1 Introduction Lidar has wide applications in aerospace, industry, and medicine. Compared with microwave and millimeter-wave radar, it has the following unique advantages: (1) high operating frequency and short wavelength; (2) high accuracy in measuring distance, velocity, and angular position; (3) small size, light weight, and high mobility, which is beneficial for airborne and spacecraft-borne applications. Lidar system simulation software is a product of the combination of lidar technology and computer digital simulation technology. It is a new means for the design of future lidar systems and one of the future research directions of lidar. The design of a lidar system requires the parameters of each component (including laser, detector, and optical element), atmospheric transmission characteristics, target characteristics, etc. Laser transmission in the atmosphere has complex statistical characteristics. The laser atmospheric transmission model is established based on the analysis of a large amount of experimental data. This paper uses VC software to model and simulate lidar. 2. LiDAR Principle The basic principle of lidar is: a transmitter emits a laser beam of a certain power, which is transmitted through the atmosphere and radiated onto the target surface. The echo reflected back from the target surface is received by the receiving device, and the useful information in the echo is extracted by signal processing. The basic problem of lidar system performance analysis is: under a certain transmission power, under the influence of environmental factors and system parameters, determining the receiving power and signal-to-noise ratio at the receiving end. The laser emits a Gaussian pulse waveform, which is expanded and pointed at the target by a two-dimensional optical scanning system. The echo signal reflected back from the target is detected by a high-sensitivity silicon avalanche diode (Si-APD). The output of the APD is sent to the computer for processing by a high-speed data acquisition card. The laser pulse energy, repetition frequency, laser emission, scanning waveform of the optical scanner, scanning start and stop, and acquisition rate of the data acquisition card are all controlled by the computer. 3. LiDAR Simulation Model 3.1 LiDAR Equations In general, the most important indicators to consider for a lidar system are: operating range, range resolution, imaging rate, and image resolution. Then, based on the above indicators, the parameters such as laser pulse repetition frequency, pulse energy, pulse width, data acquisition rate, scanning waveform, and scanning frequency are determined. Direct detection lidar systems can generally be divided into four parts: the transmitting system, the receiving system, the signal processing system, and the image display system. The simplest method for lidar simulation is functional simulation. The basis for this method is the lidar distance equation, namely: As can be seen from formula (1), there are many factors that affect the performance of lidar systems. In addition to the transmitting power, there are also atmospheric transmission characteristics of the laser, such as the effects of atmospheric turbulence, clouds and fog, and water and dust in the air. For different types of targets (such as point targets and extended targets), the amplitude and phase of the echo signal will change significantly. In lidar system simulation, the most important task is to establish a mathematical model describing the environment. The most important physical phenomenon in lidar simulation is the scattering of lidar signals, which includes both target and clutter aspects. In some cases, it also includes multiple scattering effects, attenuation, refraction, and dispersion. Generally, the model should be established based on the function of the lidar system. For imaging lidar, it may be necessary to measure the target's distance, velocity, and position. Based on the intensity and range images of the formed targets, the targets are further identified, classified, and recognized. According to the above basic principles, the established lidar system model software is divided into six modules: system transmission module, system receiving module, target reflection module, and transmission medium module. This model is built using VC language. 3.2 Atmospheric Transmission Simulation Model The atmospheric transmission environment has a significant impact on the performance of the lidar system; rain, fog, snow, and other weather conditions will degrade lidar performance. When designing lidar simulation software, we should consider the impact of weather factors and the atmospheric environment on lidar performance. In practice, we used the existing atmospheric software LOWTRAN to calculate the loss of the laser beam during atmospheric transmission. LOWTRAN is a broadband, narrowband, and line-by-line calculated atmospheric radiation transmission model and its corresponding application software developed and manufactured by the U.S. Air Force Geophysical Laboratory (AFGL). The atmospheric transmission in the model uses the U.S. standard LOWTRAN model database. Based on the target type and atmospheric environmental parameters, the atmospheric attenuation coefficient is obtained by calling the LOWTRAN database. 