It is precisely because of the noise present in the signal is relatively large and a signal distortion occurs, which makes very low possibility solely on the basis of the threshold value to precisely locate the time t0.
It is because the signal in the presence of noise interference and signal distorted, which allows to solely on the basis of threshold to accurately find the time t0 probability is very low.
Because of noise in the signal interference and signal distortion occurred, which made the mere possibility of thresholds to accurately find the time t0 is very low.