Wind resource and site assessment - Wind Power Generation and Wind Turbine Design - page 55

Figure 2: The principle of a three-beam phased array Sodar [ 7 ] .
two others, tilted to the vertical and perpendicular to each other's planes produce
the three-dimensional velocity fi eld (Fig. 2).
One of the drawbacks of a Sodar is its dependency on temperature fl uctuations
which in turn means that a Sodar is at a disadvantage under neutral atmospheric
conditions, which are related to high wind speed situations [8]. Another type of
Sodar, the bistatic, reacts on velocity inhomogeneities rather than temperature
fl uctuations and does not exhibit this problem. However, there is no commercially
available bistatic Sodar and the principle requires separate, spatially separated
transmitter and receiver, which makes it less practical than conventional Sodar.
The signal-to-noise ratio of the Sodar is also known to deteriorate with height,
resulting in a reduced number of valid signal returns. Thus a measured profi le is
diffi cult to interpret as the profi le might be based on a different number of mea-
surements for each height.
At the same time, due to the tilt of the beams, the measurement takes place in three
non-overlapping volumes. Assuming a typical tilt angle of 17°, the distance between
the tilted volumes and the distance between the vertical and a tilted volume is pre-
sented in Table 1 for a number of altitudes. As a consequence, the interpretation of
a measured profi le becomes diffi cult because the measurement volume changes
with height, especially in complex terrain.
Since the Sodar is an acoustic system, the presence of noise sources can infl u-
ence the Sodar's function. The source of noise could be rain but any other noise,