# VIIRS Day/Night Band--Correcting Striping and Nonuniformity over a Very Large Dynamic Range
## 4 Discussion
### Changing the DNB Calibration Process to Reduce Striping  
By taking statistics for dark scenes over many orbits of data, a mean nightglow profile could be estimated from the dark noise via the Photon Transfer Curve (PTC) method and this could then be subtracted off [1]. In this way dark DNB imagery for scenes with no lunar illumination could be produced automatically by the calibration process that is both uniform and radiometrically accurate. This could also be done as an offline process for users who work with very dark scenes. The NASA VCST has reported a 6% to 7% bias when comparing HGS gain computed using the cal sector with the HGS gain determined from the monthly Earth-view cross-calibration [17]. They attribute this to the stray light in the Earth-view cross-calibration [10]. We have shown that there is actually stray light in both the cal sector data and in the Earth-view data, either can cause bias in the cross-calibration, and these levels should be compared to determine which is worse. The Earth-view  
Figure 23: Gain correction generated from HMM algorithm for Full moon scenes collected on 7 October 2014.  
stray light has been well characterized and an estimate is used in the twilight where cross-calibration occurs [22]. The stray light in the cal sectors have also been extensively characterized. Evaluating these data we have found that in general, the Earth-view data used by the original cross-calibration process has a better signal-to-stray-light ratio than does the SD illuminated by Earthshine. This is not because the stray light is stronger in the SD cal sector, but rather because the Earthshine signal is weaker compared with the Earth-view signal for the same solar angles. Nevertheless, if the stray light discrimination techniques described above were implemented, the SD signal could potentially yield better gain ratios and cause less striping. Also, an improvement to the Earth-view method would to be to perform the HGS/MGS cross-calibration using only the aggregation zones on the left half before nadir (beginning of scan). This would reduce stray light in the gain ratio computation because the calibration modes are reflected on each side of the scan, and because of the angle that the satellite path makes with the terminator, the left side transitions from MGS to HGS sooner, when the stray light is much less [12; 13]. The RSBAutoCal was designed to use only cal sector data, because it was presumed that automating the Earth-view cross calibration would be too complicated or would require putting the DNB in a non-operational mode [15; 21]. Actually the LGS and MGS data could be enabled to transmit at the same time that the instrument is switched to night mode for every orbit without any special operational procedure. There is a misconception that this requires putting the instrument into a non-operational mode, or that there must be a loss of operational data while doing this. The loss of daytime RSB does occur if in fact the day-to-night mode transition is enabled earlier to allow for LGS-to-MGS cross-calibration [12]. However, if the SD cal sector is used instead only for this MGS/LGS gain ratios, then the night mode can be enabled at the normal time, along with the MGS and LGS data transmission, as long as this transmission is automatically turned off before the SD is illuminated (otherwise the M4 and M5 bands cal sector data would not be transmitted). Allowing MGS and LGS to transmit for the first 35 min of the night mode for every orbit would provide more than enough data for per-orbit MGS to HGS gain ratio updates. Such a change, of course would require that flight operations change the per-orbit command sequence. This data could also be used for MGS offsets for any pixels where there is no electric lights, stray light, solar illumination or lunar illumination and for any LGS pixels at night.