A NEW LOOK AT THE POINT TARGET REFERENCE SPECTRUM FOR BISTATIC SAR

In this paper, an omega-K imaging algorithm to focus the raw data of translational invariant (TI) bistatic synthetic aperture radar (BSAR) is proposed. The method utilizes a point target reference spectrum of generalized Loffeld's bistatic formula (GLBF). Without the bistatic deformation term, GLBF is the latest development of Loffeld's bistatic formula. It is comparable in precision with the method of series reversion (MSR), but it has a much simpler form than MSR and a similar form to a monostatic case. Based on the spatial linearization of GLBF, the Stolt transformation relationship is derived. The method can consider the linear spatial variation of Doppler parameters, which is always ignored in previous publications about bistatic omega-K algorithms. This method can handle the cases of TI BSAR with high squint angles and large bistatic degrees. In addition, a compensation method for the phase error caused by the linearization is discussed. Numerical simulations and experimental data processing verify the effectiveness of the proposed method.

This paper concentrates on the focusing for hybrid bistatic SAR configurations, consisting of a space borne/airborne carrier configuration with considerable differences in velocities and altitudes of transmitter and receiver. Therefore the direct implementation of a bistatic SAR focusing algorithm based on Loffeld's bistatic formulae (LBF) cannot be readily applied. An extension of the bistatic point target reference spectrum, based on LBF is derived. Simulated focusing results verify the proposed approach.

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°.

In this paper, we develop a range Doppler algorithm (RDA) for the tandem bistatic synthetic aperture radar (BiSAR) configuration. The key step is to use an analytical tandem bistatic spectrum. The proposed algorithm is able to handle reasonably high squint by compensating secondary range compression in the two-dimensional wavenumber domain, and is more robust to focus bistatic data compared with Neo's RDA based on the spectrum of method of series reversion (MSR) in the tandem configuration particularly when the baseline-range ratio is large. The effectiveness of the proposed algorithm is verified by simulations.

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.

Frequency-Modulated Continuous-Wave (FMCW) Synthetic Aperture Radar (SAR) combines FMCW and SAR techniques which makes a lightweight, high-resolution and cost-effective imaging sensor which can work all weather, day and night. However, the failure of stop-and-go approximation leads to additional range-azimuth coupling terms in the Point Target Reference Spectrum (PTRS) of FMCW SAR. To exploit the influence of these terms in FMCW SAR circumstances, this paper presents the remarkable differences between the spectrum formulations of FMCW SAR and pulsed SAR, and validates the significance of the additional range-azimuth coupling terms introduced by the variation of slant range during the long pulse durations. Simulation experiments proved that ignoring these terms will cause serious reduction of image quality.

A novel autofocus method for synthetic aperture radar (SAR) image is studied. Based on a quadratic model for the phase error within each sub-area (narrow strip × sub-aperture) after a wide range swath is subdivided into narrow range strips and long azimuth aperture into sub-apertures, an objective function for estimation of the error is derived through the principle of minimum entropy. There is only one unknown variable in the function. With the Chebyshev approximation, the function is approximated as a polynomial, and the unknown is then solved using the method of series reversion. Curve-fitting methods are applied to estimate phase error for an entire scene of the full-swath by full-aperture. Through simulations, the proposed method is applied to restore the defocused SAR imagery that is well focused. The restored and original images are almost identical qualitatively and quantitatively. Next, the method is implemented into an existing SAR data processor. Two sets of SAR raw data at X- and Ku-bands are processed and two images are formed. Well-focused and high-resolution images from plain and rugged terrain are obtained even without the use of ancillary attitude data of the airborne SAR platform. Thus, the studied method is verified.

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.

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.

The speed and direction of a missile shifts sharply in the dive phase, making the azimuth frequency modulation (FM) rate change with the azimuthal position, leading to azimuth ambiguities and image distortion. To solve this problem, a modified nonlinear chirp scaling (NLCS) algorithm was adopted to compensate for the azimuth FM rate. First, the geometric configuration and echo signal model of the spaceborne missile bistatic synthetic aperture radar (SAR) were built, and then the Doppler frequency correction was performed, and the 2-D spectrum of the signal was derived by the method of series reversion. Next, range migration correction and range compression were finished in the 2-D frequency domain. Following this, a modified NLCS algorithm was proposed to solve the space variance of Doppler phase problem. After compensating for the azimuth FM rate, the azimuth compression focusing was completed and the imaging result was obtained. Finally, by comparing the calculation amount, imaging effect, and performance index with the traditional NLCS algorithm, it can be concluded that the algorithm reduced the calculation amount by 1.0128 × 108 floating point operations per second (FLOPs) compared with the traditional NLCS algorithm, and the azimuth focusing effect of the edge point was greatly improved. Its resolution, peak sidelobe ratio (PSLR), and integrated sidelobe ratio (ISLR) were improved by 0.87 m, 3.32 dB, and 1.79 dB, respectively, which proved the effectiveness and feasibility of this method.

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.

This paper focuses on the bistatic synthetic aperture radar (SAR) data processing in a spaceborne/airborne hybrid bistatic configuration. Due to the extreme differences in platform velocities and slant ranges, the airborne system operates in the inverse sliding-spotlight mode, while the spaceborne system works in the sliding-spotlight mode to achieve a tradeoff between azimuth scene size and azimuth resolution. In this extreme bistatic configuration, our original bistatic formula shows a limitation of accurately describing the bistatic point-target reference spectrum, owing to the assumption of equal contributions of transmitter and receiver to the total Doppler spectrum. We extend our previous formula using the weighting operation where the weighting factor is the ratio of the azimuth time-bandwidth product (TBP) of the platform to the total azimuth TBP. In this paper, the bistatic-deformation and azimuth-dependent range-cell-migration terms were removed with phase multiplications performed blockwise in range-azimuth subsections. The remaining quasi-monostatic term shows the characteristic of the conventional monostatic SAR besides an additional azimuth-scaling term. For the monostatic characteristic, any precision monostatic SAR processing algorithms can handle. In this paper, we prefer the wavenumber-domain algorithm (also known as Omega-K), since it can accurately correct the range dependence of the range-azimuth coupling, as well as the azimuth-frequency dependence. For the azimuth-scaling term, an inverse scaled Fourier transformation is performed to correct it. Finally, a hybrid spaceborne/airborne simulation experiment is conducted to validate the proposed processing procedure.

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.

Using Legendre polynomials and the derivation method of the implicit function, this paper proposes a range-Doppler algorithm for multireceiver synthetic aperture sonar. Based on Legendre polynomials, the two-way slant range is first expanded into a power series with respect to the slow time. Then, the derivation method of the implicit function is exploited to deduce the point of stationary phase and point target reference spectrum (PTRS). Based on this PTRS, the paper presents an imaging algorithm, which uses the range-dependent sub-block processing method to cancel the space-variant coupling between the range and azimuth dimensions. Simulation results and real data processing are presented to validate the proposed method.