GROUND-BASED REMOTE SENSING SYSTEMS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 108

and other emitters of high pitched tones should be avoided, if possible. Lastly, because
the beeping or chirping can disturb people living nearby, the sodar should be sited at
least 500 m from homes in open, flat terrain, and at least 350 m from homes in other
terrain.
In theory, because a laser beam is more tightly focused than sound waves, lidar
is less susceptible than sodar to interference (echoes) from nearby obstacles. This
attribute may make it possible to use lidar in locations that would be troublesome
for sodar and to obtain a better match to anemometer data by placing the system
closer to the reference meteorological mast. Nevertheless, it is preferable to keep the
device's “measurement cone” as unobstructed as possible, especially when it comes
to moving objects such as tree branches, guy wires, and anemometers. While some
lidar devices can tolerate the blockage of a significant portion of their field of view,
this may reduce data recovery and increase the error margin in the observed wind
speeds. Some lidar units can be rotated so that their beams are directed away from
obstacles.
The horizontal wind speed derived from both sodars and lidars can be biased in
complex terrain. This is because the radial readings are spaced increasingly far apart
as the measurement height increases, and in complex terrain, the vertical component
of the flow may not be homogeneous over the sampling volume. The bias can be
as much as 5% in very complex terrain but is usually much less. This feature of
remote sensing measurements, as well as possible corrections for it, is an area of
active research (Section 8.6).
How much sodar or lidar data must be collected at a site and over what period
depends on the wind conditions and the objectives of the study. Where the system is
the sole source of wind measurement, at least a year of data collection (12 continuous
months) is recommended, just as for monitoring towers. In the more common situation
where there is a reference meteorological tower at the site, a much shorter period will
usually suffice. Ideally, in this case, the data collected should span a representative
range of speeds and directions. This can be accomplished in 4-6 weeks at most
locations. The precision achieved can be estimated by comparing the speeds with
simultaneous measurements at a nearby reference mast or by observing the number
of samples in the important direction and speed bins. To further improve confidence
in the observed profile, measurements should be taken at different times of year,
especially at sites where strong seasonal variations in the wind resource, including
speed, direction, and shear, are expected.
8.4 DATA COLLECTION AND PROCESSING
The primary outputs at each height for both sodar and lidar systems are the horizontal
wind speed and direction, vertical wind speed, and their associated standard deviations.
In addition, some indicator of signal quality, such as the signal-to-noise ratio (SNR),
as well as the maximum height of reliable data, is usually provided. Understanding the
definitions and thresholds for these parameters is useful for establishing appropriate
data screening procedures and for identifying suspect data periods.