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Using spectrogram function to find direction of incoming ULF emission via SSTF

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Hi,
So this is a very specific question, since I'm so lost for so long. I'm working with geomganetic data with X, Y and Z components. What I know is:
  1. I have to use spectrogram() function to each component to find its frequency domain, and only pick those in ultra-low frequency (ULF) range.
  2. Use Single Station Transfer Function (SSTF) to find the azimuthal angle (direction of the incoming signal).
From spectrogram() function, I got complex numbers. I'd like to ask, how do I use the SSTF to find direction from spectogram data?
This is a segment of my code:
for j=1:size(H_spec_mu1,1)
XX=H_spec_mu1(j,:);
YY=D_spec_mu1(j,:);
ZZ=Z_spec_mu1(j,:);
XY_mat=real([XX;YY]);
ZZ_mat=real(ZZ');
AB=(inv(XY_mat*XY_mat'))*(XY_mat*ZZ_mat);
azim_amp(j,i,l)=sqrt(AB(1)^2+AB(2)^2);
azim_theta1=atan2d(AB(2),AB(1));
if azim_theta1<0 azim_theta1=360+azim_theta1; end
azim_theta(j,i,l)=azim_theta1;
end

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