Two novel approaches for measuring the ionospheric total electron content (TEC) from single-polarized spaceborne SAR data were proposed in the mentioned paper, i.e., the TEC autofocus method and the systematic alternate up/down pulse methodology. The up/down pulse methodology indicates that ionospheric TEC could be estimated using the difference between range-compressed phase distributions of the up and down chirps. However, it can be shown theoretically that the difference between the up and down chirps after compression is not expected. Thus, the estimation of TEC from alternating up versus down chirp combinations might not be feasible.

This letter presents a novel extended space-time coding waveform scheme for multiple-input-multiple-output synthetic aperture radar implementation. The extensions include the fulfilling of the two Alamouti periods in one transmit duration minimizing the time-variant channel effect and the new form of the basic orthogonal waveforms. The main advantages of the proposed waveform scheme include ghost image prevention, cross-channel interference mutual cancelation, simultaneous transmission via orthogonal Tx channels, free selection of PRF, etc. The proposed techniques are verified by a simulation for the case of point-like targets.

This letter investigates the performance of a waveform-separation approach employing a posteriori digital beamforming (DBF) in elevation for multidimensional waveform encoding SAR sensing. The signal-to-noise ratio scaling factor in elevation and full-Doppler-band range ambiguity-to-signal ratio are defined to give global assessment of the DBF approach. Constraints for the selection of elevation subaperture number and their relevant influence on image quality are investigated. By theoretical analysis and simulation, the subaperture number can be appropriately selected by a tradeoff between the data volume and imaging performance. Further strategies for data reduction and performance optimization are briefly discussed.

Spaceborne synthetic aperture radar (SAR) systems operating at lower frequencies, such as P-band, are significantly affected by Faraday rotation (FR). FR angle (FRA) estimation is affected by additive noise, system imbalance, and cross-talk. Two new FRA estimators are proposed from linearly polarized covariance matrix data. Assumptions of these estimators are looser than reciprocity of the true scattering matrix, which is assumed for all the pre-existing estimators. The new estimators are validated by ALOS PALSAR full-pol data processing. Simulation results show that one of the new methods is sensitive to system amplitude imbalance, but possesses particularly high robustness to system phase imbalance. Hence, the proposed estimators can be candidates for FRA estimation according to the type and magnitude of the errors.

Ionospheric Faraday rotation (FR) introduces polarimetric distortion to trans-ionosphere signals. Spaceborne low-frequency wide-bandwidth synthetic aperture radar (SAR) signals may be distorted by FR effect because it is highly dispersive. Ionospheric polarimetric dispersion (PD) effects on spaceborne SAR imaging are modeled and analyzed. PD effect is included in polarimetric SAR system impulse response matrices. In particular, PD effects on linearly polarized and circularly polarized SAR systems are analyzed by system impulse response matrices. Theoretically, PD will introduce amplitude modulation to SAR spectrums and hence distort linearly polarized SAR imaging; for circularly polarized systems, it introduces relative image shift and phase errors between images of different channels. Numerical evaluations and simulations show that PD effects will introduce noticeable effects on future P-band wide-bandwidth SAR systems.

Terrain observation by progressive scans (TOPSAR) solves the scalloping effect of ScanSAR by steering the radar beam in the azimuth direction. The steering enables TOPSAR to achieve more qualified images than ScanSAR with the same coverage and resolution. Nevertheless, when TOPSAR is used for district observation, the steering enables it to get higher resolution and/or wider coverage. This letter compares the resolution limits of TOPSAR and ScanSAR and then proposes four possible modes of using TOPSAR for district observation. For each mode, the steering angle is discussed, and a design result is provided.

When bistatic synthetic aperture radar (SAR) works in alternating bistatic/ping-pong mode, it can perform phase referencing without a synchronization link between different satellites. The key to this is to extract a compensation phase from the bistatic echoes, including oscillator phase differences. An echo-domain phase synchronization algorithm using the correlation of bistatic echoes is proposed. Because of its larger equivalent synchronization frequency, the echo-domain algorithm has higher phase synchronization accuracy than the algorithm in the image domain. Experiments using simulated bistatic echoes with real single-stripmap SAR echo validate the proposed echo-domain phase synchronization algorithm.

To address the new requirements for InSAR raw-signal simulation for hardware-in-the-loop simulation application, a real-time raw-signal simulation algorithm with high accuracy is proposed. The raw signal is expressed as the convolution of the transmitting signal and the scene modulation signal (SMS), and the convolution is then implemented using the fast Fourier transform. The SMS is calculated by the position approximation in the time domain, and an interpolation technique is used to maintain the accuracy of the fast approximation algorithm. The SMS is filtered to the radar sampling frequency for real-time implementation. The approximation error of the fast algorithm is analyzed quantitatively to obtain the minimum interpolation factor. Ground-based hardware-in-the-loop simulation results are provided to validate the algorithm.

Recently, a synthetic aperture radar (SAR) tomograhic focusing method based on compressive sensing was proposed. This focusing method can reduce the required number of measurements and achieve satisfying elevation resolving ability. First, we briefly review this novel focusing method and prove the applicability of compressed sensing (CS) for SAR tomography theoretically using the latest improvement of CS. Then, we apply this focusing method to the 3-D reconstruction of Angel Stadium with Envisat-ASAR data. Both the theoretical analysis and satisfying results of real data processing confirmed the applicability of this SAR tomograhic focusing method.

