Analysis Of Imaging Geometry And Radiometric Characteristics Of Bistatic Radar

Bistatic radar is a radar system whose transmitter and receiver are spatially separated.This separation improves the system's reliability,flexibility,and invisibility,making it attract considerable and increasing attention in the remote sensing field over the last decade.Compared to monostatic radar,for which only the backscattering is measured,bistatic radar expands the scattering measurements in aspects of the angular region,conducive to analyzing target scattering mechanism and greatly enhances the capability of remote sensing over terrain and sea.In imaging,bistatic radar can find a distinct bistatic angular to increase the radar cross-section(RCS)to make these targets visible in the final image.It can also avoid strong backward scattering signals,thereby improving the dynamic range the signal-to-noise ratio(SNR)of the image.However,its flexible configuration makes the imaging geometry and radiation characteristics more complicated than those in monostatic mode.Therefore,in this dissertation,we studied the imaging geometry and radiation characteristics of bistatic radar.Firstly,we analyzed the characteristics of the bistatic range history,including the relationship with the two-dimensional resolution derived by the gradient method.We proposed the fast time-domain imaging algorithm based on the fundamentals of bistatic radar and classic time-domain imaging algorithms.Secondly,we analyzed the imaging characteristics and optimized the geometry according to desired imaging performance in forward bistatic radar with a large bistatic angle and a long baseline.Finally,we discussed the radiation characteristics closely related to the quantitative bistatic radar application were studied,and the antenna radiation pattern effect.In summary,the main research contents and conclusion of this dissertation include the following there parts:(1)A modified fast bistatic imaging algorithm(Modified Cartesian fast decomposition back projection,MCFFBP)in the Cartesian coordinate system has been proposed,based on the basic principles of bistatic synthetic aperture radar(SAR)imaging and classic time-domain imaging algorithms.The proposed algorithm,using sub-aperture and azimuthal sub-image,applies two-stage azimuth spectrum compressing filters and performs fusion images progressively in the Cartesian coordinate system to achieve a high-precision and efficient focusing.First,the spectral expression of the subaperture image was derived,and the factors that affect the azimuth broadening of the wavenumber spectrum were analyzed through the stationary phase approximation.The matched filter was designed to relieve the dependence of the algorithm's sampling rate on the imaging scene.Then,the details of the proposed algorithm and the corresponding computational complexity analysis were provided.Finally,simulated results were given to analyze the compression ability and noise tolerance of the two-stage compressor,and the comparison imaging results were shown to validate the MCFFBP algorithm.(2)Based on the forward bistatic radar observation's potential and limitations,the chaos particle swarm optimization(CPSO)was used to find the optimal angle combinations under a given imaging performance,which provides the best bistatic geometric configuration scheme.This part mainly concerns the imaging geometric properties and the SNR the bistatic observation geometry,using the system parameters of SAOCOM-CS(Satellites for Observation and Communications-Companion Satellite)to explore the potential and limitations of the forward bistatic radar observation.The results indicated that the range resolution deteriorates in the forward bistatic radar imaging,such as specular forward imaging geometry.In addition,due to the different imaging projection rules between backward and forward bistatic radar,the ghost point is produced in the forward imaging.The forward bistatic imaging geometry must be taken into consideration to avoid the problems mentioned above.For a given application requirement with the desired imaging performances,the motion parameters' design can be considered a question of solving the nonlinear equation system(NES).Then the improved CPSO was introduced to solve the NES and obtain the optimal solutions.And the simulated imaging results were used to test and verify the effectiveness of CPSO.The results help to deepen understanding of the constraints and properties of bistatic radar imaging.The conclusions provide guidelines for the optimal configuration of the bistatic radar's geometric parameters with large angles and long baselines.(3)The antenna pattern effect was studied.There are two aspects of antenna pattern effect.One is the influence on bistatic imaging,and the other is on estimating the bistatic scattering coefficient.In this dissertation,we took the Gaussian antenna pattern as an example,without loss of generality,to evaluate such influences.In the first part: first,according to the imaging geometry,the antenna radiation pattern model containing the beam pointing error was given.Then three types of beam pointing errors,namely,linear,quadratic,and sine(high frequency/low frequency),were considered in the elevation direction and the azimuth direction of the antenna pattern,respectively.Finally,the influences of these errors on bistatic radar imaging results were discussed.The results show that the three types of pointing errors affect the pixel amplitude and the sine error has the greatest influence,followed by the quadratic and linear errors.Also,the influence on azimuth profile is more severe than that in the range profile.Moreover,the side lobe rises significantly under the high-frequency error,and the main lobe widens obviously under the low-frequency error.In the scattering coefficient estimation of rough surface scattering,the bistatic scattering coefficient was derived under Kirchhoff's approximation.Then,we discussed the error caused by half-power area instead of illumination integral under three radar observation geometries: monostatic,GPS,and airborne-airborne case.Finally,using the SAOCOM-GPS mission and DLR-ONERA system,we conducted the error analysis.The main conclusions are the following: 1)For the monostatic case,it leads to a 1.592 d B scattering coefficient error;2)For the bistatic GPS case,the scattering coefficient error can be controlled within the expected threshold for an exemplary configuration and antenna size;3)For the airborne-airborne case,taking the DLR-ONERA system as an example,it will lead to a 3d B error.Because the aircraft's attitude is prone to atmospheric turbulence,we also considered the effect of attitude on the illumination integral in the airborne-airborne case.The result proves beneficial for the error correction of measured bistatic scattering coefficient and for configuring the bistatic measurements.