Hybrid space-airborne bistatic SAR geometric resolutions

ABSTRACTBistatic synthetic aperture radar (SAR) is a new type of SAR system which is capable of achieving multi‐angle target detection and flexible configuration. For spaceborne bistatic SAR, when operating at low frequencies, it is susceptible to the ionosphere. The ionospheric Faraday rotation effect will cause a rotation of polarisation plane of electromagnetic waves, leading to amplitude and phase errors between different polarisation channels of SAR. This effect can be compensated by estimating the Faraday rotation angle. Currently, Faraday rotation angle estimation methods for spaceborne SAR are all applicable to monostatic SAR. However, due to the different signal propagation paths between bistatic SAR and monostatic SAR, the Faraday rotation angle estimation method differs for bistatic SAR. This paper proposes a Faraday rotation angle estimation method for bistatic spaceborne SAR, which is applicable to the alternating bistatic mode. The effectiveness of the method is verified through simulation experiments and the performance of the method is analysed. Under different signal‐to‐noise ratios (SNR) and filtering window sizes, the estimation error range is 0.02°‐0.26°.

With appropriate geometry configurations, bistatic Synthetic Aperture Radar (SAR) can break through the limitations of monostatic SAR on forward-looking imaging. Thanks to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications. However, the current BFSAR imaging algorithms always involve the time-consuming interpolation to carry out range migration correction. This paper develops a Chirp Scaling type imaging algorithm, which just needs the complex multiplication and FFT, for BFSAR with two parallel moving stations. The simulation results validate the effectiveness of the method.

The bistatic configuration of frequency-modulated continuous wave (FMCW) synthetic aperture radar (SAR) offers considerable capabilities, reliability and flexibility in designing the compact size and low cost remote sensing systems. Furthermore, the spacial separation between transmitter and receiver avoids the antenna isolation problem. A signal model is proposed here to accurately formulate the bistatic slant range history in FMCW SAR configuration, where the motion during the long pulse duration is taken into account. Moreover, based on the preceding model, an optimization approach is presented to deal with the double square root (DSR) term, and thus an accurate FMCW bistatic point target reference spectrum (BPTRS) is derived. The proposed spectrum can not only address the signal characteristics of bistatic FMCW SAR, but also significantly simplify the bistatic FMCW SAR processing, which will really push the applications of FMCW SAR in the remote sensing field. Furthermore, based on the proposed signal model, the application of an improved range-Doppler algorithm (RDA) is further proposed to focus the bistatic FMCW SAR data. Simulation experiments are carried out to verify the accuracy of this novel formulation and highlight the performance of the proposed focusing approach.

With appropriate geometry configurations, bistatic Synthetic Aperture Radar (SAR) can break through the limitations of monostatic SAR for forward-looking imaging. Thanks to such a capability, bistatic forward-looking SAR (BFSAR) has extensive potential applications. This paper develops a frequency-domain imaging algorithm for translational invariant BFSAR. The algorithm uses the method of Lengendre polynomials expansion to compute the two dimensional point target reference spectrum, and this spectrum is used to perform the range cell migration correction (RCMC), secondary range compression and azimuth compression. In particular, the Doppler-centroid and bistatic-range dependent interpolation for residual RCMC is presented in detail. In addition, a method that combines the ambiguity and resolution theories to determine the forward-looking imaging swath is also presented in this paper.

Due to the fact that the range history of bistatic synthetic aperture radar (SAR) doesn’t follow the hyperbolic function, the accurate analytical spectrum of bistatic SAR cannot be obtained directly by the principle of stationary phase (POSP). In this paper, we approximate the range history of bistatic SAR by the hyperbolic function, and propose a novel two-dimensional (2-D) analytical spectrum model for bistatic SAR. The huge advantage of this novel 2-D analytical spectrum is that the equivalent radial velocity of any scatter point in the observed scene, with respect to the radar platform, is not only the same but also invariant with the equivalent range. Further, based on this novel spectrum model, the wavenumber domain algorithm (WDA) could be applied to image the whole observed scene. Finally, the simulation and realistic data imaging results also validate that this novel spectrum model is correct and effective.

