Line of Sight Wind Speed Calculation
At each sample interval \(t\), the LIDAR Beam Direction is updated and then the wind module is interrogated to obtain the wind speed along the direction of the beam. This section outlines the calculation process that is used for each beam. The line of sight speed reported is relative to the motion of the mounting component.
The focal distance(s) can be set directly in the user interface, or by the external controller. This is the distance from the LIDAR instrument along the beam direction, at which the wind module is interrogated to obtain a line of sight wind speed.
In reality, a single precise wind speed at the focal point is not recorded, but rather the weighted average of velocities around the focal point according to a weighting function. For a pulsed beam, this can be specified by the user, as a lookup table. For a continuous beam, the weighting function is defined by the lens area and beam wavelength (see the Theory manual for further details).
If there is more than one LIDAR beam, then the LIDAR beam sampling can be made sequential, in which case a measurement is only taken from a single beam per LIDAR sample. The order upon which the readings are taken is determined by the order in which the lidar beams were inputted in the user interface (top to bottom is first to last). If simultaneous beam sampling is selected, then wind speed measurements will be taken from all beams on every LIDAR sample.
In order to compute the line of sight velocity reading for beam the following algorithm is used:
The Beam Direction is updated based on the mounting component, the LIDAR axis and the scanning pattern. This determines the direction of the LIDAR beam in the inertial frame (global frame).
The focal position of the beam is computed (in the global frame) using the mounting location of the lidar instrument, the distance along the beam and the direction of the beam.
The wind velocity is computed by interrogating the wind module at the focal position. The velocity of the mounting component is subtracted from this value, if the “correct for mounting velocity” dialog box is not checked.
The dot product of the lidar beam direction and the wind velocity is computed to get the line of sight velocity.
If a weighting function is defined, then steps 2 to 4 are repeated at each distance from the focal point and the result is multiplied by the corresponding weighting. The product of the line of sight speed and weighting is summated and then normalised to compute a final line of sight velocity.
Note
When the wind velocity is computed for line of sight calculation the user should be aware that:
- If switched on then Tower Shadow will be included in the calculation.
- If using Turbulent Wind consider using Evolving Turbulence.
- If using Transient Wind consider including the Gust Propagation option.
Last updated 10-09-2024