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Re: harmonic analysis



Hi Ramesh,
Here are several possibilities; one of them may apply to the analysis you
are trying.

Any FFT analysis makes some underlying assumptions, for example that
the time series continues beyond the endpoints, with the same periodicities.
If there is a discontinuity between the data at the start and end of the data,
this will result in false signals in the FFT, and those will be present when
the code inverts the FFT back to a time series.  These effects will discussed
in any good reference on Fast Fourier Transforms

Also note that the result of the function FFT_INVERSE will always be
on an abstract axis, with indices 1, ... NT.  You need to take this into
account when comparing to a time series whose indices may be different.
I recently made an example where the original series was defined for
t=0:11, and then took the FFT, and inverted it.  The result had time
indices t=1:12, which of course looked shifted when I compared it to
the original time series.

And last, we recently found a bug in the definition of the FFT frequency
axis, where the axis is the wrong length for some series.  This also could
be the cause of what you are seeing, and it will be fixed in the next release
of Ferret.

Note that we have linked a more detailed document about the FFT code to
our web pages at
http://ferret.pmel.noaa.gov/Ferret/Documentation/FFTpack_notes/FFTPACK_notes.html

Ansley Manke
 

"K. V. Ramesh" wrote:

Dear ferret users,

   I am performing  FFTA and FFTP of a SST monthly time
series(12 datas in time)in ferret. Then I am trying to reconstruct
the original SST time series. I am getting ~-45 deg lag in the
reconstructed SST. cane somebody can give some help to to solve
this.

At the same time if I use daily data (365 pts in time) is
working fine.

Thanks in advance.

Cheers,
Ramesh****************************************
K.V. Ramesh,
Scientist "C",
Indian National Centre For Ocean Information Services (INCOIS),
Hyderabad,
Andhra Pradesh, India.
Pin Code:500033
mail:ramesh@incois.gov.in,kvram55@yahoo.com
*************************************

--
Ansley Manke  Pacific Marine Environmental Laboratory  Seattle WA
 

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