GES DISC DAAC Data Guide: Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Level 3 Data
SeaWiFS Level 3 Monthly Chlorophyll a Global Composite Browse Imagery
The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) is an eight-channel visible light radiometer dedicated to global ocean color measurements which are used to detect and analyze patterns of biological activity in the marine environment. The mission parameters of SeaWiFS allow coverage of more than 90% of the ocean surface every two days. SeaWiFS will map global ocean color at a resolution of 4.5 kilometers, and it also provides regional data at a resolution of 1 kilometer. SeaWiFS is the follow-on mission to the Coastal Zone Color Scanner (CZCS), and the predecessor to several ocean color satellite sensors scheduled for deployment in the years 1998-2002.
The requested form of acknowledgment for research publications utilizing SeaWiFS ocean color data is: "Ocean color data used in this study were produced by the SeaWiFS Project at Goddard Space Flight Center. The data were obtained from the Goddard Earth Sciences Distributed Active Archive Center under the auspices of the National Aeronautics and Space Administration. Use of this data is in accord with the SeaWiFS Research Data Use Terms and Conditions Agreement."
Table of Contents:
- Sea-viewing Wide Field-of-view Sensor
- This dataset consists of satellite measurements of global and regional ocean color data obtained by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), in orbit on the OrbView-2 (formerly "SeaStar") platform. The concentration and predominant identity of substances and particles in the euphotic (lighted) zone of the upper ocean influences the apparent color of the ocean, which can range from deep blue to varying shades of green and ruddy brown. Living phytoplankton (which contain chlorophyll and associated photosynthetic pigments), inorganic sediments, detritus (particulate organic matter), and dissolved organic matter all contribute to the color of the ocean.
- A quantitative determination of global ocean primary productivity is crucial to understanding the ocean's role in the global carbon cycle. The SeaWiFS mission parameters were designed with this goal as a fundamental consideration. In addition, SeaWiFS will refine remote-sensing measurements of phytoplankton chlorophyll and associated pigments, organic matter, and suspended particulate matter in the oceans. SeaWiFs will provide the first continuous observations of global ocean color, anticipating ocean color data from several future ocean remote sensing missions.
The Coastal Zone Color Scanner (CZCS) went far beyond its original status as a proof-of-concept mission. During its eight years of operation (November 1978-June 1986) the CZCS clearly demonstrated that measurements of ocean color from space were possible, and also proved that this technology could be used to characterize the distribution of biological productivity in the surface ocean. CZCS mission constraints, however, prevented quantitative determination of global ocean primary productivity.
NASA's ocean biogeochemistry research program, of which SeaWiFS is a critical element, has established a set of science goals. Data from SeaWiFS is expected to be used in the following ways:
- Goal I: Determine the spatial and temporal distributions of phytoplankton blooms, along with the magnitude and variability of primary production by marine phytoplankton on a global scale.
- Goal II: Quantify the ocean's role in the global carbon cycle and other biogeochemical cycles.
- Goal III: Identify and quantify relationships between ocean physics and large-scale patterns of biological productivity.
- Goal IV: Understand the fate of fluvial nutrients and their possible effect on marine carbon budgets.
- Goal V: Identify the large-scale spatial and temporal distribution of spring blooms in the global oceans.
- Goal VI: Acquire global data on marine optical properties, accompanied by an improved understanding of processes associated with mixing along the edges of eddies and boundary currents.
- Goal VII: Advance the scientific applications of ocean color data and the technical capabilities required for data processing, management, and analysis, in preparation for future missions.
One of the primary goals of the SeaWiFS Project is the following stringent objective:
"To achieve radiometric accuracy to within 5% absolute and 1% relative, water-leaving radiances to within 5% absolute, and chlorophyll a concentration to within 35% over the range 0.05 - 50.0 mg m-3."
- Level 3 data consists of geophysical parameters binned to a 9x9 km (81 km2) global, equal-area grid at daily, 8-day, monthly, and annual intervals. The Level 3 geophysical parameters consist of five normalized water-leaving radiances (radiance data corrected for atmospheric light scattering and sun angles differing from nadir), and seven geophysical parameters derived from the radiance data. The following table lists the 12 SeaWiFS Level 3 geophysical parameters.
Normalized water-leaving radiances at:
Chlorophyll a concentration
Diffuse attenuation coefficient at 490nm, K(490)
Chlorophyll a / K(490) (integral chlorophyll)
Epsilon of the aerosol correction at 765 and 865 nm
Aerosol optical thickness at 865 nm
Angstrom coefficient, 510-865 nm
In addition to the Level 3 binned data, Standard Mapped Image (SMI) products are created for five Level 3 binned data products. The SMI products are image representations of five Level 3 geophysical parameters: chlorophyll a concentration, Angstrom coefficient 510-865 nm, normalized water-leaving radiance at 555 nm, aerosol optical thickness at 865 nm, and K(490). Each SMI product corresponds to the equivalent Level 3 binned data file, i.e., a daily SMI product represents the data from the Level 3 binned data file from the same day. The HDF data object containing the geophysical parameter data is a byte-valued, two-dimensional array for an Equidistant Cylindrical projection of the globe.
[ See Glossary of Terms for definitions. ]
- Technical aspects of visible wavelength remote sensing of the ocean surface are discussed in this document.
- The primary precursor dataset to the SeaWiFS dataset is the eight-year archive collected by the CZCS. Other ocean color datasets are from the Ocean Color and Temperature Scanner (OCTS) and the Modular Optoelectronic Scanner (MOS). The Moderate Resolution Imaging Spectroradiometer (MODIS) is slated to begin operations in 1998, and several additional ocean color sensors are slated for launch in the period 1997-2002. Data from the Advanced Very High Resolution Radiometer (AVHRR), primarily used to observe sea surface temperature but also employed to observe turbid water masses, can be correlated with ocean color data. Sea surface wind datasets (derived either from remote sensing, meteorological instruments, or meteorological observations) can also be used in concert with ocean color data.
