Bryan A. Franz

The continuity of ocean color measurements from SeaWiFS to MODIS

The Ocean Biology Processing Group (OBPG) at NASAs Goddard Space Flight Center is responsible for the processing and validation of oceanic optical property retrievals from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS). A major goal of this activity is the production of a continuous ocean color time-series spanning the mission life of these sensors from September 1997 to the present time. This paper presents an overview of the calibration and validation strategy employed to optimize and verify sensor performance for retrieval of upwelling radiances just above the sea surface. Substantial focus is given to the comparison of results over the common mission lifespan of SeaWiFS and the MODIS flying on the Aqua platform, covering the period from July 2002 through December 2004. It will be shown that, through consistent application of calibration and processing methodologies, a continuous ocean color time-series can be produced from two different spaceborne sensors.

Moderate-Resolution Imaging Spectroradiometer ocean color polarization correction

The polarization correction for the Moderate-Resolution Imaging Spectroradiometer (MODIS) instruments on the Terra and Aqua satellites is described. The focus is on the prelaunch polarization characterization and on the derivation of polarization correction coefficients for the processing of ocean color data. The effect of the polarization correction is demonstrated. The radiances at the top of the atmosphere need to be corrected by as much as 3.2% in the 412 nm band. The effect on the water-leaving radiances can exceed 50%. The polarization correction produces good agreement of the MODIS Aqua water-leaving radiance time series with data from another, independent satellite-based ocean color sensor, the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS).

Utility of MODIS-Terra for Ocean Color Applications

The Moderate Resolution Imaging Spectroradiometer (MODIS) is currently flying on both the Terra and Aqua satellite platforms. The Ocean Biology Processing Group (OBPG) at NASA Goddard Space Flight Center is producing operational ocean color products from the MODIS-Aqua sensor; however, documented uncertainties and instabilities in the prelaunch and on-orbit characterization have inhibited the production of similar products from MODIS-Terra. In particular, the radiometric response of the 412-nm band has degraded by more than 40% over the 7-year mission lifespan, with similar though less extreme changes in the longer wavelengths. Furthermore, the degradation trends are significantly different between the two mirror sides, which is likely a result of asymmetric damage done to the mirror during prelaunch testing. These effects contribute to uncertainty in our knowledge of instrument response versus incidence angle on the mirror and sensitivity with respect to polarization of the observed radiance. In this paper, we examine the impact of apparent MODIS-Terra instrument characterization errors on the derived ocean color products and show that residual errors in the current operational calibration give rise to significant cross-scan artifacts, mirror-side differences, and detector-to-detector striping in the retrieved water-leaving radiances. In addition, we describe OBPG efforts to reduce these artifacts through statistical and vicarious instrument characterization, and show the quality of the resulting water-leaving radiance retrievals relative to those derived from MODIS-Aqua.

SIMBIOS Program in Support of Ocean Color Missions: 1997-2003

The NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Program had a worldwide, ongoing ocean color data collection program, as well as an operational data processing and analysis capability. SIMBIOS data collection takes place via the SIMBIOS Science Team. In addition, SIMBIOS had a calibration and product validation component (Project Office). The primary purpose of these calibration and product validation activities were to (1) reduce measurement error by identifying and characterizing true error sources, such as real changes in the satellite sensor or problems in the atmospheric correction algorithm, in order to differentiate these errors from natural variability in the marine light field; and (2) evaluate the various bio-optical and atmospheric correction algorithms being used by different ocean color missions. For each sensor, the SIMBIOS Project reviews the sensor design and processing algorithms being used by the particular ocean color project, compares the algorithms with alternate methods when possible, and provides the results to the appropriate project office.

