Ground Moving Target Imaging for the Geosynchronous-Low Earth orbit Bi-Static Multi-channel SAR System

Focusing on the issue of echo data acquire approach in bistatic SAR system. In this paper, the key technology of echo sampling window synchronization is introduced, the effect of echo sampling window synchronization error to the echo data and image performance are analyzed, an adaptive echo sampling window synchronization approach based on amplitude demodulation and phase compare is proposed, which could automatically adapt for the echo pulse repeat interval(PRI), and establish a high accurate synchronized echo sampling window, then a high precise phase bias compare algorithm for the proposed approach is introduced. At last the performance and residual error for the presented approach are analyzed to illustrate the efficiency of the proposed approach. This paper provides a theoretical and engineering reference for the design of echo sampling window synchronization in bistatic SAR system.

Bistatic Synthetic Aperture Radar (BSAR) with GPS transmitter is in a focus of consideration. Bistatic topology, satellite transmitter of opportunity and stationary receiver, combined with inverse synthetic aperture radar (ISAR) technology with GPS P code modulated waveform for imaging of a moving are presented. Kinematical equations and mathematical model of the deterministic BSAR signal are created. It is proven that the signal formation and image reconstruction can be regarded as direct and inverse space transforms. Stages of image reconstruction, including phase correction, cross-correlation range compression and Fourier transform azimuth compression are defined. The effectiveness of models and algorithms are illustrated with results of numerical experiments.

Chapter 1 - Overview of bistatic SAR

Existing imaging methods about Bistatic SAR(BiSAR) mostly consider small scene with single channel and ignore the negative effects of range ambiguity. In order to address this problem, a wide-swath imaging method based on multiple-input multiple-output(MIMO) BiSAR system is proposed in this paper. Firstly, the element pulse coding(EPC) MIMO BiSAR model is established by modulating both of elements and pulses. After that, the spatial filtering technique is utilized to separate signals from different range regions. Then, the Doppler preprocessing that based on the prior configuration information is performed to eliminate the Doppler centroid ambiguity. Finally, classical imaging method is performed, and the wide-swath imaging results can be obtained. Numerical simulations verify the proposed method.

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.

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.

Side-looking SAR cannot image targets in flight direction. Introduced here is the bistatic missile-borne forward-looking SAR system, as an efficient approach to forward-looking SAR imaging. According to the spatial geometry and the model of echo signal, the signal spectrum derived by the method of series reversion, the phase compensation factors and the realized step of the proposed algorithm are given. Several simulation and processing results are presented to demonstrate the validity of the proposed algorithm.

The paper presents results from developing and testing a bistatic SAR system for ground surveillance in the VHF band (25-83 MHz). It is based on two radars developed by FOI: The monostatic LORA system installed in an aircraft and the bistatic and passive CARINA system installed on a helicopter. CARINA is based on direct RF sampling of two receiver channels (2 × 14 bits @ 250 MSPS) together with continuous non-interrupted data storage capability. The system has recently been developed by FOI and is described in detail. System architecture, installation in the helicopter platform and results from early trials in bistatic SAR mode are described. Results show forest clutter can be significantly reduced in bistatic SAR geometries compared to the signature of a small truck (van).

This paper analyzes the synchronization of a bistatic synthetic aperture radar (SAR) system based upon a geostationary transmitter and small receive-only SAR systems mounted on airplanes or unmanned aerial vehicles (UAVs), namely GEO Spaceborne-Airborne Bistatic SAR. This paper will analyze and present our solution of the space synchronization, time synchronization, frequency synchronization and phase synchronization for the GEO Spaceborne-Airborne bistatic SAR respectively. It will first introduce the GEO Spaceborne-Airborne bistatic SAR system and geometry model briefly. Then possible solutions of the system synchronization will be discussed. It will introduce the synchronization scheme based on the direct path between the transmitter and receiver. It uses synchronization modules to realize the time and frequency synchronization. A mathematical model considering the time and frequency synchronization error is given and a phase error analysis is performed. Range and azimuth compressions of a single target are implemented by simulations.

