GES DISC DAAC Data Guide: Coastal Zone Color Scanner (CZCS) Instrument Guide
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Figure 1. The Coastal Zone Color Scanner Instrument
This document contains information about the Coastal Zone Color Scanner (CZCS) instrument which was flown aboard the NIMBUS 7 platform and which collected ocean color data from November 1978 to June 1986. An overview of the instrument design and construction is provided as well as a discussion of its calibration, manufacture and history. Additional sources of information on this instrument are given, along with a list of relevant acronyms and a glossary of technical terms used in this document.
For information about the CZCS data products and their formats, refer to the CZCS Dataset Guide. For more information about the NIMBUS 7 platform, refer to the NIMBUS 7 Platform Guide.
Table of Contents:
- Coastal Zone Color Scanner, CZCS
- The Coastal Zone Color Scanner Experiment (CZCS), launched aboard NIMBUS-7 in October 1978, was the first instrument devoted to the measurement of ocean color and flown on a spacecraft. Although other instruments flown on other spacecraft had sensed ocean color, their spectral bands, spatial resolution and dynamic range were optimized for land or meteorological use and had limited sensitivity in this area, whereas in CZCS, every parameter was optimized for use over water to the exclusion of any other type of sensing.
The content of water, be it organic or inorganic particulate matter or dissolved substances, affects its color. Open ocean water, containing very little particulate matter, scatters visible light as a Rayleigh scatterer in the deep blue color range. As particulate matter is added to the water, the light scattering characteristics and the color are changed. Phytoplankton, for instance, have specific absorption characteristics and normally change the water to a more greenish hue, although some phytoplankton can also cause red, yellow, blue-green or mahogany colors. Inorganic particulate matter in the water, such as outflow from rivers, has different colors than organic material. By sensing the ocean color with very high signal-to-noise ratios, CZCS provided a mechanism for analyzing that color for the content of the water. The ocean color data collected by CZCS were thus used to map chlorophyll concentration in water, sediment distribution and gelbstoffe concentrations and its infrared channel was used to determine the temperature of coastal waters and ocean currents.
CZCS was a conventional mulit-channel scanning radiometer utilizing a rotating plane mirror at a 45 degree angle to the optic axis of a Cassegrain telescope. The rotating mirror scanned 360 degrees however, only data centered plus or minus 40 degrees of the spacecraft nadir were collected for ocean color measurements. During the rest of the scan, the instrument acquired a view of deep space and of internal instrument sources for calibration of the various channels. Further details about the instrument's optics may be found in Section 2 of this document.
Reflected solar energy was measured by CZCS in six channels: Channel 1, .433-.453 microns (blue) for chlorophyll absorption, Channel 2, .510-.530 microns (green) for chlorophyll correlation, Channel 3, .540-.560 microns (yellow) for yellow substance (Gelbstoffe used to indicate salinity), Channel 4, .660-.680 microns (red) for aerosol absorption, Channel 5 .700-.800 microns (far red) for land and cloud detection and Channel 6, 10.5-12.5 microns (infra-red) for surface temperature. CZCS measurements have 8 bit resolution. The scan width was 1556 km centered on nadir and the ground resolution was 0.825 mk at nadir. CZCS collected data intermittantly until June, 1986.
- The most important objective of the Coastal Zone Color Scanner mission was to determine if satellite remote sensing of color could be used to identify and quantify material suspended or dissolved in ocean waters. Specifically CZCS attempted to discriminate between organic and inorganic materials in the water, determine the quantity of material and discriminate between different organic particulate types.
Being satellite mounted, CZCS was able to provide measurements of ocean color over large geographic areas in short periods of time in a way that was not previously possible with other measurement techniques, such as from surface ships, buoys and aircraft. These measurements allowed oceanographers to infer the global distribution of the standing stock of phytoplankton for the first time.
A "proof-of-concept" experiment, CZCS also showed that satellite ocean color measurements could be reliably used to derive products such as chloroplyll and sediment concentrations and provided justification for future ocean color missions such as SeaWiFS and MODIS. The algorithms developed to analyze CZCS data were a considerable step forward from those available earlier. and included corrections for atmospheric backscatter, limb brightness and gelbstoffe. Ground truth campaigns led to empirical correlation of ocean color and biomass. CZCS also showed the need for good radiometric calibration and stability and the necessity of sufficient ground truth data to verify sensor and algorithm performance over time.
- Ocean color. chlorophyll absorption, chlorophyll correlation, gelbstoffe, surface vegetation, surface temperature.
Since NIMBUS 7 flew from south to north in daylight, the CZCS scan mirror was positioned to aft of the satellite when the spacecraft was south of the subpolar point and forward of the spacecraft when it was north of the subpolar point. Tilt and gain setting information were transmitted with the CZCS data and are included in the data product metadata records.
