Research On Semi-active Laser Target Azimuth Detection For Low Speed Aircraft In Terminal Trajectory

In order to meet the needs of the new modern battlefield,and considering the problem that the drop distribution scale of conventional low speed aircraft with no control method,such as the individual rocket,is large after extending the range of fire,the low cost guidance modification of the conventional low speed aircrafts needs to be conducted.The method is to apply the ballistic correction technology of the missile to the low speed aircraft and reduce the drop distribution scale of them by one or two corrections in the flight trajectory,and improving the battlefield efficiency.However,taking the small caliber and a certain launch overload restriction of the low speed aircraft into account,as well as the instability characteristics of terminal trajectory,it cannot directly use the mature missile guidance technology.In addition,the semi-active laser target detection and guidance technology has the advantages of strong anti-electromagnetic interference ability,high target positioning accuracy,low cost and fast speed,which is a feasible solution to the contradiction between the cost and guidance precision of the uncontrolled low speed aircraft.Therefore,it is of great practical significance to study the target detection technology based on semi-active laser for low speed aircraft,which is of great significance to speed up the low cost guidance modification process of conventional low speed aircraft,and the development of new low speed guided aircraft.In this paper,the low speed aircraft is the object of study,and the process of semi-active laser target azimuth detection is studied.Based on the ground transmission environment of the laser,the ability of the laser detection system to capture the laser signal is studied.For the background noise and the circuit noise,the measurement accuracy of quadrant photodectector is studied.The influence of terminal trajectory characteristics on the semi-active laser detection system is studied by calculating the trajectory of the low speed aircraft.The specific research contents are as follows:The target detection system structure for the low speed aircraft is defined to the strapdown type,and the optical system structure is defined to the refraction type.The measurement model of the azimuth deviation angle of the quadrant photodetector is established,and the influence of the quadrant photodetector deadband on the measurement is studied.According to the six degree of freedom trajectory model of the low speed aircraft,the spatial state of the aircraft on the terminal trajectory is analysed.The power equation of the laser signal captured by the optical system of the semi-active laser target detection system is proposed.Based on the concept of bidirectional reflection distribution function,the reflection cross section formula of semi-active laser system is deduced.The influence of different slope angle and different laser reflection angle on the target reflection cross section is calculated and analyzed,when the target surface is inclined.Based on the theory of Mie scattering,the transmission efficiency of aerosol particles on laser transmission path is studied.Based on the Gamma-Gamma atmospheric turbulence theory,the intensity fluctuation effect of the strong ground turbulence field on the laser signal captured by the laser detection system is studied.The models of inverted parabolic laser signal and Poisson distribution environment background noises captured by the semi-active laser target detection system are established,and based on the Poisson photon capture mode,the Cramer-Rao lower bounds of laser pulse peak power in single quadrant of the quadrant photodetector are generated.Based on the laser pulse signal captured by the quadrant photodetector,the model of ideal voltage pulse signal produced by the transimpedance amplifier is established.Considering the white noise model of the circuit that obeys the Gaussian distribution,the statistical distributions of the target position deviation angle and the target direction deviation angle calculated by the normalized sum-difference algorithm of quadrant photodetector are studied based on Monte Carlo method,under the conditions of different signal distribution types in the time domain,different ideal signal widths at half maximum and different signal to noise ratios.The model of target capture domain for the semi-active laser detection system is established,and the optimal field of view angle and the optimal starting trajectory point for the target detection are studied.The ideal defocused optical system model of the strapdown laser detection system is established,and a defocused optical system with large field of view and short focal length is designed.The error compensation method for the optical system is studied by optical simulations.The target position deviation angle and the target direction deviation angle of the strapdown semi-active laser detection system in terminal trajectory are calculated reversely,and the accurate azimuth deviation information is extracted by the mothod of Savitzky-Golay first order smoothing filter.Considering the limitation of the trajectory correction capability of the low speed aircraft,the moment and direction of the trajectory correction of the low speed aircraft are designed by analyzing the variation characteristics of the target azimuth deviation angle calculated by the strapdown laser detection system in terminal trajectory.Considering the characteristics that the quadrant photodetector is rotating with the aircraft longitudinal axis and spots’ centers are far away from the center of the quadrant photodetector in the early and middle terminal trajectory,the programmable gain method for single quadrant circuit channel is proposed based on the prediction of the spot’s center at the next detection time.According to the system requirements,the overall system scheme of the strapdown semi-active laser target detection circuit and the sub circuit systems such as the pulsed laser detection circuit system and the pulsed signal acquisition and processing system are designed.The optical system and the circuit system are processed,and a plane target is set as the target,so the test platform for detecting target azimuth deviation angle is set up.The echoed laser detection experiment,the target azimuth deviation angle measurement and statistical experiment are conducted.The experimental results show that the analogue measurement accuracy of target position deviation angle is better than 0.2°.