The principle of getting PDoA in DW1000

Hi everyone.

I saw the paper ‘Angle of Arrival Estimation Using Decawave DW1000 Integrated Circuits’ from the Decawave, talking about how to get PDoA of two DW1000s. And in Section 2-F, it introduces the procedure of calculating the PDoA, as follow:
My question is, why we need SFD phase here. Can anyone give some insight information?

Best Regard!