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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 then1. converted to radiances linearly (or sometimes non-linearly) that can then be2. 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/
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