A bistatic synthetic aperture radar uses a separated transmitter and receiver flying on different platforms to achieve benefits like exploitation of additional information contained in the bistatic reflectivity of targets, reduced vulnerability for military applications, forward looking SAR imaging or increased RCS. A particular constellation, where the transmitter is in space and the receiver near or on the earth surface (e.g. aircraft, tower) is called a hybrid bistatic SAR system. Besides technical challenges, like the synchronization of transmitter and receiver, the overlap of the two antenna footprints is of vital importance. Due to the extreme platform velocity differences, SAR modes with flexible steering of the antenna beams are necessary. The sliding spotlight mode offers such a beam steering, where the antenna footprint velocity can be chosen slower or faster than the platform velocity. If both transmitter and receiver use this mode it is called double sliding spotlight mode, which will be investigated in this paper using the example of the satellite TerraSAR-X and the airborne SAR system PAMIR. Several aspects like the ground resolution, Doppler frequency and Doppler-bandwidth will be analyzed.

Range cell migration correction (RCMC) is the key process in the imaging formation of the bistatic SAR. Due to the existence of double-square-root in the bistatic range equation, the RCMC in the general bistatic SAR configuration is more complicated than the traditional monostatic method. In this letter, the range cell migration of bistatic SAR system is analysed in detail, and a novel data transformation is derived. From the derivation, we find that the original keystone transform is only a special case of our proposed transformation, so that we called it the generalized keystone transform. Compared with the original keystone transform, this generalized method can correct more complex range cell migration nicely. After the generalized keystone transform processing, the range cell migration is eliminated closely, and the focused image with full range and Doppler resolution can be obtained using the fast Fourier transform. Finally, a hybrid spaceborne/airborne bistatic SAR numerical simulation is performed to validate the proposed processing procedure.

Space-borne-ground-based Bistatic SAR systems are becoming increasingly interesting, and the motivation is threefold: they call for specific signal processing algorithms and allow for a better characterization of the signal properties and propagation path; angular diversity introduced by the bistatic angle and the receiver attitude opens new perspectives for target characterization; modular and configurable receiver hardware permits diverse applications with little effort. In this context, this paper presents the main hardware characteristics of a dual-band Bistatic SAR system developed at the University Politehnica of Bucharest. The system consists of a C/X-band SAR receiver with satellite emitter of opportunity. We discuss the system properties, applications and we shortly revisit the ongoing experiments and measurements.

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.

Bistatic forward-looking synthetic aperture radar (BFL-SAR) is a kind of bistatic SAR system that can image forward-looking terrain in the flight direction of an aircraft. Until now, BFL-SAR imaging theories and methods have been researched for stationary targets. Unlike the stationary target, the motion of a ground-moving target (GMT) induces unknown range cell migration and additional modulation of the azimuth signal. Thus, to finely image the GMT, one must obtain its velocity parameters accurately, but they are usually unknown. In this paper, a novel GMT imaging and velocity estimation method, which is based on mismatched compression, is proposed for BFL-SAR without a priori knowledge of the GMT's velocity parameters. The main idea behind mismatched compression is to use a presumed azimuth reference function for performing correlated operation with the azimuth signal of the GMT. In general, the Doppler parameters of the presumed azimuth reference function are different from those of the GMT's azimuth signal because the velocity parameters of the GMT are unknown. Therefore, the correlation operation referred to earlier is actually mismatched compression, and the resulting image is shifted and defocused. The shifted and defocused image is utilized to get the real Doppler and velocity parameters of the GMT. The advantage of this method is that not only the GMT can be well focused but also the GMT's velocity can be simultaneously obtained. In addition, this method needs only monochannel antenna. The proposed BFL-SAR GMT imaging and velocity estimation method is validated by numerical simulations.

Launched in June 2010, TanDEM-X is an interferometric mission with the main goal of providing a high-resolution, global and unprecedentedly accurate digital elevation model (DEM) of the Earth by means of single-pass X-band SAR interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system was operated in pursuit monostatic mode. During that time, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes were conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this article includes results of the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focussing on the analysis of the experimental results. Even in the absence of essential synchronisation and calibration information, bistatic images and interferogramswith similar quality to pursuit monostatic have been obtained. Some months later, with TanDEM-X already in its operational DEM-acquisition phase a further challenging bistatic acquisition carried out in cooperation with DLR's airborne radar F-SAR has been carried out. The objective was to acquire fully polarimetric interferometric data with a high range of available bistatic angles. A dedicated commanding of both TanDEM-X and F-SAR was required, including irregular sampling schemes, partially missing bistatic echo reception and bistatic synchronisation, which pose a number of technological challenges in SAR data processing before the calibrated bistatic SAR images are obtained. This article reports about these two set of experiments.