# VIIRS Day/Night Band--Correcting Striping and Nonuniformity over a Very Large Dynamic Range
## 1 Introduction
### On-Orbit DNB Calibration Process
#### 1.2.1 Original DNB Calibration Process  
The original process for offset was devised based on pre-launch testing of the instrument and is described in full in the VIIRS SDR Calibration Algorithm Theoretical Basis Document [11]. In these tests it was found that the offsets observed in the space view were not consistent with the offsets observed in the Earth-view for any of the gain stages, and in addition, the Earth-view has a fixed-pattern noise that is not constant in the scan direction, even within a single aggregation zone. The possible reason for this discrepancy will be discussed later in the section _Causes of DNB non-uniformity_. Because of these issues, an alternative offset measurement was needed and it was decided that dark Earth scenes over the Pacific would be used to determine an offset for each of the 4064 pixels within a scan. Since the MGS and LGS are normally not transmitted at night, it was decided that once every lunar month around the time of new moon a special process would be run that transmits the MGS and LGS in addition to the HGS data. Any bright lights from islands or ships are then filtered out as outliers and the remaining data are averaged to produce an offset table for every detector, HAM side, gain state and in-scan pixel. Other statistical tests are also done to determine whether the data has the characteristics of dark noise [11; 12].  
The gain calibration process for the LGS is similar to the other Reflective Solar Bands (RSB) on VIIRS. Data are collected for the DNB as it views the three cal sectors. The mean counts of the SV sector is subtracted from the SD view, and using knowledge of the SD reflectance, attenuation screen transmittance and solar geometry, a reference radiance is calculated to determine the gain in radiance per digital count. For the DNB, however, there is a complication in that it has 36 Aggregation Sequence Numbers (ASN), 32 of which are used in the Earth-view. To accommodate this the DNB cal sectors undergo a 72 scan cycle with a scan for each HAM side and ASN combination. [13].  
Because the solar-illuminated SD radiance saturates the MGS and the HGS, these require a cross-calibration process. As with the offset calibration, the special mode that sends MGS and LGS is enabled, but for cross-calibration this needs to be done around the terminator region as the Earth scene transitions from night to day or day to night. It can be performed at any time during the lunar month, but because the SD is illuminated by the sun only during the night-to-day transition, this process is always performed only during the day-to-night transition, so as not to interfere with transmission of SD and SV cal sector data used for RSB gain calibration. The data simultaneously collected for HGS, MSG and LGS in the Earth-view are used to determine gain ratios between these stages. LGS gains from the SD view along with the gain ratios between each of the stages are then used to construct calibration gain tables for all the stages, ASNs and mirror side combinations [12; 13].