Implementation of OCTS Processing within the OBPG

NASA Ocean Biology Processing Group
October 2006

Introduction

The 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 Oceans ftp site. This document provides some details on the processing approach, data formats, and initial quality assessment.

Processing Approach

Source 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.

Calibration

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.

Level-2

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).

Table 1: Standard Level-2 Product Suite for CZCS

name	description
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

Level-3

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).

Quality Assessment

A detailed analysis of temporal trends relative to SeaWiFS was performed. The SeaWiFS data is not contemporaneous (years 2000-2001 versus OCTS 1997-1998), so an exact match is not expected, and the spectral bands differ at 510 and 555 differ from OCTS bands at 520 and 565, respectively. However, the trends provide some indication of relative bias and temporal stability. Results show some indication of temporal trend in OCTS water-leaving radiances relative SeaWiFS. No effort has been made to remove temporal degradation in the OCTS visible bands, so this may be an instrument artifact, but it may also be related to the onset of the 1997 El Nino. Clear-water (oligotrophic) chlorophyll retrievals are in very good agreement between the two missions, but OCTS is biased low in the higher chlorophyll waters, relative to SeaWiFS 2000-2001 retrievals.

References

Wang, M., A. Isaacman, B.A. Franz, and C.R. McClain (2002).

Ocean-color optical property data derived from the Japanese Ocean Color and Temperature Scanner and the French Polarization and Directionality of the Earth s Reflectances: a comparison study. Appl. Opt., Vol. 41, No. 6, 974-990

O'Reilly, J.E., and 24 Coauthors (2000).

SeaWiFS Postlaunch Calibration and Validation Analyses, Part 3. NASA Tech. Memo. 2000-206892, Vol. 11, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Center, 49 pp.

Curator: OceanColor Webmaster

Authorized by: gene carl feldman