VIIRS-NPP Reprocessing 2012.2

VIIRS-NPP Reprocessing 2012.2

1. Introduction

An effort is currently underway to assess the capacity of the VIIRS instrument to support NASA Ocean Biology and Biogeochemistry Program science (e.g., Turpie et. al 2012). The NASA Ocean Biology Processing Group (OBPG) is applying calibration methods and algorithms based on lessons learned from SeaWiFS and MODIS with the goal of producing a suite of ocean color products from VIIRS that are consistent in content, format, and quality with heritage sensors. These products are being distributed as evaluation products for assessment by the NPP Science Team and the research community. The initial NASA processing of VIIRS and details on the differences between NASA products and the operational VIIRS ocean color products produced by NOAA are described here.

In this reprocessing, called 2012.2, several significant changes were made relative to previous NASA VIIRS ocean color processing.

  1. the vicarious calibration is now using data from the Marine Optical Buoy (MOBY) as the reference source.
  2. the temporal calibration was updated to include the latest solar diffuser measurements.
  3. the temporal calibration was removed from the Level-1 product and is now applied in the Level-1 to Level-2 processing.

2. The Pseudo Level-1A Concept

A primary change in this reprocessing is that instrument calibration is now applied in two parts. Prelaunch radiometric calibration is applied to produce a Level-1 product, and then postlaunch temporal calibration is applied during Level-1 to Level-2 processing. As such, the Level-1 product is not fully calibrated, and is now referred to as pseudo Level-1A. This is similar to the approach used for SeaWiFS. It results in a more stable Level-1 product for the user community, since on-going instrument calibration assessment and improvement efforts are primarily focused on the instrument temporal calibration, and it provides the OBPG with a much more efficient platform for testing instrument calibration changes, since the resource-intensive Level-0 to Level-1 processing does not have to be repeated.

As before, the VIIRS Level-0 (Raw Data Record, RDR) files are processed to Level-1 using the ADL software (Algorithm Development Library, provided by the University of Wisconsin, developed by Raytheon). The difference in the 2012.2 reprocessing is that we are now providing ADL with a unity temporal calibration look-up table (F-LUT) to produce the pseudo Level-1A. The ADL code follows the official IDPS code used by NOAA to process VIIRS data, and thus the format of Level-1A files produced by NASA through this process is similar to NOAA Science Data Record (SDR) format. Since NASA Level-1A are not fully calibrated, however, they differ significantly in content from the NOAA SDR products. Also, to minimize digitization error, NASA Level-1A radiances, reflectances, and brightness temperatures are now stored as floating-point values rather than scaled integers, and associated scaling attributes are no longer included.

3. Instrument Calibration

NASA is deriving a continuous temporal calibration based on the on-board calibration measurements for the visible and near-infrared bands (M1-M7, 410-862), and then reprocessing the full mission to produce a continuously calibrated Level-2 product.

In Level-0 to Level-1A processing, the calibration is based on results from the prelaunch characterization (e.g. spectral response, polarization sensitivity, response versus scan angle, etc.). In Level-1A to Level-2 processing, the prelaunch-calibrated radiances are multiplied by trending coefficients that track the on-orbit change of the radiometric gains. The solar diffuser is currently the main source for gain trending, which is independently validated with lunar measurements. Trending is currently performed over time with respect to the first light measurement, across detectors (detector destriping), and between mirror sides (mirror-side correction). The measurements of a Solar Diffuser Stability Monitor (SDSM) are used to correct for reflectance degradation of the solar diffuser (MODIS approach). The gains are assumed to be a linear function of the measured counts after dark current subtraction. The response versus scan is corrected based on prelaunch measurements. Temperature correction may be added at a later date. The gains are calculated independently for each mirror side, gain state, and detector. See Eplee et al. (2012) for more details on the current OBPG calibration approach.

The figure below shows the change in radiometric response for bands M1-M4 (410-551nm) and M4-M7 (551-862nm) as derived from the OBPG solar diffuser analysis. The filled circles show the independently-derived changes from analysis of lunar observations, which clearly confirm the large and rapid degradation observed in the longer wavelengths. The rapid degradation is due to tungsten oxide contamination of the primary mirror, when exposed to ultraviolet light. Prior to 2 January 2012, the instrument was stowed several times in an effort to assess this effect. After 2 January 2012, the instrument was fully powered-up (including thermal bands) and continuously exposed. The degradation after that time is smooth and tractable, but the instrument radiometric responsivity has degraded by as much as 30% in one year, along with an associated loss in signal to noise. This raises concerns about the long-term viability of the instrument for ocean color.

solar_lunar_compas_1129.png solar_lunar_compbs_1129.png

Another concern is the growing discrepancy in the lunar calibration measurements relative to the solar calibration trends. This discrepancy is being investigated by both the VIIRS Calibration Support Team (VCST) and the OBPG. A complicating factor in the solar calibration is that the solar diffuser reflectance has degraded substantially since launch, with largest decrease of ~15% in the shortest wavelengths. Unlike MODIS, the solar diffuser on VIIRS is continuously exposed, so accelerated degradation is to be expected. Error in the correction for solar diffuser degradation may be the reason for the solar to lunar discrepancy. The OBPG is currently evaluating alternative temporal calibration strategies, including transition to a lunar based temporal calibration, but the solar calibration is still required to capture detector and mirror-side relative changes.

4. Vicarious Calibration

As for SeaWiFS and MODIS, the OBPG applies an additional vicarious calibration to VIIRS during Level-2 processing (Franz et al., 2007). Band M7 (862nm) is assumed to be correctly calibrated from prelaunch measurements. Band M6 (748nm) is adjusted using match-ups from the South Pacific Gyre, to force the aerosol type retrievals to match, on average, the aerosol type observed at the Tahiti AERONET site. The calibration of bands M1-M5 (410nm to 671nm) is then adjusted to produce retrievals that match, on average, a surface reference.

