Specifications

Section 6. Data Table Declarations and Output Processing Instructions
A sub-interval is specified as a number of scans. The number of scans for a
sub-interval is given by:
Desired sub-interval (secs) / scan rate (secs)
For example if the scan rate is 1 second and the data interval is 60 minutes, the
standard deviation is calculated from all 3600 scans when the sub-interval is 0.
With a sub-interval of 900 scans (15 minutes) the standard deviation is the
average of the four sub-interval standard deviations. The last sub-interval is
weighted if it does not contain the specified number of scans.
Measured raw data:
S
i
= horizontal wind speed
Θ
i
= horizontal wind direction
Ue
i
= east-west component of wind
Un
i
= north-south component of wind
N = number of samples
Calculations:
s
n
Θ
u
s
2
North
East
U
s
1
s
3
s
4
FIGURE 6.4-3. Input Sample Vectors
In Figure 6.4-3, the short, head-to-tail vectors are the input sample vectors
described by
and
s
i
Θ
i
, the sample speed and direction, or by Ue
i
and Un
i
,
the east and north components of the sample vector. At the end of output
interval T, the sum of the sample vectors is described by a vector of magnitude
U and direction Θu. If the input sample interval is t, the number of samples in
output interval
T
is
N
T
t
=
/
. The mean vector magnitude is
UU
N
= /
.
Scalar mean horizontal wind speed, S:
S=(Σs
i
)/N
where in the case of orthogonal sensors:
S
i
=(Ue
i
2
+Un
i
2
)
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