## 2 Methods  
### Description of the Time-Gated Long-Range Imaging LiDAR System  
The time-gated long-range imaging LiDAR system is configured in a bistatic setup comprising three parts: the transmitter, the receiver, and the associated system control components, as shown in Figure 1a,b. A 532 nm wavelength pulsed laser illumination was provided by a medium repetition frequency nanosecond pulsed all-solid-state laser (AOML-500, Nanjing Institute of Advanced Laser Technology, Nanjing, China). For the measurement reported here, the repetition frequency of the laser was set to 10 kHz and the illumination beam had a maximum average optical power of 1.7 W on exiting the laser head, corresponding to approximately 170 \(\upmu\)J per pulse. The pulse width of the laser was 6.4 ns. This selected output was used on a pair of scanning mirrors (SM with 10 kHz), passed through a half-wave plate (HWP), a polarizing beam splitter (PBS), and relay optics, and exited the transmitter head through the camera lens objective (SIGMA 150-600 mm S version, Sigma Corporation, Kawasaki, Japan).The probe laser was scanned across the scene of interest by a galvano mirror pair. In addition, 50% of the incoming natural light from the transmitter head is reflected into a monitoring channel (CCD in the transmitting light path as shown in the left panel of Figure 1b). Its primary function is to assist the LiDAR equipment in delineating the illuminated area, compensating for the limited long-distance observation capabilities of human eyes. As shown in the right panel of Figure 1b, the scattered optical return from the distant target went through the same receiver lens (SIGMA 150-600 mm S version, Sigma Corporation, Kawasaki, Japan) and relay optics, passed through the polarizing beam splitter (PBS), a linear polarizer (LP), and a narrow bandpass filter (1.2 nm@ 532 nm), and coupled into a gated photomultiplier tube (gated-PMT, H11526-20 NF, Hamamatsu Photonics K.K., Hamamatsu, Japan). Similarly, 50% of the incoming natural light from the receiver head is reflected into another monitoring channel. The polarization beam splitter and the linear polarizer were used to orientate the polarization in order to suppress the solar background  
Figure 1: (**a**) Layout of the long-range active imaging. SM, scanning mirror; HWP, half-wave plate: PBS, polarization beam splitter; CCD, charge coupled device; Gated PMT, gated photomultiplier tube; BPF, narrow bandpass filter (1.2 nm); LP, linear Polarizer; LM, lens mount; FPGA, field programmable gate array; OSC, oscilloscope. (**b**) The demonstration prototype, where the left panel is the device configuration of the transmitting light path, while the right panel is the one of the receive light path. (**c**) Sequence control diagram. (**d**) 20 m active imaging results indoor. illumination. Such a bistatic system can effectively suppress the noise caused by optical back reflections from the components. The associated system control unit for the pulsed laser source, scanning mirrors, the gated-PMT detector, the acquisition module, and the universal stage was needed for sequence control, as shown in Figure 1c, realized by a Field Programmable Gate Array (FPGA) device. Moreover, the system can gain further noise reduction benefits from such a design. The sequence control logic is demonstrated in Figure 2. Firstly, calibrate the universal stage with the help of the monitoring channel and then standby. Then, instruct the scanner to move to the designated position and remain on standby. Subsequently, generate a specific quantity of laser pulses. Next, simultaneously activate the gated-PMT detector and the acquisition module after a designated duration of time, with the option to adjust the gate opening time based on the system's distance from the objective. In this step, one can adjust the gate width for the acquisitions to optimize the signal to noise (SNR). Finally, apply data preprocessing.