Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Readme

Production and distribution of this ocean color data set are funded by NASA's Mission To Planet Earth (MTPE) Program. The data are not copyrighted; however, the data may only be used for the purposes of scientific research by Authorized Users for five years following the launch of SeaWiFS (see Data Access Information). We request that when you publish data or research results utilizing these data, please acknowledge as follows: "The authors would like to thank the SeaWiFS Project (Code 970.2) and the Distributed Active Archive Center (Code 610.2) at the Goddard Space Flight Center, Greenbelt, MD 20771, for the production and distribution of these data, respectively. These activities are sponsored by NASA's Mission to Planet Earth Program." The use of trade names in this document does not imply endorsement by the U.S. Government, NASA, or the Goddard DAAC. The DAAC does not provide support for commercial software products or packages.

Data Set Information

This data set consists of satellite measurements of global and regional ocean color data obtained by the Sea-viewing Wide Field-of-view Sensor, 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.

The first instrument to collect scientific data on the color of the ocean was the Coastal Zone Color Scanner (CZCS), an instrument on the NIMBUS-7 satellite, which operated from November 1978 to June 1986. The operational parameters of the SeaWiFS mission were based on the heritage of the CZCS mission and were designed to improve the acquisition and accuracy of ocean color data for global and regional study of ocean biology and related physical oceanographic phenomena.

The temporal range of the data set begins in September 1997, with a nominal mission duration of five years. All of the data products from SeaWiFS are stored in the Hierarchical Data Format (HDF), which was developed by the National Center for Supercomputing Applications (NCSA) at the University of Illinois.

The SeaWiFS mission is a public-private partnership between NASA and Orbimage Inc. For that reason, access to the data during the mission is restricted to Authorized Users who will use the data for scientific purposes. To become an Authorized User, an individual investigator must submit a "Dear Colleague" letter to the SeaWiFS Project stating the purpose of research utilizing SeaWiFS data.

Satellite and instrument information

SeaWiFS was launched August 1, 1997 by a Pegasus XL launch vehicle. Data acquisition commenced on September 4, 1997. SeaWiFS acquires approximately 15 pole-to-pole orbital swaths of data per day, and approximately 90% of the ocean surface is scanned every two days.

Nominal orbit parameters for the OrbView-2 satellite:

Orbit	Sun-synchronous
Nominal altitude	705 km
Equator Crossing	Noon +/- 20 min., descending node
Inclination	98 deg 12 min
Orbital Period	98.9 min

Nominal operating parameters for SeaWiFS:
(LAC stands for Local Area Coverage; GAC stands for Global Area Coverage)

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

Nominal radiometric parameters for SeaWiFS:

Band	Center Wavelength (nm)	Primary Use
1	412 (violet)	Dissolved organic matter (incl. 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

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
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.

A brief description of ocean color measurements
When visible light from the Sun illuminates the ocean surface, it is subject to several optical effects. Foremost among these effects are light reflection and absorption. Reflection beneath the water surface is generally inefficient, returning only a small percentage of the light intensity falling on the ocean surface. Absorption selectively removes some wavelengths of light while allowing the transmission of other wavelengths.

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:

http://disc.sci.gsfc.nasa.gov/OCDST/classic_scenes/11_classics_radiation.html

SeaWiFS Mission Data Collection Strategy
SeaWiFS bands can only view the sunlit Earth. The useful data collection range is limited to solar zenith angles less than 75 degrees, which corresponds to 40 minutes per orbit.

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 onboard the satellite for downlink twice a day. A limited amount of LAC resolution data is also recorded onboard the satellite for regions of special interest. GAC recording and HRPT direct broadcast are both scheduled for the full 40 minutes per orbit.

Data Set Organization

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:

http://oceancolor.gsfc.nasa.gov/DOCS/ocformats.html

(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
A typical SeaWiFS file name is in the following format:

Syyyydddhhmmss.<suffix>

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

Data Description
[ The file sizes given are uncompressed volume. Data files are shipped in compressed form. ]

Level 1A LAC (HRPT) data:

File contents:	Raw radiance counts for eight SeaWiFS bands; calibration and navigation data; instrument and spacecraft telemetry.
Resolution:	1.13 km
Data granule:	One downlink session during satellite overpass, corresponding to the area viewed by the satellite when it is above the station's acquisition horizon boundaries.
Granule size:	58-70 MB
Browse product:	Level 1A pseudo-true color image from band 1,5, and 6

Level 1A GAC data:

File contents:	Subsampled raw radiance counts for eight SeaWiFS bands; calibration and navigation data; instrument and spacecraft telemetry.
Resolution:	4.5 km
Data granule:	One global (north-to-south) orbital swath
Granule size:	19.1 MB
Browse product:	Level 1A pseudo-true color image from bands 1,5 and 6

Level 2 GAC data:

File contents:	Derived geophysical values, corresponding to parent Level 1A data file.
Resolution:	4.5 km
Data granule:	One global (north-to-south) orbital swath
Granule size:	21.9 MB
Browse product:	Chlorophyll a concentration
SeaWiFS geophysical data values (11):	Normalized water-leaving radiances at 412, 443, 490, 510, 555, and 670 nm Chlorophyll a concentration K(490) Angstrom coefficient, 510-865 nm Epsilon of aerosol correction at 765 and 865 nm Aerosol optical thickness at 865 nm

