# VIIRS Day/Night Band--Correcting Striping and Nonuniformity over a Very Large Dynamic Range
## 1 Introduction
### VIIRS Design  
The VIIRS design was primarily based on the MODIS instrument heritage. However, unlike MODIS, VIIRS also has the DNB, which has more stringent stray light exclusion requirements. Thus, VIIRS was specifically designed to suppress stray light [6]. The first optical element in the MODIS optical path is a scan mirror used to produce its cross-track scanning. VIIRS instead uses an afocal, three-mirror anastigmat telescope, where the entire telescope assembly rotates in order to produce cross-track scanning. One reason the rotating telescope design was chosen rather than a scan-mirror was to suppress stray light. This is because the primary mirror in the telescope is recessed deeper inside the scan cavity. As the telescope rotates it takes a view of the three calibration (cal) sectors: the Space View (SV) that points out to space just above the mesospheric limb, and occurs just before the start of the Earth-view; the Black Body (BB) view that occurs after the telescope sweeps across the Earth and is inside the instrument; and the Solar Diffuser (SD) that is illuminated by the sun through a solar attenuation screen for during a period of about 2 to 3 min per orbit when the solar geometry is right [7]. A rotating mirror relays light from the telescope to the detectors, and since it rotates at half the rate of the telescope, it is called the Half-Angle Mirror (HAM). The DNB has a wide spectral bandpass, nominally between 500 and 900 nm. It is capable of a dynamic range in radiance of about eight orders of magnitude at nadir and seven orders of magnitude at the edges of its 3000 km wide swath. It achieves this dynamic range via a charge-coupled device focal plane array that is sectioned into four stages, where each stage essentially functions as a separate imager even though they all reside on the same detector array. The array has much higher detector resolution than what is required for the DNB's 750 m pixel footprint, but it combines detector signals electronically in the read-out circuit, aggregating them as a function of scan position. Using variable aggregation over 32 symmetric zones about nadir, it compensates for perspective and foreshortening in a way that results in an almost constant 750 m horizontal ground resolution [8]. The four stages for the DNB are the Low-Gain Stage (LGS), Mid-Gain Stage (MGS) and two redundant High-Gain Stages (HGS). The LGS is used for daytime scenes, the MGS for twilight scenes near the Earth's terminator, and the HGS for late twilight and nighttime scenes. Because of the complexity of the DNB circuitry it is important here to define some terms. The detector array actually has hundreds of detectors in each dimension that are aggregated into 16 virtual along-track detectors. Because only the aggregated output of these many detectors is transmitted to the ground, we will always refer to the virtual detectors as \"detectors.\" When we do refer to the actual preaggregated detectors, we will use the word \"subpixels.\" In this paper three terms are used to describe detector aggregation--aggregation mode, aggregation zone and aggregation sequence number. The terms are defined here in the way that they are usually used on the VIIRS program. \"Aggregation zone\" is used when referring to a specific area of the swath in the cross-track direction relative to nadir. \"Aggregation mode\" refers to the specific method of aggregation, that is, the number of subpixels that are aggregated in each dimension in forming a single DNB pixel. \"Aggregation sequence number\" refers to the aggregation mode numbering specifically in relation to the order that the aggregation modes are cycled through in the cal sector data. Therefore, this term will not be used when referring to data in the Earth-view. The aggregation zones are numbered symmetrically about nadir, with Aggregation Mode 1 at nadir and Aggregation Mode 32 at edge of scan. This is opposite the Aggregation Sequence Number, so that Aggregation Mode 1 is Aggregation Sequence Number 32 and vice versa.