Re: [cf-satellite] Calibration Look Up tables

• To: cf-satellite@xxxxxxxxxxxxxxxx
• Subject: Re: [cf-satellite] Calibration Look Up tables
• From: John Caron <caron@xxxxxxxxxxxxxxxx>
• Date: Thu, 27 May 2010 09:48:50 -0600

On 5/26/2010 2:25 PM, Ken Knapp wrote:

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

Hi Ken:

Some details in above example: Seems like this could be

  byte image(lat,lon);

(always nice to reduce file size by 4x).

  byte image(lat,lon);
    image:_Unsigned = "true";

byte image(lat,lon)
    image:long_name = "GOES Water vapor channel";
    image:units = "digital number";
    image:CF\:lookup = "table_1";
    image:_Unsigned = "true";

• References:
  • [cf-satellite] Calibration Look Up tables
    • From: Ken Knapp