# High Spatial and Temporal Resolution Bistatic Wind Lidar
## 2 System Description
of both Doppler shifted modulation peaks; due to the used modulation scheme, the phases are periodic over the beam's length. While the vertical wind velocity component directly generates an absolute Doppler shift to the backscattered light, the horizontal wind velocity component must be computed by means of correlated frequency offsets between all three receiving channels. Overall, due to the sharp angle between the transmitter beam and the receiver beam, the detection of the vertical wind velocity component is orders of magnitude more sensitive than that of the horizontal wind velocity component [18]. With an average beam diameter of 10 mm between heights of 100 m to 200 m, the average transit time of a particle passing the measurement volume at a speed of 10 m \(\cdot\) s\({}^{-1}\) amounts to 1 ms. Both the reference laser's maximum bandwidth and the frequency resolution of the signal processing were chosen accordingly. The optical set-up is constructed using single-mode fibre optics internally, as shown in Figure 5. A narrow-bandwidth (\(<\)1 kHz) laser reference of a wavelength of 1550 nm is modulated using an acousto-optic modulator (AOM), amplified (up to 30 W CW) using an erbium-doped fibre amplifier (EDFA), and coupled into the transmitter beam optics. The received light is fed into fibre couplers, where it is coherently mixed (heterodyned) with the light of the reference laser; the resulting beat frequencies are the Doppler frequencies, converted down into a lower frequency range. Each optical mixing product is fed into a balanced photodetector (PD), where it is converted to an electrical signal that is ready to be processed. The signal processing is performed in two steps: a field-programmable gate array (FPGA) pre-processes the signals at a high data rate, and a CPU post-processes these signals at a lower data rate. Both units are contained inside a common MicroTCA system carrier, transferring data via a high-speed backplane protocol. The FPGA has four analogue-digital converters (ADC) and four digital-analogue converters (DAC) associated with it, all interfaced using a fixed-latency protocol to ensure deterministic phase relationships within the coherent signal chain. The AOM modulation signal is provided using one of the available DACs, and two spectral peaks at 75 MHz \(\pm\) 12.5 MHz are added to the  
Figure 5: The optical set-up is constructed using fibre optics. A narrow-bandwidth laser reference is modulated, amplified and fed into the transmitter beam optics. The light scattered back into the receiving beam optics is fed into fibre couplers, where it is coherently mixed with the reference laser’s light. Each optical mixing product is fed into a balanced photodetector, where it is converted to an electrical signal. transmitted light intensity. These are necessary for determining the Doppler polarity, signal phase and, accordingly, the measurement height by means of spectral signal processing. Three of the ADCs are used for digitising the received signals, as shown in Figure 6. The electrical signals from each balanced photodetector are subtracted and digitised using one dedicated ADC sampling at 300 MHz. Using the FPGA, the signal is mixed with a complex valued 75 MHz reference signal. The signal is then low-pass filtered and subsampled at a rate of 50 MHz. Finally, the signal is transmitted to the CPU, where it is transformed into the spectral domain using a fast Fourier transform (FFT) with a block length of 32,768 samples, taking about 1 ms to complete. Further signal processing, such as filtering and correlating, typically increases the total computing time per sampling block to about 7 ms (which is only about 10% of the theoretically achievable real-time performance).