Overview of SeaWiFS Data Processing and Distribution
Donna Thomas and Bryan Franz Sept 2005
The Ocean Biology Processing Group (OBPG) is responsible for the production and distribution of the ocean color data products from SeaWiFS (Sea Viewing Wide Field of View Sensor). This document provides details about the processing flow of SeaWiFS data. Figure 1 below gives an overview of the processing of SeaWiFS data within OBPG which begins with Level 0 and culminates in Level 3 Binned and Mapped Browse data which are available for download from the Ocean Color Web and archived in an FTP repository.
Figure 1: Processing flow from Level-0 through Level-3.
II. Level-1A Processing
The OBPG first processes SeaWiFS Level 0 to Level 1A using l1a_gen, by appending calibration data, navigation data,instrument telemetry information, and selected spacecraft telemetry information to form a Level 1A
HDF file that contains raw radiance values for each of the eight SeaWiFS bands.
The input Level 0 data used to produce the Level 1A can be of two spatial resolutions - either LAC (Local Area Coverage), which is 1.1 km at nadir, or GAC (Global Area Coverage), which is 1.1 km resolution but with pixels spaced at 4.4km intervals.
Four types of Level 1 SeaWiFS data are processed by OBPG: GAC, LAC, HRPT, and MLAC.
GAC (Global Area Coverage) data are subsampled and recorded onboard the spacecraft and subsequently downloaded twice a day at Wallops and NASA/Goddard. This data have effective resolution of about 4.5 kilometers along the center of the swath. For GAC data, individual products are generated from each Level-0 GAC recording period (the Earth data collection portion of an orbit). Each such GAC product thus constitutes one scene.
LAC (Local Area Coverage)data are recorded at full 1.1 kilometer resolution for selected parts of the world and downloaded with the GAC data. For recorded, full-resolution (e.g., LAC) data, each scene is comprised of a continuous recording of one data type. LAC data is broadcast to
HRPT (high resolution picture transmission) sites around the world operating as ground-receiving stations.
HRPT (High Resolution Picture Transmission) Direct Broadcast data have the same basic format and resolution as the LAC data, but they are collected by ground stations within range of the OrbView-2 spacecraft transmitter. For HRPT data, each scene is comprised of one satellite pass.
MLAC data (merged LAC) contain all available SeaWiFS HRPT and LAC data for a given orbit. This is done by consolidating all SeaWiFS 1-km-resolution data which have been collected by various HRPT stations, as well as the LAC data recorded onboard the spacecraft into Level 1A files on a per orbit basis. In geographic regions with a high concentration of HRPT stations, the data collected from multiple stations will frequently overlap. Any duplicate scans resulting from that overlap are evaluated through a series of quality tests, and only the "best quality" scan is stored in the MLAC file. The quality tests look at pixel-to-pixel variance between duplicate scans, and bit-error counts derived from sync words. This process significantly reduces the transmission noise inherent in HRPT data stream, and elimination of duplicate observations makes it possible to bin the MLAC data with the same algorithms and software used for SeaWiFS GAC or MODIS. Level-1A or Level-2 MLAC data is identical in format to SeaWiFS LAC and HRPT, except that the scan times may be discontinuous due to gaps in the HRPT/LAC geographic coverage. For MLAC data, each product contains the best available full-resolution data for a single orbit, without duplication.
Of the Level 1 and 2 SeaWiFS data, only GAC and MLAC data are distributed via the Ocean Color Web.
III. Level-2 Processing
Each Level 2 scene corresponds to a level 1A scene. Level-2 processing is performed using the Multi-Sensor Level-1 to Level-2 (MSL12) code, which is developed and maintained by the OBPG. MSL12 is used for thestandard processing of all ocean products distributed through the ocean color web browsers and ftp sites. This software is capable of retrieving oceanic optical properties and various derived products from the observed top-of-atmosphere (TOA) radiances collected by a variety of ocean remote sensing radiometers, including SeaWiFS, MODIS, OCTS, MOS, OSMI, POLDER, and CZCS.
Full documentation, updated code source, sensor files, and output product descriptions are available.
Before computing Level 2 data, pixels are eliminated if they contain clouds, sun glint, or other abnormalities. For pixels that pass these screens,
an atmospheric correction is applied to subtract the atmospheric scattering components from the total radiance to obtain the water-leaving radiances for bands 1-5.
Ancillary meteorological data (U and V winds, surface pressure, water vapor) and Ozone data (O3) are used for atmospheric correction in Level 2 processing. EPTOMS Ozone is the primary source for
ozone used within OBPG. The Ancillary Data is Quality Controlled and made avaible to the public by ftp from the Ocean Color Web Site.
Other Level 2 processing steps include data navigation so that land mask may be correctly placed, computational steps to derive the geophysical products, and tests for anomalous conditions in the data
(masks and flags).
IV. Level-3 Processing
Level 3 data processing involves both temporal and spatial binning and utilizes Level 2 GAC data as input in a process called l3bin. The Level 2 data is spatially binned before it can be temporally binned. Pixels containing valid Level 2 data are mapped to a fixed spatial grid composed of 9 x 9 km2 resolution elements (bins). The Level 2 GAC data is sub sampled by a factor of two to produce the Daily Level 3 product. This processing takes every valid measurement of water-leaving radiance falling within the lat/lon boundaries of a given grid square and compiles it within that bin. During the space binning step, a pixel from a parent Level-2 product can be excluded from binning as determined by the existence of any of the following conditions:
A bit in the parent Level-2 product's l2_flags corresponding to the pixel is set (equals
1) and the algorithm name for that bit has been specified to be used for exclusion by
an input parameter to the space binner.
The pixel's scan line is within a tilt_ranges range in the parent Level-2 product for
which the corresponding tilt_flags is 3 (changing tilt angle) or -1 (unknown state)
Note that for condition 2, the entire scan line of that pixel will be excluded.
To time bin 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 time binning step "l3bin" is used to combine scene bin products, generated by the space binner, into daily products. The day bin products are combined into 8-day and monthly products, while the monthly bin products are binned into yearly products.
The Level 3 binned data set consists of geophysical parameters binned to a 9x9 km (81 km2) global, equal-area grid at daily, 8-day, monthly, and annual intervals. The parameters are the 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. Each Level-3 binned data product will be stored in multiple HDF files. Each multi-file product includes a main file containing all product-level metadata and data for each bin that are common to all the binned geophysical parameters. In addition, each product includes 12 subordinate files (class = DataSubordinate ), each of which contains data of one binned geophysical parameter for all bins.
Subordinate files must be read in conjunction with the associated main file.
The Level-3 standard mapped image (SMI) products are image representations of binned data products and are produced at daily, 8day monthly and yearly time intervals from the equivalent binned product. Five SMI products are generated from each binned data product, one for each of the following geophysical parameters: chlorophyll a concentration, angstrom coefficient 510 to 865 nm,
normalized water-leaving radiance at 555 nm, aerosol optical thickness at 865 nm, and diffuse attenuation coefficient at 490 nm. Thus, each SMI product represents data binned over the period covered by the parent product. The mean is used in each case to obtain the values for the SMI grid points from the binned data products. Each SMI product contains one image of a geophysical
parameter and is stored in one physical HDF file.
V. Data Distribution
SeaWiFS Level 1A and L2 MLAC and GAC data are available on the Ocean color browser, as are the
Level 3 Mapped Daily, Weekly, Monthly, Annual and Climatological data.
The Mapped data and Binned data files are available from the
VI. Additional Information
1). Latest reprocessing configuration
2). MODIS Aqua processing overview document
3). Standard Operational Products