Campbell CR850, Operator'S Manual

Page: 193 / 566

Section 7. Installation
193
Standard Deviation of Direction
Figure 59: Standard Deviation of Direction
The Taylor Series for the Cosine function, truncated after 2 terms is:
For deviations less than 40 degrees, the error in this approximation is less than
1%. At deviations of 60 degrees, the error is 10%.
The speed sample can be expressed as the deviation about the mean speed,
Equating the two expressions for Cos ( θ ') and using the previous equation for s
i
;
Solving for ( Θ
i
') 2 , one obtains;
Summing ( Θ
i
') 2 over N samples and dividing by N yields the variance of Θ u.
Note The sum of the last term equals 0.
The term,
is 0 if the deviations in speed are not correlated with the deviation in direction.
This assumption has been verified in tests on wind data by Campbell Scientific;
the Air Resources Laboratory, NOAA, Idaho Falls, ID; and MERDI, Butte, MT.
In these tests, the maximum differences in