There are 2 types of calibration.
- Calibrating the distance measurement: In this case the ceilometer is turned on its side and aimed at a hard target. In this case the ceilometer is aimed at a tree 4450 ft away. The exact distance of the tree was surveyed using Google Earth. The ceilometer was aimed using a telescopic sight. The backscatter profile is an almost perfect replica of the laser pulse, delayed by the time taken for the laser pulse to go to the target and back.
In reality, the calibration of the ceilometer is based on the well known speed of light and if the timing crystal inside the ceilometer is accurate and stable, the distance calibration is stable and accurate and should never need to be checked in the service life of the ceilometer.
Clouds are not solid reflectors, and the backscatter comes from a range of scatterers inside the cloud, so the backscattered laser pulse is broadened and flattened. The height of the cloud is defined as a threshold in the backscatter profile which has been determined will result in a correct reading for most types of cloud.
2. Calibrating the ceilometer constant. For this , there is a clever method as described in a paper by O’connor et al, which utilises the known Lidar Ratio of 18.8 in stratocumulus cloud ( SC).