# High Spatial and Temporal Resolution Bistatic Wind Lidar
## 2 System Description  
The PTB high-resolution bistatic lidar is mounted on a custom-designed trailer, providing increased mobility and suitability for long-term outdoor use. The hinged cover is closed during field operation but may be opened during development and servicing, as shown in Figure 3. The sensitive optical set-up and the signal processing unit are located under the cover. To prevent vibrations during transport and operation, the optical set-up is mechanically decoupled from the trailer using air suspension. Further, to prevent the mechanical expansion and torsion of the optics carrier aluminium profile, the entire set-up is climatised to about 20 \({}^{\circ}\)C using a 2 kW water-cooling unit. The transmitting laser amplifier is encased separately and water-cooled to reduce the excessive heating of the remaining set-up. Additional fans ensure sufficient air convection, and a dehumidifier prevents condensation forming on the optical windows inside the cover. The optical receivers are located at a radius of 1 m around the central transmitter in order to provide both sufficient backscattered light intensity (quasi-backwards; i.e., in a quasi-vertical direction) and sufficient resolution for determining the horizontal wind velocity component. Each receiver includes a servo-driven focus lens and a tilting mirror, as shown in Figure 4. The tilting mirror is driven by a servo-piezo ensemble in order to provide sufficient resolution for the accurate positioning of the Gaussian beams at measurement heights of up to 250 m. As the transmitter does not need to be tilted, it only  
Figure 2: (**a**) A conventional monostatic lidar uses a single combined transmitter and receiver, resulting in a large measurement volume which needs to be multiplexed spatially and temporally by sequentially tilting the beam and sampling the wind speed. (**b**) The PTB bistatic lidar uses three separate receivers (_RX1_, _RX2_, _RX3_) around a central transmitter (TX), facilitating the simultaneous measurement of the three wind velocity components with high spatial and temporal resolution. consists of a focus lens and a fixed mirror. Optically adjusting and focusing all the receivers and the transmitter into the small measurement volume is achieved by means of a specially developed scanning algorithm, successively approximating motor positions based on the received light intensity and measurement height. The PTB high-resolution bistatic lidar uses coherent detection, processing the interference of the transmitted and backscattered light of aerosols carried along the flow [15]. While bistatic lidar technology generally poses the problem of low backscattered signal intensity, the received Doppler spectrum is concentrated into a significantly narrower bandwidth compared to that of conventional monostatic lidar. This is due to the more uniform motion of aerosols within the small measurement volume, allowing the detection of Doppler peaks with an improved signal-to-noise ratio (SNR) [16]. At the receiver, the backscattered light from multiple aerosols with slightly different Doppler frequencies is detected [17]. Correlation techniques are used during signal processing, ensuring that only Doppler frequencies emitted by the same aerosol are evaluated [18]. The measurement height is first roughly determined by the theoretical height, which is set using the optomechanical actuators, and it is finely determined by means of the difference in coherent phases  
Figure 4: Principle of the optomechanical set-up: Each receiver consists of a servo-driven focus lens and a tilting mirror which is driven by a servo-piezo ensemble in order to provide sufficient resolution for accurate positioning at measurement heights up to 250 m. As the transmitter does not need to be tilted, it only consists of a focus lens and a fixed mirror. Figure 3: The PTB high-resolution bistatic lidar is mounted on a custom-designed trailer with a hinged cover (opened here), under which the optical set-up and the signal processing unit (SPU) are located.