Research On Achievement Of Wide-swath And High Resolution SAR Image By Using Distributed Small Satellites

A new conceptual implementation of spaceborne synthetic aperture radar (SAR) system in which several small satellites fly in constellation forming a highly sparse array was presented in the mid-1990s. Small spaceborne system has many advantages, such as light weight, small size, short development time, low cost, flexible launch and so on. Further more it can batch produced. Consequently, forming a constellation of small satellites not only has far lower cost than a conventional spaceborne SAR system but also better performance.The formation flying distributed small satellites can provide multiple and long baselines in single-pass observation mode, thus greatly improving the performance of interferometric SAR (InSAR) and ground moving target indicator (GMTI). The coherent combination of several SAR images obtained from different observing angles can improve the image resolution and provide accurate geometric information. Furthermore, combining a broad illumination source with multiple small receiving antennas placed on separate formation-flying micro-satellites, we can obtain high resolution SAR images of wide areas. However, several challenging problems are also introduced by the constellation SAR regime at the same time. These problems include high sparseness of the array, range or Doppler ambiguities, many kinds of error sources and so on. These problems should be carefully considered to ensure that the advantages of the constellation can be achieved.In this doctoral dissertation, the approaches to reconstruct ground scean with wide swath and high resolution and the problems of error estimation are studied. The main returns of this doctoral dissertation are listed as follows:1. The small satellites in constellation distribute sparsely along specific orbit, which is to say that they form a multi-channle sampling system. And for a single small satellite, it is under sampling in spatial domain. Combining multiple small satellites in constellation to increase the spatial sampling rate the distributed spaceborne SAR system can restrain range or Doppler aliasing. This idea is accordant with the fundamental thought of time-domain multi-channel sampling method. The research of multi-channel sampling and its signal reconstruction method can provide the theory foundation for resolving Doppler ambiguity. Introducing the multi-channel delay sampling idea and combining the idea of frequency band segmentation in frequency multi-channel sampling method, we propose a new reconstructing method which is robust to delay error and channel gain error. The method retrieves the complete bandwidth by using adaptive beamforming thechnique in frequency domain. Due to the adoption of robust adaptive processing, the proposed method can retrieve the complete bandwidth of broad band signal robustly, even when the error exists.2. The minimum antenna area constraint cannot be satisfied by individual small satellite in constellation. Range and/or Doppler ambiguity will be introduced by small antenna inevitably. To obtain the SAR image with wide swath and high resolution, we must restrain Doppler aliasing firstly. Compared with time-domain multi-channel sampling system, distributed spaceborne SAR system will introduce more error sources, such as timer error, beam pointing error, frequency synchronization error, baseline error, channel error, yawing (it distorts the along track linear formation of the sub apertures) and so on. In this dissertation all kinds of errors existing in the real system is classified according to the impact on the performance of Doppler aliasing restraining, approaches to compensate the errors are proposed and a method of restraining Doppler aliasing which is robust to remanent errors is presented. Finally all the methods above are verified by using a set of airborne multi-channel measured data.3. Linear array along the track (or cross-track baseline is very long) is the best configuration for the distributed spaceborne SAR system, if only SAR and GMTI need to be implemented. The samples will not be affected by the terrain fluctuation in this case, because the linear array along the track does not have the ability of resolving the height of terrain. However, this type of configuration can not achieve height measurement (InSAR). Not only can constellation InSAR system achieve wide swath and high resolution two-dimensional SAR image, but also it can obtain the terrain height information. Nevertheless, highly sparse three-dimensional array brings new challenges for data processing, where the violent change of beam pattern (array steering vector) along with the height and range of the terrain make it difficult to obtain sufficient independent and identically distributed (i.i.d) samples for adaptive processing (to restrain Doppler aliasing). The envelope registration of broadband array coupling with the sampling aliasing of the echo is the other important challenge generally, which makes it more difficult to resolve Doppler ambiguity and register the envelope. To overcome these difficuties, this dissertation proposes a novel method of reconstructing wide-swath and high resolution three-dimenssional topography. The method obtains sufficient samples in high resolution SAR image domain and acquires the height information of all aliasing components in SAR image by using joint space-image subspace projection technique. According to the height information the envelope of the every aliasing component is further registered, and all aliasing components in the pixel are extracted by using adaptive beam forming technique. Finally reconstructing three-dimensional topography is finished.4. There are many error sources exist in the distributed spaceborne system, yawing and beam pointing error is the main factor which affect SAR focusing. It is the basis of wide-swath and high resolution SAR imaging, GMTI and InSAR to avoid the affection of the error sources above to focus the SAR data. Starting with complex phase-degraded SAR image, two novel SAR autofocus algorithms are presented in the dissertation. They estimate phase error coefficients by using the total viriation and DCT of the signal. Compared with other SAR autofocus algorithm based on image domain, these two methods above are of less computational complexity and easy to implement.