Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 57

3.5.4.2 Range Determination by Beam Focusing

A different method for range determination is beam focusing (Fig. 3.12 ). Range
attribution is made by the assumption that the overall majority of the backscat-
tered light is from the focal range. Such wind LIDARs (Fig. 3.13 shows a currently
available beam-focusing Doppler-LIDAR for a height range of up to about 200 m)
emit a continuous-wave (CW) beam, and interrogation of the wind speed at a given
range is achieved by focusing, rather than by the time-of-flight method of pulsed
systems. The interferometer of the system cannot distinguish between air motion
towards and away from the LIDAR, and this leads to an ambiguity of 180 ◦ in the
derived value of wind direction. This is easily resolved with reference to a simple
wind direction measurement at a height of a few metres. Several different measur-
ing heights between 5 and 150 m above ground can be programmed. The profile
of the three-dimensional wind vector is yielded by scanning a cone with 30 ◦ half
angle once per second (Banakh et al. 1995 ). Hence the diameter of the measure-
ment volume is 173 m at 150 m height. The probe length increases roughly as the
square of the height; as an example the vertical resolution is ~ 10 m at 100 m

height. Strong reflections from particles and other moving objects outside the focal
range (e.g. due to smoke, fog or birds) can lead to spurious Doppler returns (Harris
et al. 2001 ), but these effects can be recognised and mitigated by signal processing
techniques. An intercomparison exercise between a beam-focussing wind LIDAR
and cup anemometers at heights between 40 and 100 m above ground on the wind
energy test range of the Danish research centre Risø in Høvsøre has yielded very
good agreement (Smith et al. 2006 ). An application for wind energy assessment is
described in Kindler et al. ( 2007 ).

±

Fig. 3.12 Schematic of a beam-focussing Doppler wind LIDAR. Due to geometrical reasons the
focal length is increasing with increasing distance from the instrument