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Technical Description
Revision R09, November 2016 15
4 Technical Description
The CHM 15k Cloud Height Meter is intended to measure clouds at a maximum
height of fifteen kilometers, to determine cloud layer penetration depth and ver-
tical visibility.
The measured data set is transmitted by digital standard interfaces. The CHM
15k uses the Lidar technique (Lidar:
Li ght d etection a nd r anging, refer to chap-
ter 9.1) to emit short light pulses into the atmosphere. These are scattered back
by aerosols and air molecules. The pulse flight time and intensity of a backscat-
tered light signal are then analyzed. Using this data as input, aerosol or cloud
layers can be assigned in terms of height and visibility levels can be determined.
The CHM 15k Nimbus working is based on a photon counting method. Com-
pared to analog measurement techniques, the benefits of this method are very
high detection accuracy and sensitiveness which makes it particularly suited for
applications using lidar technique. This equally refers to the useful signal and to
potential "noise signals", for example, background light.
Engineering design features of the CHM 15k
The CHM 15k Cloud Height Meter is:
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of compact design
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operational in climatic conditions as specified in Table 1
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of modular setup
4.1 Setup of CHM 15k
The CHM 15k is enclosed by a double-shell casing of corrosion-resistant alumin-
ium. The purpose of the outer shell is to reduce and prevent potential external
influences such as:
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solar radiation
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wind
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rain
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snow
from effecting the inner casing shell that carries the measuring unit. There is a
chimney effect between the outer shell and the inner shell to support this pro-
cess.
The top cover protects the inner casing shell from dirt and precipitation.
An opening for laser beam outlet and inlet is machined into this cover. A parti-
tion panel inside the top cover separates the emitter module from the receiver
module and an air deflector in the interior cover space directs the air stream
from the two fans onto the glass panels in the inner casing shell.
The inner casing shell contains all equipment parts for CHM 15k operation. The
cable ports for data line, current supply, grounding and power supply of the
external fans are designed as compression glands. For pressure equalization, the
inner casing shell includes a pressure-balancing component with a Goretex
membrane.
Integrated into the top wall of the inner casing shell is a split-area vision panel of