------- Forwarded Message Date: Tue, 16 Apr 2002 11:49:35 -0400 To: address@hidden From: "Peter Romanov" <address@hidden> Subject: GOES Imager ch.1 calibration Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from quoted-printable to 8bit by ssec.wisc.edu id g3GFsX006824 Sender: address@hidden Precedence: bulk Hello, Does anybody know what calibration coefficients are used in McIDAS to convert GOES Imager ch.1 count into albedo value ? It looks like this conversion is slightly different from what is recommended by NOAA. The NOAA procedure to convert raw count to albedo is as follows (http://www.oso.noaa.gov/goes/goes-calibration/goes-vis-ch-calibration.htm): R=m(X-Xsp) A=kR where R is reflectance, X is a raw count value, Xsp is a space count and A is albedo. In particular, for GOES -8 Imager m = 0.5501873 [W/(m2-sr-m-count)] k = 1.92979 x 10-3 [(m2-sr-µm)/W] Xsp= 29 hence, A=1.06(X-29) However with this formula I am getting albedo values, which are 0.3 to 0.4% less than the ones calculated in McIDAS. It seems, that the reason for the difference is in the lower value of the space count (around 26) used in mcidas. Of course the 3 counts difference is small considering the noise and sensor degradation with time, but nevertheless, what is the right way to do the conversion ? Thanks, Peter ================================================ Peter Romanov NOAA/NESDIS/ORA, WWB 712 5200 Auth Road Camp Springs, MD 20746 USA tel: (301) 763 8042 fax: (301) 763 8108 ================================================ ------- End of Forwarded Message From: John Benson <address@hidden> Date: Tue, 16 Apr 2002 11:59:54 -0500 (CDT) To: address@hidden Subject: Re: GOES Imager ch.1 calibration In-Reply-To: <address@hidden> Warning: this is not a definitive answer. It is merely a discussion of where to look. The module kbxgvar.dlm in the McIDAS package is where the calibration is being performed. A quick reading of the code suggests (but does not prove) the following: 1. Tables which originate in the block0 record are being used unless they produce strange results, in which case pre-launch tables would be used instead. 2. It appears to me that the tables are expressed separately for each sensor, but that McIDAS is averaging them and using the average values regardless of which sensor is in use. 3. Early revision data on this module reveals the names Dave Santek and Bob Krauss. One of these folks might yet remember what is going on in the visible. I have actually spent some time in the internals of kbxgvar, but only to add new satelites. And most of the in-house work which has vetted the calibrations were done by people with much more interest in the correctness of the IR results than in any small discrepancies in the visible. Especially since McIDAS does nothing to correct for path length through the atmosphere or even solar angle. --johnb
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