GROUND-BASED REMOTE SENSING SYSTEMS - Wind Resource Assessment: A Practical Guide to Developing a Wind Project - page 109

The recording interval should be compatible with that being used by other mea-
surement systems with which the sodar or lidar readings will be compared (typically
10 min). Other averages, such as 60-min or daily means, can be calculated later, if
desired. Clocks in the data recorders of all systems should be synchronized.
Sodar systems record a complete wind profile at each moment of time over a range
of heights and at intervals determined by the software settings. The pulse repetition
rate (or duty cycle) of the sodar is determined in part by the maximum measurement
altitude. Increasing the altitude can reduce the number of valid data samples included
in each recording interval. For example, for one common sodar type, setting the
maximum altitude to 200 m typically results in about 15% fewer samples per 10-min
recording interval than does setting the maximum altitude to 150 m. Since the SNR
is related to sample size, this setting may influence data quality and data recovery,
depending on the atmospheric conditions.
The elevations of the lidar range gates (height intervals over which data are
recorded) can be programmed by the user, but the number of reporting levels is
currently limited to between 5 and 10, depending on the model. Given the limited
number of reporting elevations, the reporting heights should be chosen carefully. For
example, two of the lidar range gates could be chosen to correspond with the top
two anemometer heights on the reference wind monitoring tower to enable a direct
comparison of the observed speeds. A third could be set at the expected turbine hub
height, and the remainder spaced across the rotor plane.
8.5 COMPARISONS WITH CONVENTIONAL ANEMOMETRY
Since turbine power curves are currently defined with respect to wind speeds measured
by IEC Class I anemometers, it is important that any source of bias in sodar or lidar
readings with respect to such anemometers be understood and quantified.
Without the adjustments described below, sodar speeds can read 5-7% lower than
anemometer speeds. Lidar speeds, too, can differ by up to 4-6% from cup anemome-
ter readings at some sites. If the anemometers on the reference mast are not IEC
Class I models, then it is equally important to understand their dynamic response
characteristics and, if necessary, correct their readings to the IEC Class I standard.
The main factors responsible for biases between sodar, lidar, and cup anemometers
are discussed below. Once these factors are accounted for, sodar and lidar speed
measurements should usually fall within about 2% of concurrent measurements from
a nearby anemometer at the same height.
8.5.1 Beam Tilt (Sodar)
The tilt angle of the acoustic beam emitted from a phased-array sodar varies slightly
with the speed of sound through air, which is a function of temperature. Such variations
can affect the accuracy of the derived speeds. Most sodar manufacturers address
this issue by continually measuring the temperature at the sodar unit and computing
the beam geometry. Temperature readings from a nearby mast can also be used in