Campbell Scientific CSAT3 3-D Sonic Anemometer - Manual (Page 28)

CSAT3 Three Dimensional Sonic Anemometer
8.1.1 Single-Measurement Mode
In the single-measurement mode, the anemometer makes one measurement per
trigger. With this approach, the anemometer’s high frequency response is
limited only by the geometry of the anemometer head, thus minimizing the
possibility of under-estimating high-frequency signal variations and
covariations. However, this approach also aliases high-frequency information
to lower frequencies. This aliasing is apparent in spectra from an upwards tail,
when compared to the -5/3 power relationship, at frequencies approaching the
Nyquist frequency (one-half the sample rate). This aliasing does not
compromise the variances and covariances (and therefore, fluxes) computed
from aliased data. The variance and covariance calculations are not frequency
dependent, they simply measure a signal’s total variation and two signal’s total
covariation, respectively.
8.1.2 Oversample Mode
In the oversample mode, the CSAT3 makes wind measurements at 60 Hz and
then block averages those measurements to 10 or 20 Hz output. The output is
synchronized to any one of the three triggers. The oversampling modes
minimize aliasing by limiting the anemometer’s high-frequency response. The
60 Hz data is filtered with a sin x/x function.
The oversampled modes maintain synchronization to the master trigger. In the
six-times oversampled mode, the CSAT3 interpolates six minor triggers that
are centered on the 10 Hz master trigger. In the three-times oversample mode,
the CSAT3 interpolates three minor triggers that are centered on the 20 Hz
master trigger. For compatibility with the existing single-sampled modes, the
block-averaged output data have a fixed delay of two periods of the master
trigger.
9. Data Pipeline Delay
When the CSAT3 is triggered to take a measurement, it determines the times of
flight for all three transducer pairs. This process takes between 12 to 18 mSec,
depending on the Execution Parameter. After determining the times of flight,
the CSAT3 corrects these times of flight for transducer delays, and then
calculates the u
x
, u
y
, and u
z
components of wind speed. In addition, it corrects
each of the three sonic paths speed of sound for the effects of wind blowing
perpendicular to the sonic path. The three corrected speed of sound values are
then averaged.
All the CSAT3 output signals (SDM, RS-232, and analog) have the two
measurement pipeline delay. The wind speeds and speed of sound that are sent
to the data acquisition system were measured by the anemometer two triggers
ago (see FIGURE 8-2). To optimize system performance, the CSAT3 uses
parallel processing techniques. These techniques cause a two measurement
delay between the trigger and data output. This delay applies at all trigger
frequencies.
When the covariance is found between the turbulence data from the CSAT3
and other scalar sensors, the data from each sensor has to be aligned in time.
This can be done online with a datalogger.
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