Hi, i have a question regarding the fft, my supervisor gave me a Matlab
code, in the code there are something that i couldn't understand due to my
poor understanding of DSP, 

   fu1=fftshift(fft(fftshift(u1)));
   det=sum(fu1(ps:pe));
   amp(jp)=real(det);
   pha(jp)=angle(det);

from the code here i don't know what is fftshift(fft(fftshift(u1))) mean,
i knew that fftshift is to shift the zero frequency component to the
centre,according to the help files in Matlab, but what is the purpose of
doing this? Also, i couldn't understand the combination of
fftshift(fft(fftshift(u1))), why it has to be in this order?