Operating principle - CL61

The ceilometer operating principle is based on LIDAR technology where short, powerful laser pulses are sent out in a vertical or near‑vertical direction. The height measurement is based on the time‑of‑flight measurement. Time‑of‑flight is the time needed for a short pulse of light to traverse the atmosphere from the transmitter of the ceilometer to a backscattering particle and back to the receiver of the ceilometer.

The time delay between the launch of the laser pulse and the detection of the backscatter signal indicates the cloud base height.

The general expression connecting time delay and backscattering height is:

$h= ct/2$

Table 1. Operating principle formula

Variable	Description
h	Backscattering height
c	Speed of light (c = 3.00 × 108 m/s)
t	Time delay

The receiver sees a reflection from 15 000 m (49 212 ft) after t = 100 µs.