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:

File contents:	Binned geophysical parameters, corresponding to Level 2 GAC data values*
Resolution:	9 km
Data granule**:	12 global, equal-area grids, 1 corresponding metadata file
Granule size:	Daily, 55 MB; Weekly, Monthly & Annual, 250 MB
Browse Product:	Chlorophyll a Standard Mapped Image (SMI)

* 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:

File contents:	Image representation of binned data products
Resolution:	9 km
Data granule:	One global image
Granule size:	8 MB
SMI products:	Chlorophyll a concentration Angstrom coefficient, 510-865 nm Normalized water leaving radiance at 555 nm Aerosol optical thickness at 865 nm Diffuse attenuation coefficient at 490 nm

Data Access Information

All of the data types described above can be accessed and ordered using the Goddard DAAC's SeaWiFS Data Browser:

http://disc.sci.gsfc.nasa.gov/data/dataset/SEAWIFS/

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
All SeaWiFS data acquired from the start of instrument operations through March 11, 1998, is free and available to the public without restrictions on usage or application. Data acquired after March 11, 1998, is ONLY available for scientific research to Authorized Users who have registered with the SeaWiFS Project. Any individual scientist who will be conducting research that employs SeaWiFS data must be a registered Authorized User. A single Authorized User cannot obtain data to be used anonymously by members of a research group; all of the members of the group must be Authorized Users. Researchers or research groups who do not follow this policy are at risk of full revocation of use and access to SeaWiFS data. Any Authorized User who distributes data to non-Authorized Users is also at risk of revocation of use and access to SeaWiFS data.

Reading data on tape/unpacking transferred data:
Data are available on 4 mm (DAT), high or low density 8 mm (Exabyte), and 6250~bpi 9-track tapes. Tapes are created with UNIX utilities "dd" and "tar" on a Silicon Graphics 440 system. The no-swap device and a block size of 63.5 KB are used, which translates to a blocking factor of 127. Tapes may be requested in "dd" or "tar" file format. The data are archived and distributed in compressed format. Each tape distributed by the Goddard DAAC contains printed paper labels with the names of the files it contains in the order they were written. Files are compressed using the standard UNIX "compress" command, indicated by a ".Z" appended to the data file name. An ASCII header file on each tape lists the files on the tape.

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: tar -xvbf <filename>.tar 127

where xvbf are tar command key arguments as follows:

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:

tar -xvf <filename>.tar

To read a "dd" format tape on a computer with a UNIX operating system, use the command:

dd if=<dev> of=<filename> bs=65024

where if=<dev> specifies the tape drive with "no rewind" option (e.g., if=/dev/8mm1nr for the DAAC computer.)
of=<filename> specifies the desired output file name
bs=65024 indicates the block size in bytes

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:
HDF is the standard data format of the entire Goddard EOSDIS Version 0 (V0) and the SeaWiFS Project. HDF was developed by the National Center for Supercomputing Applications (NCSA) Software Development Group. The HDF group also supplies HDF utilities that allow file manipulation and conversion on a variety of platforms with UNIX-based operating systems.

Additional explanation of HDF can be found at the HDF Web site:

http://hdf.ncsa.uiuc.edu/

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.

http://hdf.ncsa.uiuc.edu/

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:

http://www.rsinc.com/NOeSYS/hdf.cfm

Processing/Analysis Software Packages:
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 available for download via FTP.

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)
Disk: 3 GB
Tape Drive: 4MM(DAT) or 8mm Exabyte
Display: 19'' Console or X-terminal, 1280x1024 resolution, 8-bit, 256 colors.

The SeaDAS software requirements:

Operating System: IRIX 5.3, IRIX 6.2, IRIX 6.3 (SGI) Solaris 2.4, Solaris 2.5 (SUN)
Languages: C (SGI V3.19, SUN V 3.0.1), FORTRAN(SGI V 4.0.2, SUN V 3.0.1), IDL 5.1 or 5.2
Software Libraries: HDF 4.1r1 (included in SeaDAS).

SeaDAS PC Linux version:
SeaDAs for Linux/PC has been developed and tested under the following environment:

Gateway 2000 PC with Pentium II 300 MHz CPU
Redhat Linux 5.2 (SeaDAS 4.0 for Linux will be for Redhat Linux 6.0)
IDL 5.1 or 5.2

More information on the current version of SeaDAS

SeaDAS is available for download via anonymous FTP from:

ftp://seadas.gsfc.nasa.gov/seadas/

Other packages:

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.

Contacts:

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: info@rsinc.com

Windows Image Manager also works with SeaWiFS HDF data and allows conversion to other formats:
http://spode.ucsd.edu

HDF Explorer is a low-cost package for Windows PCs.

References

All of the volumes (hard copy only) in the SeaWiFS Technical Report Series (NASA Technical Memorandum 104566) may be ordered online. See "The SeaWiFS Technical Report Series" under SeaWiFS at

http://disc.sci.gsfc.nasa.gov/OCDST/OB_Documentation.html

The following sites also have information regarding SeaWiFS and SeaWiFS data:

SeaWiFS Project Home Page

SeaDAS Home Page

Summary and Samples of SeaWiFS Operational Data Products

http://oceancolor.gsfc.nasa.gov/DOCS/ocformats.html