Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 46

technique. The use of, e.g., eight different frequencies halves the minimal accept-
able signal-to-noise ratio compared to single-frequency sounding. Moreover, the
multi-frequency mode improves the accuracy of instantaneous values of measured
parameters and significantly increases the reliability in recognizing noisy echo-
signals. Further details on the principle of multi-frequency SODARs are given in
Kouznetsov ( 2009 ).
Like RADAR beams SODAR beams also have side lobes. Therefore, fixed
echoes can severely influence and disturb SODAR measurements. This requires
a good site selection for these instruments. SODARs have to be sited away from
obstacles like buildings, trees, and electric cables.

3.3.1 SNODAR

A special SODAR development for extreme cold environments is a SNODAR
(Surface layer NOn-Doppler Acoustic Radar), which is designed to measure the
height and turbulence intensity of the atmospheric boundary layer on the Antarctic
plateau. This is for example of relevance for astronomers wishing to plan future opti-
cal telescopes there. SNODAR works by sending an intense acoustic pulse into the
atmosphere and listening for backscatter off inhomogeneities resulting from tem-
perature gradients and wind shear. The theory of operation is very similar to that of
the well-known underwater sounding techniques of SONAR. SNODARs are mono-
static acoustic radars with a minimum sampling height of 5 m, a range of at least
200 m, and a vertical resolution of 1 m. SNODARs operate at frequencies between 4
and 15 kHz. Such high frequencies propagate relatively well in the low temperature
of the Antarctic atmosphere (Bonner et al. 2008 ).

3.4 RASS

A radio-acoustic sounding system (RASS) operates acoustic and electromagnetic
sounding simultaneously (Marshall et al. 1972 ). This instrument is able to detect
acoustic shock fronts of the acoustic pulses and to determine their propagation speed
from the Doppler shift of the backscattered electromagnetic waves. This propagation
speed is equal to the speed of sound, which in turn is a known function of air tem-
perature and humidity. Two different types of RASS have been realised (Engelbart
and Bange 2002 ): a Bragg-RASS and a Doppler-RASS.

3.4.1 Bragg-(Windprofiler) RASS

A Bragg-RASS is a windprofiler (see Section 3.1.1 above) with an additional acous-
tic emitter. When the Bragg condition is fulfilled (Fig. 3.6 ), i.e. the wavelength of
the sound waves λ a is half the one of the electromagnetic waves λ e , then there is