The bistatic SAR has forward-looking imaging capabilities if a opportunistic illuminator is selected to form the synthetic aperture. This would enable it to perform some inspection tasks that traditional SAR cannot perform. The system resolution of a bistatic SAR is related to its configuration. Due to the special configuration of the bistatic forward-looking SAR, the traditional system trajectory design method is no longer applicable. Therefore, a trajectory design method used for bistatic forward-looking SAR is proposed in this paper. First, the original trajectory of the receiver is determined according to the system which is always cannot be changed. Then, the slant range and the pitch angle of the transmitter are determined at each time. Next, the resolution of the transmitter under different squint angles at different times is calculated. Then, the available area of squint angle is obtained by drawing the results into a diagram. Finally, a position curve of the squint angle that is contained within the available area is found, and then the position of the transmitter and the resolution are calculated based on the position curve. The method can make the resolution of the forward-looking SAR system meet the requirements at all times under the condition that the changes to the original trajectory are at the minimum. The simulation results verify the validity of the proposed method.

Low-earth-orbit (LEO) synthetic aperture radar (SAR) can achieve advanced remote sensing applications benefiting from the large beam coverage and long duration time of interested area provided by a geosynchronous (GEO) SAR illuminator. In addition, the receiving LEO SAR system is also cost-effective because the transmitting module can be omitted. In this article, an imaging method for GEO-LEO bistatic SAR (BiSAR) is proposed. First, the propagation delay characteristics of GEO-LEO BiSAR are studied. It is found that the traditional "stop-and-go" propagation delay assumption is not appropriate due to the long transmitting path and high speed of the LEO SAR receiver. Then, an improved propagation delay model and the corresponding range model for GEO-LEO BiSAR are established to lay the foundation of accurate imaging. After analyzing the sampling characteristics of GEO-LEO BiSAR, it is found that only 12.5% sampling data can be acquired in the azimuth direction. To handle the serious sub-Nyquist sampling problem and achieve good focusing results, an imaging method combined with multireceiving technique and compressed sensing is proposed. The multireceiving observation model is first obtained based on the inverse process of a nonlinear chirp-scaling imaging method, which can handle 2-D space-variant echo. Following that, the imaging problem of GEO-LEO BiSAR is converted to an L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> regularization problem. Finally, an effective recovery method named complex approximate message passing (CAMP) is applied to obtain the final nonambiguous image. Simulation results show that the proposed method can suppress eight times Doppler ambiguity and obtain the well-focused image with three receiving channels. With the proposed method, the number of required receiving channels can be greatly reduced.

Bistatic synthetic aperture radar (SAR) processing requires precise knowledge about geometrical parameters of the flown bistatic constellation, whereas estimation of these parameters is even more important than in the monostatic case. As it is impossible to separate the individual semimonostatic parameters from the bistatic raw data, the searched parameters are derived from the tracks of the moving platforms. For this reason, global positioning system (GPS) and inertial navigation system (INS) position and velocity data of transmitter and receiver are combined in an optimal way by a Kalman filter approach. As a consequence of model-based interpolation, we can easily determine varying parameters like Doppler centroid frequency, azimuth velocity, and the bistatic parameters a2 and a0 referring to the illuminated scene at each time instant. Accurately determined position and velocity of the transmitter and receiver enable a first-order bistatic motion compensation (MC), which is also described in this paper. In verifying our approach, GPS/INS tracks and raw data of a bistatic airborne SAR experiment flown in 2003 were used, where the data were provided by Forschungsgesellschaft fu umlr Angewandte Naturwissenschaften e.V., Wachtberg, Germany. The focusing was done by a scaled inverse Fourier transformation processor for nearly parallel trajectories for transmitter and receiver. This paper focuses on analyzing the trajectories and antenna beams of bistatic missions to obtain accurate processing parameters and MC functions.

Based on the analysis of the airborne bistatic synthetic aperture radar (SAR) imaging geometric mode, an extended nonlinear chirp scaling algorithm is employed to simulate and verify the imaging effect of the bistatic SARs. A gradient theory-based two-dimensional resolution bistatic SAR model is proposed, and the constraints of the multi-platform flight trajectory parameters meeting the imaging accuracy of the bistatic SAR are analyzed. Finally, through the bistatic SAR imaging simulation of cooperative flight trajectories under various situations, the spatial configuration constraint envelope between the flight vehicles to achieve the optimal resolution is revealed. The results of this paper will provide a theoretical reference for the SAR application in formation flight control.

