Campbell CR10 PROM Instruction Manual download pdf (Page 13)

CSAT3 THREE DIMENSIONAL SONIC ANEMOMETER

internal trigger then it will take measurements at

this rate until it is changed.

Note that the power consumption of the

anemometer is dependent on the Execution

Parameter and the trigger rate (see Figure 8-1).

Where an external trigger source is used it is

important to make sure the trigger rate is

greater than or equal to the Execution

Parameter. This is because, as discussed

above, the Execution Parameter is used to

define how much the wind speed could have

changed from the previous reading. If the

anemometer is triggered too slowly it is possible

that under gusty conditions the wind speed

could change to the extent that the CSAT3 sets

its search window to a point in time which

misses the ultrasonic pulse. This will cause it to

give erroneous readings.

Conversely if the trigger rate is much higher

than the Execution Parameter the CSAT3 will

consume more current than necessary and at

higher rates the CSAT3 will simply not be able

to take measurements at the speed required,

because it is analyzing wider time windows than

necessary and it will run out of time. The latter

will cause the CSAT3 to output 61503 as the

diagnostic word, indicating an overrun (see

Section 10.2 and B.4).

If the SDM or PC fails to provide a trigger to the

CSAT3, it will enter a lost trigger state after

three seconds. The data acquisition system will

then have to reissue the acquire signal

command (see Section 10.1 and B.1).

8.1 SAMPLING MODES

The CSAT3 can be configured to make

measurements in either a single-measurement

mode or a oversampled mode. In the single-

measurement mode the CSAT3 makes one

measurement synchronously with the trigger

provided by the CSAT3 internal trigger, SDM

communications, or RS-232 communications.

In the oversampled mode, the CSAT3s internal

trigger, SDM communications, or RS-232

communication trigger several measurements.

The average of these measurements is output

at the trigger rate.

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. However, as an

option for the oversampled modes,

experimenters can select block averaged output

data that have a fixed delay of a one master

trigger. Contact Campbell Scientific for details

on changing this parameter.

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