- Dr. Charles McClain, Project Scientist
- Goddard Space Flight Center, Code 970.2
- Greenbelt, MD 20771
- (301) 286-5377
- Email: firstname.lastname@example.org
- Dr. Wayne Esaias, MODIS Oceans Team Leader
- Goddard Space Flight Center, Code 971
- Greenbelt, MD 20771
- (301) 614-5709
- Email: email@example.com
- Dr. Stanford Hooker, Field Program Manager
- Goddard Space Flight Center, Code 971
- Greenbelt, MD 20771
- (301) 286-9503
- Email: firstname.lastname@example.org
- Dr. Gene Feldman, Data System Manager
- Goddard Space Flight Center, Code 610.2.3
- Greenbelt, MD 20771
- (301) 286-9428
- Email: email@example.com
- Dr. Gene Feldman
- Goddard Space Flight Center, Code 610.2
- Greenbelt, MD20771
- (301) 286-9428
- email: firstname.lastname@example.org
- Frederick Patt
- SeaWiFS Project, Code 970.2
- Goddard Space Flight Center
- Greenbelt, MD 20771
- (301) 286-2866
- Email: Frederick.S.Patt.email@example.com
- Sea-Viewing Wide Field-of-view Sensor (SeaWiFS) Project
- See above
- SeaWiFS data is primarily used to determine concentrations of chlorophyll in the oceanic water column. These values may be used to derive phytoplankton concentrations and oceanic primary productivity. The ocean optical data from SeaWiFS can also be used to determine light attenuation in the oceanic water column, which provides information on suspended sediment concentrations and other parameters. Ocean color distribution can be used to investigate the forces influencing trophic productivity in the world's oceans.
- Ocean color remote sensing is based on the principle that particulate and dissolved substances suspended in water will interact with incident light. Where concentrations of particulate matter and dissolved substances are low, conditions typical for the open ocean, water molecules scatter light similar to the way that the atmosphere scatters light, producing a characteristic deep blue color. The scattering of light by particulates and the absorption of light by dissolved substances will alter this color. Chlorophyll, the photosynthetic pigment found in phytoplankton, absorbs strongly in the red and blue regions of the visible light spectrum and reflects in the green. As the concentration of phytoplankton increases, the color of the water will therefore appear increasingly green. The absorption of light by chlorophyll can be quantified to determine the concentration of chlorophyll in water, allowing estimation of phytoplankton abundance in a given area.
The relationship between light absorption and chlorophyll concentration may be complicated by the presence of light-scattering inorganic particulate matter in the water. Particulate matter concentrations generally increase in coastal regions, such that the water color near the coast trends from green to brown or reddish-brown. Even though chlorophyll may be present in higher concentrations near the coast, the presence of particulate matter makes it more difficult to extract the amount of light absorption due soley to chlorophyll. In addition, certain classes of phytoplankton form hard mineral shells that scatter light very effectively, such that the water color can appear shade of aquamarine or milky white.
SeaWiFS measures light intensity in several bands. The measurements allow quantification of light absorption and subsequent estimation of chlorophyll and suspended matter concentrations. SeaWiFS improves on the CZCS mission by having better bands for atmospheric correction (i.e., removing the effect of light scattering by the Earth's atmosphere), which will particularly aid the estimation of chlorophyll and suspended matter in coastal regions.
- The primary optics of SeaWiFS consist of an off-axis folded telescope and a rotating half-angle mirror. Radiation backscattered by the Earth's surface and atmosphere is collected by the telescope and reflected onto the mirror, and the beam path is then directed through beam splitters (dichroics, which transmit some wavelengths and reflect the rest) to separate the radiation into four wavelength regions. Spectral bandpass filters are used to narrow these regions to the 20 nm requirements of the eight SeaWiFS spectral bands, and the radiation then falls on silicon detector elements. The electronics module amplifies the detector signal, performs analog-to-digital conversion and time delay and integration for data transmission. Instrument calibration utilizes an on-board solar radiation diffuser and lunar observation. The instrument may be tilted forward or backward 20 degrees along the spacecraft orbital trajectory to minimize the effects of sun glint.
- The OrbView-2 satellite (formerly called "SeaStar") orbits in a sun-synchronous, descending node orbit at an altitude of 705 km. The orbital period is 98.9 minutes, with an inclination of 98.217 degrees. Local time of descending node is 12:05 PM + 15 minutes. The satellite was launched on August 1, 1997 into a 305 km orbit, and 32 orbit-raising burns performed over the next month raised the orbit to its final altitude.
The satellite has a three-axis stabilized system consisting of orthogonal magnetic torque rods for roll and yaw control and two momentum wheels for pitch stabilization. The satellite is equipped with sun sensors, horizon sensors, and magnetometers.
The propulsion system consists of two subsystems, a reaction control system and a hydrazine propulsion system. The reaction control system uses nitrogen and provides third stage stabilization during the launch. The hydrazine propulsion system is used for raising the orbit from the nominal 278 km parking orbit to the 705 km sun-synchronous operational orbit. In addition, it is used for orbit trim requirements over the life of the mission. The spacecraft employs four Hamilton Standard one pound thrusters.
Redundant global positioning system (GPS) receivers are used for orbit determination, an essential component of satellite and data navigation (Earth location). The orbit state derived from GPS is included in the spacecraft health telemetry.
