GRWG VIS/NIR Web Meeting 2018-07-05
GSICS Web Meeting on VIIRS as a Reference
- Changyong Cao (NOAA) - NOAA NPP VIIRS calibration
Changyong Cao (NOAA) - NOAA CLASS VIIRS data availability, and other options for GSICS
- Dave Doelling (NASA) – Using VIIRS I or M band as the calibration reference
Guest Chair: Dave Doelling (NASA)
CMA: Scott Hu, Li Yuan
EUMETSAT: Sebastien Wagner, Tim Hewison
JMA - Masaya Takahashi, Yusuke Yogo and Kazuki Kodera
KMA: Dohyeong Kim, Minju Gu, Eunkyu Kim, Tae-Hyeong
NASA: Dave Doelling, Amit Angal
NOAA: Changyong Cao, Likun Wang, Fangfang Yu, Fred Wu, Srirish Uperty, Manik Bali
Changyong Cao (NOAA) presented powerpoint slides highlighting the VIIRS calibration improvements, such as low and high gain consistency, removal of incremental forward calibration adjustments, reexamination of the in orbit and ground characterization calibration activities, where the band absolute calibration may have been adjusted by up to 2%. Many questions were asked during the presentation. All of the VIIRS geolocation has been corrected for terrain. The sea-level geolocation is not saved during production, due to storage limitations. DCC is used for low gain and ocean color for high gain derivation. The M1-M4 degradation seen in V0 has been corrected. The M5 and M7 band calibration has been adjusted by 0.985 and 0.98 based on extensive inter-comparisons with other datasets, such as Aqua-MODIS and Landsat OLI. In reprocessing, NOAA incorporates the lunar looks in addition to the solar diffuser for RSB stability. Unfortunately, as time progresses the stability of the lunar and solar diffuser is diverging. A small seasonal cycle was also discovered in the f-factor and was removed, that did not rely on the ocean color group, but was an inhouse mitigation effort. V2 uses a Kalman filter to combine the solar diffuser, lunar, DCC, SNO, and time series to determine the forward processing gain. Changyong mentioned that NPP was a joint NOAA and NASA mission, but all future and NOAA-20 JPSS VIIRS instrument are NOAA missions. NOAA does provide NASA money for some calibration activities. Also, NASA flight does provide Jack funding for VIIRS calibration effort. However, NASA funding of these efforts is not guaranteed. NOAA will provide the official VIIRS datasets. Changyong encourages any type of VIIRS calibration feedback.
NOAA VIIRS data availability:
Changyong (NOAA) stated that the NOAA CLASS archive has very stringent dataset requirements for archival. Currently, only the VIIRS Version 0 is available at NOAA CLASS. VIIRS version 2 will be archived at NOAA CLASS in the future but will take 1-2 years. Changyong recommended that we use the processing-on-demand feature at STAR and the University of Maryland facility. It is not ready at this time. In the meantime, Changyong will allow access to the DCC, desert, and SNO VIIRS V2 granules, that their group has used for their calibration activities, when they become available in this Fall. Changyong also mentioned that when improvements are made, that V2 is the baseline dataset and that correction factors will be supplied that will convert the V2 radiances to future versions such as V3 etc. radiances. Changyong stated that the VIIRS raw data is 1/10 the volume of the calibrated dataset. However, applying calibration modules to the raw data requires a lot more work to verify proper implementation and he did not recommend this path for general users. NASA (Raj Bhatt) will process the VIIRS V2 DCC granules in order to provide all GEO domains their unique DCC reference response, in the same manner as with the Aqua-MODIS DCC reference response. CMA (Scott) has requested VIIRS V2 data over Tibetan glacier sites and will email the coordinates of the sites to Sirish Uprety or Fangfang Yu.
The selection of VIIRS M or I band as the calibration reference:
Dave Doelling (NASA) presented a strategy for selecting VIIRS I or M bands if they cover the same wavelength range. This applies to the M5 and I1 (0.65µm), M7 and I2 0.86µm) and M10 and I3 (1.61µm) bands. It was noted that only the M5 and I1 bands have differing spectral bands. I bands have more radiometric noise than the M bands, but if I1 pixels are averaged into the FOV of a M band pixel, they should have comparable noise. There is greater SBAF uncertainty if the two inter-calibration bands differ, and some examples were given in the presentation. Sirish stated that initially the VIIRS I1 and M5 calibration differed by 1.5%, where M5 was brighter. In VIIRS V2, the M5 band calibration was lowered by 0.985 (slide 12) making the I1 and M5 calibration consistent. There is an action item given to Changyong to document the consistency of the M5 and I1 band calibration. Fangfang also added that when she compared the GOES-16 0.65µm calibration, based on multiple methods, it most resembled the I1 calibration. The guidance given to GSICS members is to use the M band for all VIIRS bands, except for the 0.65µm wavelength, where members should select the M or I band based on spectral response function consistency. For example, for GOES-16 band 2, should use the I1 band, whereas for OCLI should use the M5 band.
GSICS VIS/NIR sub-group has recommended that the NOAA NPP-VIIRS V2 as the new RSB calibration reference.
It is recommended to use the M band that most closely resembles the spectral response function of the sensor to be inter-calibrated. For the 0.65µm wavelength, use either the I1 or M5 band that is most consistent with the given sensor.
: NASA (Raj Bhatt) will process the VIIRS V2 DCC granules in order to provide all GEO domains their unqiue DCC reference response, in the same manner as with the Aqua-MODIS DCC reference response.
: NOAA (Changyong or Sirish) to make available the VIIRS V2 granules over the DCC, desert, and SNO sites used in their V2 calibration efforts, when ready this Fall.
: NOAA (Changyong or Sirish) to provide CMA(Scott) VIIRS V2 data over Tibetan glacier sites after emailing the latitude and longitudes of the region of interest.
: NOAA (Changyong) to document the consistency of the VIIRS V2 M5 and I1 band calibration.
: NOAA (Changyong) to write a journal article of the VIIRS V2 calibration that GSICS members can reference. Until then the readme file is available.
: NASA (Dave) to prepare the documentation to allow the VIIRS V2 L1B
dataset as a GSICS deliverable.