The ToT is measured w.r.t. a specific point on leading edge. The time counter is started by transmit pulse. Time counter stops when the voltage reaches a pre-specified value for received pulse. This is measured on more steep leading edge/rising slope. In case of ideal return there will be no error in ToT measurment. However, due to different amplitude returns (different slopes of leading edges of return pulses) from the targets with different reflectivity and topology different ToT will be measured notwithstanding the targets being at same distance from the sensor. This is called range walk Figure 11 shows how the ToT is measured for an ideal return (middle line) and for returns from targets of different reflectivity. The ToT measured for ideal return is without error, however, the range walk is introduced for other returns (lower line).
Figure 11: Time measurement by constant fraction
The error due to range walk needs to be eliminated. Some approaches for this will be discussed in following paragraphs.