Two telemetry streams are transmitted. The first is real-time LAC data merged with spacecraft health and instrument telemetry at 665.4 kbps. This is transmitted at L-band with a frequency of 1702.56 MHz. The other telemetry stream consists of stored GAC and selected LAC, along with spacecraft health and instrument telemetry, at 2.0 Mbps. This is transmitted at S-band with a frequency of 2272.5 MHz. The command system uses S-band with an uplink of 19.2 kbaud at 2092.59 MHz.
- The primary mission objective of SeaWiFS and the Orbview-2 satellite is to obtain a continuous five-year record of ocean radiance observations.
Nominal operating parameters for SeaWiFS:
Scan Width 58.3 deg (LAC); 45.0 deg (GAC)
Scan Coverage 2,800 km (LAC); 1,500 km (GAC)
Pixels along Scan 1,285 (LAC); 248 (GAC)
Nadir Resolution 1.13 km (LAC); 4.5 km (GAC)
Scan Period 0.167 seconds
Tilt -20, 0, +20 deg
Digitization 10 bits
- Remote sensing instruments measure electromagnetic energy that is either reflected or emitted from objects and surfaces. This measurement technique can be termed either radiometry or photometry, depending on the wavelength range of the energy being measured. Radiometry refers to measurement of electromagnetic radiation, ranging from X-rays to radio waves. Photometry refers specifically to measurement of energy in the human optical wavelength range. The terms "spectral radiometry" or "spectral photometry" refer to measurements of energy defined per unit of wavelength.
- Refer to the schematic diagram.
- Santa Barbara Research Center (SBRC)
Due to the stringent radiometric objectives of the SeaWiFS Project, SeaWiFS underwent an extensive prelaunch calibration program. Calibration was performed at Hughes SBRC, and included an open air observation of the Sun for solar calibration purposes. (The preflight solar calibration is described in Chapter 3 of Volume 19 in the SeaWiFS Technical Report Series, NASA Technical Memorandum 104566.) The prelaunch characteristics of SeaWiFS were analyzed in detail to provide a comprehensive understanding of the sensor's radiometric response. SBRC employed a 100cm Spherical Integrating Source (SIS) which with a spectral shape equivalent to a 2,850 K blackbody for calibration purposes. Despite the approximate three-year hiatus between instrument completion and spacecraft integration, the calibration of the instrument was essentially unchanged over that time.
For further information, see Volume 22 of the SeaWiFS Prelaunch Technical Report Series, "Prelaunch Acceptance Report for the SeaWiFS Radiometer".
IFOV, nadir, zero tilt: 1 - 1.21 km
Fore and aft pointing: 0, +20, -20 deg, 40 degree tilt change within 30s
Band tolerances: Band edges +/- 2 nm, stable to less than 1 nm
Out-of-band response: Less than 5% of within-band value for 100% reflectance
Band co-registration: Co-registration within 0.3 pixel
Sensitivity: SNRs to exceed: Band 1, 499; Band 2, 674; Band 3, 667; Band 4, 640; Band 5, 596; Band 6, 442; Band 7, 455; Band 8, 467.
Absolute radiometric accuracy: 5%
- SeaWiFS does not carry calibration lamps, but will rely on views of a solar radiation diffuser and the moon for radiometric calibration. The solar radiation diffuser is viewed once per orbit (near the southern terminator) to monitor sensor calibration over several orbits. The moon is viewed via a spacecraft maneuver to monitor calibration over months or years. Lunar views take place when the lunar phase is few days prior to or past full moon.
- Data validation is accomplished by comparing data from the sensor to ocean optical data obtained during a series of calibration cruises, both prelaunch (commencing in 1992) and soon after the launch of the satellite. The data validation process also utilizes data from the Marine Optical Buoy (MOBY) moored off of the island of Lanai, Hawaii. MOBY is a moored in- and above-water optical radiometer that can transmit data to a receiving station on Lanai, allowing frequent comparison to data from SeaWiFS.
- Telemetry from the instrument is either transmitted directly to ground High Resolution Picture Transmission (HRPT) stations or recorded on the instrument for later transmission during downlink sessions to GSFC. Direct broadcast data is 1 km resolution LAC data. Recorded data is either 1 km resolution LAC data (primarily for calibration and validation) and 4.5 km resolution GAC data. The GAC data is used for the production of the global data set. Data is placed on disk and then processed to Level 1A, Level 2, and Level 3 products by the SeaWiFS Project. The data are then transmitted to the Goddard DAAC for archive and distribution. (HRPT data are processed to Level 1A by the receiving station, then sent to the SeaWiFS Project and subsequently to the DAAC.)
- SeaWiFS data consists of ocean radiances in 8 spectral bands and derived geophysical products.
- Spatial coverage is global, with full GAC coverage (approximately 90% of the ocean surface) every two days. Cloud cover prevents viewing of the entire ocean surface on this time scale, so clouds are therefore apparent in a daily Level 3 product, but in the 8-day and monthly binned products the influence of clouds is significantly reduced. Coverage along the equator is slightly degraded due to instrument tilt to avoid sun glint effects.
SeaWiFS Level 3 Daily Browse Image
SeaWiFS Level 3 Weekly Browse Image
- Level 3 Binned Geophysical Products:
Global, equal-area 9x9 km grid (81 km2)
Level 3 Standard Mapped Image Products:
Approximately 1° x 1° on an Equidistant Cylindrical map projection
Level 3 Browse Product:
Approximately 78 km at the equator (the browse image is subsampled by a factor of 8 from the chlorophyll a SMI product).
- The binned product is mapped to an global equal-area grid. (See below.) The SMI and browse products are mapped to an Equidistant Cylindrical Projection of the globe.
- Level 3 binned data are stored in a representation of a global, equal-area grid whose grid cells, or "bins", are approximately 9x9 km.
