Vol. 82, No. 18, May 1, 2001
Data Analysis System Developed for Ocean Color Satellite Sensors
Karen Baith, Robert Lindsay, and Gary Fu, General Sciences Corporation, Laurel, Md., USA; and Charles R. McClain, NASA Goddard Space Flight Center, Greenbelt, Md., USA
Copyright 2001 American Geophysical Union
A primary objective of the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) program [Hooker et al., 1992] at the NASA Goddard Space Flight Center is to provide rapid and easy access to all SeaWiFS data products and exploit the data set to its maximum potential. To accomplish this, the project provides extensive documentation and a variety of services to the user community, including the SeaWiFS Data Analysis System (SeaDAS). This philosophy and approach is the result of the research community's experience with the proof-of-concept Nimbus-7/Coastal Zone Color Scanner (CZCS) mission. The CZCS data set was not exploited by the research community until several years after the CZCS ceased operating in 1976 because the data and processing code were generally not available. Processing software for CZCS data was developed by individual researchers. Unix and PC versions of one such package, SEAPAK [McClain et al., 1989], was distributed to the research community beginning in 1989 and provided the foundations for the SeaDAS development effort.
SeaDAS is a comprehensive software package for processing, displaying, and analyzing all SeaWiFS data products and it serves a range of users, including individual scientists, SeaWiFS ground stations, and operational or commercial users. It is designed to accurately replicate the standard data products--for example, geophysical fields and data formats--generated by the SeaWiFS Project by using the default input values, but it also allows processing flexibility in the algorithms applied, the map projections used, and other aspects of processing and analysis that let users customize their data products. Flexibility is enhanced by providing executable programs for those who only need the basic capabilities as well as source code for those who wish to modify the code to insert alternative algorithms. The SeaDAS support group is co-located with the SeaWiFS Project to help ensure close coordination with the SeaWiFS Project's development activities. The SeaDAS software is freely available and can be downloaded from the SeaDAS Web site (http://seadas.gsfc.nasa.gov).
Since SeaDAS development began in 1993, versions of SeaDAS have been released periodically, even before the launch of SeaWiFS, to prepare the community for SeaWiFS data. Version 4 was released in May 2000 to coincide with the third SeaWiFS data reprocessing. A reprocessing is executed when significant improvements in the data products result from enhancements in the sensor calibration, atmospheric correction, or bio-optical algorithms. During its development, SeaDAS was expanded and generalized to provide processing for three additional satellite sensors: the CZCS, the Ocean Color and Temperature Sensor (OCTS), and the Modular Optoelectronic Scanner (MOS) as well as display and analysis support (but not processing support) for the Moderate Resolution Imaging Spectroradiometer (MODIS) ocean data products and Advanced Very High Resolution Radiometer sea-surface temperature (AVHRR SST) data. The support of international ocean color data sets is possible because the Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project [Fargion and McClain, 2001] provides the processing code for those data sets. SIMBIOS is also co-located with the SeaWiFS Project.
SeaDAS Functionality
The Interactive Data Language (IDL) software product from Research Systems, Inc., is an integral part of SeaDAS. It is a portable, high-level interpretive programming language that can be used to develop graphic user interfaces (GUIs), scientific graphics, or any standard analysis application quickly and with minimal code and development time relative to that required by low-level programming languages such as C. Users do not need to know the IDL programming language to run SeaDAS. However, users who do know IDL can use their own IDL programs within the SeaDAS batch scripting environment. Also, IDL is not required for executing the satellite data processing programs.
The SeaDAS software package contains both processing programs as well as a full suite of interactive display and basic analysis tools. SeaDAS is not designed to provide an extensive data analysis capability because these applications can be easily developed using IDL. Instead, the SeaDAS effort focuses primarily on satellite data processing and display. The majority of the underlying processing code consists of C and FORTRAN programs developed by the SeaWiFS and SIMBIOS Projects. As a convenience to the user, SeaDAS provides GUIs from which to run these processing programs interactively as well as command-mode capability and includes detailed documentation. Figure 1 provides an example of a SeaDAS display.
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The SeaDAS tool kit includes many navigation, display, analysis, and output functions. Navigation functions include data registration, mapping to standard projections, overlaying of coastlines, plotting of in situ data, and determining latitude/longitude location. General display functions include data scaling, color bar definition, annotation, zooming, roaming, and color palatte manipulation. General analysis functions include bathymetry generation, simple arithmetic functions, contour plots, profile plots, scatter plots, and histograms. Either data or latitude/longitude values (ASCII, HDF, and binary flat files formats) or displayed images (GIF, TIFF, and Postscript formats) can be output.
SeaDAS 4.0: New Features
Enhancements in SeaDAS Version 4 allow a much broader community to use the SeaWiFS data set. Version 4 allows a more diverse suite of data products to be derived and marks a major departure from the previous versions of SeaDAS. The SeaWiFS Level-2 processing program was replaced by the multi-sensor Level-2 processing program (msl12). In addition to multi-sensor support, this new Level-2 processing program also expanded the Level-2 output parameter suite from a fixed number of 12 products to a variable number of user-selected products chosen from a set of 176 possible products. Subsequently, the processing and display programs were also modified to handle the new generalized Level-2 format. Due to the wide scope of this restructuring, several of the main processing programs were rewritten in the C programming language with emphasis on simplifying program logic and incorporating internal documentation. Supporting a larger number of data products is consistent with SeaWiFS' effort to expand the mission's archive product suite to include additional oceanic, atmospheric, and terrestrial geophysical fields.
