Cloud height measurement
The main principles
The Laser ceilometer is used to measure cloud height using the LIDAR principle (acronym for Light Detection and Ranging).
A laser (the emitter) emits short pulses of light. The optic system focalizes all the light into a collimated light beam and transmits it vertically from the ground into the atmosphere. The particles and the molecules contained in the clouds will then scatter the light. Only a part of this light will actually reach the detection device. This reflected light is then focalized in its turn by a second optic system to a photodiode (the receiver), then converts it into an electric signal. As the speed of light is a well known parameter, the distance from the reflexion point (the height of the clouds) can be calculated from the laps of time which separates the transmission and the reception of the light radiation.
The ALC 30 provides the latest state-of-the-art technical development in the field of laser source and signal processing. The transmitter unit uses a pulsed Microship Laser technology LIDAR (Light Detection and Ranging), where the wavelength is particularly suited to all weather conditions.
An experiment carried out by METEO FRANCE has shown all the advantages of this technology. Short but powerful pulses are emitted from the instrument vertically (or practically vertical depending on the angle of the tilting part of the instrument) the height of the clouds is then determined thanks to the reception of light reflected off from the clouds or rain. This experiment has shown that a wavelength situated at 1 535 nm improved the quality of the measurement when there was rainfall, and it ensured total ocular security.
As well as this, by choosing this wavelength meant we could use the pulsed Microchip Laser Technology which was extremely reliable (high MBTF).
Depending on the sun’s direction, the ceilometer should be tilted so as to prevent the sun damaging the laser.