About
The Visible Infrared Imaging Radiometer Suite (VIIRS) - aboard the NOAA-21, S-NPP, and NOAA-20 satellites - provides views of the entire globe in 5 higher-resolution channels (“I-Bands”: 375m pixels at nadir); in 16 moderate-resolution channels (“M-bands”: 750m pixels at nadir); and in a panchromatic “Day/Night Band” (also 750m pixels at nadir). For each of these channels, the raw Sensor Data Records are geometrically* corrected to produce VIIRS Imagery products (rectangular granules roughly 3,000x600km along the orbit track) which are then thinned and reformatted to produce the NetCDF product that AWIPS receives.
Observations from the Day/Night Band undergo additional processing to ensure a nearly-uniform contrast despite large differences between day and night reflectances. For this reason, the VIIRS Imagery product for the Day/Night Band is usually called “Near-Constant Contrast” (NCC).
NOAA prepares a "thinned" version of the NCC product (and additional VIIRS products) expressly for AWIPS - as documented in the NDE to AWIPS Interface Control Document (Sept. 2012) available here. These "thinned" products are what are made available on SBN.
VIIRS’ spatial resolution (375m or 750m per pixel) is finer than the 1km to 2km resolution of most channels of the GOES-R Advanced Baseline Imager (ABI). VIIRS bands are near the 0.5km resolution of ABI’s Channel 2 (0.64µm).
Each VIIRS imagery channel measures radiances in a portion of the electromagnetic spectrum, from Visible to Near Infrared (IR), Short-, Medium-, and Long-wave IR. The chart below compares VIIRS’ spectral coverage (red) with that of ABI (blue).
As for timing, the NOAA-20, NOAA-21, and S-NPP satellites are on a low-earth, sun-synchronous orbit slightly more than 100 minutes long; each crosses the equator at 13:30 local solar time in the northbound direction (ascending node), and at 01:30 local solar time in the southbound direction (descending node). NOAA-20 and S-NPP are at opposite points on this orbit. NOAA-21 is directly between them, one quarter-orbit ahead of S-NPP. Together they provide VIIRS imagery for most of the globe at least six times per day (three in the daytime; three more at night). In mid- and northern latitudes, given the overlap among VIIRS swaths, many CONUS locations are sensed 8 or 9 times a day (between the three satellites); and parts of Alaska 16-20 times per day.
VIIRS Imagery products from NOAA-21 and S-NPP use the same WMO headers, so when received via SBN, they are stored in files with similar names (based on the headers). However, data files from NOAA-21 and S-NPP embed the original filename in ASCII format, which includes either "npp" for S-NPP or "n21" for NOAA-21. This allows them to be distinguished from each other.
Limitations
Temporal Resolution: Daylight observations occur every 22 hours. There is a 25- to 50-minute gap each between the midday NOAA-20, NOAA-21, and S-NPP viewings of a CONUS location. The next pass occurs at night, 10 hours later.
Latency: NESDIS ground systems only begin to receive the raw data 20 to 80 minutes post-observation, often resulting in latency greater than an hour.
AWIPS Product Availability: AWIPS sites are currently receiving only one of the VIIRS Imagery data products from NESDIS: the Day-Night Band “Near-Constant Contrast.” Additional VIIRS Imagery bands may be made available, depending on forecaster needs and dissemination capacity.
