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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.
Although the LUT approach would generalize linear and non-linear calibrationwith the same construct, the tables would get pretty large - 16bit not too bad, but the next size up is 32 ( can NetCDF have types in between ?). LUTs should perhaps be recognized if they are present, but not required considering Tim's
point, and that there's quite a bit of data already where the conversion is linear.
Why not include a few conversion coefficients (and the equations)?
Linear (scale/offset) is well understood, I guess there'd have to be way to represent/parse
and apply equations in general.
As was stated, the conversion from scaled integers to say radiance is often linear.
I can add that this is the case for MODIS.
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
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