# Presentation: 265866

## NPOESS Preparatory Project (NPP) VIIRS Calibration Maneuvers

- May 15, 2008

- Ocean PEATE Team

## Background 

- NPP Deputy Project Scientist for Sensors (Butler) is coordinating instrument requests for calibration maneuvers for the NPP Project Office.
- VIIRS requests were presented by McClain (lunar roll) and Xiong (solar diffuser yaw and deep space pitch) in February.  These slides are excerpted from those presentations.
- All instrument requests are currently being reviewed by Project and spacecraft engineers.
- NPOESS platforms will not perform calibration maneuvers.

## Slide 3

## Maneuvers Proposed for VIIRS

- Lunar Roll Maneuvers
- Near monthly
- Solar Diffuser Yaw Maneuvers
- One early in the mission and repeat as needed
- Each set consists of 12-16 yaws (in consecutive orbits)
- Deep Space Pitch Maneuvers
- Two pitch maneuvers: early and late in the mission

## Lunar Calibration Roll Maneuvers

- Moon is the most reliable calibration source
- Observations at _constant_ phase angle most optimal
- Provided unprecedented stability (~0.1 %) for SeaWiFS (7° phase angle).
- MODIS performs monthly roll maneuvers to maintain 55° phase for operational calibration (optimal given viewing & maneuver limitations).
- VIIRS lunar calibration considerations** **(10:30 am or 1:30 pm orbits)**:**
- Without roll maneuvers, only 4-5 monthly lunar views through SV port capture entire moon (but not at constant phase).
- 9 potential views/yr with phases between 40°-70° (daylight side of orbit) with _<_0.5° roll.
- 8 views/yr at 55° with roll maneuvers up to 15°.
- No lunar views during 3 consecutive months each year even with roll maneuvers.
- Benefits all EDRs that use VIIRS RSB (VIS/NIR/SWIR)

## VIIRS Lunar View through SV

- VIIRS

- NADIR

- Pitch

- Yaw

- Roll

- VIIRS Space View Port
- Aperture ~ 0.85o

- ~62o

- 65o

**Recommendation:  Optimize lunar calibrations by rolling the spacecraft**
**                                 **** ****to collect data at constant 55****° phase angle.**

## SeaWiFS Lunar Calibration Time Series

## Solar Diffuser Yaw Maneuvers

- Characterize system-level SD BRF and SD screen transmission
- SD BRF characterization uncertainty (1.3-1.5%) is the largest component for the VIIRS RSB calibration uncertainty (2%)
- Pre-launch BRF characterization is performed at limited wavelengths, viewing geometry, and beam size
- The SD screen is characterized at a component-level, not coupled with the SD panel or integrated with the sensor for a system-level characterization
- Yaw maneuvers enable a system-level BRF and SD screen characterization using sensor spectral bands and detectors under full on-orbit illumination conditions (stray light impact analysis)

## Solar Diffuser Yaw Maneuvers (cont)

- Characterize SD degradation uniformity
- SDSM views the SD at a fixed angle while the sunlight incident angles to the SD vary with observations
- Using observations from repeated yaws (with each performed in a short period time) enables characterization of SD on-orbit degradation uniformity, thus improve RSB on-orbit calibration quality
- Benefits all EDRs that use VIIRS RSB (VIS/NIR/SWIR)

## Deep Space Pitch Maneuvers

- Make deep space observations through the Earth View with sensor in its nominal operation mode
- Characterize thermal emissive band (TEB) system-level response versus scan angle (RVS)
- Pre-launch measurements were made at ambient conditions with limited angle of incidence
- Repeated pitch maneuvers track TEB RVX changes on-orbit
- Benefits all EDRs that use VIIRS TEB (MWIR/LWIR)
- SST benefits the most

## Conclusion

- The combined VIIRS teams (NICST and VOST) have proposed a comprehensive program of calibration maneuvers
- All maneuvers have substantial heritage (MODIS and SeaWiFS) and have proven to be essential to achieving the radiometric consistency and stability required for climate quality Ocean products.