- Full-time operation of SeaWiFS began on September 18, 1997, such that the first complete daily Level 3 product is on September 19, 1997. Daily Level 3 products prior to September 18 have partial global coverage. Full-time operation of SeaWiFS obtains approximately14.5 orbital swaths of data per day.
- Level 3 Binned data products: daily, 8-day, monthly, annually
Level 3 Standard Mapped Image products: daily, 8-day, monthly, annually
- SeaWiFS Level 3 data products are derived from the Level 2 GAC product, which in turn is derived from the Level 1A GAC data. The SeaWiFS Level 1A and Level 2 Guide Document describes Level 1A and Level 2 SeaWiFS data in detail. A brief summary is provided below.
- SeaWiFS measures visible wavelength radiances for eight 20nm wide bands. The bands are centered on the wavelengths given in the following table, along with the primary use of data for that band.
Band Center Wavelength Primary Use
1 412 (violet) Gelbstoffe
2 443 (blue) Chlorophyll absorption
3 490 (blue-green) Pigment absorption (Case 2), K(490)
4 510 (blue-green) Chlorophyll absorption
5 555 (green) Pigments, optical properties, sediments
6 670 (red) Atmospheric correction (CZCS heritage)
7 765 (near IR) Atmospheric correction, aerosol radiance
8 865 (near IR) Atmospheric correction, aerosol radiance
- SeaWiFS Level 3 data consist of the following geophysical products. The units of measurement for the geophysical products are included in this table.
|SeaWiFS Level 3 Geophysical Products
radiance @412 nm
radiance @443 nm
radiance @490 nm
radiance @510 nm
radiance @555 nm
radiance @670 nm
||Angstrom coefficient, 510-865 nm
||Chlorophyll a concentration
||Diffuse attenuation coefficient
at 490 nm
[chlorophyll a divided by K(490)]
||Epsilon of the aerosol correction
at 765 and 865 nm
||Aerosol optical thickness
at 865 nm
- Goddard Space Flight Center Distributed Active Archive Center (DAAC)
- SeaWiFS is planned as a five-year mission. The current data begins on September 18, 1997 and continues to present.
- The browse images provided in this dataset document provide a visual representation of SeaWiFS Level 3 data.
- Level 3 Binned data products: 13 files (1 main and 12 subordinate files)
Level 3 Standard Mapped Image products: 1 file (for a single parameter)
For the creation of a Level 3 binned data product, every valid measurement of water-leaving radiance which falls within the latitude and longitude boundaries of a given grid square is compiled within that bin. For the daily Level 3 product, the net result is a subsampling of the Level 2 GAC data by a factor of two, as no time binning is required. For the 8-day, monthly, and annual Level 3 products, all of the valid measurements for the given time period and grid square are compiled in the same bin and the weighted mean of all observations is generated. The weight is based on the number of valid pixels used in the binning process. The binning procedure is described in detail in Volume 32 of the SeaWiFS Technical Memorandum Series, "Level-3 SeaWiFS Data Products: Spatial and Temporal Binning Algorithms". (See References.)
- All SeaWiFS data is available in Hierarchical Data Format (HDF), a data format developed by the National Center for Supercomputing Applications (NCSA). HDF is a "self-describing" data format, which means that all of the information necessary to examine the data in an HDF file is contained within the file.
HDF has several different "data models" which are used to store data products. The data models that are used to store data are Scientific Data Sets (SDS), Raster Image Sets, Vgroups, and Vdatas. Global Attributes contain data that is applicable to the entire data file. An entire HDF file may be visualized schematically as a set of objects containing different data variables. Vgroups act as directories to data arrays, and they can contain SDS objects. Vdatas are list objects with data organized into fields within each Vdata, where each field is identified by a unique field name.
HDF Structure of SeaWiFS Level 3 Binned Data
Each SeaWiFS Level 3 binned data file actually consists of a main file and 12 subordinate data files. Each of the 12 subordinate data files contains the data for one of the 12 Level 3 geophysical parameters. Both the main file and the subordinate files are in the Vgroup "Level-3 Binned Data".
There are three Vdatas in the Level 3 main file: SEAGrid, BinIndex, and BinList. Each of the Vdatas contains several fields that describe the geographic binning scheme. The information in the level 3 main file is essentially the metadata in common to all of the geophysical parameters as well as metadata appropriate to Level 3 SeaWiFS data. Each of the 12 geophysical parameters is in the Vgroup "Level-3 Binned Data" and is a separate Vdata. There are two fields in a Vdata for a given geophysical parameter, indicated by the suffix sum or sum_sq. These fields are stored as 4-byte floating point quantities in the subordinate files.
(Refer to the PDF document SeaWiFS OPERATIONAL ARCHIVE PRODUCT SPECIFICATIONS for information on the variables of each of these Vgroups.)
SeaWiFS Level 3 Global Attributes:
Mission and Documentation
L2 Flag Names
Period Start Year
Period Start Day
Period End Year
Period End Day
Percent Data Bins
HDF Structure of SeaWiFS Level 3 Standard Mapped Image Data
The Standard Mapped Image (SMI) products have a much simpler HDF structure than the Level 3 binned data. Each file consists of global attributes, the byte-valued data array corresponding to one of the five geophysical parameters that are made into an SMI product, and a palette array. The "Mission and Documentation" and "Data Time" global attributes do not change from the corresponding binned data file. However, "Data Description" is now called "Scene Coordinates", and the "Data Description" fields are expanded. The scaling information in the "Data Description" category is required to convert the values in the array to actual geophysical parameter values. The scaling equations for each parameter are found in:
"SeaWiFS Science Algorithm Flow Chart document", Michael Darzi, Publisher: Greenbelt, Md.: [Springfield, Va.: National Aeronautics and Space Administration, Goddard Space Flight Center ; National Technical Information Service, distributor, 1998]
SMI Scene Coordinates (Note: "SW" stands for "southwesternmost". This data element is used to locate the grid squares.)
