Geo-location and the use of a terrain model

[Adam]

I copy my mail (without attachment) I sent to some of you this morning
here:

Hi,

As Trygve I also "enjoyed" myself a little on my way home looking into
the sample data and documentation.

I stumbled over a few writings which I think are relevant to our
discussions last week:

----------------------------------------------------------------------------------
D34862-03E_CDFCB-X_Volume_III_PR.pdf
Page 287

Document page number = 300.

"""
While the geolocation angles included with VIIRS SDRs are with respect
to the Geoid, the calibrated TOA reflectance is calculated using the
terrain corrected solar zenith angle. Note that radiances are not
corrected for out of band contributions.
"""
----------------------------------------------------------------------------------

D34862-01F_CDFCB-X_Volume_I_PR.pdf
Page 235-236

Document page number = 249-250

"""
This column is provided to indicate which terrain corrected GEO
corresponds to a particular SDR. SDR Geolocations delivered with SDRs
are not the terrain corrected geolocations. However, users who would
like the terrain corrected version of a Geolocation may order a
corresponding granule of any EDR listed in this column.
"""


In same document on page 328:

"Table A-8, Geolocation Identifiers"



GIMGO : VIIRS Image Bands SDR Ellipsoid Geolocation Data GITCO : VIIRS
Image Bands SDR Terrain Corrected Geolocation Data GMODO : VIIRS
Moderate Bands SDR Geolocation Data Provided in Degrees
(D34862-03: NPOESS CDFCB-X Volume III) GMTCO : VIIRS Moderate
Bands SDR Terrain Corrected Geolocation Data
----------------------------------------------------------------------------------


The plot attatched shows the differences in geolocation between the
Ellipsoid and the terrain corrected geolocation for the VIIRS M-bands.
I simply read the two sample files GMODO and GMTCO and plotted the
differences.


So, to me it looks like we will get both (maybe dependent on the
configuration of the CSPP) and can choose to take the one which is not
terrain corrected.

Then, as Trygve also concluded, we may need to better understand the
statement "the calibrated TOA reflectance is calculated using the
terrain corrected solar zenith angle". This implies that for the
shortwave channels the reflectances have been corrected (normalised)
by the solar zenith angle. I am rather used to a practise where you
apply the normalisation by sun-zenith yourself afterwards. I would
prefer it that way, but apparently this is not how it is for NPP/
VIIRS.
Does anybody know more or have any comments?


I was considering if we should create a google-group on NPP (NPP
preprocessing and understanding the data).
Are you in for that idea?

-Adam