# VIIRS Day/Night Band--Correcting Striping and Nonuniformity over a Very Large Dynamic Range
## 3 Results
### Evaluating Causes of Striping and Nonuniformities
#### 3.2.1 Stray Light in the Cross-Calibration  
The VIIRS SDR Cal/Val Team performed analysis of the DNB gain ratios using the twilight Earth-view data as described above in _Original DNB Calibration Process_. The gain ratios changed in the first month by as much as 1.7%, with the biggest changes in the aggregation zones near the edges of the swath. The largest changes were in the MGS/LGS gain ratios. For most of the aggregation modes the gain ratios did not change by more than 0.5% over one month [33]. The analysis also revealed that differences in the details of the methodology for calculating the gain ratio look-up tables could change the amount of striping seen in the DNB radiances produced using those tables [34]. There were fluctuations in the gain tables reported during the first year [10] as the calibration methodology was being refined.  
We mentioned in the discussion of the RSBAutoCal that during the day there is enough signal in the SD cal sector to produce a gain ratio between the MGS and LGS, and in the SV and BB there is enough signal to determine a gain ratio between the MGS and HGS. Ideally none of these signals should exist if VIIRS were a perfect optical design, so these signals are, strictly speaking, all stray light contamination. After all, the SV should be as dark as space, and the black body should obviously be black for visible and near-infrared wavelengths, and further there is a baffle to prevent solar illumination from striking the BB.  
Unfortunately, the SD HGA and HGB signals saturate through most of the daytime, but there are two brief periods per orbit over the twilight region where they do not. However, these are the same periods where direct solar light enters the scan cavity and creates stray light [23]. As can be seen in Figure 7 the resulting HGA/HGB ratios are not consistent for two different day-to-night transitions, a clear sign of stray light issues [19]. These are during the period when the satellite is passing over the terminator from the day to night (the satellite is still in full sunlight). In the first example (a), the ratios at the beginning are around 0.955, which is probably close to the true HGA/HGB gain ratio. However, as the satellite moves deeper into twilight the ratio drops to 0.925. The actual response of the detectors would never change so suddenly, so this drop is apparently due to the difference between the stray light on HGA relative to HGB.  
The same is true regarding the difference between stray light on MGS relative to HGA or HGB. In the second day-to-night transition the ratios drop off sooner, possibly because the Earthshine signal is weaker here. For the night-to-day transition, there are only a few unsaturated high gain scans, and many orbits would be required to cover all the ASNs. Therefore, using the SD for determination of gain ratios may be difficult. It may be possible, however, to use the HGA/HGB ratio as a discriminator to detect stray light contaminated data used for HGS/MGS gain ratio computation. Also, by comparing the SV and BB sneak-path stray light signal to the SD signal, a signal-to-stray-light ratio could be computed, and used for filtering. However, neither of these discrimination techniques are used in the current RSBAutoCal algorithm or the Land SIPS automatic calibration. Therefore, not taking stray light contamination into account could lead to striping and bias in the HGS calibration.  
Figure 7: HGA/HGB ratio for two day-to-night transitions. Each of the 16 detectors has a different color/symbol combination, and this plot illustrates the spread (or striping) by detector.  
Because of these inconsistencies, the authors believe that if BB and SV data were to be used in cross-calibration to determine gain ratios in RSBAutoCal, this would make striping and non-uniformities even worse than they now are, and would also cause more calibration bias. The VIIRS SDR Cal/Val Team has reported problems with this method, and it therefore has not been activated in the RSBAutoCal [20].