SW Point Latitude
SW Point Longitude
SMI Data Description
Number of Lines
Number of Columns
File Naming Conventions for SeaWiFS Level 3 Data
The naming conventions for SeaWiFS Level 3 data describe the parameter, the binning period, and the file type. Even though the file name appears complex, it encapsulates all the necessary information to identify the file.
For a daily Level 3 binned data file generated on January 1, 1998, the file name for one of the 12 subordinate files would appear as follows, showing the Julian date (001) for the file:
The two-digit designation (##) in the suffix ranges from 00 to 11, denoting the geophysical parameter. The DAAC appends the suffix "main" to the designate the Level 3 main file and to distinguish it from the subordinate (geophysical parameter) files. Thus, the corresponding main file name would be:
The following table lists each suffix and the corresponding geophysical parameter.
|Geophysical Parameters and
Corresponding Level 3 File Name Suffix
For the 8-day, monthly, and annual Level 3 binned files, the file name will show the start date and end date of the binning period, and a file type designation. The file type designations are "8D" for the 8-day binned product, "MO" for the monthly product, and "YR" for the annual product. As an example, for an 8-day binned product with a start date of January 1, 1998 and end date of January 7, 1998, the file names would be
For the SMI files, the binning period convention is the same as for the binned products. The SMI products are designated with the suffix "L3m", and the file type convention (DAY, 8D, MO, YR) is also the same for the binned products, as the SMI files correspond to a parent binned product. The five parameters are designated with the file name terminators "CHLO", "A510", "L555", "T865", and "K490" for chlorophyll a concentration, Angstrom coefficient 510-865 nm, normalized water-leaving radiance at 555 nm, aerosol optical depth at 865 nm, and K(490), respectively. As an example, for the chlorophyll a SMI product corresponding to the above binned product, the file name would be
- The process of deriving accurate geophysical values from remote sensing radiance data is conceptually simple yet operationally complex. In principle, the instrument in space detects the intensity of light at various wavelengths of the electromagnetic spectrum. In the case of SeaWiFS, all of the wavelengths it detects are in the narrow segment of the spectrum that is visible to the human eye. The sole function of the instrument and its associated electronics is to quantify the light intensity, translate it into digital form, append data that allows the data to be navigated (i.e., determine the location on Earth from where the light originated), and send it to an Earth-based receiving station.
The remainder of the data analysis takes place on Earth. Algorithms developed on the basis of radiative transfer physics and both oceanographic and meteorological observation are employed to accurately extract the faint signal of backscattered light radiating from the ocean surface from the pervasive influence of scattered light in the atmosphere, an effect that is accentuated by the presence of atmospheric aerosol particles. The CZCS employed the assumption that no light radiated from the ocean surface at 670 nm, and thus all of the light detected was due to Rayleigh scattering from air molecules and aerosol scattering. SeaWiFS improves on this scheme by detecting light at 765 and 865 nm, as a small amount of light may actually radiate from the ocean at 670 nm. Furthermore, the atmospheric correction scheme used by SeaWiFS more accurately reproduces variable atmospheric conditions (Gordon and Wang 1994).
Once the radiance signal has been corrected for atmospheric light scattering, the signal is then corrected for the solar zenith angle to derive normalized water-leaving radiances. Normalized water-leaving radiances are subsequently used in algorithms to produce geophysical values. These algorithms were developed through oceanographic research into the optical characteristics of oceanic surface waters. As the most significant influences on the optical nature of oceanic waters are the presence of chlorophyll in phytoplankton and the presence of suspended particles, the algorithms use the water-leaving radiances to calculate the values of the related geophysical parameters. The geophysical parameters are calculated from the radiance values on a pixel-by-pixel basis, allowing the values to be mapped to Earth coordinates.
Several different methods have been employed to allow an accurate continuous assessment of instrument calibration. These methods were previously described in Section 5. The data analysis utilizes observations of the onboard solar diffuser and of the nearly-full moon for onboard instrument calibration. Data from the Marine Optical Buoy (MOBY) moored off of Lanai, Hawaii, is used to monitor the accuracy of "system calibration", which refers to the interaction of sensor data and scientific data processing to derive geophysical values that approximate reality.
- SeaWiFS Level 0 data is digitized at 10 bits for transmission to ground stations. The primary data elements in Level 0 data are the raw radiance counts for all eight bands, accompanied by spacecraft and instrument telemetry. Processing to Level 1A appends calibration and navigation data to the file, as well as instrument and selected spacecraft telemetry. There are several different forms of Level 1A data: HRPT LAC, recorded LAC (which includes several types of calibration data), and GAC. A single GAC file consists of a swath data recorded from one north-to-south orbital pass, and constitutes one HDF file. A single HRPT file contains all of the scans received by the ground station while the satellite was above the station's receiving horizon. Recorded LAC scans, which are usually recorded for calibration and validation purposes as well as for regions of special research interest, contain the number of scan lines ordered by Mission Operations to cover the designated region.
Processing to Level 2 requires several additional steps. The data is navigated so that land masks may be correctly placed. Ancillary meteorological data and ozone data is used for atmospheric correction. The computational steps described earlier are employed to produce normalized water-leaving radiances and derived geophysical products. Each Level 2 data file is one HDF file, and corresponds exactly in temporal and spatial extent to the parent Level 1A file. Note that only Level 1A GAC data is processed to Level 2. Recorded LAC and HRPT LAC data is not processed to Level 2, but the SeaWiFS Data Analysis System (SeaDAS) which is discussed further in the "Related Software" section, will be capable of processing Level 1A data to Level 2 geophysical products.