3.3 Target Cross Section Simulation Model The target laser cross section (LCS) is defined as: 3.4 Laser Radar Signal-to-Noise Ratio Analysis The output signal-to-noise ratio of the laser radar after passing through the detector is defined as: the root mean square value of the peak power of the signal to the noise power. The noise equivalent power of the direct detection laser radar system can be calculated by equation (5), and its output signal-to-noise ratio is: [b]4. Function Introduction[/b] We developed the laser radar system simulation software using the Visual C++ 6.0 visual development tool. The laser radar system simulation includes five major modules: system transmission module, atmospheric transmission module, target reflection module, system receiving module, and detector module. The modular software architecture makes it easy to expand the functions. Users can use the interactive graphical user interface (GUI) to select environmental variables and configure various module parameters in the simulation software to simulate the impact of various environmental and system conditions on the performance of the laser radar. It can conveniently and intuitively estimate and simulate the key characteristic data of the laser radar (such as echo power, received signal-to-noise ratio, etc.). The general simulation software framework of laser radar is divided into three parts, namely, calculating echo power, calculating signal-to-noise ratio, and signal processing simulation. The functions of each part include calculation, graphical display, and saving simulation results. Each part is implemented through multiple dialog windows. Users can select and set system parameters, transmission media, and environmental variables of the lidar through the dialog windows to simulate the lidar system under various conditions. 5. Simulation Results By selecting and configuring various module parameters in the visualization interface of the simulation software, it is possible to simulate and estimate the key characteristic data and performance of the lidar under various weather environments and system conditions. Figures 2 and 3 show the simulation interface diagrams of the direct detection lidar echo power signal-to-noise ratio. The parameters set for each module are as follows: 5.1 System Transmitter Module: Transmit power: 10,000,000W; Wavelength: 1.06 Å; Transmit aperture: 0.2m; Aperture transmittance constant: 0.84; Transmit efficiency: 0.950; 5.2 System Receiver Module: Receive aperture: 0.5m; Receive efficiency: 0.900; 5.3 Target Reflection Module: Target type: Extended target; Hemispherical reflectivity: 0.100; 5.4 Transmission Medium Module: One-way distance: 100,000m; Weather: Clear (visibility 23.5), atmospheric transmission efficiency calculated by LOWTRAN: 0.5139; 5.5 Detector Parameters: Noise bandwidth: 50MHz; Detector dark current: 50nA; Detector load resistance: 1MΩ; Absolute temperature: 293K; Amplifier equivalent input resistance: 16MΩ; Detector responsivity: 1.1A/W; Calculation Results: Beam divergence angle: 0.004452 (m rad); Receiving area: 0.19634 m²; Spot area: 0.155664; Scattering cross section: 0.062; Atmospheric attenuation: 5.7824 dB; Total transmission attenuation: 128.50397 dB; Dark current noise: 8.0 10⁻¹⁹; Thermal noise: 5.1 10⁻²⁰; Amplifier noise: 1.87 10⁻²³; Shot noise: 1.2 10⁻¹⁵; Background noise: 8.51 10⁻²² Conclusion The research and design of lidar systems is a complex task. The development of computer simulation technology is crucial for the research and design of lidar systems and has become an important aspect of lidar system design and research. This paper studies lidar simulation models, simulating atmospheric transmission models, noise models, and transmission and reception models to simulate the impact of various environmental and system conditions on lidar performance. System simulation can directly identify specific problems within the system. Solving these problems allows for a comprehensive and in-depth consideration of each subsystem in the research of the lidar system, and it also identifies potential difficulties that may be encountered during system development, thus providing theoretical and technical guidance for the actual system development. Simulation results demonstrate that simulation software can simulate and estimate key characteristic data of lidar, but simulation results still need to be verified by comparing them with actual test results. Further research should include refining and modifying the lidar simulation model based on actual experimental data. (Edited by: He Shiping)