Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 52

Fig. 3.9 Ceilometer
the height of the cloud base (ceiling). Today, they can frequently be found at air-
ports and in their surroundings. Figure 3.9 shows a ceilometer, which is about a
metre high. The emitted and received signal pass through the window on the top
side of the instrument.
A closer investigation of the lower dynamic range of the backscatter intensity in
the recent years has proven that the backscatter profiles carry useful information on
the vertical aerosol distribution in the atmosphere up to a height of several kilome-
tres above the instrument. Range determination for the profiles is done via the signal
travel time. Two different types of ceilometers are in use: those with two optical axes
(one for the emitted light beam and one for the receiving telescope) and those with
one optical axis where the emitted beam is sent through a small hole in the mirror
of the receiving telescope. Instruments with one optical axis are able to yield useful
information already from a distance of about 30 m. Instruments with two optical
axes have an insufficient overlap in the lower 150 m so that backscatter information
from this height range cannot reliably be interpreted (Münkel et al. 2007 ).
3.5.2 Differential Absorption LIDAR
The basic idea of differential absorption LIDARs (DIALs) is the emission of
light pulses at two neighbouring frequencies (Weitkamp 2005 ). The frequencies
are chosen such that one frequency is optimally absorbed by the sought trace