## 3 FMCW LiDAR Research Progress
### The Future Direction of FMCW LiDAR Technology  
With the rapid advancement of FMCW LiDAR technology, significant progress has been observed in key areas such as ranging accuracy, signal processing, frequency modulation linearity, narrow linewidth technology, and system integration. Despite these achievements, challenges remain, particularly in applications under extreme conditions and in the development of high-performance systems. First, although improvements in ranging accuracy have been substantial, maintaining this precision in extreme environments continues to be problematic, necessitating support from novel materials and advanced technologies to ensure stable operation under diverse climatic conditions. Second, enhancements in signal processing have bolstered radar stability and data handling capabilities, supporting the use of LiDAR technology in sectors like industrial monitoring and security. Additionally, research in frequency modulation linearity and narrow linewidth technologies is pushing the performance boundaries of LiDAR systems, especially in demanding applications such as autonomous driving and long-range detection. These advancements in technology have not only facilitated the widespread application of LiDAR systems in terrestrial environments but also demonstrated unique potential for use in space environments. FMCW LiDAR systems aboard spacecraft offer precise distance measurement and target detection, owing to their superior range accuracy and signal processing capabilities, which are well-suited to the extreme temperatures and radiation conditions encountered in space. Specifically, this technology can be utilized in Earth observation satellites to achieve high-precision surface monitoring and atmospheric sensing, thereby enhancing the accuracy and reliability of satellite data. The U.S. National Aeronautics and Space Administration's (NASA) [PERSON] et al. [69] have developed a fully fiber-optic linear FMCW coherent LiDAR system designed to support NASA's new space exploration programs, including crewed and robotic missions to the moon and Mars. Compared to traditional pulse laser altimeters or rangefinders, this fully fiber-optic coherent LiDAR system offers a significant advantage by directly measuring platform velocity through the extraction of Doppler frequency shifts, rather than relying on time-of-flight measurements. This approach avoids errors introduced by terrain features such as hills, cliffs, or slopes. The Doppler measurement precision is two orders of magnitude higher than that of pulse laser altimeter velocity estimates. The system comprises frequency-stabilized fiber lasers, high-power fiber amplifiers, fiber-coupled telescopes, fiber optics, balanced detectors, and processors. The fiber amplifiers amplify the laser signal and split it into three parts to distribute power to three telescopes on the sensor's optical head. These telescopes collect ground signals and transmit them via fiber optics to three pairs of balanced detectors. The outputs from the detectors are processed by a receiver based on a Pentium Dual Core processor, which can either store timing data for post-processing or provide real-time distance and velocity measurements. The system utilizes an FMCW laser emitter to produce linearly modulated triangular waveforms and employs optical mixing to use a portion of the emitted beam as the local oscillator (LO) for the optical receiver. Mixing the LO field with the delayed received field yields a time varying IF that is directly related to the target distance. The system enables high-resolution distance measurements and, through a multi-channel reception setup, provides precise horizontal and vertical velocity measurements. Experimental results indicate that the LiDAR achieves a distance measurement accuracy of 1 cm and a velocity measurement accuracy of 1 cm/s. In conclusion, although current FMCW LiDAR technology has achieved multiple milestones, further research and innovation in materials, technology, and design are essential to broaden its applications.