Nominal operating parameters for SeaWiFS:
Nominal radiometric parameters for SeaWiFS:
Bands 1-6 have 20 nm bandwidth; bands 7 and 8 have 40 nm bandwidth.
Notes: Gelbstoffe (German for "yellow substance") describes amorphous, high molecular weight organic matter with a somewhat polymeric nature. It absorbs strongly in the blue region of the spectrum.
The term "Case 1" (and also "Case 2") refers to a water "type" defined by optical characteristics. Case 1 water is clear, open-ocean water, and Case 2 is generally coastal, higher productivity, turbid water.
K(490) is the diffuse attenuation coefficient at 490 nm, a measure of optical clarity.
Brief instrument overview
A brief description of ocean color measurements
In the ocean, light reflects off particulate matter suspended in the water, and absorption is primarily due to the photosynthetic pigments (chlorophyll) present in phytoplankton. The net result of these optical interactions is light radiating from the ocean surface, the "water-leaving radiance". Radiometers are instruments that measure the radiance intensity at a given wavelength of light. The measured radiance may then be quantitatively related to various constituents in the water column that interact with visible light, such as chlorophyll. The concentration of chlorophyll, in turn, may be used to calculate the amount of carbon being produced by photosynthesis, which is termed primary productivity.
SeaWiFS is a spectroradiometer, which means that it measures radiance in specific bands of the visible light spectrum. The advantage of observing the oceans with a space-based spectroradiometer is the global coverage that a satellite provides. The disadvantage is that interfering optical effects, primarily light scattering in the atmosphere, must be accounted for to provide an accurate measurement of the water-leaving radiance.
For a more detailed description of how ocean color measurements are accomplished, refer to the SeaWiFS Technical Report Series Volumes 5 and 25, "Ocean Optics Protocols for SeaWiFS Validation" and "Ocean Optics Protocols for SeaWiFS Validation, Revision 1", or read the online discussion at:
SeaWiFS Mission Data Collection Strategy
SeaWiFS acquires data at a nadir resolution of 1 km per pixel (LAC resolution). This data is broadcast continuously and can be recorded by any HRPT station within range. The data is subsampled at 4 km resolution (GAC resolution), which is recorded on board the satellite for downlink twice a day. A limited amount of LAC resolution data is also recorded on board the satellite for regions of special interest. GAC recording and HRPT direct broadcast are both scheduled for the full 40 minutes per orbit.
SeaWiFS data at the Goddard Distributed Active Archive Center (DAAC) are available in HDF, the data format used by the SeaWiFS Project and the Earth Observing System (EOS). HDF is a self-describing, platform independent format. Tools for analysis of this data are described under "Data Access Information" below. The use of HDF allows a large amount of metadata, including calibration, navigation, mission information, and data quality indicators, to be included with each data file. A complete description of the contents of SeaWiFS HDF files may be found in the PostScript document "SeaWiFS Archive Product Specifications", available at:
(The DAAC will have a copy of this document in June.)
There are two distinct types of SeaWiFS data, LAC and GAC. GAC data is the global SeaWiFS data product, obtained at 4 km resolution over most of the world's oceans. LAC data is at 1 km resolution, and is available for specific regions from High Resolution Picture Transmission (HRPT) ground stations, or for sites of interest designated by the SeaWiFS Project. The latter category utilizes the limited on-board memory designated for LAC data. The satellite continuously broadcasts HRPT LAC data, which can be received by any SeaWiFS ground stations within receiving range.
SeaWiFS data is archived according to the standard remote sensing definitions of Level 1a, Level 2, and Level 3 data. Level 1a data consists of raw radiances measured at the satellite, and also includes calibration and navigational data along with selected instrument and spacecraft telemetry. Level 2 data consists of derived geophysical parameters produced using the Level 1a radiances as input data. Level 2 parameters are only produced for GAC data. Level 3 data is global gridded data that has been statistically collected into daily, weekly, monthly, or annual grid cells, corresponding either to 9 x 9 km equal area grid squares (binned product) or 0.09 x 0.09 degree squares (standard mapped image product). The exact contents of each data level are given below.
In addition to the SeaWiFS data described below, ancillary ozone and meteorological data that are utilized in geophysical product algorithms are also available from the DAAC. These products are also in HDF.
