VNP02DNB stands for the Suomi National Polar-orbiting Partnership (SNPP) platform-derived NASA VIIRS panchromatic Day-Night band (DNB) calibrated radiance product. The DNB is one of the M-bands with an at-nadir spatial resolution of 750 meters (across the entire scan). The panchromatic DNB's spectral wavelength ranges from 0.5 µm to 0.9 µm. It facilitates measuring night lights, reflected solar/lunar lights with a large dynamic range between a low of a quarter moon illumination to the brightest daylight. The DNB attempts to maintain a nearly constant 750-m resolution over the entire 3060 km orbital swath by resorting to an on-board aggregation method, which also renders the calibration of the DNB a challenge. Stray-light and other sources of noise (lunar illuminance, twilight, clouds, noisy scan-edges, etc.) affect the DNB quality, and warrant correction. Considered a major advance in low-light imaging, the VIIRS DNB owes its heritage to the Operational Linescan System (OLS) visible sensors that were part of the Defense Meteorological Satellite Program (DMSP) platforms since the 1970s. The image dimensions of the 750-m DNB product measure 3232 lines by 4064 pixels.
Traceability for reference-based instrument calibration and measurement uncertainty
Traceability relates to how an instrument's uncertainty measurements were calibrated using reference sources whose absolute radiometric performance is well-known and understood relative to the International System of Units (SI). This usually happens in the pre-launch calibration phase using a national laboratory's standard sources, and later corroborated as part of the on-orbit validation activities in the post-launch phase.
The SNPP VIIRS instrument's pre-launch radiometric calibration was accomplished through measurements that are traceable to the National Institute of Standards and Technology (NIST), and its post-launch on-orbit characterizations are based on data derived from the VIIRS' on-board calibrators.
VIIRS Level-1B measurement uncertainty
The NASA VIIRS Characterization Support Team has developed VIIRS L1B measurement uncertainty algorithms that are designed to empirically derive measurement uncertainty values from the output L1B products to compare against the recommended values in the VIIRS Sensor Specifications that are detailed in the VIIRS ATBD. These measurement uncertainty algorithms are slated for implementation by the end of 2022. Subsequently, we should have data-derived measurement uncertainty values as part of the L1B product itself.
Radiometric accuracy forms a key requirement that enables us to derive good quality L2 retrievals, and provide the basis for higher-level products. The SNPP VIIRS instrument's radiometric accuracy is characterized for each of its three spectral band groups:
Reflective Solar Bands: Based on the VIIRS Sensor Specifications, the VIIRS reflective solar bands' calibration uncertainty for spectral reflectance over a scene with typical radiance is defined at less than 2%.
Thermal Emissive Bands: Given a uniform scene with 267 K brightness temperature (for image and emissive bands), the specified calibration uncertainty for spectral radiance is defined at 5% for Image band-4 and 2.5% for Image band-5.
Day-Night Band: The DNB's defined radiometric calibration uncertainty for the effective in-band radiance over a uniform scene is gain-state dependent.