# VIIRS Day/Night Band--Correcting Striping and Nonuniformity over a Very Large Dynamic Range
## 1 Introduction
### On-Orbit DNB Calibration Process
#### 1.2.2 DNB RSB Automatic Calibration Process  
The VIIRS thermal emissive bands have always been calibrated by performing offset and gain corrections on a per-scan basis, therefore making the process completely automatic. From the beginning it was hoped that RSB calibration could eventually be automated in the same way [14], but the once-per-orbit SD view complicated gain calibration. However, fully-automated RSB calibration has recently been implemented as part of IDPS and is referred to as the RSB Automated Calibration (RSBAutoCal). For the other RSB bands, the RSBAutoCal process is relatively straightforward. The per-orbit SD events used to compute time dependent, per-detector gains are temporally filtered to produce smoothed gain corrections [15]. For the DNB the monthly updates of both the gains and the offsets have presented an almost insurmountable obstacle to automate. Nevertheless, much work has been done to achieve that goal, and the development of that process is summarized here. The use of cal sector data is key to achieving an automated calibration process. Shortly after the DNB became functional in January 2012, the VIIRS SDR Calibration/Validation (Cal/Val) Team began analyzing the cal sector data. All the cal sectors were found to be contaminated with stray light. However, during the periods when the satellite went into the Earth's shadow, there was no stray light signal, and it was suggested that averages of the data from these periods could be used to determine offset [5; 16]. This has become the basis for automated DNB offset correction. Still, the absolute offset could not be determined because of differences between the Earth-view and the cal sector, but a pitch maneuver was performed which filled the entire Earth-view sector with a view of deep space. This deep-space view has become the offset reference for the Earth-view, and the changes to offsets from the cal sector data are taken with respect to this [17; 18]. The SDR Cal/Val Team also noticed during the daytime portions of the orbit that the SD cal sector signal was about two orders of magnitude higher than either the SV or BB signals. Since during the day the attenuation screen is not exposed to any direct sunlight, it was hypothesized that this strong signal was due to Earthshine (solar reflection off the Earth) coming through the attenuation screen and illuminating the SD. Looking for a technique to provide gain ratios using only the cal sector data, it was suggested that this daytime SD signal could be used for LGS-to-MGS cross-calibration and possibly MGS-to-HGS cross-calibration as well [5; 19]. Concerns were raised, however, about whether sneak-path stray light might be contaminating the MGS-to-HGS gain ratios [20]. These ideas were incorporated into the operational VIIRS calibration algorithm at IDPS in December 2013 [15], but operationally it was placed in \"manual mode\" (as described in the previous section) until it could be fully tested off-line. In August 2015 the VIIRS RSBAutoCal was placed in automatic calibration mode for all bands except the DNB [20]. At the time of this writing, the RSBAutoCal has not yet been put into automatic mode for most of the DNB calibration, so its effects are not seen in any of the images shown here. Nevertheless, the process will be discussed here with considerations of how it would affect nonuniformity in the DNB. The RSBAutoCal algorithm takes the gain ratio process a step further by providing the option of using not only the SD sector data, but also the BB and SV data to produce gain ratios for cross-calibration [21]. Both the BB and SV are predominantly stray light [5; 15; 22], so this method is based on the assumption that stray light illuminates the HGA, HGB, MGS and LGS sections of the CCD focal plane array equally. This assumption will be discussed later in the paper. The NASA VCST has also incorporated DNB calibration algorithms similar to the RSBAutoCal [17].