The primary operation that is performed to create SeaWiFS Level 3 data products is data binning. Binning is used to reduce the total volume of data, creating reduced resolution files which are more useful for global or basin-scale research. To create a binned data product, all of the valid measurements of water-leaving radiance which fall within the latitude and longitude boundaries of a given grid square are compiled within that bin. Any pixel values that are masked are excluded, with the net result that over longer binning periods, the influence of clouds is selectively eliminated. The daily Level 3 product is only a spatial binning of the Level 2 GAC data by a factor of two, to produce a 9x9 km global data product. No time binning is required for the daily Level 3 product, and thus the cloud mask will still be apparent. For the 8-day, monthly, and annual Level 3 products, all of the valid measurements for the given time period and grid square are compiled in the same bin and the weighted mean of all observations is generated. The weight is based on the number of valid pixels used in the binning process. Because the number of valid water-leaving radiance measurements increases with longer binning intervals, the influence of clouds will be lessened, producing cloud-free images. Note also that the variability of ocean color in a given area will be averaged out over longer time intervals.
- The SeaWiFS Project will periodically reprocess the entire dataset during the course of the mission as the algorithms for the calculation of the geophysical products are refined. The current status of the algorithms may be found in: "SeaWiFS Science Algorithm Flow Chart", Michael Darzi, Publisher: Greenbelt, Md. [Springfield, Va. National Aeronautics and Space Administration, Goddard Space Flight Center, National Technical Information Service, distributor, 1998]. Other processing changes may be required as the calibration of the instrument or the sensor operating environment vary over time.
Summaries of the processing changes which were implemented in each of the SeaWiFS data reprocessings can be accessed on the SeaWiFS Data Reprocessing Web page.
- Numerous effects can lead to anomalous radiance conditions which influence the calculation of normalized water-leaving radiances and derived geophysical parameters. In particular, highly turbid waters, coccolithophore blooms, and sun glint provide anomalously high water-leaving radiances. Clouds and ice are "bright targets" than can influence adjacent pixels. Low sun angles also affect water-leaving radiances.
Flags and masks that are used in SeaWiFS data processing are described in detail in the Level 1A and Level 2 Data Set Guide Document. Flags are set for data pixels that do not pass quality tests, indicating anomalous and possibly erroneous data. The masks in the Level 3 product correspond to the masks in the Level 2 products and correspond to one of five conditions: high Lt, land, clouds or ice, sun glint, or atmospheric correction failure. The field flags_set in the BinList Vdata describes all grid squares for which flags were set in the parent Level 2 data file.
- Similar to other optical remote sensing instruments, SeaWiFS data will be affected by the presence of clouds, particularly on a daily basis. The weekly and monthly binned data products substantially reduce the influence of clouds, but with a concomitant loss of temporal resolution of oceanic features that change through time.
- The binning procedure gives particularly high weight to limited numbers of observations. In areas where cloud cover or sea ice is pervasive, a single valid pixel obtained during the temporal binning period can give a biased indication of the actual parameter value for that region. As more observations are compiled, this variability is reduced. Thus, data from areas known to be affected by considerable cloud cover or sea ice should be interpreted with caution.
- It is anticipated that the data from SeaWiFS will be augmented by data from the Moderate Resolution Imaging Spectroradiometer (MODIS), launched in December 1999 on the Terra platform. Several other countries have ocean color sensors in development that are slated for launch in the period 1998-2002. The Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) project was created for the purpose of intercalibration of ocean color data obtained by different ocean color instruments.
- Goddard DAAC Ocean Color Data Support Team
James Acker, Team Lead
NASA Goddard Space Flight Center
Greenbelt, MD 20771
Goddard DAAC Helpdesk
NASA Goddard Space Flight Center
Greenbelt, MD 20771
- NASA Goddard Space Flight Center DAAC
- SeaWiFS data is obtained from the Goddard DAAC by establishing a SeaWiFS data subscription with the DAAC. Use of the data browser allows researchers to select appropriate data products and selection of data transfer options, either magnetic tape or FTP. The Ocean Color Data Support Team must be contacted at firstname.lastname@example.org to create a data subscription. Data subscriptions automatically retrieve selected data products as they are received from the SeaWiFS Project and prepare them for transfer on magnetic tape (by mail delivery) or by FTP. Currently available tape formats are 8mm EXABYTE tape (8200, 2.5 GB or 8500, 5 GB) or 4mm DAT tape (90m, 1.6 GB).
- The Goddard DAAC is the designated archive and distribution center for all ocean color data obtained by NASA remote sensing missions.
- The SeaWiFS project is a "data buy" mission, for which NASA contracted with Orbital Sciences Corporation to build and launch the satellite. In return, Orbital Sciences Corporation was granted the opportunity to sell data from SeaWiFS for commercial applications. From September 18, 1997 to March 11, 1998, data from SeaWiFS was unrestricted. After March 11, 1998, the data is restricted to SeaWiFS Authorized Research Users solely for scientific research purposes. The data is subject to a two-week distribution embargo for normal research applications. Real-time data access is granted to researchers and selected HRPT stations for specific research needs. After five years, all SeaWiFS data will be unrestricted.
In order to restrict data access to SeaWiFS Authorized Research Users, user names and passwords are issued from the Goddard DAAC to each individual research user after they have provided the necessary documentation to the SeaWiFS Project.
- SeaDAS was specifically developed for the processing and analysis of SeaWiFS HDF data. The following describes the what SeaDAS can do, as well as providing a path to obtain the software.
