Geoscience Reference
In-Depth Information
3.5.4 Doppler Wind LIDAR
In the following, currently available optical remote-sensing methods based on the
analysis of the Doppler shift are presented. Doppler wind LIDARs are special
backscatter LIDARs (see Section
3.4.1
). In recent years special Doppler wind
LIDARs for measurements in the near-surface layer from about 40 m up to about
200 m have been developed and are presently used for wind energy assessments (for
a review see: Emeis et al.
2007
). Different methods for range determination will be
addressed.
3.5.4.1 Range Determination by Signal Delay
Range-resolved remote-sensing systems transmit signals in pulses, which are then
scattered by atmospheric inhomogeneities or suspensions (e.g. aerosol, droplets),
sending a small fraction of the transmitted energy back to the receiver. Distance to
the measurement volume is determined from the time of flight of the signal pulse.
Overviews of the state of the art of LIDAR techniques for wind and turbulence
measurement using signal delay for the range determination have been given by
Hardesty and Darby (
2005
) and Davies et al. (
2003
). Two common approaches
to determining the frequency of the return signal for Doppler analysis are coher-
ent (heterodyne) detection and direct (or “incoherent”) detection, which will be
described in this section.
Mie Scattering: Coherent Doppler LIDAR
Coherent or heterodyne detection of wind speeds using Doppler LIDAR relies on
the ability to generate two signals having a precise offset in frequency. The signal
transmitted to the atmosphere comes from a primary laser, variously called a “mas-
ter oscillator” (MO), a “pulse laser” (Grund et al.
2001
), or a “power oscillator” (PO;
e.g. Post and Cupp
1990
). Generally, the second or reference signal comes from a
second, low-power, continuous-wave laser, or “local oscillator” (LO), although in
some systems the frequency difference is produced by splitting the signal from the
MO and offsetting the frequency of one of the two resulting signals. The wavelength
of coherent LIDARs has typically been in an eye-safe region of the infrared spec-
trum, between 1.5
m(CO
2
source), where
atmospheric scattering is dominated by aerosol. In a coherent LIDAR receiver the
backscattered return signal from the atmosphere is optically mixed with the coher-
ent reference-frequency radiation from the LO at the system detector. The detector
senses the beat frequency between the return signal and the reference signal (het-
erodyne detection), and the Doppler shift is calculated from this beat frequency. A
detailed description of the configuration, design characteristics, trade offs, and data
processing issues of a coherent Doppler LIDAR system is provided in Grund et al.
(
2001
).
μ
m (solid-state lasing source) and 10.6
μ
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