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[McIDAS #AMH-341410]: 2 questions



Hi Ioan,

re:
> the created: time value is surprising indeed.:)) I hope SSEC can clarify
> that.

After poking through code, I was reminded that the image creation time is
its start time.  The creation time in the example I sent was the time that
IMGCOPY created the AREA file in my local MYDATA/IMAGES dataset, and this
_is_ correct.

There are a couple of other words in an AREA file header that are described
as being (word reference is 1-based):

    W46.  Actual image start year and Julian day, yyyddd
    W47.  Actual image start time, hhmmss; in milliseconds for POES data

Since SSEC is setting the nominal time of the image:

     W4.  YYYDDD:   Nominal Year and Julian day of the data
     W5.  HHMMSS:   Nominal Time of the data

to the value of 'time_coverage_start', so including the actual start time
again would be a (little) bit redundant.

Also, there is no formalism for storing the image scan end time in the
AREA file header.

re: why is the scanning mode important to you?

> Well, long story short...GOES satellites have been giving us some trouble
> because, from our perspective, they are/were unstable when it comes to their
> scanning schedules. Of course this might be because we don't really have
> any hurricanes in Europe and we are used to very stable scanning (MSG).

Interesting.  I have never assumed that the the start of a scan would
be fixed to a particular minute much less second.  I am a bit surprised
that MSG's scanning schedule is so fixed.

re:
> Internally we use NetCDF to store data and we split the time dimension into
> 2 dimensions (date and time) (DFB/DAY day from beginning) and SLOT/TIME
> (just a whole number representing the number or position of the  image in
> the range of total number of images a specific instrument would acquire
> during during a day)

OK.

re:
> We do this for a number of reasons, most important  being the
> simplification of the computations we do using the satellite data (solar
> radiation estimation).

OK.

re:
> While the day/dfb business is straightforward, the slot is not.
> Whenever the current or past GOES instrument changed its scan mode or
> regime to RSO or SRSO, some images are shifted (SH coverage is taken at 05
> instead of 07 for example) and the process we call slot mapping was storing
> suddenly CONUS images instead of SH ones:))
> This resulted  in temporal holes in our  radiation database  for South
> America and complaining customers:))

Interesting.

re:
> I however, fixed all these issues when I started with Solargis so now we
> kind of have everything under control.
> For current GOES these mapping involve using the coverage of the data so we
> don't get tricked by the change of the scan regime.
> 
> For GOES16 I designed some functions which take as input the nominal time
> of an image and the scan regime and return the corresponding slot number
> for that given image in the day.

OK, I understand.

re:
> This functions will work even if I do not know the scan regime because I
> hard-coded the scan regimes into the code and even if GOES 16 will move to
> M6 scan regime I will store only let's say 96 images in our archive and
> ignore the others by using the M3 scan regime.

I think I understand...

re:
> It is very important however for us to know the exact time when a pixel was
> scanned to model the solar radiation at a given location precisely. this is
> why i need the end of scan time, to compute the scan time for every line in
> the image

Question:

- why can't you simply use the start scan time for your computation?

  This is the type of thing I have done in the past when helping a user...

re:
> In the implementation i use (I did some stats on CLASS data) :
> channel_no, scan time duration (s)
> 1 636.647916667
> 2 636.647916667
> 3 636.648958333
> 4 636.644791667
> 5 636.648958333
> 6 637.211458333
> 7 637.775
> 8 636.645833333
> 9 637.204166667
> 10 637.771875
> 11 636.645833333
> 12 637.204166667
> 13 637.769791667
> 14 636.645833333
> 15 637.216666667
> 16 637.784375
> 
> 
> 
> 
> I am attaching my ABI slot mapping module  in case you want to understand
> exactly how it works

OK, got it.  I am still curious why using the scan time beginning is not
as useful as using the scan time end.  Again, the McIDAS AREA format does
not have a formalism for storing scan time end.

Cheers,

Tom
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Ticket Details
===================
Ticket ID: AMH-341410
Department: Support McIDAS
Priority: Normal
Status: Closed
===================
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