As shown in Figure 9 the time of travel in pulse ranging is measured between the leading edges of transmitted and received pulse. The range measured is given by:
Further, the range resolution and maximum range are given by:
In case of pulse ranging the resolution of range measurement depends only on the resolution of ToT measurement, which is limited by the precision of the clock on the sensor. The maximum range that can be measured in pulse ranging depends upon the maximum time that can be measured, as shown above. However, in practice the maximum range that can be measured depends upon energy of the laser pulse. The received signal should be of sufficient strength to be distinguished from the noise for detection. This in turn depends upon the divergence, atmosphere, reflectivity of target and detector sensitivity. In addition, the Rmax also depends upon the pulse firing rate (PFR), i.e. number of pulses being fired in one second, which will be understood in later paragraphs.
Figure 9: Time of travel measurement between transmitted and return pulse
It is clear from the above discussion that in airborne LiDAR pulse ranging is mostly employed. The discussion in rest of this document will thus be about pulse ranging only.