User manual

Code: ENIGMA III User Manual
Edition: 1 Revision: 4
Date: 19/12/2008
Page: 27/97
2.6 DFT processing.
The Doppler power spectrum may be estimated from the Discrete Fourier Transform (DFT) of the
complex signal.The DFT decomposes the observed data into a sum of sinusoids having amplitude and
phase that will exactly reproduce the observed data. These N discrete components are adequate to
reconstruct the entire continuous spectrum as long as the complex data samples are taken at a rate equal
to or greater than the bandwidth of a signal (Which is typically 500kHz - 2MHz).
The autocorrelations are calculated using the Wiener–Khinchin theorem, which relates the autocorrelation
function to the power spectral density via the Fourier transform:
The advantage of measuring the full Doppler spectrum is that spectral impurities, such as ground clutter,
can be easily suppressed by algorithms. When highly coherent spectral components (e.g., clutter) are
present, the correlation usually will not decay to zero within N samples.
Processing stages of the DFT algorithm:
1) applying window function
on the input data;
2) forward DFT;
3) filtering;
4) inverse DFT, which produces
autocorrelations.
Parameter: AcfMode
GAMIC Proprietary
Illustration 19: DFT processing