Monthly Archives: August 2015

Calibrating a Ceilometer

There are 2 types of calibration.

  1.  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.

8200-CHS HARD TARGET AT 4450

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).

A technique for autocalibration of cloud lidar

8200-CHS Ceilometer multi level clouds

The 8200-CHS has the capability of detecting up to 4 simultaneous layers but certain types of standard messages display only 3 layers .

If the lowest layer is optically dense,  the returns from beyond the first  layer are extinguished ,  and the layers above it are not detected until a gap in the lower layer is detected.   If the cloud is not optically dense,  the return from a higher level cloud is not significantly attenuated when passing back through the lower layer,  so both layers can be detected.

Here is a case where multiple layers are  at 6620 and 7290 are both detected.