Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 44

Fig. 3.5 Phased-array sodar for wind and turbulence profile measurements up to several hundreds
of meter above ground. Left : outside view showing the cuffs protecting the instruments from ambi-
ent noise. The ragged top of the cuffs is made in order to reduce the formation of side lobes. Right :
View to the inside of the cuffs showing part of the sound transducer array on the bottom
detectable for human ears at a distance of several hundreds of metres. Thus, siting
of SODARs close to residence areas and office buildings has to be avoided or need
the explicit approval of the people living and working there.
Doppler-SODAR instruments for the measurement of wind profiles work with
the Doppler-beam-swinging (DBS) technology similar to windprofilers. Here, three
to five beams are emitted one after the other into three to five different directions.
One of the three or five directions is always the vertical direction, the other direc-
tions are tilted by 15-20 ◦ from the vertical. The azimuthal directions of the tilted
beams usually differ by 90 ◦ . Because the following beam cannot be emitted before
the backscatter of the previously emitted beam has been received in order to avoid
disturbances, one measurement cycle of a SODAR lasts six to ten times the time
a sound pulse needs to travel up to the maximum vertical range of the instrument.
This maximum vertical range depends on the emitted power and the used sound
frequency and ranges from about 200 m at 4500 Hz to about 1 km at 1500 Hz. The
frequency dependence is due to the different absorption of sound waves of different
frequency in the atmosphere. The higher the frequency is the stronger is the absorp-
tion. And 4500 Hz-SODARs with a maximum range of up to 200 m are frequently
called MiniSODARs. For a three antenna SODAR operating at 1500 Hz, this means
a length of a measurement cycle of about 20 s. The measurement of the wind speed
is based on the analysis of the Doppler shift between the emitted and the received
signal. The height of the reflecting air volume is computed from the travel time of
the sound signal. The minimum range gate is around 20-30 m above the instru-
ment, the vertical resolution depends on the pulse duration and is in the order of
5-20 m.
Using an emitting power of about 1 kW leads to a backscattered signal with
a power of just of 10 − 15 W. This is below the hearing threshold of the human ear