GROUND-BASED REMOTE SENSING SYSTEMS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 104

Another advantage of remote sensing is that the devices can be deployed and moved
relatively easily, so that the wind resource can be sampled at a number of locations
within a project area, often at less cost and in less time than with tall towers. In
some cases, they can be deployed at sites where it is impractical or prohibited to
erect towers. Typically, the period of measurement, when the systems are paired with
long-term meteorological towers, is from a few weeks to a few months, or however
long is deemed adequate to obtain a statistically representative sample of atmospheric
conditions.
Both sodar and lidar measure the wind very differently from conventional anemom-
etry. The differences between these measurement systems must be considered when
comparing wind characteristics derived from them. One difference is that they mea-
sure the wind speed within a volume of air rather than at a point. Another is that
they record a vector average speed rather than a scalar average speed. Remote sensing
units also behave differently from anemometers under precipitation, in turbulence,
and where vertical winds are significant, and their performance can be affected by
variations in temperature, complex terrain, and other factors.
The next two sections discuss current industry-accepted practices and techniques for
integrating sodar and lidar into wind resource assessment programs. Areas of active
research and development are also mentioned. Some providers of remote sensing
equipment are listed in Appendix A.
8.1 SODAR
Sodar operates by emitting acoustic pulses (audible chirps or beeps) upward and
listening for the backscattered echoes. The scattering is caused by turbulent eddies
(small-scale fluctuations in air density) carried along by the wind. The movement
of these eddies causes a Doppler frequency shift—the same effect that makes an
ambulance siren seem to change pitch as the ambulance approaches and passes an
observer. The frequency shift is analyzed by software, which determines the radial
wind velocity along the transmitted pulse; the horizontal and vertical wind veloc-
ities are then derived from the radial velocities based on the acoustic beam angle
and direction. The timing of return echoes establishes the height at which the scat-
tering occurred. Most sodar devices used for wind resource assessment measure
the wind from 30 m up to about 200 m above ground in increments of 5-20 m.
Figure 8-1 illustrates a sodar in operation, and Figure 8-2 shows two particular sodar
models.
A typical sodar system is equipped with a series of speakers that function as
transmitters and receivers, an onboard computer containing the operating and data
processing software (including self-diagnostics), a power supply, and a combination
data storage and communications package. Some sodars are trailer-mounted so they
can be easily moved, and the trailers are partially enclosed for security and protection
from the weather. The power supply must be sized to maintain continuous operation
of the sodar and communications equipment. If the sodar is operated off-grid, a battery
recharging system such as a diesel or gas generator, solar panels, or wind generator