Study On Imaging Algorithms For Geosynchronous SAR And Medium-Earth-Orbit SAR

Synthetic aperture radar (SAR) can work all day and all weather to obtain the large areaand high resolution images of the surface and of the earth through the coverage of the plants.In recent years, with the development of the technology and the expansion of the applicationsin SAR field, much tougher requirements for the growing number of observation missions areprovided on the performance index of the SAR system, such as military mapping,oceanography, intelligence reconnaissance, disaster management and so on. Most of theseapplications demand both higher spatial resolution and better temporal accessibility to getfiner imaging performance, which brings a new challenge for the system design in revisitperiod, swath width and so on.However, all the current spaceborne SAR systems operate in low earth orbit at a heightof500km-1000km. Because of the low orbit height, the small instantaneous view field andlong revisit time show increasingly more obvious disadvantages in low earth orbit SAR (LEOSAR). Especially, with the recent rapid development of space ballistic missiles and laserweapons, a severe challenge for battlefield survivability of LEO SAR have been posed, itsapplication is largely limited. One solution to this problem is to adopt geosynchronous SAR(GEO SAR) and medium earth orbit SAR (MEO SAR). Because higher vantage points canimprove both the spatial resolution and the temporal accessibility, GEO SAR and MEO SARhave the large instantaneous view field and short revisit time, and have a lot of advantages inthe earthquake and volcano prediction, disaster management, ocean applications, and so on. Inaddition, due to the high orbit altitude, GEO SAR and MEO SAR have strong anti-strikeability and battlefield survivability. Moreover, the ultrawide synthetic aperture makes itpossible to trace the moving target and to perform high resolution imaging. Thus, the GEOSAR and MEO SAR will be one of the trends in the future’s spaceborne SAR for wide-swathand high resolution detection.Due to its the high orbit height, whose characteristics are different from LEO SAR, theimaging geometry of GEO SAR or MEO SAR is more complicated, and the syntheticaperture time is also very long which will result in a curved synthetic aperture trajectory, thusthe “Stop-and-Go” assumption and conventional imaging methods of LEO SAR will loseeffect in GEO SAR and MEO SAR. Based on all the above reasons, this dissertation consistsof two parts，one is about the imaging methods of GEO SAR, and the other is imagingmethods of MEO SAR. Most of the conclusions and analysis methods of GEO SAR are alsoapplicable to the MEO SAR. The main research results are as follows: 1. In order to derive the expression of the two-dimensional spectrum for the echosignal, two potential thoughts are analyzed and compared. One is to solve equations to obtainthe stationary phase point and the other is to use the method of series reversion (MSR) to getthe stationary phase point. After the comparison, it is easy to find that MSR has advantages ofhigh accuracy, strong portability and easiness of realization to meet the demand of the highresolution GEO SAR imaging, paving the way for the research of the following highresolution algorithm.2. The method for parameters design and the characteristics of the GEO SAR areresearched systematically. Firstly, the orbit model is constructed and the properties of themotion of the GEO SAR are analyzed. The Doppler property of the GEO SAR target isdiscussed in detail. The changes of the satellite attitude have significant effects on the GEOSAR imaging. By deducing the Doppler parameters expression with the satellite attitudevariety, the impact of satellite attitude changes to the GEO SAR imaging is obtained, theformula is significative for analysis of performance and preliminary design for spaceborneSAR. Based on the above analysis of the characteristics, the parameters’ constraint relation isdeduced to find a parameters design method. In the end, as for the China’s geomorphiclocation, the designed parameters of low resolution and high resolution are given.3. The subaperture imaging algorithm for GEO SAR is studied. At first, the features ofthe targets’ echo signals are analyzed. Then, a subaperture imaging model is established,which takes the “Stop-and-Go” assumption error and the space-variance of equivalent radarvelocity and the equivalent squint angle into account. Two subaperture imaging algorithms areproposed based on the imaging model. One is the improved SPECAN algorithms, which isefficient and can satisfy the demand of real time imaging. The other is improved chirp scaling(CS) algorithm, which performs range cell migration correction accurately and suits forwide-swath imaging. These two algorithms can be considered as an extension of the SPECANand the CS algorithms from LEO SAR case to the GEO SAR case, which overcome thelimitation of height. At last the effectiveness of the proposed algorithm is validated by thesimulations.4. As for the problem of high resolution imaging of full aperture, this dissertationproposes an approximate OMEGA-K algorithm and an improved CS algorithm which arebased on the MSR based two-dimensional spectrum. Because of the higher orbit of GEO SAR,the integration time is long, and the relative motion between the satellite and the earth alsobecomes more complicated. Thus, the linear trajectory model and imaging algorithm of LEOSAR are not suitable for GEO SAR. With regard to this issue, this paper establishes the rangeequation for long synthetic aperture time by using the high order approximation based on the characters of the GEO SAR movements. Then, the two-dimensional spectrum is derived bythe method of series reversion. The approximate OMEGA-K algorithm is a two-dimensionalfrequency domain imaging algorithm, all the operations of which are composed of the fastFourier transform and the phase multiplication, so the algorithm is efficient. The improved CSalgorithm has the advantages of high accuracy and relatively low computation load, since nointerpolation is required during the whole processing steps. The range migration correction isimplemented by the chirp scaling operation and numerical approximation of the GEO SARparameters.5. According to the problem of high resolution imaging of full aperture, a high-ordermodified hyperbolic range equation is proposed for MEO SAR. Incorporating with anadditional linear component and quartic component, quartic Taylor series expansion of it hasexactly the same value as that of the actual range history of MEO SAR. Then, thetwo-dimensional spectrum based on high-order modified hyperbolic range is analyticallyderived by using an approximate azimuth stationary phase point, based on the MSR, theaccuracy of the two-dimensional spectrum is analyzed which is exactly equal to quartic phaseterm. In order to design the imaging the space-variance of each parts of the two-dimensionalspectrum is analyzed, and the imaging flowchart is given. And current available MEO SARalgorithm can be implemented directly with little modification. Simulation results show thatthe proposed range equation and the two-dimensional spectrum are accurate which can givefine resolution imagery with the entire aperture.