VIIRS AF Products

M Band Product

The fire icons () on the map represent daytime detections made by VIIRS (theoretical probability of detection is shown below) for the preceding day and thus each fire icon is a detection from the past 24 hours. Clicking a fire icon in the map will display the center coordinates for the corresponding nominal VIIRS pixel (750m, at-nadir). You can change the date of the detections you want to see with the upper left corner drop-down window or turn off the detections with the slide bar.

The color-coded satellite icon show the center of the satellite granule.

  • Red for previous 24 hours

Click a satellite icon to view an RGB M-band (5, 4, 3, respectively) quicklook with fire hot spots and lat/lon graticules overlaid. Right click to “save image as”. (M-bands and corresponding wavelength centroid – 5: 0.672µm; 4: 0.555µm; 3: 0.488µm)

You can also download just the RGB as a GeoTIFF, as well as the M-band detections for the entire day (cover all of CONUS) as an ASCII file or KMZ.

VIIRS Moderate-bands Active Fire Algorithm

The current standard IDPS VIIRS active fires (AF) product is based on the MODIS Collection 4. In keeping with MODIS, the algorithm is a hybrid thresholding and contextual algorithm using radiometric signals from 4 micron and 11 micron bands (M13 and M15, respectively) and additional bands and a suite of tests for internal cloud mask and rejection of false alarms. The product consists of simple file containing primarily latitude & longitude data for those pixels classified as thermal anomalies. The data is distributed in 4-85 second granules in HDF 5 format. Note, this is not the same as MODIS which is HDF4.

Figure 1: The above figure shows how the primary fire channels (bright red) used by VIIRS, MODIS, and AVHRR align. The two primary channels for VIIRS are M13 and M15, the 4 and 11 micron channels, respectively, while the additional channels (dark red polygons) are used for contextual test (e.g. cloud screening).

Spatial Resolution:

M bands horizontal sampling interval at nadir is 742m (along track) x 259m (along scan). The sensor scan parameters used in the aggregation design scheme are:

  • VIIRS scan extends from –56.059° to +56.059°
  • Along-track horizontal sample interval is 0.742 km at nadir
  • Cross-track horizontal sample interval is 0.259 km at nadir
  • 16 along-track pixels per scan.

Sub-Pixel Aggregation

  • 3-pixel aggregation at scan angles less than 31.59°
  • 2-pixel aggregation at scan angles from 31.59° to 44.68°
  • 1-pixel aggregation at scan angles greater than 44.68°

The VIIRS ‘M’ and ‘I’ band detectors are rectangular, with the smaller dimension projecting along the scan. At nadir, three detector footprints are aggregated to form a single VIIRS “pixel.” Moving along the scan away from nadir the detector footprints become larger both along track and along scan due to geometric effects and the curvature of the Earth. The effects are much larger along scan. The aggregation scheme is changed from 3x1 to 2x1 at 31.589° in scan angle. A similar switch from 2x1 to 1x1 aggregation occurs at 44.680°. The pixel growth multiplier is limited to approximately 2, both along track and along scan, compared with a growth factor of 6 along scan which would be realized without the use of the aggregation scheme.

Bowtie deletion:

As VIIRS is carried in the track direction by sa tellite motion, it scans th e earth in a direction perpendicular to the track directi on. Since the angular spac ing of samples in the scan is constant, the ground footprint of the samples is minimum at nadir and is larger in both scan and track directions at points in the scan other than nadir. At large angles from nadir, the swath width in the track direction is sufficiently large that detectors at the edges of the swath overlap the preceding or succeeding swath. This is referred to as the bowtie effect and can be seen in the image below. In VIIRS data, in order to minimi ze data rate, some of th is redundant data is not transmitted. This is referred to as bowtie deletion:

  • 0 pixels deleted at scan angles less than 31.59 degrees
  • 2 pixels deleted at scan a ngles from 31.59 to 44.68 degrees
  • 4 pixels deleted at scan a ngles greater than 44.68 degree

The actual number of pixels in each aggregation angle zone ( both before and after the 3:2:1 aggregation whether on-board or on the ground) is given in Table 5. All calculations of cell sizes ar e based on the following orbit pa rameters, 6371 km Earth radius and 833 km orbit altitude and are considered nominal performance. Note: Bowtie correction has been performe d for the resampled (geographic projection) GeoTIFF and PNG datasets provided on this website.