Raw signal simulation is a useful tool for synthetic aperture radar (SAR) system design, mission planning, processing algorithm testing, and inversion algorithm design. Time and frequency synchronization is the key technique of bistatic SAR (BiSAR) system, and raw data simulation is an effective tool for verifying the time and frequency synchronization techniques. According to the two-dimensional (2-D) frequency spectrum of fixed-receiver BiSAR, a rapid raw data simulation approach with time and frequency synchronization errors is proposed in this paper. Through 2-D inverse Stolt transform in 2-D frequency domain and phase compensation in range-Doppler frequency domain, this method can significantly improve the efficiency of scene raw data simulation. Simulation results of point targets and extended scene are presented to validate the feasibility and efficiency of the proposed simulation approach.

The paper introduces an algorithm for Ultrawideband Ultrawidebeam (UWB) bistatic Synthetic Aperture Radar (SAR). The algorithm works in time-domain and therefore inherits time-domain characteristics such as unlimited scene size, local processing and manageable motion compensation. The proposed algorithm is not limited by any configuration of bistatic SAR. The algorithm processes the UWB bistatic SAR data on a subaperture and subimage basis. This means, instead of backprojecting directly the SAR data to a ground image plane, the algorithm handles the data in two stages: beam forming and local backprojection. The algorithm is named Bistatic Fast Backprojection (BiFBP) and has been tested successfully with the simulated UWB bistatic SAR data.

The technology of Synthetic Aperture Radars (SAR) is growing at a rapid pace in the last decade. The advancements in SAR opens doors to explore more scientific problems which were considered impactical in the past. The study of meteorological targets using spaceborne SAR platforms is an emerging field of study. In the past, spaceborne monostatic SAR for observing precipitation echoes was studied. In this paper, a bistatic architecture of the SAR system is considered for studying the precipitation target using signal simulations. The precipitation radar parameters on-board the GPM core satellite is considered for the simulation study. The architecture of the bistatic SAR considered is introduced and the various factors considered for a bistatic SAR is studied in detail. This simulation work currently being researched on shows the initial results. The results presented will aid in understanding the precipitation echoes observed from spaceborne SAR platforms.

As having the imaging ability of area in front of flight direction, forward-looking synthetic aperture radar (SAR) has become a hot topic in areas of SAR research. Nevertheless, existing imaging algorithms of forward-looking SAR are all based on the assumption that the platform is moving in low velocity. When this assumption is not satisfied, the imaging result of existing algorithms will show focus out effect and deviation of imaging position from real position. Aiming at this problem and taking airborne forward-looking SAR system using single transmitting and multiple receiving antennas as the research object, this paper analyzes the feasibility of forward-looking SAR imaging of high velocity platform, and proposes a forward-looking SAR imaging algorithm suitable for high velocity platform according to its spatial geometry and the model of echo signal. Finally, the raw echo data of an array of point targets is simulated, and the focused images are obtained using the algorithm proposed in this paper. Imaging results demonstrate the validity of the algorithm. (6 pages)

Synthetic aperture radar (SAR) raw data simulator is an important tool for parameter-optimizing and algorithm-testing, particularly for those complicated configurations in which real raw data is difficult to obtain. As a new and special imaging mode, bistatic forward-looking SAR with constant acceleration (BFCA-SAR) can perform two-dimensional imaging for targets in the straight-ahead position over mono-static SAR. But there exist more complicated square roots and high-order terms in range history owing to high velocities and accelerations from both platforms. In addition, space variances in phase terms of two-dimensional frequency spectrum (2-D FS) make it difficult to gain echo data accurately. In this paper, a fast scene raw data simulator for BFCA-SAR based on quantitative analysis and effective correction of phase space variance is proposed. With high precision, our method can generate raw data more efficiently than traditional algorithms.

For high quality synthetic aperture radar (SAR) processing, Doppler centroid estimation is an essential procedure. According to the range migration characteristic of bistatic forward-looking SAR, an adaptive iterative scheme of Doppler centroid estimation is proposed in this paper. This scheme utilizes the fact that, in the range compressed and azimuth time domain, the range migration slope of point target response is proportional to the Doppler centroid for BFSAR. It estimates Doppler centroid by searching for the range migration slope based on the minimum waveform entropy. Simulation results validate the effectiveness of this method.