The CZCS data were transmitted from the spacecraft to ground receiving stations at a rate of 800 kbs either in real time or via playback of the onboard tape recorder. The tape recorder was utilized when the spacecraft was out of the range of ground tracking stations. Due to the power demands of the various on-board experiments the CZCS sensor was operated on an intermittent schedule and collected data only two hours per day on the average.
The infrared temperature sensor (channel 6, 10.5-12.5 microns) never functioned satisfactorily, and was useful only in the first year (1979) in a relative sense. This analysis was possible until about 16 August 1979, and somewhat in the following periods: 3 April 1980 to 10 September 1980; 28 May 1981 to 20 Aug 1981; and 24 June 1982 to 5 August 1982. During the above periods, there may be some useful relative information (only within a given scene), but any equations provided should be treated with considerable caution.
The detector lost sensitivity rapidly above its operation temperature, and eventually saturated only a few degrees above its operation point. The date ranges given above indicate when the detector was within the range 119.1 to 121.1 K. The reasons for the failure were never determined with any degree of confidence. Three potential reasons were: contamination of the cooler by an unknown substance or substances, delamination of the silver-teflon tape on the annulus, or unidentified thermal loads of about 1 watt due to glint or scatter. Because of the variability, no correction algorithms were ever developed. Most of the CZCS team was concerned with problems in the visible data, and good atmospheric corrections were precluded because CZCS lacked the split thermal channels.
Subsequent to August 1982, all attempts to restore some useful operation to the thermal channel were discontinued. The NIMBUS Project chose to include meaningless data rather than write new code to insert zeros.
Sometime in 1981 it was also determined that the sensitivity of the other CZCS sensors was degrading with time, in particular channel 4. Sensitivity degradation was persistent and increased during the rest of the mission. In mid-1984 NIMBUS-7 Mission personnel experienced turn-on problems with the CZCS system which were related to power supply problems and the annual lower power summer season of NIMBUS-7. Also spontaneous shut down of the CZCS system began occurring. These also persisted for the rest of the mission. From March 9, 1986 to June, 1986 the CZCS system was given highest priority for the collection of a contemporaneous data set of ocean color. It was turned off in June at the start of the low power season with the intention of turning it back on in December when power conditions would be more favorable. Attempts to reactivate the CZCS system in December 1986 met with failure. The CZCS sensor was officialy declared non-operational on 18 December 1986. (ESRIN, 1995)
- Coastal Zone Color Scanner
- CZCS was a multi-spectral radiometer with a recorded scan width of 1566 kilometers centered on spacecraft nadir. The scanner actually scanned through 360 degrees, but the electronics limited the high data rate sampling to 39.34 degrees about nadir. The scanner looked either forward or aft of spacecraft nadir in increments of 2 degrees up to twenty degrees to avoid surface sun glint. The ground resolution of the instantaneous field of view was 0.825 kilometers at nadir and degraded somewhat as the instrument scanned away from nadir on either side. The viewing geometry of the instrument is shown in Figure 2.
Figure 2. CZCS Viewing Geometry and Earth Scan Pattern
CZCS had six spectral bands, five sensing backscattered solar radiance and one sensing emitted thermal radiance. Figure 3 illustrates the optical method by which discrimination of the spectral bands was achieved. The incoming beam was first split by a dichroic beam splitter, one portion of the beam going through a set of depolarizing wedges to a small polychromator where the radiance was dispersed and detected by five silicon diode detectors in the focal plane of the polychromator. Radiance in the 10.5 to 12.5 micron spectral band was reflected off the dichroic and then imaged onto an infrared detector of mercury cadmium telluride cooled to approximately 120 degrees Kelvin using a radiative cooler. Table 1 shows the center wavelengths, the spectral bandwidths and the minimum signal-to-noise ratio specified for the instrument at its most sensitive gain setting, that is, the gain setting used for the darkest targets. The first four channels were selected to cover the so-called 'hinge point'. These channels were meant to look at water only and to saturate when land or clouds were in the field of view. The spectral response of channels 1 through 5 are shown in Figure 4.
Figure 3. CZCS Optical Arrangement
Table 1. CZCS Performance Parameters
Figure 4. CZCS Spectral Response for Channels 1 Through 5
Channel 5 had the same spectral response as channel 6 of the Landsat multi-spectral scanner series. The gain of channel 5 was fixed and set to produce the same percentage of maximum signal over land targets as the Landsat channel 6. However, the actual radiance for saturation was higher since NIMBUS 7 crossed the equator at noon wheras Landsat crossed the equator at 9:30 local time.
