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![Figure 1. The Coastal Zone Color Scanner Instrument](https://webarchive.library.unt.edu/eot2008/20090512175618im_/http://daac.gsfc.nasa.gov/oceancolor/images/CZCS1_small.gif)
Figure
1. The Coastal Zone Color Scanner Instrument
Summary: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](https://webarchive.library.unt.edu/eot2008/20090512175618im_/http://daac.gsfc.nasa.gov/oceancolor/images/CZCS_Sensor_fig2.gif)
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](https://webarchive.library.unt.edu/eot2008/20090512175618im_/http://daac.gsfc.nasa.gov/oceancolor/images/CZCS_Sensor_fig3.gif)
Figure
3. CZCS Optical Arrangement ![Table 1. CZCS Performance
Parameters](https://webarchive.library.unt.edu/eot2008/20090512175618im_/http://daac.gsfc.nasa.gov/oceancolor/images/CZCS_Sensor_table1.gif)
Table
1. CZCS Performance Parameters ![Figure 4. CZCS Spectral
Response for Channels 1 Through 5](https://webarchive.library.unt.edu/eot2008/20090512175618im_/http://daac.gsfc.nasa.gov/oceancolor/images/CZCS_Sensor_fig4.gif)
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.
-
- 1.
"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
-
-
Change History
- Version 2.0
- Version baselined on addition to the GES Controlled Documents List, September 1, 1996.
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