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

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