NASA Ocean Biology Processing Group
Implementation of OCTS Processing within the OBPG
IntroductionThe Ocean Biology Processing Group (OBPG) completed a reprocessing of the ADEOS Ocean Color and Temperature Scanner (OCTS) mission data set on 17 October 2006. This is an update to the previous NASA/NASDA collaborative reprocessing performed in 2002, which was based on Wang et al. (2002). The new OCTS products are being distributed through the Level-2 and Level-3 browsers of the Ocean Color Web (OCW) and the associated Ocean Color Data Distribution site. This document provides some details on the processing approach, data formats, and initial quality assessment.
Processing ApproachSource Data
The OBPG was provided with the OCTS GAC data in a Level-1A, HDF4 format. The GAC data is subsampled 1-km data, which includes every 4th pixel and every 5th line. As such, we do not have a full record of the observed radiance field, so identification of potential straylight from bright sources may be compromised. In addition, the design of the OCTS sensor results in observations that are not co-registered within each band. Thankfully, NASDA's Level-1A generation included a co-registration step using a nearest neighbor algorithm. However, this neccessarily produces individual scan lines that consistute a mix of detectors (OCTS has 10 detectors distributed along-track for each band). If the detector-to-detector relative calibration is imperfect, this can give rise to systematic artifacts across scan and from scan to scan. The coregistratioin and geolocation is as provided by NASDA.
The OBPG worked closely with NASDA/JAXA personnel to verify implementation of the prelaunch calibration and detector relative calibrations. Since OCTS lacks any functional onboard calibration capabilities to track sensor degradation, the OBPG developed vicarious techniques to assess and improve the temporal calibration. A final vicarious calibration was derived based on a bio-optical model and inversion of the atmospheric correction algorithm. More details are provided here.
The OBPG incorporated OCTS processing capabilities into the Multi-Sensor Level-1 to Level-2 code (MSL12). The Level-2 format is very similar to that of SeaWiFS or MODIS ocean products, which are also produced with MSL12. The processing algorithm is effectively identical to SeaWiFS Reprocessing 5.1. As in the 2002 NASA OCTS reprocessing, a spatial filtering scheme was applied to reduce detector relative calibration artifacts (striping and vertical banding). A list of the standard Level-2 products is presented in Table 1. The chlorophyll algorithm is the OC4 form from O'Reilly et al. (2000), but tuned for OCTS bands at 443, 490, 520, and 565 nm (O'Reilly, private communication) using the latest NOMAD dataset (Werdell & Bailey, 2005).
|chlor_a||chlorophyll-a concentration (OC4)|
|nLw_412||normalized water-leaving radiance at 412 nm|
|nLw_443||normalized water-leaving radiance at 443 nm|
|nLw_490||normalized water-leaving radiance at 490 nm|
|nLw_520||normalized water-leaving radiance at 520 nm|
|nLw_565||normalized water-leaving radiance at 565 nm|
|nLw_670||normalized water-leaving radiance at 670 nm (reflectance model)|
|tau_865||aerosol optical thickness at 865 nm|
|epsilon||aerosol epsilon at 765 relative to 865|
|angstrom_520||aerosol Angstrom coefficient at 520 relative to 865 nm|
The Level-2 products were binned to daily, 8-day, and monthly Level-3 products using the same software and formats currently employed for MODIS, SeaWiFS, and CZCS processing within the OBPG. The Level-3 binned data was generated at a spatial resolution of approximately 9.2 x 9.2-km, and mapped to equirectangular projections at 0.088-deg resolutions (like SeaWiFS).