Validating VIIRS Imagery from NOAA-20

VIIRS Imagery from the JPSS satellites is the highest spatial resolution (375 m) multi-spectral imagery of any operational weather satellite to date. The polar-orbit and wide/overlapping swaths allow global coverage of the entire world every 12 hours. SNPP is now accompanied by NOAA-20 (JPSS-1 before reaching orbit), launched in November 2017, providing a second VIIRS image 50 minutes in advance of VIIRS from SNPP. Both satellites occupy a polar orbit at the same local time, but a half orbit difference, on opposite sides of the earth. This allows a double view of any location at VIIRS wavelengths including the unique Day-Night-Band (DNB) and/or Near Constant Contrast (NCC) Imagery.

The Provisional validation level for VIIRS Imagery from NOAA-20 (JPSS-1) occurred at launch plus 90 days, much of which was orbit raising time and initial engineering checkout before the nadir doors were opened and the cryoradiator cooled the focal plane arrays to allow nominal imagery at about launch plus 48 days. This left only about 40 days for Imagery checkout.

Since each VIIRS instrument is unique, there was some hope that the stray light issue seen in the SNPP DNB would be reduced. However, stray light is largely similar between VIIRS on SNPP and VIIRS on NOAA-20, with notable exceptions. Software will be used to compensate for some of the stray light artifacts. The Look-Up Table for Near Constant Contrast (NCC) imagery will be updated at some point after the VIIRS (Sensor Data Record (SDR)) Team characterizes the DNB first.

Another known issue with VIIRS on JPSS-1 is the non-linearity in the edge of swath DNB that was encountered in testing of VIIRS before launch. The non-linearity is dealt with thru additional aggregation of pixels at the end/edge of the DNB swath. That resulted in an extended granule (needed to keep file size constant), which includes measurements beyond the normal end of scan, but on only one side of the swath. This causes nadir to be offset from the center of the DNB granule. The NCC product, derived from the DNB, was be able to handle this extended granule and nadir offset, to produce imagery that is not unlike that from SNPP.

Regardless of any pre-launch testing that was done, the real test of Imagery occurred after launch, when realtime data became available. The Imagery Team continues to work on the validation of VIIRS, expecting to meet Validated status at launch plus 9 months, which will be after this abstract has been submitted and before this presentation will be given.

Disclaimer: The views, opinions, and findings contained in this article are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration (NOAA) or US Government position, policy, or decision.