Design Of LFMCW Altimeter Based On FPGA

LFMCW radar altimeter has a long history of development.Compared with other types of altimeters,it has advantages of low radiation power,simple design,high precision,no distance blind area,light weight,small size and no weather influence.Based on these advantages,LFMCW radar altimeter is widely used in military fields such as aerospace,aviation and navigation.In recent years,with the application environment of altimeter becoming more and more complex and people's requirements for altimeter performance getting higher and higher,it is of great significance to develop a radar altimeter with strong compatibility,high accuracy and good real-time performance.This thesis focuses on the design of radar altimeter based on FPGA.The signal processing methods in the altimeter are analyzed in detail and the overall design of the system is given.Finally,the verification of the radar altimeter is completed.In this thesis,the basic structure and altimetry principle of LFMCW radar is introduced.On this basis,the surface target echo model is simulated and analyzed.Constant modulation period system and constant beat frequency system are analyzed and compared in this thesis The influences of fence effect,noise and doppler effect on the accuracy of altimeter measurement are also introduced in the thesis.The principle of various spectrum analysis methods are deeply explored.These methods are simulated by matlab under single target and multiple targets.A multi-channel constant beat frequency working system is used in the system.All digital signal processing is completed by FPGA independently.FPGA is suitable for analysis and verification of this system because of its powerful capability of parallel signal processing and great flexibility.In this experiment,the self-test module,power control module and height calibration module are added to the altimeter system,which can improve the system's adaptability,stability and versatility effectively.The height from 4m to 10000 m is divided into five non-uniform intervals.Each interval corresponds to different constant beat values.When the aircraft is flying at low altitude,a smaller constant beat frequency value is adopted,which solves the problem of low sensitivity caused by insufficient sampling points.When the aircraft is flying at high altitude,the higher beat frequency value is adopted,which overcomes the shortcoming of poor real-time performance in the system due to the long modulation period.In order to calculate the beat frequency value accurately,various spectrum analysis methods are realized by FPGA.The resource utilization of the three spectrum refinement methods in the FPGA is compared and analyzed.The performance of these methods was compared and analyzed by altimeter simulation test platform.Combining Chirp-Z transform and ZFFT transform as the signal processing method of altimeter system.This method not only solves the problem of poor sensitivity at low altitude and poor real-time performance at high altitude,but also reduces the resource utilization rate in the FPGA.The speed of digital signal processing in the system is also improved.The project cost is reduced effectively.Finally,The functionality of the system was verified by the flying test on the drone,and the requirements of various design indicators were met.