SeaWiFS File Naming Convention
S represents SeaWiFS, and the subsequent digits represent the Greenwich Mean Time (GMT) year, day of the year (Julian calendar), and hour, minutes and seconds of the start of the first scan line.
The suffix describes the actual data type:
Level 1A data:
L1A_GAC Level 1a GAC data L1A_LAC Level 1a LAC data L1A_SOL Solar calibration data L1A_LUN Lunar calibration data L1A_TDI Time delay and integration (TDI) check L1A_IGC Intergain calibration check L1A_BRS Level 1a browse data L1A_Hxxx HRPT data, where xxx is a three-letter code for a particular HRPT station. HWFF is for Wallops Flight Facility.
Level 2 data:
L2_GAC Level 2 GAC data L2_BRS Level 2 Browse data
Level 3 data, binned product:
L3b_DAY.main Binned product main file L3b_DAY.xff Binned product subordinate file (one of 11 geophysical parameters)
All of these files also have 11 subordinate files, with the same naming convention as above:
L3b_8D 8-day binned product L3b_MO Monthly binned product L3b_YR Annual binned product
Level 3 data, standard mapped image product:
L3m_DAY_CHLO Daily chlorophyll a L3m_DAY_A510 Angstrom coefficient, 510-865 nm L3m_DAY_L555 Daily normalized water leaving radiance at 555 nm L3m_DAY_T865 Aerosol optical thickness (tau) at 865 nm L3m_DAY_K490 Daily K(490)
There are also SMI files corresponding to the 8-day, monthly, and yearly binned products. The suffixes 8D, MO, and YR are inserted in place of DAY in the format shown above.
Level 3 browse products:
L3_BRS_DAY Daily browse product L3_BRS_8D 8-day browse product L3_BRS_MO Monthly browse product L3_BRS_YR Annual browse product
Level 1A LAC (HRPT) data:
Level 1A GAC data:
Level 2 GAC data:
Notes: The CZCS-like pigment concentration parameter uses SeaWiFS bands to generate a product similar to the pigment concentrations derived from CZCS data, for the purpose of intercomparison with the CZCS data archive.
The epsilon of the aerosol correction is an atmospheric correction parameter representing the ratio of the aerosol reflectances at 765 and 865 nm. CZCS estimated aerosol reflectance using a single-scattering model; SeaWiFS accounts for multiple scattering effects with several different aerosol models.
Level 3 Binned Data Products:
* There is one change: the ratio of chlorophyll a concentration to K(490) is an additional binned product. This product is also called "integral chlorophyll".
** The Level 3 Binned data products consist of 1 main and 12 subordinate HDF files. Each subordinate HDF file corresponds to one SeaWiFS binned geophysical data product. The main file contains the metadata describing the geophysical data in each of the subordinate files.
Level 3 Standard Mapped Image (SMI) Products:
All of the data types described above can be accessed and ordered using the Goddard DAAC's SeaWiFS Data Browser:
For Level 1A HRPT LAC, Level 1A GAC, and Level 2 GAC data, the browser allows users to specify spatial and temporal search criteria. Spatial criteria may be entered using either an interactive map or by entering numerical latitude and longitude values. Temporal search ranges are entered by year, month, and day. Level 1A LAC data from individual HRPT stations and Level 3 binned and SMI data are accessed in a similar fashion through the browser, though only temporal searches are required for these data categories. Note that HRPT data availability from the DAAC is dependent on data exchange agreements and data transfer arrangements that are negotiated by the SeaWiFS Project with HRPT station operators.
After the browser has identified data files meeting the specified search criteria, users may examine browse images before ordering individual data files. Data may be obtained by File Transfer Protocol (FTP), or on tape in either 4mm or 8mm format via mail delivery from the DAAC. Users with slow or uncertain network links to the DAAC should consider acquiring the data on tape, which also applies to users who are requesting large volumes of data.
Authorized User Status
Reading data on tape/unpacking transferred data:
To read a "tar.Z" format tape on a computer with a UNIX operating system:
First uncompress the file by typing "uncompress <filename>.tar.Z".
When the uncompression is finished, type the command:
The fields in < > are system specific and may specify a device, such as a tape drive, or a file directory. The specific parameters depend on your local workstation configuration (e.g., this will be "/dev/8mm1nr" if you read the tape off the 8mm1 tape mdrive on the DAAC computer with the "no rewind" option). 127 is the blocking factor.
