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
--
Timothy J. Schmit
National Oceanic and Atmospheric Administration (NOAA)
National Environmental Satellite, Data, and Information Service (NESDIS)
Center for SaTellite Applications and Research (STAR) -- Formerly, ORA
Advanced Satellite Products Branch (ASPB)
tim.schmit@xxxxxxxxxxxxx or Tim.J.Schmit@xxxxxxxx
(608) 263-0291 (voice)
(608) 262-5974 (facsimile)