The SeaDAS Web site, http://seadas.gsfc.nasa.gov, is updated with current operating information for SeaDAS 4.0. System configuration and hardware requirements, and information on how to obtain SeaDAS 4.0, are given below and are also found on the SeaDAS Web site.
The SeaDAS software system was written for the specific purpose of analyzing and processing SeaWiFS HDF data. SeaDAS is a comprehensive image analysis package for all SeaWiFS data products and ancillary data (wind, surface pressure, humidity and ozone) from NMC (National Meteorological Center and TOVS (TIROS Operational Vertical Sounder). All SeaDAS source code is free and available for download via FTP.
The Interactive Data Language (IDL) from Research System Inc. (RSI) is used to build all the GUI and display related programs in SeaDAS. SeaDAS 4.0 is released with a blanket purchase of IDL-Runtime, so users do not have to acquire IDL or IDL-Runtime at their expense. SeaDAS includes the Hierarchical Data Format (HDF) libraries from National Center for Supercomputing Applications (NCSA) which are also required to build certain SeaDAS programs. IDL, C, FORTRAN77, and IMAKE from the vendors are required only if modifications to the source code and user-defined versions of the executables are desired.
Suggested Hardware Requirements:
- Platform: SGI 02, SUN UltraSparc workstations, or PC
- Memory:192 MB (regular users), 384 GB (HRPT users)
- Disk: 9 GB (actual SeaDAS installation requires ~330 MB without demo files and ~950 MB with demo files). Additional disk space is required for storing original data and processed data files.
- Tape Drive: 4MM (DAT) or 8mm Exabyte (for DAAC data)
- Display: 19" Console or X-terminal with 20 MB memory, 1280x1024 resolution, 8-bit, 256 colors
- Operating Systems: SGI: IRIX 6.3, or IRIX 6.5 SUN: Solaris 2.6 or Solaris 2.7
- Required Software: IDL-Runtime, IDL 5.1 or 5.2
- Languages: C (SGI V3.19, SUN V 3.0.1), FORTRAN (SGI V 4.0.2, SUN V 3.0.1), IDL 5.2 or 5.3 (IDL 5.1 may work but has not been re-tested)
- Software Libraries: HDF 4.1r1 (included in SeaDAS)
SeaDAS PC Linux Version
SeaDAS 4.0 for Linux/PC has been developed and tested under the following environment:
"Generic" PC with Pentium II 350 MHz CPU
Redhat Linux 6.0
IDL 5.1.2L for PC, IDL-Runtime
- SGI: IRIX 6.3 and6.5
- Sun: Solaris 2.6 and 2.7
- PC: RedHat Linux 6.0
Obtaining SeaDAS 4.0
SeaDAS is available for download via anonymous FTP from seadas.gsfc.nasa.gov. The /seadas directory contains the following compressed tar files:
- seadas_data1.tar.Z, seadas_data2.tar.Z, seadas_data3.tar.Z: SeaDAS required data files for L1, L2, and L3 processing
- seadas_demo.tar.Z: sample files for testing, and demonstrations
- seadas_irix6.3.tar.gz: SeaDAS for SGI IRIX 6.3 operating system
- seadas_irix6.5.tar.gz: SeaDAS for SGI IRIX 6.5 operating system
- seadas_solaris2.6.tar.gz: SeaDAS for SGI IRIX 6.3 operating system
- seadas_irix2.7.tar.gz: SeaDAS for SGI IRIX 6.3 operating system
- seadas_rhlinux6.0.tar.gz: SeaDAS for PC, RedHat Linux 6.0 operating system
Connect to the SeaDAS ftp site to download program files.
Note, some users, especially outside the U.S. have had trouble with the size of large SeaDAS files. Smaller "split" sections of the files have been created using the UNIX "split -20000" command. The split files can be found in the /seadas/split directory.
To put the split files back together (concatenate the files), use the following commands:
cat seadas_src.tar.Z?? > seadas_src.tar.Z (for source tar file), OR
cat seadas_data.tar.Z?? > seadas_data.tar.Z (for data tar file)
To put the pieces back together, and uncompress them, and unTAR the TAR file in one step:
cat seadas_src.tar.Z?? | zcat - | tar xvf - (for source files) OR
cat seadas_data.tar.Z?? | zcat - | tar xvf - (for data files)
SeaDAS can also be created on 4mm (DAT) or 8mm tape for those users who do not have Internet access or who have substantial difficulty with FTP of these large files. Please send your request to email@example.com.
Other HDF Software
SeaWiFS data has been successfully opened and examined using the Fortner Research prototype HDF Browser, available for download. The Research Systems Inc. software products Transform and Noesys (descriptions cban also be found at the SciSpy Web site) have been used on SeaWiFS data files, and Noesys has been used to transfer SeaWiFS data to the EASI/PACE Geographical Information System (GIS) software package.
Two other software packages, HDF Explorer and Windows Image Manager, have also been used with SeaWiFS data files. Links to the sites where more information can be obtained are below. Windows Image Manager offers the capability of converting SeaWiFS data to many other image formats.
Windows Image Manager
- Aiken, J., G.F. Moore, C.C. Trees, S.B. Hooker, and D.K. Clark, 1995: The SeaWiFS CZCS-Type Pigment Algorithm. SeaWiFS Technical Report Series, Volume 29, NASA Technical Memorandum 104566, S.B. Hooker and E.R. Firestone Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 34 pages.
- Barnes, R.A., W.L. Barnes, W.E. Esaias, and C.R. McClain, 1994: Prelaunch Acceptance Report for the SeaWiFS Radiometer. SeaWiFS Technical Report Series, Volume 22, NASA Technical Memorandum 104566, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 32 pages.
