The vortex electromagnetic (EM) wave carrying orbital angular momentum (OAM) offers a new degree of freedom for synthetic aperture radar (SAR) imaging. Because vortex EM waves have helical wavefronts, the vortex EM wave-based SAR echo contains the three-dimensional (3D) information of the target. In this paper, an OAM-based synthetic aperture radar interferometry (InSAR) technique is proposed to obtain 3D target information accurately without the existence of baseline. First, a vortex EM waves' SAR imaging model is established, and an improved range-Doppler algorithm is proposed correspondingly. Subsequently, the scheme of the OAM-based InSAR without the physical baseline is proposed. Compared with the conventional InSAR, the OAM-based InSAR can avoid the baseline decorrelation and reduce the requirement of the platform. Besides, the processing procedure of the OAM-based InSAR is simplified, which avoids the image registration and the interferogram flattening. The simulation results demonstrate the effectiveness of the proposed technique. Besides, the height estimation accuracy of the OAM-based InSAR was analyzed, in terms of interferometric phase error and OAM mode error. The height estimation accuracy can be improved by increasing the OAM mode difference appropriately. The OAM, which is completely independent of time, frequency, and polarization, offers a new scheme for the InSAR. INDEX TERMS Vortex electromagnetic wave, orbital angular momentum, SAR, InSAR.

In recent years, multi-input multi-output (MIMO) synthetic aperture radar (SAR) systems, which can promote the performance of 3D imaging, high-resolution wide-swath remote sensing, and multi-baseline interferometry, have received considerable attention. Several papers on MIMO-SAR have been published, but the research of such systems is seriously limited. This is mainly because the superposed echoes of the multiple transmitted orthogonal waveforms cannot be separated perfectly. The imperfect separation will introduce ambiguous energy and degrade SAR images dramatically. In this paper, a novel orthogonal waveform separation scheme based on echo-compression is proposed for airborne MIMO-SAR systems. Specifically, apart from the simultaneous transmissions, the transmitters are required to radiate several times alone in a synthetic aperture to sense their private inner-aperture channels. Since the channel responses at the neighboring azimuth positions are relevant, the energy of the solely radiated orthogonal waveforms in the superposed echoes will be concentrated. To this end, the echoes of the multiple transmitted orthogonal waveforms can be separated by cancelling the peaks. In addition, the cleaned echoes, along with original superposed one, can be used to reconstruct the unambiguous echoes. The proposed scheme is validated by simulations.

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