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20050113: NVXOPT.FOR



>From: Owen Cooper <address@hidden>
>Organization: Aeronomy Laboratory/NOAA
>Keywords: 200501131844.j0DIikv2015187 McIDAS navigation

Hi Owen,

I was away at the AMS conference in San Diego last week and unable to
reply before now.

>I'm working on extracting cloud top height from GOES IR imagery, by 
>cross referencing brightness temperatures with the temperature fields 
>from the GFS model.

OK.

>IR pixels at high latitudes appear colder than they truly are due to 
>attenuation of the IR radiation through the atmosphere at high satellite 
>zentih angles.  This will make the clouds seem higher in altitude than 
>they really are.

Right.

>Tony Schreiner at U. of Wisc said the Mcidas subroutine 
>NVXOPT.FOR can correct for the zentih angle problem.

There is no NVXOPT.FOR function in the McIDAS distribution either from
SSEC or from Unidata.  There is, however, an entry point named NVXOPT
in each McIDAS navigation module:  

nvxmsgt.dlm   nvxairc.dlm   nvxgraf.dlm   nvxtiro.dlm   nvxlamb.dlm
nvxgmsx.dlm   nvxmsg.dlm    nvxgvar.dlm   nvxmoll.dlm   nvxgoes.dlm
nvxlalo.dlm   nvxtanc.dlm   nvxradr.dlm   nvxmsat.dlm   nvxdmsp.dlm
nvxsin.dlm    nvxmerc.dlm   nvxrect.dlm   nvxps.dlm

Here are a couple of snippits from the McIDAS Programmer's Reference Guide:

http://www.ssec.wisc.edu/mug/prog_man/2003/access-6.html#30949

that refers to what NVXOPT is for:

nvxopt - performs special navigation services and is module-dependent

Special services                      Modules that support them

oblate radius                         nvxmerc, nvxps, nvxrect

orbital period                        nvxdmsp

parallax correction                   nvxgoes, nvxgvar, nvxmsat

satellite location/subpoint           nvxdmsp, nvxgoes, nvxgraf, nvxgvar,
                                      nvxlamb, nvxmerc, nvxmsat, nvxps, nvxradr,
                                      nvxrect, nvxsin, nvxtiro

view angles (satellite, solar, etc.)  nvxgoes, nvxgvar

So, it looks like Tony was talking about the special services offered
by the copy of NVXOPT that is contained in nvxgoes, nvxgvar, and nvxmsat.

>How can I find out more about this subroutine and how to apply it?

I am afraid that you will need to read code.  I would also send a note
back to Tony and ask him for some detailed pointers on how to use
NVXOPT to "correct for the zenith angle problem".

>I 
>haven't had any luck searching through the support e-mails or the user's 
>manual

The misdirection was being told that there was an NVXOPT.FOR routine.

>I thank you for any information you can send my way,

I hope that the above helped...

Cheers,

Tom
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