The basic principle of pulsed laser ranging is shown in Figure 1.6. Laser pulses are incident to the ground and the reflected pulses are collected by a detector (receiver in the figure). The range can be determined by measuring the time of flight according to Eq. (1.3.1), which in this case takes the form
Figure 1.6. The basic principle of pulsed laser ranging (taken from (Wehr and Lohr 1999)).
while the properties of the reflecting surface are determined by analyzing the reflected intensity. The time of flight is equal to the delay time of the arrival of the reflected pulse as shown in Figure 1.6. It is measured as the time difference of the leading edges of the transmitted and reflected pulses using the time counter of the laser. The emitted and the reflected pulses are shown in the figure. The range resolution depends on the resolution of the photon time of flight, that is, from Eq. (1.3.1)
An important parameter in LiDAR operation is the maximum unambiguous (measurable) range . This is determined by the maximum time of flight that the instrument can handle:
To achieve higher , should be large. However, larger means that the pulses stay longer in flight, which makes them more susceptible to attenuation. This energy loss limits the achievable maximum unambiguous range.
Accuracy is another important parameter of a LiDAR system. It is given by
for pulse ranging where is the rise time of the pulse and is the signal-to-noise ratio, which depends on several factors like the power of received signal, background radiation, sensitivity of the detector, etc. Figure 4 of Ref. (Wehr and Lohr 1999) outlines the relation of several parameters and their influence on the signal-to-noise ratio and the accuracy. If the received signal power is low that only thermal noise is important, the square root of is proportional to the received (reflected) optical and the ranging accuracy can be written as
where is the noise input bandwidth, which is inversely proportional to the rise time and is the peak reflected optical power. Since it is hard to measure the reflected optical power, for practical purposes one can replace it with the transmitted power ,
One can also define an average transmitted power using the formula,
where is the duration of the pulse while is the pulse repetition rate.