Developing the VIIRS/DNB Lunar Reflectance Product

1. Developing the VIIRS/DNB Lunar Reflectance Product Steve Miller (Steven.Miller@colostate.edu) Updated: 27 July 2012

2. Lunar Spectral Irradiance Model VIIRS Day/Night Band (DNB) data are provided as calibrated radiances, allowing in principal for quantitative applications. The lunar source is highly variable across the ~29.5 day lunar cycle…  We must account for this variation in order to exploit the data quantitatively. Example animation of the lunar cycle A lunar irradiance prediction model (Miller and Turner, 2009) allows for conversion from DNB radiance to reflectanceunits. R = I / [cos(m)Em] R = DNB reflectance, I = DNB radiance, m = lunar zenith angle, Em = lunar irradiance Reflectance enables quantitative applications lunar measurements, as well as ‘constant contrast’ imagery. Lunar irradiance model output, showing highly variable magnitude as function of lunar phase Miller and Turner, 2009. IEEE Trans. Geosci. Rem. Sens., 47(7), 2316-2329.

3. Reflectance Near ‘Lunar Terminator’ VIIRS DNB Reflectance VIIRS DNB Radiance • The lunar model can be used to produce a form of near constant contrast (NCC) imagery. • Applicable to night-only (i.e., to lunar observations at different times in the lunar cycle, and especially in the lunar terminator region). • Not applicable to the day/night terminator where solar signal is present. • Performance near the lunar terminator (28 June 2012, South Africa, around first-quarter Moon) shown here…  • In this scene the Moon is setting in the west at the time of the DNB nighttime overpass. SOUTH AFRICA 85 Lunar Zenith Angle70 0100

4. VIIRS ‘Near Constant Contrast’ (NCC) Operational Product The VIIRS Environmental Data Record (EDR) imagery product suite includes a ‘Near Constant Contrast’ (NCC) product that is intended to provide a consistent appearance to DNB imagery both day and night, and across the terminator. The NCC product maps the DNB Sensor Data Records (SDRs) to the coarse-grid Ground-Track Mercator (GTM) map and then attempts to normalize the DNB top-of-atmosphere radiances (which have ~7 orders of magnitude dynamic range from day to night) in a way that yields a scaled quantity that is similar to a reflectance. Uses a set of look-up tables describing gain as a function of solar and lunar zenith angles, surface reflectance functions, and lunar radiance as function of lunar phase to compute a downwelling TOA source radiance. The algorithm then transforms DNB radiances to a unitless16-bit scaled NCC output. Scale and offset coefficients supplied in the EDR are applied to this output to yield a value between [0,1] at each pixel in the remapped GTM grid.

5. NCC Product Shown below is an example of the NCC performance for a day/night terminator (non-lunar) case. The NCC, when it is working, appears to be doing a good job in extending constant contrast into the twilight portion of the granule swath. SVDNB NCC Curtis Seaman, CIRA • For nighttime scenes, currently the NCC product only works around the time of Full Moon. When this bug is fixed we will do comparisons against the lunar model-based product.

6. Other Notes • Reflectance in the expected range [0,100] for the DNB reflectance product for the low lunar illumination example shown is encouraging, given uncertainties in the model and the amplifying effects of the cosine scaling. • However, more tests are needed, including comparisons against stable targets observed during the day and sampling across the full lunar cycle. This is work in-progress. • Cosine weighting (lunar zenith angle) accounts for West/East normalization of the cloud and surface feature brightness. Lunar irradiance model provides the reflectance [0-100] scaling factor. • Day/Night terminator crossings for Suomi-NPP and the future JPSS (all 1330 LTAN orbits) will occur mainly at high latitudes (e.g., poleward of 60 degrees, particularly in the winter hemisphere). • The de-scoped 0530/1730 satellite would have encountered cross-terminator swaths for much of its orbit.