FAQ


What is spatial resolution of the VIIRS AF product?

Through an aggregation scheme (see below) designed to reduce the pixel size growth with increasing scan angle, the pixels at nadir are 0.742 km x 0.776 km (along-track x along-scan).

What is the VIIRS swath width?

3040 km during normal operations.

What is the aggregation scheme and why?

The intention of the pixel aggregation was to provide a more uniform pixel size regardless of the scan angle. The official text:
The VIIRS 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. At around 32° in scan angle, the aggregation scheme is changed from 3:1 to 2:1. A similar switch from 2:1 to 1:1 aggregation occurs at 48°. The VIIRS scan consequently exhibits a pixel growth factor of only 2 both along track and along scan (1.6 x 1.6 km at 56.063 degree scan angle), compared with a growth factor of 6 along scan which would be realized without the use of the aggregation scheme.

Why is the along-track swath so narrow?

NPP was designed to reduce latency and therefore the nominal swath is roughly 85 seconds. The images provided here have been aggregated to about 5 minutes to better coincide with MODIS. See examples below:

85 second swath:

5 minute swath:

 

What is the striping at the edges of the swath/image?

As VIIRS is carried in the track direction by satellite motion, it scans the earth in a direction perpendicular to the track direction. Since the angular spacing 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 MODIS image example below. In VIIRS data, in order to minimize data rate, some of this redundant data is not transmitted and thus referred to as “bowtie deletion”.

As VIIRS is carried in the track direction by satellite motion, it scans the earth in a direction perpendicular to the track direction. Since the angular spacing 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 MODIS image example below. In VIIRS data, in order to minimize data rate, some of this redundant data is not transmitted and thus referred to as “bowtie deletion”.

A feature of the VIIRS flight software that trims the ‘M’ band and ‘I’ band data in the along-track direction as a function of along-scan aggregation zone will be enabled. This will result in some of the samples in the overlap area to be excluded from the data that are delivered to the ground. The VIIRS Command, Telemetry, Science and Engineering Data Description [EDD154640-101] describes this “bow-tie deletion” feature, which is illustrated in the figure below.

- 0 pixels deleted at scan angles less than 31.59 degrees

- 2 pixels deleted at scan angles from 31.59 to 44.68 degrees

- 4 pixels deleted at scan angles greater than 44.68 degrees

 

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 the table below:

What does Beta data mean?

  • Early release product.
  • Initial calibration applied.
  • Minimally validated and may still contain significant errors (additional changes are expected).
  • Available to allow users to gain familiarity with data formats and parameters.
  • Product is not appropriate as the basis for quantitative scientific publications, studies and applications.

What does Provisional data mean?

  • Product quality may not be optimal.
  • Incremental product improvements are still occurring as calibration parameters are adjusted with sensor on-orbit characterization (versions will be tracked).
  • General research community is encouraged to participate in the QA and validation of the product, but need to be aware that the product validation and QA are ongoing.
  • Users are urged to consult the SDR product status document prior to use of the data in publications.