FAQ
CHM 15k: On-site calibration & on-site check of the calibration - How to do? How often to do?
Lufft is offering a cloud simulator tool to check the instrument quality in field.
The simulator generates different light pulses and background light levels to test the detection unit and the data acquistion path of our CHM units. The starting point is the laser itself. The laser pulse is detected by the simualtor and time shifted LED pulses are generated. By comparision with the instrument specific production protocol the user can easily see differences in the calibration. The simulator takes a test time of 30 minutes in maximum and should be repeated once per year to verify the status of the system.
In case the instrument is showing an error or the results of the simulator are out of specs, the LOM (laser optic module) of the CHM instrument shall be exchanged. It can be repaired and recalibrated in lab.
Furthermore, on-site calibration in field can be done to achieve absolute backscatter values from aerosol layers. The absolute calibration can be done in two different ways:
1) comparision with another lidar instrument, which should have a sensitivity 10 times better than the ceilometer and should have at least the same range resolution of 5m.
2) clear sky calibration and specific rain cloud calibration ( both methods will be implented by the European Met services shortly. There is an ongoing project to deliver the algorithms. Please check:
http://www.toprof.imaa.cnr.it/index.php/short-term-scientific-mission/8-1-short-term-scientific-mission
http://www.toprof.imaa.cnr.it/images/toprof/working_group/TOPROF_WG1_2015_UpdatedWorkPlan.pdf
Advantage: The Rayleigh calibration and the cloud calibration method do not need additional equipment.
The on-site calibration takes 24 h to 1 week to be able to compare different cloud and aerosol layer situations. In a rainy season, in permanent low cloud, brown cloud or in foggy places the on-site atmospheric calibration might not work at all and our cloud simulator check of the lab calibration gives you the best result.
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