Simulation And Implementation Of FMCW Radar Altimeter

FMCW radar altimeter is widely used.It has the advantages of simple structure design,low transmission power and high accuracy.At present,the FMCW altimeter is stable in flatter terrain.In complex undulating terrain,there will be sudden changes and inaccuracies in height.It is difficult to collect radar signal echo data by test flight,which limits the research and application of the algorithm.This paper focuses on the echo models of complex terrain radar.Based on the simulation model,the search process and tracking process of the altimeter radar system are analyzed.Then the search algorithm is optimized and the tracking algorithm is verified.And the system is implemented in FPGA and experiments are performed in the air.First of all,.the article performs complex terrain modeling.Through the appropriate undulation model and interpolation method,a terrain model that meets the resolution requirements is obtained,and flat terrain and complex terrain are established.The terrain is divided into scattering units,and the unit scattering characteristic model is established.The area of the scattering units is calculated.The altimeter transmission signals and echo signals modulated by the sawtooth wave are analyzed to realize the ranging model of point target and area target.And the point target echo,the flat terrain echo and the complex terrain echo are compared.Complex terrain echoes have severe spectrum splitting and obvious spectrum broadening.Secondly,the method of measuring the sensitivity of the radar altimeter and the factors affecting its accuracy are described.According to the characteristics of the constant beat frequency system,the workflow of the radar altimeter is divided into a search state and a tracking state.In the search state,the radar altimeter echo beat frequency range is large.The traditional spectrum front detection method works well on flat terrain,but it cannot perform altitude measurement on complex terrain.OCOG algorithms are currently used in pulsed radar time-domain analysis.Based on the OCOG algorithm and the characteristics of the constant-beat frequency system,this algorithm is applied to the echo-beat spectrum analysis.The OCOG algorithm is used to calculate the echo beat spectrum of complex terrain.It is verified that the method can reduce the influence of complex terrain on the altimeter.In the tracking state,the radar altimeter echo beat frequency is fixed within the frequency range of the constant beat,so the fixed spectrum range is refined to improve the measurement accuracy.This article uses the ZFFT method and the CZT method to conduct a comparative analysis through simulation.The ZFFT method has higher resolution in narrower frequency spectrums,but this method requires more points and is difficult to apply at low altitudes.The CZT method can refine the frequency spectrum in any frequency range and can be applied to full heights.Finally,according to the system index requirements,the appropriate microwave components and power boards are selected on the hardware,and the signal processing board is designed.According to the radar altimeter signal processing process,the FPGA is used to implement microwave component control,altitude information extraction,and voltage monitoring.The tracking accuracy and system sensitivity of the ZFFT and CZT methods are analyzed in the tracking state.Through laboratory tests,the CZT method has good accuracy and practicality.And the system function of the altimeter was proved by twice data of the flying test.