The 10.5 to 12.5 micron channel measured equivalent blackbody temperature as seen by the sensor with a noise equivalent temperature difference of less than 0.35 degree Kelvin at 270 degrees Kelvin. Atmospheric interference with this channel, principally from weak water vapor absorption in the 10.5 to 12.5 micron range produced measurement errors of several degrees. Temperature gradients, however, were seen quite well because of the extremely low noise equivalent temperature difference of this sensor.
- Ball Brothers Research Corporation, Broomfield Colorado, now Ball Aerospace and Technologies Corporation.
Prelaunch calibration of CZCS was achieved utilizing a 76 cm diameter integrating sphere as a source of diffuse radiance for channels 1 through 5 and a blackbody source for channel 6. The integrating sphere was specially constructed for calibration of CZCS and was itself calibrated from a standard lamp from the National Bureau of Standards using a spectrometer and another integrating sphere to tranfer calibration from the lamp to the sphere. This same type of sphere was also used in calibration of Landsat mulitspectral scanners and the Advanced Very High Resolution Radiometer (AVHRR).
In addition to the sphere and blackbody, a collimator was also used to calibrate CZCS in vacuum testing. Calibration was transferred from the primary calibration standard, the sphere and blackbody, to the collimator using the instrument itself.
In-flight calibration of the first five CZCS channels was to have been accomplished using a built-in incandescent light source. This in-flight calibration source was calibrated using the instrument itself as a transfer against the reference sphere output. CZCS carried two incandescent light sources to provide redundancy. However, because the light from these lamps "did not pass through the entire optical train and its intensity was too low to provide even a useful calibration in the blue...it was used only to monitor the long term stability of the internal optics and detectors" (Evans & Gordon, 1994). Channel 6 was calibrated in flight by viewing the blackened housing of the instrument whose temperature was monitored and by viewing deep space during the 360 degree rotation of the scan mirror.
Operationally the CZCS sensor was calibrated "vicariously" by forcing the sensor output plus algorithms to yield ship-measured water leaving radiances where available. This calibration began early on when it was realized that discrepencies appeared when the atmospheric correction algorithm was applied. At about the same time, it was noticed that the instrument sensitivity was declining with time. This led to empirical modifications to the sensor radiance equation described below.
The sensor radiance Lt(i) in channel i was orginally given by this version of the sensor radiance equation:
Lt(i) = S(i,G)*DC=I(i,G,),
where DC was the digital output of the instrument (0-255), and S(i,G) and I(i,G) were the slope and intercept respectively. The radiometric sensitivity of the instrument in channels 1-4 was adjusted by varying the gain of the amplifiers (G) between the detectors and the digitizer. The gains and their selection criteria are described in reference 4. below. (Ball Aerospace, 1979). Based on the ship-based water leaving radiance measurements, the sensor radiance equation was eventually modified to include correction factors for initialization and degradation in sensitivity:
Li(i) = g(i,t)*k(i,G)*S(i,G)*DC + I(i,G),
where g(i,t) corrects for loss of sensitivity over time and k(i,G) is the initiliazing factor relating prelaunch calibration to on-orbit calibration. Recent studies also indicate the existence of short term sensitivity fluctuations throughout the mission, (Evans & Gordon, 1994)
- Because the albedo of the ocean is very low compared to land or cloud, atmospheric backscatter contributes a major portion (80-100%) of the radiance measured at the sensor. "Therefore if the water-leaving radiance is to be determined with an absolute accuracy of 10%, the error in the sensor calibration cannot exceed ~1%." (Evans & Gordon, 1994).
For accurate vicarious calibration, both subsurface and atmospheric optical measurements are required. Unfortunately, these data were only collected during the initial validation field experiments. Over most of the mission, surface pigment concentration measurements collected infrequently along ship tracks contemporaneous with satellite overpasses were the only source of available ground truth data.
- A retrospective analysis of the entire CZCS dataset by Evans and Gordon, 1994, has attempted to identify and quantify short term fluctuations in sensitivity and to estimate the radiometric state of the sensor for the entire mission. The experience with CZCS has shown that good radiometric calibration and stability are neccessary requirements for an ocean color sensor and that an extensive ground truth data collection field campaign is needed to monitor the performance of the sensor and algorithms over time and under varying atmospheric and water conditions.
"The Nimbus 7 Users' Guide", prepared by the Landsat/Nimbus Project, Goddard Space Flight Center, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland, August, 1978.
2. "Coastal zone color scanner 'system calibration': A retrospective examination." R.H. Evans & H.R. Gordon, Journal of Geophysical Research, Vol.99. No. C4, pages 7293-7307, April 15, 1994.
