Doppler Laser Anemometry

Doppler laser wind measurement is a laser wind measurement method based on the Doppler shift of the echo signal, and Doppler laser wind measurement radar is mainly divided into direct detection Doppler laser wind measurement radar and coherent detection Doppler laser wind measurement radar. The wind measurement lidar emits laser light, which is pointed at the detected area with the orientation set by the system, and due to the movement of aerosol particles under the action of wind field, the atmospheric echo signal is Doppler shifted, and the radial wind speed is measured by its Doppler shift. The relationship between Doppler shift and radial wind speed is as follows：

Where: V is the radial wind speed; Δf is the Doppler shift; λ is the laser wavelength. The actual wind speed of the detected area is then obtained by the wind field vector synthesis method.

1 Doppler laser wind measurement principle

The Doppler laser wind measurement radar consists of a laser and its accessories, a transceiver system, a signal processing and display system, and a control system.transmitter system, signal processing and display system and control system, whose structure is shown in Figure 1 ^[2].

Figure 1 Composition of Doppler wind measurement lidar

The laser emitted from the pulsed laser in Doppler laser wind radar is expanded by a collimating mirror and then enters the detected area of the atmosphere at the system-set azimuth and elevation angles. In the atmosphere, aerosol particles and molecules move in the direction of the wind field, and then the backscattered signal undergoes a Doppler shift, i.e., the laser return signal frequency is shifted relative to the laser emission signal. The atmospheric scattering echo signal is received by the telescope, and the received echo signal is processed by the corresponding signal to obtain the radial wind speed of the measured wind field ^[3].

2 Implementation of three-beam wind measurement

A single beam at a fixed azimuth elevation can only invert the radial wind speed of the measured wind, but not the exact wind vector in its entirety. Thus, we need a laser beam at fixed azimuth intervals in a short period of time to measure the radial wind velocity in multiple directions to obtain the inverse wind field.

Figure 2 Three beam scanning method

The Doppler laser anemometry radar three-beam sweep is shown in Figure 2. shown in ^[4]. The fixed elevation angle of the lidar is a, generally a = 45°, and the The azimuth angle interval is the same angle in a shorter period of time, generally azimuth angle β = 120°, a laser beam is emitted. as shown in Figure 2. According to the Doppler The wind field measurement principle of Doppler lidar, we can get the laser beam A₁ The radial wind speed at point A is V₁ ,Radial wind velocity at point B was V₂ measured by laser beam A₂, and radial wind velocity at point C was V₃ measured by laser beam Α₃. The actual measured wind speed V can be obtained by wind field vector synthesis, as shown in FIG. 3

Figure 3 Vector synthesis of wind speed

However, the three-beam method of wind measurement has its own disadvantages, the measurement accuracy is not high, and the error increases with the increase of the detection distance.

First of all, when we measured V, we assumed that the atmospheric environment and wind speed at the points A, B and C corresponding to the measured radial velocity V₁, V₂ and V₃ were the same. The vector combination of the three radial velocities at this point gives us our V.

When the atmospheric environment of the three beam method is different and the wind velocity is different, the accuracy of the measurement will be decreased. At the same time, when the elevation Angle of lidar is unchanged, the farther the atmospheric region detected, the farther the distance between points A, B and C3 of radial wind speed measured, so the error of wind speed measurement gradually becomes larger. That is, the farther the detection distance, the greater the error, the worse the resolution.