On-orbit calibration of SeaWiFS: revised temperature and gain corrections

The NASA Ocean Biology Processing Groups Calibration and Validation (Cal/Val) Team has used SeaWiFS onorbit lunar and gain calibration data, in conjunction with mission-long trends of global ocean color data products, to diagnose and correct recently emergent residual drifts in the radiometric response of the instrument. An anomaly analysis of the time series of global mean normalized water-leaving radiances, the atmospheric correction parameter ∈, and chlorophyll show significant departures from the mission-long trends beginning in January 2006. The lunar time series trends for the near infrared (NIR) bands (765 nm and 865 nm) show significant periodic departures from mission-long trends beginning at the same time. ∈ is dependent on the ratio of these two bands; trends in this parameter would propagate through the atmospheric correction algorithm to the retrieved water-leaving radiances. An analysis of fit residuals from the lunar time series shows that the focal plane temperature dependencies of the radiometric response of the detectors for these two bands have changed over the 9+ year mission. The Cal/Val Team has used these residuals to compute a revised set of temperature corrections for data collected starting 1 January 2006. The lunar calibration data and a mission-long ocean color test data set have been reprocessed with the revised temperature corrections. The reprocessed data show that the trends in the NIR bands have been minimized and that the departures of the water-leaving radiances, ∈, and chlorophyll from the mission-long trends have been greatly reduced. The anomaly analysis of the water-leaving radiances in the 510 nm band still shows a residual drift of -2.9% over the mission. The anomaly analysis of the ∈ time series shows a residual drift of +2.8% over the mission. A corresponding drift is not observed in the lunar calibration time series for the NIR bands. The lunar calibration data are obtained at a different set of instrument gains than are the ocean data. An analysis of the mission-long time series of on-orbit gain calibration data shows that the gain ratios for the NIR bands change -0.76% (765 nm) and +0.56% (865 nm) over the mission, corresponding to a -1.3% drift in the band ratio. The lunar calibration time series for the NIR bands have been corrected for this gain drift, and the change in radiometric response over time has been recomputed for each band. The mission-long ocean color test data set has been reprocessed with these revised corrections for the NIR bands. The anomaly analysis of the reprocessed water-leaving radiances at 510 nm shows the drift to have been essentially eliminated, while the anomaly analysis of epsilon shows a reduced drift of +2.0%. These analyses show the sensitivity of ocean color data to small drifts in instrument calibration and demonstrate the use of time series of global mean geophysical parameters to monitor the long-term stability of the instrument calibration on orbit. The two updates to SeaWiFS radiometric calibration have been incorporated into the recent reprocessing of the SeaWiFS mission-long ocean data set.

Correction of subframe striping in high resolution MODIS ocean color products

The MODIS (Moderate Resolution Imaging Spectroradiometer) scanner makes subframe measurements in some of its bands to increase the spatial resolution from its standard 1km resolution to 500m or 250m. This is achieved by sampling a detector of a high resolution band at twice (or four times) the sampling rate of the 1km bands. This paper shows that a calibration equation nonlinear with radiance and specific to the individual subframes will reduce striping in the images. The effects are significant for low radiance levels like those encountered over ocean scenes. A preliminary calibration correction is derived with two approaches: first from prelaunch measurements, then from on-orbit data. The results of the two methods are qualitatively similar.

SeaWiFS on-orbit gain and detector calibrations: effect on ocean products

The NASA Ocean Biology Processing Groups Calibration and Validation Team has analyzed the mission-long Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) on-orbit gain and detector calibration time series to verify that lunar calibrations, obtained at nonstandard gains and radiance ranges, are valid for Earth data collected at standard gains and typical ocean, cloud, and land radiances. For gain calibrations, a constant voltage injected into the postdetector electronics allows gain ratios to be computed for all four detectors in each band. The on-orbit lunar gain ratio time series show small drifts for the near infrared bands. These drifts are propagated into the ocean color data through the atmospheric correction parameter epsilon, which uses the 765/865 nm band ratio. An anomaly analysis of global mean normalized water-leaving radiances at 510 nm shows a small decrease over the mission, while an analysis of epsilon shows a corresponding increase. The drifts in the lunar time series for the 765 and 865 nm bands were corrected. An analysis of the revised water-leaving radiances at 510 nm shows the drift has been eliminated, while an analysis of epsilon shows a reduced drift. For detector calibrations, solar diffuser observations made by the individual detectors in each band allows the response of the detectors to be monitored separately. The mission-long time series of detector calibration data show that the variations in the response of the individual detectors are less than 0.5% over the mission for all bands except the 865 nm band, where the variations are less than 1%.

New results of ground-target-based calibration of MOS on IRS

The success of the Modular Optoelectronic Scanner MOS on the Indian Remote Sensing Satellite IRS-P3 during the 6 years mission time has been based on its sophisticated in-orbit calibration concept to a large extent. When the internal lamp and the sun calibration failed in September 2000 we tested the possibility of ground target based (or vicarious) calibration of the MOS instruments to continue the high data quality. This is essential for future watching of global changes of the ocean coastal zones (phytoplancton, sediments, pollution, etc.) using spectral measurements of the VIS/NIR MOS spectral channels. The investigations have shown the suitability of a part of the Great Eastern Erg in the Sahara desert for this purpose. The satellite crosses this very homogeneous area every 24 days. Because of the good cloudfree conditions we can use 6 - 8 overflys a year for calibration. The seasonal variability of the surface reflectance is very small so that we obtain relative calibration data of sufficient accuracy even without ground truth measurements for most of the channels. The trend of this vicarious calibration corresponds very well with the previous trend of the failed lamp and sun calibration. Dfferences between the three methods will be discussed. In the paper we will also present the results of a comparison between SeaWiFS and MOS data of comparable spectral channels from the Great Eastern Erg area. They confirm the suitability of this area for calibration purposes too.