In the previous reprocessings of NASA VIIRS, the vicarious calibration reference was derived from a sea surface reflectance model and a climatology of chlorophyll-a concentration (Werdell et al. 2007). In this 2012.2 reprocessing, the vicarious calibration is based on measurements from the Marine Optical Buoy (MOBY) near Lanai Hawaii (the same reference currently used for SeaWiFS and MODIS).

The vicarious calibration gains derived in this manner are:

Wavelength (nm)

410

443

486

551

671

745

862

1238

1601

2257

Gain

0.9627

1.0008

1.0092

0.9747

1.0147

1.041

1.0

1.0

1.0

1.0

It must be emphasized that these gains are only applicable to the OBPG instrument calibration and the standard NASA atmospheric correction algorithm. If NOAA SDRs are processed through the NASA algorithms, it is recommended to set the vicarious calibration to unity, as the NOAA SDRs include an absolute calibration to the solar diffuser that may largely resolve the biases captured by NASA through vicarious calibration.

No attempt has yet been made to calibrate the shortwave infrared (SWIR) bands.

5. Processing Algorithms and Derived Products

For the NASA SDR to Level-2 processing, the OBPG is using the standard NASA atmospheric correction, i.e., the same algorithms and software that are currently used in standard processing of MODIS, SeaWiFS, and other ocean color sensors. See the initial NASA processing of VIIRS for details.

For this reprocessing, the product suite has been expanded to include photosynthetically available radiation (PAR) and particulate organic and inorganic carbon (POC and PIC), which are currently standard MODIS and SeaWiFS products. The full list of products now contained in the NASA VIIRS Level-2 product includes:

  1. remote sensing reflectance, Rrs, in 5 visible channels (410-671nm)

  2. aerosol optical thickness at 862nm, aot_862, using the models of Ahmad et. al 2010

  3. aerosol Angstrom exponent at 443nm, angstrom, using the models of Ahmad et. al 2010

  4. chlorophyll-a concentration, chlor_a, using the OC3 algorithm

  5. diffuse attenuation at 490nm, Kd_490, using the KD2 algorithm

  6. particulate organic carbon, POC, using the algorithm of Stramski et al. 2008

  7. particulate inorganic carbon, PIC, using the combined algorithm of Balch and Gordon

  8. photosynthetically available radiation, par, from the algorithm of Frouin et al. 2003

All derived product algorithms have been updated to account for the VIIRS spectral band-passes and effective band centers. Updates for PIC and PAR were provided by NASA NPP Science Team PIs Balch and Frouin, respectively.

6. File Formats and Naming Conventions

The NASA VIIRS pseudo Level-1A and Level-2 products are distributed through the Level-1/2 Browser and the via the online the Ocean Color Archive for direct access and bulk download. As previously noted, the pseudo Level-1A products produced by NASA are similar to standard NOAA SDR format. The Radiance, Reflectance, and BrightnessTemperature fields, however, are output as 32-bit floating-point values, and the associated scaling attributes (i.e., RadianceFactors, ReflectanceFactors, and BrightnessTemperatureFactors) are therefore not included.

Each Level-1A or Level-2 file spans ~85-seconds of observation time, thus there are over 1000 granules in a day. This granularity may eventually be changed, but the OBPG is currently constrained to match the granularity of the source (RDR) data. Each band within an SDR is stored in a separate HDF5 file, as is the geolocation data. The NASA processing code requires all 16 M-bands and the geolocation file. To simplify the data distribution and ensure that all bands are present, the OBPG is distributing the Level-1A as a tar file that contains the full suite of bands for the granule (excluding higher resolution I-bands). The tar file mimics the standard naming convention used for other NASA ocean color missions: Vyyyydddhhmmss.L1A_NPP.tar, where yyyy is year, ddd is day, hhh is hour, mm is minute, and ss is second, and the date/time indicates the observation time of the first line of the granule. An example is shown below.

% tar tf V2012145214452.L1A_NPP.tar

  1. README.txt
  2. GMTCO_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  3. SVM01_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  4. SVM02_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  5. SVM03_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  6. SVM04_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  7. SVM05_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  8. SVM06_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  9. SVM07_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  10. SVM08_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  11. SVM09_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  12. SVM10_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  13. SVM11_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  14. SVM12_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  15. SVM13_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  16. SVM14_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  17. SVM15_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5
  18. SVM16_npp_d20120524_t2144540_e2146182_b02972_obpg_ops.h5

The Level-2 files produced by NASA follow the same general naming convention (e.g., V2012145214452.L2_NPP_OC), and the format is the same HDF4 format used for all other NASA ocean color sensor products. The OBPG is also distributing VIIRS Level-3 products derived from the NASA Level-2 processing. The Level-3 products are binned to 4.6-km spatial resolution (identical to MODIS), composited to daily, 8-day, monthly, and seasonal products, and distributed in the same binned and mapped HDF4 formats as other NASA'sensors. The Level-3 binned and mapped products are available from the evaluation tab of the Level-3 Browser and directly from the Ocean Color Archive.

NOTE: To support evaluation activities by the NASA NPP Science Team, the OBPG is also distributing a set of Level-3 products derived from the standard NOAA Level-2 (EDR) products. These files are clearly identified on the Level-3 browser, and the filenames contain NPPE in place of NPP.

7. Results

8. Additional Information

  1. Documentation of NOAA operational algorithms and product formats

  2. Access to NOAA operational data distribution (CLASS)