# High Spatial and Temporal Resolution Bistatic Wind Lidar
###### Abstract  
The high-resolution bistatic lidar developed at the Physikalisch-Technische Bundesanstalt (PTB) aims to overcome the limitations of conventional monostatic lidar technology, which is widely used for wind velocity measurements in wind energy and meteorology applications. Due to the large measurement volume of a combined optical transmitter and receiver tilting in multiple directions, monostatic lidar generally has poor spatial and temporal resolution. It also exhibits large measurement uncertainty when operated in inhomogeneous flow; for instance, over complex terrain. In contrast, PTB's bistatic lidar uses three dedicated receivers arranged around a central transmitter, resulting in an exceptionally small measurement volume. The coherent detection and modulation schemes used allow the detection of backscattered, Doppler shifted light down to the scale of single aerosols, realising the simultaneous measurement of all three wind velocity components. This paper outlines the design details and theory of operation of PTB's bistatic lidar and provides an overview of selected comparative measurements. The results of these measurements show that the measurement uncertainty of PTB's bistatic lidar is well within the measurement uncertainty of traditional cup anemometers while being fully independent of its site and traceable to the SI units. This allows its use as a transfer standard for the calibration of other remote sensing devices. Overall, PTB's bistatic lidar shows great potential to improve the capability and accuracy of wind velocity measurements, such as for the investigation of highly dynamic flow processes upstream and in the wake of wind turbines. wind lidar; Doppler lidar; bistatic; metrology; traceability; wind energy; meteorology +
Footnote †: journal: Applied Physics Letters  
16 July 2021  
Accepted: 17 August 2021  
Published: 19 August 2021  
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**Published: 19 August 2021  
**Published: 19 August 2021**  
flow [11; 12]. Figure 1 illustrates this problem of the monostatic measurement principle resulting from the calculation of the horizontal velocity based on the difference of varying radial wind velocities measured at different locations and times within a single measurement cycle. The measurement beam within the measurement cycle is tilted in different directions around a circle with a height-dependent diameter of up to 100 m. Due to the dependence of the measurement uncertainty on the flow conditions at the measurement site, monostatic lidar systems are generally not traceable to the SI units. They are thus not permitted to be used as transfer standards for the calibration of other remote sensing devices [13]. To overcome the aforementioned limitations and provide accurate and traceable measurements over any terrain, a _bistatic_ wind lidar system has been developed at PTB. Figure 2 highlights the difference in operation of both lidar technologies. The conventional monostatic lidar (cf. Figure 2a) uses a rotating prism above a single combined transmitter and receiver unit scanning over a conical contour at different locations and times, with the beam's measurement volume extending up to 30 m in length [14]. In contrast, the bistatic PTB lidar (cf. Figure 2b) uses three dedicated receivers arranged at a radius of 1 m around a central transmitter, with all units focused into a spatially highly resolved, ellipsoid-shaped measurement volume (at a height of 5 m: diameter 300 \(\upmu\)m, length 2 mm; at a height of 200 m: diameter 12 mm, length 4 m). This facilitates the simultaneous measurement of the three-dimensional wind velocity with exceptionally high spatial resolution, down to the scale of single aerosols. In consequence, the measurement uncertainty is largely independent of flow conditions at the measurement site.