Hi all-
The LUT is an option, not a requirement. Thus, it makes sense for older
satellite data and data that is not linear, and as described, may be
difficult to standardize given the options possible. I think there
should also be some discussion on how to store the coefficients in a
standard way to help convert DN to brightness temperatures and would
support that too, but this thread is for look up tables.
The LUT option is simply one means (like scale_factor/add_offset) of
saving space that in some circumstances (certainly not all) makes sense.
So if it doesn't make sense for MODIS, then don't use it.
-Ken
Tim Schmit wrote:
Hi,
As there are more and more bits per pixel (MODIS-=12) and at least
some ABI bands will be 14 bits, I think the LUT approach makes less
sense.
Why not include a few conversion coefficients (and the equations)?
As was stated, the conversion from scaled integers to say radiance is
often linear.
The conversion from radiance to Planck is a few constants (how many
depend on the implementation method). For example, does one use
central wavenumber and 2 band correction values, or use the same band
correction values with 2 other constants (FK1, FK2).
Tim
Tom Whittaker wrote:
...
tom
---------- Forwarded message ----------
From: Ken Knapp <Ken.Knapp@xxxxxxxx>
Date: Wed, May 26, 2010 at 3:25 PM
Subject: [cf-satellite] Calibration Look Up tables
To: cf-satellite@xxxxxxxxxxxxxxxx
Raw satellite data are generally stored as integers (DN=digital
numbers) that are then
1. converted to radiances linearly (or sometimes non-linearly) that
can then be
2. converted to brightness temperatures.
With steps that are nonlinear, the scale factor offset doesn't work.
If a coefficient is tweaked/corrected, then the entire variable would
need to be rewritten.
Satellite data often use lookup tables to more easily and quickly
convert from DN to whatever (radiance/temperature). Updates would then
be made to calibration tables, rather than equations.
So I would propose something like the following CDL where variable
image has range from 0-255 and its attribute lookup means that the
table to convert to meaningful units is table_1
dimensions:
lat = 100
lon = 100
num_bins = 256
int image(lat,lon)
image:long_name = "GOES Water vapor channel"
image:units = "digital number"
image:lookup = "table_1"
image:valid_range = 0, 255
float table_1(num_bins)
table_1:long_name = "Brightness temperature"
table_1:units = "Kelvin"
Thoughts?
-Ken
--
Ken Knapp
Remote Sensing and Applications Division
National Climatic Data Center
151 Patton Ave
Asheville, NC 28801
828-271-4339 (voice) 828-271-4328 (fax)
IBTrACS: http://www.ncdc.noaa.gov/oa/ibtracs/
ISCCP B1: http://www.ncdc.noaa.gov/oa/rsad/isccpb1/
HURSAT: http://www.ncdc.noaa.gov/oa/rsad/hursat/
