. (**a**) Overall $\tau$ histories; (**b**) $\tau$ values at D is −0.](sensors-00-00000-g000){#sensors-00-00000-f000}  ![ACF change according to the spoofing signal in case that spoofing signal strength is larger than authentic signal strength. (**a**) $- 0 < D < 0$; (**b**) $D = 0$; (**c**) $D < - 0$.](sensors-00-00000-g000){#sensors-00-00000-f000}  ![Equation of $\tau$ and D in case that the spoofing signal strength is larger than authentic signal and *XE* is same with *XL*.](sensors-00-00000-g000){#sensors-00-00000-f000}  ![ACF change according to the spoofing signal in case that spoofing signal strength is lower than authentic signal strength. (**a**) $- 0 < D < 0$; (**b**) $D = 0$; (**c**) $D < - 0$.](sensors-00-00000-g000){#sensors-00-00000-f000}  ![Equation of $\tau$ and D in case that the spoofing signal strength is lower than authentic signal and *XE* is same with *XL*.](sensors-00-00000-g000){#sensors-00-00000-f000}  ![Su