for Sea-viewing Wide Field-of-view Sensor. It was the only scientific instrument on GeoEye's OrbView-2 satellite, and was a follow-on experiment to the Coastal Zone Color Scanner on Nimbus 7...
) was a follow-on to CZCS, launched in 1997. The MODIS
MODIS
The Moderate-resolution Imaging Spectroradiometer is a payload scientific instrument launched into Earth orbit by NASA in 1999 on board the Terra Satellite, and in 2002 on board the Aqua satellite...
/Aqua instrument currently provides ocean color data.
Technical details
The CZCS instrument was manufactured by Ball Aerospace & Technologies Corp..Reflected solar energy was measured in six channels to sense color caused by absorption due to chlorophyll, sediments, and gelbstoffe in coastal waters. The CZCS used a rotating plane mirror at a 45 degree angle to the optic axis of a Cassegrain telescope. The mirror scanned 360 degrees but only the 80 degrees of data centered on nadir
Nadir
The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous...
were collected for ocean color measurements. The instrument viewed deep space and calibration
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
sources during the remainder of the scan. The incoming radiation was collected by the telescope and divided into two streams by a dichroic
Dichroism
Dichroism has two related but distinct meanings in optics. A dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths , or one in which light rays having different polarizations are absorbed by different amounts.The original meaning of...
beam splitter
Beam splitter
A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...
. One stream was transmitted to a field stop that was also the entrance aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
of a small polychromator
Polychromator
A polychromator is an optical device that is used to disperse light into different directions to isolate parts of the spectrum of the light. A prism or diffraction grating can be used to disperse the light. Unlike a monochromator, it outputs multiple beams over a range of wavelengths simultaneously...
. The radiance that entered the polychromator was separated and re-imaged in five wavelengths on five silicon detectors in the focal plane of the polychromator. The other stream was directed to a cooled mercury cadmium telluride detector in the thermal region (10.5-12.5 micrometer). A radiative cooler was used to cool the thermal detector
Infrared detector
An infrared detector is a photodetector that reacts to infrared radiation. The two main types of detectors are thermal and photonic.The thermal effects of the incident IR radiation can be followed through many temperature dependent phenomena....
. To avoid sun glint, the scanner mirror was tilted about the sensor pitch axis on command so that the line of sight of the sensor was moved in 2-degree increments up to 20 degrees with respect to the nadir. Spectral bands at 0.443 and 0.670 micrometers centered on the most intense absorption bands of chlorophyll, while the band at 0.550 micrometers centered on the "hinge point," the wavelength of minimum absorption. Ratios of measured energies in these channels were shown to closely parallel surface chlorophyll concentrations. Data from the scanning radiometer were processed, with algorithms developed from the field experiment data, to produce maps of chlorophyll absorption. The temperatures
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
of coastal waters and ocean currents were measured in a spectral band centered at 11.5 micrometers. Observations were made also in two other spectral bands, 0.520 micrometers for chlorophyll correlation and 0.750 micrometers for surface vegetation. The scan width was 1556 km centered on nadir and the ground resolution was 0.825 km at nadir.