GROUND-BASED REMOTE SENSING SYSTEMS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 106

precipitation), and additional analyses to perform to obtain accurate results. Further
analysis may also be required in complex flow conditions to obtain readings compa-
rable to anemometer readings. It is consequently recommended that staff carrying out
the analysis receive special training or that an experienced consultant be employed to
carry out the data validation and preliminary analysis.
Air temperature and precipitation should be measured at the sodar site to facilitate
data-quality screening and improve measurement accuracy. Air temperature is needed
to accurately calculate the speed of sound, which in turn determines both the altitude
assigned to returned echoes and the estimated tilt angle of a phased-array sodar's
emitted acoustic beams. Precipitation can cause acoustic noise and scattering of sound
back to the sodar. It can also invalidate the vertical velocity measurements. For these
reasons, periods of measurable precipitation should be identified and will most likely
have to be excluded from the data analysis.
8.2 LIDAR
Lidar operates by emitting a laser light signal (either as pulses or a continuous wave)
that is partially scattered back in the direction of the emitter by aerosol particles
suspended in the air. The light scattered from these particles is shifted in frequency,
just as the sound frequency is shifted for a sodar system. This frequency shift is used
to derive the radial wind speed along the laser path. Multiple laser measurements are
taken at prescribed angles to resolve the 3D wind velocity components. The operational
characteristics, number of measurement ranges, the depth of the observed layer, and
even the shape of the measurement volume vary by lidar model type.
Two distinct types of lidar currently exist for wind resource assessment. Profiling
lidars measure the wind along one dimension, usually vertically, producing a profile
similar to that taken from a tower or sodar. These lidars typically measure wind
speeds up to 200 m above the device. Three-dimensional scanning lidars have the
capacity to direct the laser about two axes, which allows them to measure the wind
throughout a hemispherical volume. Side-scanning devices are designed to obtain a
three-dimensional grid of wind speeds over a large area, with some units having a range
of several kilometers. While the side-scanning lidars have the potential to contribute
greatly to wind resource assessment, this chapter focuses on the more widely tested
and used profiling units.
A typical profiling lidar system is equipped with one or more laser emitters and
receivers, an onboard computer containing the operating and data processing software
(including self-diagnostics), heating and cooling systems, and a combination data
storage and communications package. While most lidars come equipped to accept AC
grid power and have onboard battery backup in case of a grid outage, a remote power
supply must be acquired or custom-built for autonomous operation away from the
grid. Like sodars, lidar units can be trailer-mounted and partially enclosed; however,
most are sold by their manufacturers as stand-alone units. Figure 8-3 depicts two lidar
models.