WIND DOPPLER LIDAR
- Continuous Wave Lidar
To measure wind speed, CW lidars need to focus the laser beam at successive distances. Pulsed Lidars have a fixed focus and measure wind at all distances with the same pulse. Range resolution increases with the square of the distance in CW Lidars. So max range is limited to 300m. Pulsed Lidars have a constant range resolution, proportional to the pulse duration, whatever the distance, even 10km. However, CW Lidars can measure at very short range, while pulsed Lidars have a blind zone, about twice the range resolution.
- Direct detection Lidar
Direct detection Lidars use correlation of aerosol concentration on different lines of sight to derive transverse wind speed. They need very low noise detection and are more sensitive to direct sun light than coherent Lidars. Specific atmospheric conditions are required, with contrasted backscatter variations. Measurement time is longer than with Doppler Lidars, and up to now, turbulence parameters are not derived.
- Doppler Rayleigh Lidar
Rayleigh scattering occurs on small particles and molecules. Doppler shift is measured with complex interferometers, requiring demanding temperature management, sensitive cameras and complex image processing. They are also sensitive to sun light. Their main advantage is that they don’t need aerosol load and can be used for high altitude wind measurement in the stratosphere.
The FCR is different from the normal mode because it does not make the assumption of homogeneity of the wind that is used by all remote sensors and also by the Windcube in normal mode. FCR mode measures the complexity of the flow and is used to provide wind data non-biased directly, which requires no post-correction. The FCR uses all the radial velocities / raw measured at all altitudes, including the vertical velocities of the fifth beam, to force the entire profile of wind speed and direction.This is possible because the radial velocities / raw Windcube are measured very precisely.Other remote sensors and also the normal mode Windcube measures speed and wind direction of each altitude independently.
In regards of quality tests. All new Lidars strictly follow below quality tests :
- A stage of qualification of the instruments, in which the manufactured system follows environmental tests (vibration, waterproofness) and functional tests (quality & performance of all sub electronic, opto electronic and mechanical sub modules).
- A stage of metrological validation. After 48h and 96h, data are compared to an instrument reference; during this stage, metrological performances of the manufactured system are compared to a reference Lidar system. The metrological performances of this reference Lidar have been certified by a reference institute every year.
- An endurance phase of 10 days, in which the system is continuously operating, in order to identify any premature aging of the instrument or one of its components.