All Weather Inc 8339, User Manual, Page: 9 / 69

Model 8339 Ceilometer User’s Manual
3
2. THEORY OF OPERATION
2.1 GENERAL DESCRIPTION
The Model 8339 Ceilometer (also known as Cloud Height Indicator, or CHI) is designed for fixed
installation at airports, meteorological stations, or anywhere reliable cloud ceiling information is
required. The Ceilometer monitors sky conditions continuously and reports up to four detected cloud
bases and depths to an altitude of 25,000 feet above ground level. If the Ceilometer cannot “see”
beyond a height of 3,000 feet, meaning that vertical visibility is restricted but there is no discernible
cloud base, the Ceilometer outputs a vertical visibility value.
Vertical visibility is defined as follows.
Extinction is given by:
z
e I I

 
0
where σ is the extinction coefficient (corresponds to the water particulate density in the air). The
vertical visibility is then found by solving:

  
visibility vertical
z
dz e
I
I
0
0
05 . 0

In other words, that distance where 95% of the initial laser beam has been extinguished by scattering.
The 8339 works in accordance with the principal of optical radar, or LIDAR. A laser pulse is emitted
at a maximum rate of 1 kHz vertically into the atmosphere, and the resulting backscattering is
analyzed for water density. Clouds are identified when the water density changes abruptly. Vertical
visibility is determined by integrating the near field water density pattern. Proprietary algorithms
enable the unit to achieve superior performance while still maintaining an eye-safe laser light level.
The 8339 is comprised of modular, field-replaceable units. Internal self-test routines identify faults
and isolate them to the module level.
! WARNING !
The Model 8339 Ceilometer is a Class I laser configuration, which is eye-safe in all
conditions except when viewed through optical instrumentation, such as binoculars
or any other condensers. Never view the optical radiation through such devices or
serious eye damage may result.
2.2 THEORY OF OPERATION
A laser pulse is emitted into the atmosphere by the transmitter, and a high-speed analog-to-digital
converter is used to record the backscatter returns to the receiver. The receiver is adjustable, so that
gains and response parameters can be varied in an adaptive sense. A number of pulse bursts are
obtained, depending on the current and historical sky conditions. Using the speed of light, the altitude
of the clouds may then be determined.