Realistic simulation and high resolution image formation for point target raw data of spaceborne synthetic aperture radar

Abstract

A spaceborne synthetic aperture radar (SAR) is an all-weather active imaging sensor in which backscattered microwave pulses are collected by a satellite and processed to form day and night images of the earth’s surface, which requires a high resolution to the level of several centimeters recently. In order to make high-quality SAR images without focusing errors, very accurate SAR raw data processing is needed. Especially, critical factors that induce errors in a SAR image have to be analyzed precisely and processed accurately in SAR raw data focusing

They are mainly related to the estimation of focusing parameters such as slant range for the signal transmission (Tx) and reception (Rx) path, effective beam velocity, and Doppler frequency reflected on SAR raw data. In this thesis, for precise raw data focusing of a high-resolution spaceborne SAR (HRS-SAR), with the accurate calculation of focusing parameters based on the realistic two-way slant range model (RTW-SRM), the enhanced two-way chirp scaling focusing method is proposed considering the real two-way signal path on a curved orbit. Moreover, the realistic full-chain model-ing and simulation of a spaceborne SAR system is suggested. The realistic point target raw data simulated by it are needed and utilized to develop and verify the raw data processing methods for the HRS-SAR. The realistic spaceborne SAR modeling and simulation method is proposed, which reflects the characteristics and operations of a real spaceborne SAR system. Unlike several conventional simulations that model partially data acquisition of a spaceborne SAR for specific purposes, its full-chain modeling and simulation is carried out, which is related to SAR dynamics and attitudes, two-way antenna beam patterns, antenna beam pointing and steering, SAR hardware pulse errors, ground point target observation, and received echo signal characteristics. In particular, the proposed models simulate the raw data for multi point targets taking into account realistic spaceborne SAR operations, and reproduce main spaceborne SAR functionalities without considering system hardware choices. Through the proposed modeling and simulation, simulated point target raw data are almost identical to ones acquired by a real system. Further, the enhanced two-way chirp scaling focusing method is proposed, which can reduce error ef-fects arising from inherent characteristics of a spaceborne SAR system used to achieve raw data, so retain reasonable image quality at the same time. In the proposed method, the two-way based method is proposed to calculate focusing parameters such as a slant range, an effective beam velocity, and a Doppler frequency, which is based on the RTW-SRM using SAR state vectors consisting of the position, velocity, and time on a curved orbit. In addition, it uses ground target position vectors at the time of pulse Tx and Rx. Moreover, in the proposed method, the effective chirp rate, RCM factor, and focusing phase functions are modified on the basis of the RTW-SRM, and the additional processing load can be reduced by using a chirp scaling focusing approach. Related to proposed methods, total processing procedures are suggested with simulation results. Comparative and extensive experiments were performed on the realistic simulated point target raw data of the HRS-SAR to evaluate the performance and effectiveness of proposed methods. The experimental results showed that high-resolution point target raw data were processed accurately by proposed methods, which various effects induced from a real spaceborne SAR are reflected on. Therefore, the proposed methods are useful for accurate HRS-SAR image formation.