3. "Coastal Zone Color Scanner", European Space Research Institute, Frascati, Italy.
4. "Development of the coastal zone color scanner for Nimbus-7, vol.2: Test and performance data, Final report, rev. A, NASA contract NAS5-20900, Rep. F78-11", pages 2-22, Ball Aerospace Division, Boulder, Colorado, 1979.
The April 15, 1994 issue of the Journal of Geophysical Research (Volume 99, Number C4) contains a Special Section entitled "Ocean Color From Space: A Coastal Zone Color Scanner Retrospective."
- Atmospheric Backscatter: scattering caused by gas molecules, particles and aerosols in the atmosphere.
- Blackbody Radiation: the characteristic thermal radiation emitted by a blackbody at a specific temperature.
- Blackbody: a theoretically perfect absorber and re-emitter of all incident radiation as described by Planck's Law.
- Cassegrain Telescope: a reflecting telescope in which a concave primary mirror reflects incident light to a convex secondary mirror which in turn reflects the light back through a central perforation in the primary mirror and onto the focal plane.
- Calibration: the adjustment or systematic standardization of the output of a quantitative measuring instrument or sensor.
- Chlorophyll: any of a group of related green pigments found in photosynthetic organisims.
- Collimator: a long narrow tube with strongly absorbing or reflecting walls which permit only radiation travelling parallel to the tube axis to traverse the entire length of the tube.
- Contemporaneous: originating, existing or happening during the same period of time.
- Dichroic Beam Splitter: an optical device that separates long and short wavelength light, transmitting one and reflecting the other depending on its design.
- Depolarizing Wedge: an optical device for removing the effects of polarized light in the input of an optical instrument.
- Dynamic Range: the range between the maximum and minimum amount of input radiant energy that an instrument can measure.
- Gelbstoffe: particulate matter, usually outflow sediment from rivers, which, when suspended in water, gives it a yellowish color. (from German: "yellow stuff").
- Hinge Point: the portion of the water-leaving radiance spectrum around 500 nm that is relatively independent of the pigment concentration.
- Incandescent Light Source: light source which emits light as a result of having been heated.
- Infrared Light: electromagnetic radiation having wavelengths longer than red light (7700 angstroms) but less than radio waves (~.1 meter).
- Integrating Sphere: an optical source with a white-painted interior and a small output hole or port. Lamps on the inside of the sphere emit light that reflects many times off the white interior surface before exiting through the output port. The output of the sphere is thus uniform across the output port.
- Limb Brightness: the change in brightness of the edge or limb of a source, relative to the brightness at its center.
- Nadir: the point on the Earth directly below an orbiting satellite.
- Noise Equivalent Temperature Difference: the minimum temperature difference detectable in an instrument, in other words, where the temperature difference is equivalent to the noise in the measurement.
- Photosynthesis: the process by which chlorophyll-containing cells in green plants convert incident light to chemical energy and synthesize organic compounds from inorganic compounds, especially carbohydrates from carbon dioxide and water, with the simultaneous release of oxygen.
- Phytoplankton: drifting, often microscopic oceanic plants which conduct the process of photosynthesis.
- Polychromator: a device that separates light into several wavelengths, as opposed to a monochromator, which transmits only one wavelength of light.
- Radiometer: a device that detects and measures electromagnetic radiation.
- Rayleigh Scattering: The scattering of light waves by particles with dimensions much smaller than their wavelengths. For example the blue color of the sky and ocean is caused by Raleigh scattering from the air and water molecules respectively.
- Saturate:in this context, to exceed the highest possible response level of a detector.
- Silicon Diode Detector: a device that converts visible light input into an electrical current output.
- Spatial Resolution: the size of the smallest object recognizable using the detector.
- Spectral Band: a narrow range of the electromagnetic spectrum.
- Spectral Response: the relative amplitude of the response of a detector vs. the frequency of incident electromagnetic radiation.
- Tilt: in this context, to make the view of a satellite-mounted instrument either forward or aft of the flight direction to avoid sun glint from the ocean surface.
- Visible Light: electromagnetic radiation with wavelength in the 3900 to 7700 angstrom range.
- AVHRR: Advanced Very High Resolution Radiometer
- CZCS: Coastal Zone Color Scanner
- ESRIN: European Space Research Institute
- Landsat: Land Remote-Sensing Satellite
- MODIS: Moderate Resolution Imaging Spectrometer
- Nimbus: NASA Meteorological Satellites (1 through 7)
- SeaWiFS: Sea-viewing Wide Field-of-view Sensor
- 1 September, 1996
- Version 2.0
- Version baselined on addition to the GES Controlled Documents List, September 1, 1996.