Campbell Scientific CSAT3 3-D Sonic Anemometer - Manual (Page 25)

CSAT3 Three Dimensional Sonic Anemometer
After power-up or loss of signal, the CSAT3 enters a mode where it tests the
forward and return paths for each of the pairs of ultrasonic transducers. This is
called the “acquire” mode and takes about 10 seconds before normal output is
available from the anemometer. This delay is caused in part because the
anemometer has no recent knowledge of the responses of the transducer pairs
and in particular no past history of the wind speed and speed of sound. During
this acquire mode, it has to search across quite wide windows in time to find
the true arrival time of each received signal.
Once it has acquired the signal, it uses a tracking algorithm to adjust the start
and end times of its search window based both on the Execution Parameter (see
Section 10.3) and the last measured ultrasonic time of flight. The basis of this
adjustment is that the dynamics of air movement prevent the time of flight
changing by more than a certain amount in a given time. The result of using
this algorithm is that the width of this window in time can be kept to a
minimum and measurements can be made at a higher frequency for an overall
lower consumption of power when compared to using wider fixed windows.
Because the Execution Parameter is used as an important input to the tracking
algorithm it must be set to match the frequency at which the anemometer is
triggered. The setting of this parameter and matching it to the trigger
frequency are discussed in Section 8.
7.3 Effects of Crosswind on the Speed of Sound
The speed of sound is found by combining the out and back time-of-flight
measurements (see Eq. 5 in Appendix C). The parallel component of the wind
along the sonic axis does not affect the measured speed of sound, however, the
perpendicular component does. The effects of the perpendicular component of
wind can be accounted for online, using the measured components of wind and
simple trigonometry, or off-line using methods described by Schotanus et al.,
1983 and Liu et al., 2001. The CSAT3 has always corrected for the effects of
crosswind on the speed of sound, and as of the printing of this manual, the
CSAT3 is the only commercially available sonic anemometer that corrects the
speed of sound, and ultimately the sonic temperature, for the effects of wind
blowing normal to the sonic measurement path.
The equations derived by Schotanus et al. (1983) apply to sonic anemometers
that make speed of sound measurements from a single pair of transducers. Liu
et al. (2001) extends these equations to sonic anemometers that measure the
speed of sound on all three axes and then averages the results to a single speed
of sound as with the CSAT3. Liu et al. (2001) assume that the geometry of
each individual three-dimensional anemometer is ideal when they derive the
factors given in their Table 1.
Liu et al. (2001) recommends that CSAT3 sonic temperature
variances and sonic sensible heat flux be corrected for the effects
of cross wind. However, CSAT3 users need not make these
corrections to their fluxes because the CSAT3 performs an online
correction. Correcting CSAT3 data off-line for cross wind effects
will cause errors in the measured fluxes.
NOTE
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