Transcript 2012_DNB_Reflectance_Miller

Developing the VIIRS/DNB Lunar
Reflectance Product
Steve Miller
([email protected])
Updated: 27 July 2012
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 reflectance units.
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.
Miller and Turner, 2009. IEEE Trans. Geosci. Rem. Sens.,
47(7), 2316-2329.
Lunar irradiance model output, showing highly
variable magnitude as function of lunar phase
Reflectance Near ‘Lunar Terminator’
VIIRS DNB
Radiance
VIIRS
DNB Reflectance
The lunar model can be used to
produce a form of near constant
contrast (NCC) imagery.
SOUTH
AFRICA
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… 
85
Lunar Zenith Angle
In this scene the Moon is setting
in the west at the time of the
DNB nighttime overpass.
0
100
70
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-ofatmosphere 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.
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.
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.