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Non-linear chirp scaling (NLCS) is a feasible method to deal with time-variant frequency modulation (FM) rate problem in synthetic aperture radar (SAR) imaging. However, approximations in derivation of NLCS spectrum lead to performance decline in some cases. Presented is the exact spectrum of the NLCS function. Simulation with a geosynchronous synthetic aperture radar (GEO-SAR) configuration is implemented. The results show that using the presented spectrum can significantly improve imaging performance, and the NLCS algorithm is suitable for GEO-SAR imaging after modification.

Many present spaceborne synthetic aperture radar (SAR) systems are constrained to only two channels for ground moving target indication (GMTI). Along-track interferometry (ATI) technique is currently exploited to detect slowly moving targets and measure their radial velocity and azimuth real position. In this paper, based on the joint probability density function (PDF) of interferogram's phase and amplitude and the two hypotheses "clutter" and "clutter plus signal", several constant false alarm rate (CFAR) detection criteria are analyzed for their capabilities and limitations under low signal-to-clutter ratio (SCR) and low clutter-to-noise ratio (CNR) conditions. The CFAR detectors include one-step CFAR detector with interferometric phase, two-step CFAR detectors, and two-dimensional (2D) CFAR detector. The likelihood ratio test (LRT) based on the Neyman-Pearson (NP) criterion is exploited as an upper bound for the performance of the other CFAR detectors. Performance analyses demonstrate the superiority of the 2D CFAR techniques to detect dim slowly moving targets for spaceborne system.

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]