- Biggar, S.F., P.N. Slater, K.J. Thome, A.W. Holmes, and R.A. Barnes, 1994: "Chapter 3: Preflight Solar-Based Calibration of SeaWiFS", IN: McClain, C.R., R.S. Fraser, J.T. McLean, M. Darzi, J.K. Firestone, F.S. Patt, B.D. Schieber, R.H. Woodward, E-n. Yeh, S. Mattoo, S.F. Biggar, P.N. Slater, K.J. Thome, A.W. Holmes, R.A. Barnes, and K.J. Voss, 1994: Case Studies for SeaWiFS Calibration and Validation, Part 2, SeaWiFS Technical Report Series, Volume 19, NASA Technical Memorandum 104566, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland, 25-32.
- Campbell, J.W., J.M. Blaisdell, and M. Darzi, 1995: Level-3 SeaWiFS Data Products: Spatial and Temporal Binning Algorithms, SeaWiFS Technical Report Series, Volume 32, NASA Technical Memorandum 104566, S.B. Hooker, E.R. Firestone, and J.G. Acker, Eds., NASA Goddard Space Flight Center, Greenbelt, Maryland.
- Brown, C.W., and J.A. Yoder, 1994: Coccolithophorid blooms in the global ocean. J. Geophys. Res., 99, 7467-7482.
- Gordon, H.R., and M. Wang, 1994: Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm. Appl. Opt., 33(3), 443-452.
- Morel, A., 1988: Optical modeling of the upper ocean in relation to its biogenous matter content (Case I waters). J. Geophys. Res., 93, 10,749-10,768.
- aerosol: a suspension of fine solid or liquid particles in gas
albedo: reflective power, i.e., the fraction of incident radiation (as light) that is reflected by a surface or body
algorithm: a step-by-step procedure for solving a problem or accomplishing some end especially by a computer
ancillary: supplementary (ancillary data with regard to SeaWiFS refers to data from other sources that is used in data processing)
backscatter: the scattering of radiation or particles in a direction opposite to that of the incident radiation due to reflection from particles of the medium traversed, or the actual radiation due to this process
bathymetry: water depth measurements in a given body of water
bloom: a rapid increase in the population and concentration of phytoplankton
boundary current: large strong surface ocean currents that occur on the margins of ocean basins, usually flowing parallel to a continental coast
chlorophyll: photosynthetic pigment found in plants. Chlorophyll a is a green pigment.
coccolithophore: phytoplankton which creates external microscopic calcium carbonate hard plates (coccoliths)
descending node: the point at which an orbiting body rises through the plane of the ecliptic traveling southward
downlink: a communications channel for receiving transmissions from a spacecraft;
eddy: a feature of ocean circulation where the direction of circulation is circular or elliptical
electromagnetic spectrum: the entire range of wavelengths or frequencies of electromagnetic radiation extending from gamma rays to the longest radio waves and including visible light
euphotic: of, relating to, or constituting the upper layers of a body of water into which sufficient light penetrates to permit growth of green plants
fluvial: related to streams or rivers
gelbstoffe: Dissolved and suspended inorganic matter, commonly found in river discharge, which gives it a yellowish color (from German: "yellow substance")
inclination (orbital): the angle between the orbital plane and the Earth's equatorial plane, as measured in degrees
interpolation: to estimate values of (a function) between two known values
irradiance: the density of radiation incident on a given surface, irrespective of direction
mask: a single data value that indicates the presence of a particular condition
nadir: the point on the Earth directly below an orbiting satellite.
optical thickness: the normalized extinction coefficient due to absorption and scattering by intervening substances or particles in a direct beam of light
period: the time interval required for the completion of one orbit by a satellite
photosynthesis: the process by which chlorophyll-containing cells in 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: free-floating photosynthetic organisms existing in aquatic environments
primary productivity: the rate at which organic carbon is produced photosynthetically.
radiance: electromagnetic energy per unit time, area, solid area and spectral band, i.e., electromagnetic energy radiating in a given direction
radiometer: a device that detects and measures electromagnetic radiation in discrete spectral bands of the electromagnetic spectrum.
resolution: In a spatial sense, the size of the smallest feature recognizable using the detector.
spectral band: a narrow range of the electromagnetic spectrum.
sun glint: sunlight that is directly reflected from the water surface back to the observer or detector
terminator: the dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk
turbidity: substances or particles that obscure light transmission
visible light: Electromagnetic radiation with wavelength in the 390 to 770 nm range.
zenith: the "sky" point located directly above an Earth-based sensor.
AVHRR Advanced Very High Resolution Radiometer
CZCS Coastal Zone Color Scanner
DAAC Distributed Active Archive Center
FTP File Transfer Protocol
GAC Global Area Coverage
HDF Hierarchical Data Format
HRPT High Resolution Picture Transmission
LAC Local Area Coverage
MOBY Marine Optical Buoy
MODIS Moderate Resolution Imaging Spectroradiometer
MOS Modular Optoelectronic Scanner
NASA National Aeronautics and Space Administration
NCSA National Center for Supercomputing Applications
OCTS Ocean Color and Temperature Scanner
SBRC Santa Barbara Research Center
SDS Scientific Data Sets
SIMBIOS Sensor Intercomparison and Merger for Biological and
Interdisciplinary Oceanic Studies
SIS Spherical Integrating Source
SeaDAS SeaWiFS Data Analysis System
SeaWiFS Sea-viewing Wide Field-of-view Sensor
- James Acker, firstname.lastname@example.org
- Version 2.0
- Version baselined on addition to the GES Controlled Documents List, January 25, 1999.
- Version 2.0
- Version 2.0 editing completed June 22, 2000. Primary changes concerned new geophysical data products