Due to the separation of the radar transmitter and receiver, the channel mismatch, the time and phase synchronization error between synthetic aperture radar (SAR) central electronic equipments (CEE) is the key problems of spaceborne distributed SAR system compared with the traditional single-satellite double pass interferometric SAR (InSAR) system, which has to be considered in the SAR system design, index allocation, integration and testing, verification and validation. A hardware-in-loop simulation and evaluation approach is presented for the influence analysis of CEE payload on the InSAR performance. The hardware-in-loop simulation system includes the master and slave CEE payloads, the time and phase synchronization prototypes, the on-board computer simulator, the SAR target echo signal simulator (TESS) and other auxiliary equipments. For obtaining the influence of the error source on InSAR performance, a hardware-in-loop simulation and evaluation approach based on different hardware configuration is adopted. The experiment results validate the theoretical analysis and computer-based simulation results, and show the feasibility of the time and phase synchronization scheme, which is meaningful for the implementation of the spacebome distributed SAR system.

This paper proposes a novel anti-jamming method for multichannel SAR. This method is based on the combination of Fast-time STAP and Pulse diversity. It can be used to reject the DRFM repeater deceptive jammer. The point-target simulation is used to show the working principle of the proposed algorithm and verify the proposed algorithm.

This letter derives a 2-D point target spectrum for general bistatic synthetic aperture radar (SAR). For the bistatic configuration, the contributions of the transmitter and the receiver to the overall instantaneous Doppler are unequal due to the different slant range histories. In this letter, an instantaneous Doppler contribution ratio is proposed to represent the difference between the instantaneous Doppler contributions of the transmitter and the receiver, which varies with instantaneous Doppler and range frequency. Then, the 2-D spectrum is obtained by using the stationary phase principle and Taylor series expansion for general bistatic SAR. The accuracy of the spectrum is verified with a point target simulation of different general bistatic configurations.

An algorithm for achieving High-resolution wide-swath (HRWS) images in bistatic space-borne SAR with multiple phase centers is presented. Based on the analysis of the correlation of echos from all phase centers, a signal model is built that all echoes combined form a periodically nonuniform undersampled signal. Then the echoes with Doppler ambiguities are reconstructed to unambiguous uniform samplings which can be processed with conventional imaging algorithms. Simulation trials prove that the approach is valid and robust.

Two approximations are used in "Three-pass" D-InSAR (Differential SAR Interferometry). First, in this paper, the error caused by the approximations is deduced and defined as First-order Approximation Error. Then, the characteristics of First-order Approximation Error are discussed in theory and proved by two experiments. In the end, three viable and effective methods are put forward to correct approximation error.

The range variant bistatic deformation term of LBF is analyzed. By linearizing the difference between the nearest ranges of transmitter and receiver, the range variant bistatic deformation term is approximated as the linear function of nearest ranges summation. Based on this, the new interpolation function is presented. Then the range variant bistatic deformation is compensated by interpolation, this processing is more efficient without blockwise in range blocks. The simulation results prove its effectiveness.

This letter presents a novel adaptive multiresolution technique to estimate the local fringe frequency in synthetic aperture radar (SAR) interferometric phase images. According to the coherent summation principle, the size and the shape of the interferogram spatial resolution for local frequency estimation are adapted pixel by pixel to track the topographic changes and optimize the space-frequency resolution. We show that, comparing conventional filters with the processing window of a fixed size (such as 7 times 7) and shape (such as a square), the proposed approach avoids the loss of phase fringes and provides a more accurate representation of the scene topography. In this letter, we present a method to eliminate invalid frequency estimation. A fast frequency estimation approach modified from the method proposed by Trouve et al. is also shown. Finally, both simulated data and actual SAR data are utilized to illustrate the effectiveness of the proposed method.

Distributed spaceborne interferometric synthetic aperture radar (DS-InSAR) system includes more than one baseline, so it is also called multi-baseline InSAR system. It offers additional observation information for the derivation of position and height of object, compared to the single-baseline InSAR system. Based on the additional information it offers, a new DEM reconstruction approach of multi-baseline InSAR system is proposed. At first, theory of the approach is introduced and DEM reconstruction equations are derived. Then, error sensitivity analysis is made by first order approximation of Taylor expanding, which is validated by Monte-Carlo simulation. The results show that theoretical error analysis formula can give support for analysis and design of the system directly and the DEM accuracy is very sensitive to the interferometric phase difference error.

Differential Inerferometry (D-InSAR) for spaceborne bistatic SAR is studied. According to advantages of the system, particularities and superiorities of D-InSAR based on spaceborne bistatic SAR are discussed first. Then, solid baseline is decomposed and the model of “three-pass” D-InSAR is established. Afterwards, the theory of data fusion is introduced to D-InSAR and a new method is put forward to avoid the max detection error. A simulation experiment accompanied by very good result is shown in the end.

An application-oriented spaceborne radar advanced simulator (SBRAS) is presented in this paper. SBRAS is initiated by the technical and economical requirements to verify formation-flying distributed satellites synthetic aperture radar (SAR) scheme and simplify the instrument hardware design. The simulator develops a full flow of signal processing including formation design, SAR raw data simulation of nature scene, imaging, InSAR processing, digital elevation model (DEM) generation and performance analysis. A user-friendly GUI is provided.

The new peculiarities of the low frequency UWB SAR result in difficulty of radiometric correction and calibration for the system. The calibration techniques developed for common high frequency and narrow band SAR system cannot be applied to the low frequency UWB SAR. In this paper, based on the peculiarities of the system, the system processing model and the electromagnetic scattering model of calibrators are given, and the new radiometric correction and calibration method is proposed. The computer simulation is used to validate the method.