Research And Implementation Of Stability Tracking Of LFMCW Radio Altimeter

The LFMCW constant-beat system radio altimeter plays an important role in the navigation of modern aircraft.As the application scenarios become more and more complex,the phenomenon of instability in altitude measurement also appears in its use.When the altimeter is unstable,it seriously interferes with the normal flight and mission execution of the carrier,and even threatens flight safety.Since the constant-beat system altimeter can only measure the height normally in the tracking state,this thesis focuses on the problem of unstable height measurement,researches the reasons that affect the stable tracking of the altimeter and methods to improve the tracking performance,and uses FPGA to complete the improved signal processing algorithm.The main tasks of the thesis are:Firstly,introduced the basic structure of the LFMCW altimeter,the principle of height measurement,and the advantages of the constant beat system,and analyzed the factors affecting the stable tracking of the altimeter.Based on this,the deficiencies of the current multi-channel constant beat system to set the detection threshold according to the characteristics of the time domain signal are analyzed.It is pointed out that the unreasonable setting of the detection threshold is one of the reasons that affect the stable tracking of the altimeter.Based on this,the structure of the multi-channel constant beat system is improved,and the detection threshold is set according to the noise power to reduce the possibility of tracking instability.Then,in order to eliminate the influence of hardware factors on the analysis of the unstable tracking phenomenon under the altimeter complex terrain,the method of complex ground echo modeling is studied.Use the DEM of the real terrain to generate simulated echoes to perform closed-loop static and dynamic simulations on the improved multi-channel constant-beat system altimeter.The static simulation ignoring the doppler frequency verifies that the threshold setting method is effective,and points out that the spectrum of the beat signal on the complex ground has the phenomenon of slowing front,spectrum splitting,and spectrum broadening.Through the dynamic simulation of setting specific motion paths,it is known that when moving at high speed under complex terrain,the altimeter based on spectrum frontier tracking will lose lock,and the height measurement will be small due to the influence of doppler frequency and slope distance.In order to improve the tracking performance in a dynamic environment,the height of the smoothed short window is used to calculate the modulation time,reduce the fluctuation of the beat frequency,and improve the adaptability to complex terrain,and its effectiveness is verified by simulation.In addition,the OCOG algorithm is introduced into the improved multi-channel constant beat system according to the characteristics of the frequency spectrum.The simulation verifies that the rectangular front edge obtained by tracking the OCOG algorithm in the frequency domain can be insensitive to complex terrain changes and has good stability.At high speeds,the ranging error introduced by the doppler frequency can be reduced,and the measurement accuracy is also better than the spectrum frontier tracking algorithm.Therefore,a combination of OCOG algorithm and spectrum frontier tracking is proposed to improve tracking performance.Finally,the FPGA-based hardware platform is used to complete the design and implementation of the altimeter,including the altimeter signal processing for height measurement,and the configuration of microwave components.The altimeter tester verifies that the measurement accuracy meets the requirements,and the drone flying experiment verifies that the altimeter works stably under the current test conditions,and proves that the improved altimeter signal processing method is effective.