Design Of Range Beam Of Antenna In Spaceborne SAR

Spaceborne SAR can observe earth in all kinds of weather conditions at day and night.SAR technologis have developed quickly since 1970s in 20^th century.The key factors affecting image quality are swath width,NEσ⁰ and ambiguity etc.All of these specifications are concerned with the design of range beam of antenna.This paper describes the design of range beam of antenna in spaceborne SAR.First,based on the geometry between satellite and earth,we draw the zebra figure complying with the following three rules:(1)to avoid the shading of transmit pulse;(2)to avoid the echo of nadir;(3)to avoid the ambiguity of mainlobe of pattern.We select appropriate Pulse Repetition Frequency corresponding to large angle of view,middle angle of view and small angle of view respectively in zebra figure,and then design the beam mask along the range direction.The beam mask describes the zone of ambiguity signal distribution and the degree of affecting the ambiguity ratio.During the study of range beam,in order to avoid the ambiguity of mainlobe of pattern and obtain enough swath width,we must design the beamwidth carefully both in the condition of large angle of view and small angle of view,the beamwidth of antenna corresponding to the small angle of view is generally needed to be broadened.Gene algorithm is an appropriate method applying to broaden the beamwidth,because the algorithm has many features such as robust,globility,and non-sensitive to the initial parameter setting.The result of beam broadening shows that we can get satisfied beam parameters such as the beamwidth,sidelobe level and gain loss.During the process of beam broadening applying gene algorithm, representation of the aperture phase may have several forms,such as independent phase variables,polynomialⅠ,polynomialⅡand Legendre polynomial,different representation form has various broadening effect.Finally,we analysis the effection of the amplitude and phase errors in a practical antenna on the specifications of pattern,which can offer the guidance to engineering practice.