Study Of Bi-and MultiStatic SAR Image Formation

This dissertation addresses topics in one main area of radar imaging formation: bi-and multistatic synthetic aperture radar (SAR) image formation, especially effective bistatic SAR focusing algorithms.The concept of distributed radar satellite constellations has been proposed as a new remote sensing technique in the literature and currently under research. It is suggested that one on-orbit radar satellite can be used as an active transmitter while several other micro-satellites act as passive receivers, and the active transmitter, if necessary, can be a new launched radar satellite. This kind of spaceborne implementation offers extra advantages compared to the monostatic scheme, namely high resolution wide swath imaging, topographic imaging or DEM generation, ground moving target indication (GMTI) and current measurement. It is clear that the bistatic radar constellation is the basis of the distributed spaceborne system, and thus the bistatic SAR is the fundamental research topics. Generally, bistatic SAR imaging is more involved than the monostatic SAR case due to the considerable separation of the transmitter and the receiver for the sake of avoiding potential collision of platforms. Therefore, the bistatic SAR data can not be processed sufficiently by using the monostatic focusing algorithms, i.e., the conventional monostatic image formation algorithms are not well suited to this type of bistatic constellation. This dissertation mainly discusses the spaceborne cases, but in order to illustrate the bistatic SAR characteristics the airborne bistatic cases are also discussed. The bistatic SAR systems can be considered as Azimuth-Invariant during an aperture time, provided that the bistatic transmitter and receiver move in a constant velocity in a parallel constellation. This case is referred to as parallel bistatic SAR system for simplicity. This dissertation focuses on this kind of systems.Due to the two square rooted terms in the slant range history of the bistatic SAR, it is very difficult, if not impossible, to obtain the analytic expression of the phase history of echoed data in the 2-dimensional frequency domain by using the principle of stationary phase, which adds to the difficulty in developing fast focusing algorithms for bistatic SAR data.We propose an analytic formulation method in the 2-dimensional frequency or wavenumber domain based on the concept of the instantaneous Doppler wavenumber. In the formula, the space-variance of the two new defined parameters, i.e., the difference of the closest distances and the half quasi bistatic angle, are detailedly discussed.A computation effective bistatic Range Migration Algorithm (Bi-RMA) is proposed under some rational approximation to the space-variance of the above two parameters. This proposed bi-RMA can be used to process the wide swath bistatic SAR data sufficiently. Another benefit of the developed analytic expression lies in the fact that it becomes easier to understand the inherent relationship of the bistatic SAR and its equivalent monostatic SAR based on the formula, and the parameters influence on the resulted image can be explained in a quite straightforward way.Unfortunately, in the high squinted bistatic cases, the space-variance of the above two parameters may be so heavy that the bi-RMA can not be used to focus the data any longer. We derive a numerical method to handle this case. The two parameters, the difference of the closest distances and the half quasi bistatic angle, are calculated by using the numerical approach, respectively. Using the developed analytic expression of the echoed signal in the 2-dimensional frequency domain in combination with the obtained parameters, we get the accurate bi-RMA in the sense of numerical computation.However, the computational load of the proposed bi-RMA is relatively heavy yet due to the quasi-Stolt interpolation used in the processing. Furthermore, the interpolation is not a phase preserving operation both in the bistatic and monostatic image formation, which is not expected for the possible interferometric applications. Also on the basis of the developed frequency domain analytic expression of bistatic data, we propose a phase preserving, computation effective bistatic Chirp Scaling (bi-CS) algorithm. The linearization of the range-variant parameters is one of the most important steps.The above proposed two image formation methods are both based on the analytic expression in the 2-dimensional frequency domain and the conventional monostatic SAR focusing scheme, and the bistatic transfer function is derived to handle the data. Another type of method is to compensate the bistatic data into the equivalent monostatic data and then processed using the monostatic algorithms. Thus, the troublesome two square rooted terms problem is successfully pushed away. However, it should be make clear that one resolution cell is composite of many individual scatterers even with a very high resolution capability, and the echo data is so sensitive to the radar-target vector that, strictly speaking, the bistatic data can not be fully represented by the equivalent monostatic data. But if Born approximation is followed, it is feasible. Now the bistatic data is identical to the monostatic data at the certain squint angle since the difference between them lies in the scaling factor of radial wavenumber.In the conventional EPC (equivalent phase center) processing method, the bistatic iso-range ellipses are compensated into the iso-range circles of the equivalent monostatic one with the origin located in the middle of the baseline. It is accurate if the far-field assumption is satisfied. Actually, the compensation depends on the range lines of the imaging scene and the Doppler frequency of the EPC monostatic SAR. Due to the fact that the bistatic SAR and the EPC monostatic SAR are not identical in the Doppler frequency and we obtain the bistatic Doppler domain data by directly using FFT over the raw data, one assumption is adopted in the EPC compensation, i.e., they are identical in the Doppler frequency. Thus, the EPC method can only be used to the bistatic data from a short baseline system.The so-called DMO (Dip Move Out) technique use the SMILE concept of seismic processing as a reference. In DMO bistatic image formation algorithm, the bistatic iso-range curves (ellipse) are compensated into a series of monostatic iso-range curves (circle) which are all inscribed circles of the former ellipse. But both the centers and the radius of the circles are not identical to each other, respectively. Each sample of a bistatic snap shot can be considered as a series of monostatic different range samples along the bistatic baseline after the slant range differences are compensated respectively. Suppose that the SAR echoed signal has the property of slow fluctuation in envelop and fast fluctuation in phase, the compensation can be divided into two main steps: first, the phase differences can be fully removed in the range Doppler domain. Second, the bulk envelop difference can be compensated using compensation function of the swath center line in the 2-dimensional frequency domain, and of course the more accurate result can be obtained by using a blocking compensation in range dimension. A scaling Fourier transform approach is developed, which is proved to have the capability of accurate compensation but the computational load is too high to use.The DMO approach has several advantages in implementation. Although the radial wavenumber is changed, the azimuth Doppler of the bistatic echoes remains not distorted. Second, both the range and azimuth resolution are not changed. The last, but most important, is that the interferometric phase is not distorted, thus the bistatic interferometry is possible.Two main applications of the distributed micro-satellites radar systems, for example, the Cartwheel constellation, are the topographic imaging (finally as DEM) and GMTI by using the multi-channel SAR data from different receivers. For the purpose of interferometry processing, both high phase preserving of the image formation processing and the high coherence of the multi-channel SAR image pairs are expected. The fixed phase of the bistatic echo is analyzed as well as the affect over bistatic image caused by the terrain relief. On the basis of the DMO concept, we propose a coherence improvement pre-filtering method for the bistatic SAR system with an along track baseline. Using this method, the baseline-induced non-coherent components of the bistatic image pair can be fully removed.SNR is a challenging problem for distributed micro-satellite radars. Suppose one master satellite in a constellation, the average power of the transmitted signal is generally low, namely 1 to 3 hundred Watts. Due to the relative small antenna aperture size of the slave micro-satellites, for instance, 1-2 meters in azimuth, their received power will be quite low compare with the master satellite. On the basis of the analysis of this reduced SNR affects on the resolution and interferometric applications, we approach this problem by using the proposed coherent and non-coherent techniques over the multi-channel received echoes, respectively.