There are significant differences between both CL51 instruments below 500m. The profile of CL51CG decreases towards ground but profile of CL51RAO increases. See examples:

This behaviour improved significantly when CL51RAO was updated to TOPROF firmware, see example of July 16:


During the To-PROf meeting in Granada, it was mentioned that a constant artefact could be seen on most CHM15k instruments. It was pointed out that this is very likely due to an improperly corrected overlap.

It is visible during the Ceilinex campaign (for example the 06/07/2015 between 23:00 and 24:00)


Yann Poltera and Maxime Hervo from MeteoSwiss developed an algorithm to correct this overlap. No horizontal measurements or external references are needed.

This new overlap function calculated the 20150603 remove the artefacts observed on CHM15k ceilometers.


This algorithm still needs to be validated against other methods.