To read a "tar" format file received by FTP, use the command:
To read a "dd" format tape on a computer with a UNIX operating system, use the command:
where if=<dev> specifies the tape drive with "no rewind" option (e.g., if=/dev/8mm1nr for the DAAC computer.)
To read a tar.Z file on a PC or Macintosh computer:
Reading the file will require an application program capable of uncompressing and untarring the file. WinZip is an application that works on the Windows 95, 98 and NT operating systems; free versions are available for download on the World Wide Web. Similar applications for Macintosh (such as StuffIt Expander and tar) are capable of performing the same operations. WinZip recognizes the UNIX compression and tar format and extracts the file in uncompressed format.
A bit about HDF:
Additional explanation of HDF can be found at the HDF Web site:
HDF provides several different "data models" which can be used to store data products. The data models currently provided by HDF include Scientific Data Sets (SDS), 8-bit and 24-bit Raster Image Sets (RIS), Vdatas, and Vgroups. An SDS is a multi-dimensional array, and a Vdata is a binary table. In addition to the data models, HDF allows the inclusion of metadata with each data file. Metadata is referred to as Global Attributes, and includes such information as the mission and sensor characteristics, when and how the data was processed, the downlink station where the data was received, and similar. Along with that information, the Global Attributes also describe the start and end times of a data file, geographic location, and data quality.
The Goddard DAAC also has a discussion of HDF, HDF utilities, and links to several different software packages.
A site for information on HDF, featuring the HDF libraries for PC and Macintosh, HDF-capable software, and links to user groups, is found at:
Processing/Analysis Software Packages:
Note: The use of SeaDAS requires IDL or IDL-Runtime. 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. Additional programming using SeaDAS source code will require full IDL, which must be purchased separately.
The minimum hardware requirements for SeaDAS are an SGI Indigo2 or SUN Sparc 10. Recommended minimum system requirements are:
Memory: 192 MB (regular users), 384 MB (HRPT users)
The SeaDAS software requirements:
SeaDAS PC Linux version:
Gateway 2000 PC with Pentium II 300 MHz CPU
SeaDAS is available for download via anonymous FTP from:
Fortner Software's HDF Browser and Transform have been used to display simulated SeaWiFS HDF data files on a Pentium PC running Windows95. The browser allows a user to inspect the contents of all SDS and Vgroups in the HDF file, and will also display a raster image if it is available. The browser is a free utility that can be obtained at the URL http://www.rsinc.com/NOeSYS/hdf.cfm. In addition to the Browser, the HDF libraries (PC and Macintosh versions) are also available for download at the above URL. Fortner (now a subsidiary of Research Systems, Inc., authors of IDL) has additional software packages that can manipulate HDF files.
Fortner Software, LLC
100 Carpenter Drive Sterling, VA 20164 Phone: 703-478-0181 FAX: 703-689-9593 Email: SciSpyinfo@scispy.com
Research Systems Inc.
4990 Pearl East Circle Boulder, CO 80301 Phone:303-786-9900 FAX:303-786-9909 Email: firstname.lastname@example.org
Windows Image Manager also works with SeaWiFS HDF data and allows conversion to other formats:
HDF Explorer is a low-cost package for Windows PCs.
The following sites have information regarding SeaWiFS and SeaWiFS data:
Web Site: http://disc.sci.gsfc.nasa.gov/
User Services Office Goddard Distributed Active Archive Center NASA Goddard Space Flight Center, Code 610.2 Greenbelt, MD 20771 USA
Email: email@example.com Phone: 301-614-5224 Fax: 301-614-5268
SeaWiFS Project and SIMBIOS Project:
Dr. Charles McClain, SeaWiFS Project Scientist, Code 970.2, Goddard Space Flight Center Greenbelt, MD 20771 USA
Data production and processing:
Dr. Gene Feldman, SeaWiFS Project, Code 970.2, Goddard Space Flight Center, Greenbelt, MD 20771 USA
Science, algorithms, and data product elements:
Fred Patt, SeaWiFS Project, Code 970.2, Goddard Space Flight Center, Greenbelt, MD 20771 USA
The SeaDAS Development Group, Code 970.2, Goddard Space Flight Center, Greenbelt, MD 20771 USA