Diannong Liang

A Novel Space–Time Coding Alamouti Waveform Scheme for MIMO-SAR Implementation

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.

Digital Beamforming on Receive in Elevation for Multidimensional Waveform Encoding SAR Sensing

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.

A Two-Dimensional Spectrum for General Bistatic SAR Processing

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.

SBRAS — an advanced simulator of spaceborne radar

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.

New Faraday Rotation Estimators Based on Polarimetric Covariance Matrix

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.

Echo-Domain Phase Synchronization Algorithm for Bistatic SAR in Alternating Bistatic/Ping–Pong Mode

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.

Real-Time Raw-Signal Simulation Algorithm for InSAR Hardware-in-the-Loop Simulation Applications

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.

High-precision, fast DEM reconstruction method for spaceborne InSAR

The primary objective of the spaceborne interferometric synthetic aperture radar (InSAR) system is to develop a consistent global digital elevation model (DEM). The improvement of the processing speed in global interferometry missions while maintaining the accuracy is becoming an important issue. DEM reconstruction is the most time-consuming step in the data processing required for spaceborne InSAR. Based on a DEM reconstruction principle analysis, we exploited two basic characteristics of the mapping relation between the interferometric phase and 3-D position of target point. First, the relation between the 3-D position of the target point and the interferometric phase can be approximated using polynomials. Second, the corresponding polynomials change slowly in the local SAR images. Also, we verified the foundations of the two mentioned characteristics theoretically. Then, we proposed a fast method of DEM reconstruction based on this analysis. Detailed descriptions of the step and key parameters of the fast algorithm are provided. Finally, the repeat-pass interferometric data obtained using TerraSAR-X was used to test the presented method. The experimental results showed that a significant reduction in computation time is achieved with only a small loss in accuracy. The effectiveness and correctness of the proposed method were also validated.

Ionospheric Polarimetric Dispersion Effect on Low-Frequency Spaceborne SAR Imaging

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.

Modeling and high-precision processing of the azimuth shift variation for spaceborne HRWS SAR

One basic assumption for high-efficient SAR imaging algorithms is azimuth-shift-invariance of SAR data. The new spaceborne high-resolution wide-swath (HRWS) SAR generation will retain the same significant swath depth of the past but will achieve much higher resolution. The formerly insignificant variance of effective radar velocity along azimuth path will cause noticeable focusing blurring at azimuth scene edges. It is a new problem to accommodate the azimuth-shift variance with high efficiency and high precision. In this paper, the azimuth variation of effective velocity and main influencing factors are analyzed and accurately modeled. A highly accurate and efficient phase preserving processor with embedded azimuth variance compensation for sliding spotlight mode is proposed. The proposed processor does not add extra computing load and inherits the high efficiency of the original azimuth-invariant subaperture approach. Through careful design of the fourth-order phase terms of the processing functions, azimuth variation compensation can be achieved in company with the original baseband azimuth scaling steps. Simulation with point targets in C-band HRWS sliding spotlight mode is made to validate the focusing and phase preservation performance of the proposed algorithm.