Sunlight is the primary background light source that affects the detection and guidance performance of semi-active laser seekers. When the solar radiation intensity is high, the angle between the receiving direction of the semi-active laser seekers and the direction of sunlight is small, or strong sunlight is diffusely reflected by ground objects or scattered by the atmosphere to form background radiation, part of the solar radiation energy will be received by the four-quadrant detector of the semi-active laser seekers, resulting in reduced detection sensitivity or false alarms. This phenomenon will decrease the accuracy of semi-active laser-guided weapons and even cause them to fail. To effectively enhance the strike effectiveness of semi-active laser-guided weapon systems under strong sunlight and to meet the requirements of all-weather strike operations, it is necessary to study sunlight interference resistance technologies for semi-active laser seekers.

Under strong sunlight interference, the narrow-band filter in the optical system cannot filter out sunlight with a wavelength range overlapping with the laser echo. This part of sunlight irradiates the photosensitive surface of the detector, and the detector responds to generate an electrical signal. This electrical signal has a wide frequency band range and also overlaps with the laser echo pulse signal. This sunlight interference signal is collected by the ADC and transmitted into the FPGA, causing the target laser echo signal to be mixed with the sunlight interference signal, which will directly affect the identification of the laser pulse signal or the calculation of the target angle, as shown in Fig.4. If the target echo signal cannot be correctly identified, the semi-active laser seeker will capture and track the sunlight interference signal, leading to serious consequences.

To investigate the impact of sunlight on the detection and guidance accuracy of semi-active laser seekers, simulation analysis was conducted from three aspects: the spectral characteristics of sunlight, the frequency characteristics of electrical signals generated by detector responses, and the signal waveform characteristics. Based on the adoption of band-pass filtering technology in optical systems and signal and information processing modules, an in-depth study was conducted on the differences in physical characteristics such as frequency, phase, rise edge width, pulse width, and amplitude between target echo signals and sunlight interference signals. A template information database was established for the aforementioned physical characteristics of laser pulse signals, and specific methods were adopted for waveform feature matching. This method could effectively improve the probability of target recognition under sunlight interference conditions, thereby enhancing the capture capability and angle measurement accuracy of semi-active laser seekers. Ultimately, the method improved the strike accuracy of the semi-active laser-guided weapon system under sunlight interference conditions. Moreover, laser pulse waveform recognition technology based on waveform feature matching was adopted. Waveform matching template libraries and fitting models were established using waveform features such as frequency, phase, rise edge width, pulse width, and amplitude of the laser echo signal. Through autocorrelation operations, the laser echo signal was effectively recognized, reducing the false alarm rate of the system. Additionally, the waveform of the laser echo signal was corrected to improve the recognition probability and angle measurement accuracy of the laser echo pulse signal. The flowchart of the laser pulse waveform recognition and filtering correction algorithm based on waveform feature matching was shown in Fig.5.

The laser pulse signal recognition technology based on waveform feature matching was programmed and implemented on the FPGA of the semi-active laser seeker signal and information processing module, which could effectively identify and extract target laser pulse signals under sunlight interference conditions. The simulation results demonstrated that the laser pulse waveform recognition technology based on waveform feature matching employed in the scheme could effectively suppress sunlight interference when the solar angle was not less than 15°, ensuring that the semi-active laser seekers could normally capture and stably track targets when the solar angle was not less than 15°.

The results of the field experiment indicate that due to the sunlight interference entering the field of view, as the solar angle decreases, the signal-to-noise ratio of the system decreases, and the error range of the line-of-sight deviation angle output by the semi-active laser seekers increases. However, the system can still normally capture and track the targets. The proposed method demonstrates that the sunlight interference resistance technology can effectively suppress the impact of sunlight on the detection and guidance accuracy of the semi-active laser seekers.