Since the initial release of SeaDAS 4.0 in May 2000, several major improvements have also been provided as updates, for example, Version 4.0p2 in December. Also, in preparation for the third reprocessing and the Version 4.0 release, the SeaWiFS Project Office purchased a general IDL run-time license that allows users to run SeaDAS 4.0 without requiring the purchase of a license, although users wanting to modify existing IDL routines or add additional IDL programs do need a separate license. Finally, another refinement is the adoption of a single-source version of the SeaDAS source code that runs on Sun and SGI Unix platforms, as well as on PC Red Hat Linux systems. Previously, the PC/Linux source code was maintained separately from the Unix code, which was cumbersome and required constant testing for consistency.
SeaDAS System Requirements
Rather than supporting old versions of operating systems, SeaDAS supports the two most current versions of the operating systems for each supported platform as well as the two most current versions of IDL. Presently, SeaDAS supports Sun Solaris 2.6 and 2.7, SGI IRIX 6.3 and 6.5, PC Red Hat Linux 6.0 and 6.2 platforms, as well as IDL 5.2 and 5.3. If compiling from scratch, then the vendor C and FORTRAN compilers are also required for SGI and Sun, and the Absoft FORTRAN compiler (http://www.absoft.com) is required for the PC environment. Recommendations on minimum hardware requirements are SGI O2, Sun UltraSPARC, or a PC with a 350 Mhz processor. All platforms require a minimum of 192 Megabytes of memory (384 megabytes if processing HRPT data) and 9 gigabytes of disk space are suggested. The SeaDAS installation only requires about 330 megabytes without the demonstration files and 1.2 gigabytes with the optional demonstration files. A 19-inch monitor with a resolution of at least 1024 by 1280 pixels is suggested and X-terminals need at least 20 megabytes of local memory.
User Support and User Base
The SeaDAS development team has always worked closely with the user community to provide timely and detailed user support. Suggestions from the oceanographic user community have always driven the SeaDAS development efforts, for example, the development of PC support as well as the embedded IDL license option. Historically, the SeaDAS team has released a major version about once a year to coincide with the reprocessing of the SeaWiFS data. As an additional convenience to the user, SeaDAS provides optional interim updates between releases. These updates allow users to apply enhancements and bug fixes without waiting for a major release. These updates are announced via e-mail to the user community and are posted on the SeaDAS Web site. The SeaDAS user base of over 500 sites is primarily comprised of oceanographic institutes and universities in more than 42 countries. In the past, the user base has been split equally between U.S. users and international users; however, with the release of SeaDAS 4.0, foreign users now exceed domestic users by about 8%. This shift is attributed to the release of the embedded IDL run-time license option.
Future Plans
Currently, SeaDAS provides 8-bit color, but support for 24-bit color is being considered. SeaDAS will incorporate processing capabilities for other ocean color satellite sensors, such as the Polarization and Directionality for the Earth's Reflectances (POLDER) radiometer. The SIMBIOS Project has completed its initial ADEOS-1/POLDER processing code. A second POLDER will be launched on ADEOS-2 along with the Global Line Imager (GLI), which will also provide ocean color data. Support for the Ocean Scanning Multispectral Imager (OSMI) and the Medium Resolution Imaging Spectrometer (MERIS) is being considered.
The SeaWiFS Project is currently producing a set of "evaluation" ocean, atmosphere, and land products, such as colored dissolved organic matter, cloud optical thickness, and normalized difference vegetation index (NDVI), respectively. These products are outside the suite of the standard archive products distributed by the Goddard Distributed Active Archive (GDAAC). Each evaluation product is subject to algorithm revisions and validation but can be promoted to archive status if the research community expresses sufficient interest in it and the product can be verified to a reasonable degree of accuracy. Recently, photosynthetically available radiation (PAR) was added to the archive list, is now available from the GDAAC, and can be also be generated using SeaDAS. The evaluation products can be browsed via the "Full Resolution Mapped Images" link on the SeaWiFS home page (http://seawifs.gsfc.nasa.gov/seawifs.html). Some of the evaluation products can be generated using SeaDAS 4.0, for example, NDVI. Other evaluation products will be added in future SeaDAS updates.
The SeaWiFS and SIMBIOS Projects are working with the SeaDAS team to develop an approach to better provide SeaDAS training opportunities to the user community. For user support or information on SeaDAS, check the SeaDAS Web site (http://seadas.gsfc.nasa.gov), send a message to seadas@seadas.gsfc.nasa.gov, or call Karen Baith at 301-286-4759.
Acknowledgments
SeaDAS is supported by the NASA ocean biochemistry program under funding to C. R. McClain (P.I.). The SeaWiFS and SIMBIOS Projects provide additional support for computer hardware upgrades and system administration services.
References
Fargion, G., and C. R. McClain, SIMBIOS Project 2000 Annual Report, NASA/TM-2001-209976, NASA Goddard Space Flight Center, Greenbelt, Md., 164 pp., 2001.
Hooker, S. B., W. E. Esaias, G. C. Feldman, W. W. Gregg, and C. R. McClain, SeaWiFS Tech. Rep. Ser., Volume 1, An Overview of SeaWiFS and Ocean Color, NASA Tech. Memo. 104566, 24 pp., 1992.
McClain, C. R., J. Chen, M. Darzi, J. Firestone, and D. Endres, SEAPAK Users Guide, Version 1.0, NASA Tech. Memo. 100728, NASA Goddard Space Flight Center, Greenbelt, Md., 594 pp., 1989.
