Multiple-input Multiple-output Synthetic Aperture Radar System Research Articles

A Robust Digital Beamforming on Receive in Elevation for Airborne MIMO SAR System

The echo separation issue for multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) is usually regarded as a more technical challenge. Spotlighted as a promising solution to the echo separation, the well-known short-term shift-orthogonal (STSO) beamforming scheme has become increasingly popular. However, for airborne MIMO SAR systems, the digital beamforming (DBF) involved in the STSO scheme usually encounters more issues, e.g., the direction of arrival (DOA) mismatch induced by topography variation. Up to now, relatively less research on robust DBF processing has been conducted for airborne MIMO SAR systems. In this respect, an adaptive DBF technique, based on interference plus noise covariance matrix (IPNCM) reconstruction and desired signal steering vector estimation, has been proposed in this paper. IPNCM reconstruction and steering vector estimation can not only cope with the DOA mismatch problem, but also remove the desired signal component in the training data cells to increase the beamformer convergence rates. Consequently, the proposed approach really improves the array output signal-to-interference-plus-noise ratio (SINR). Moreover, numerous discussions and simulations are carried out to prove the effectiveness of proposed DBF technique under various disturbance environments. Compared with the current DBF techniques, the proposed method provides a bright application prospect for the STSO scheme.

MIMO-SAR Interferometric Measurements for Structural Monitoring: Accuracy and Limitations

Terrestrial Radar Interferometry (TRI) is a measurement technique capable of measuring displacements with high temporal resolution at high accuracy. Current implementations of TRI use large and/or movable antennas for generating two-dimensional displacement maps. Multiple Input Multiple Output Synthetic Aperture Radar (MIMO-SAR) systems are an emerging alternative. As they have no moving parts, they are more easily deployable and cost-effective. These features suggest the potential usage of MIMO-SAR interferometry for structural health monitoring (SHM) supplementing classical geodetic and mechanical measurement systems. The effects impacting the performance of MIMO-SAR systems are, however, not yet sufficiently well understood for practical applications. In this paper, we present an experimental investigation of a MIMO-SAR system originally devised for automotive sensing, and assess its capabilities for deformation monitoring. The acquisitions generated for these investigations feature a 180∘ Field-of-View (FOV), distances of up to 60 m and a temporal sampling rate of up to 400 Hz. Experiments include static and dynamic setups carried out in a lab-environment and under more challenging meteorological conditions featuring sunshine, fog, and cloud-cover. The experiments highlight the capabilities and limitations of the radar, while allowing quantification of the measurement uncertainties, whose sources and impacts we discuss. We demonstrate that, under sufficiently stable meteorological conditions with humidity variations smaller than 1%, displacements as low as 25 μm can be detected reliably. Detecting displacements occurring over longer time frames is limited by the uncertainty induced by changes in the refractive index.

Analysis and Optimization of a Pulse Repetition Frequency for Along-Track Distributed Multiple-Input Multiple-Output Synthetic Aperture Radar

A distributed multiple-input multiple-output synthetic aperture radar (MIMO-SAR) system combines azimuth multi-channel and digital beamforming technologies to produce data with a high-resolution azimuth and wide swath. The MIMO-SAR system uses a tight along-track formation, and the spacing between its satellites is typically 100 m. The space sampling characteristic is sensitive to changes in pulse repetition frequency (PRF). This paper presents a method for optimizing the PRF of a distributed MIMO-SAR system based on the performance of the latter. The optimized PRF adapts well to variations in satellite spacing. These results show that PRF optimization can effectively improve the performance of the distributed MIMO-SAR system.

Novel MIMO‐SAR system applied for high‐speed and high‐accuracy deformation measurement

As a novel system, multiple-input multiple-output synthetic aperture radar (MIMO-SAR) has achieved rapid development in the past years. Based on the MIMO technique, multiple transmitting and receiving antennas could be utilised to compose a specific geometrical structure and acquire a large aperture. The MIMO-SAR system can be utilised to take deformation measurement to scenes with high deformation rate and can avoid the problems of phase errors caused by the antennas' vibration on the rail. This study introduces the basic principle of the MIMO technique and the key problems in the system design. Experimental datasets are acquired with the MIMO-SAR and GB-SAR (ground-based SAR) systems simultaneously. Comparisons of the imaging results indicate that the MIMO-SAR system has a better repeat-track interferometry performance than the GB-SAR system. In addition, deformation measurement results validate the high-accuracy measurement ability of the MIMO-SAR system.

Waveform diversity design for an MIMO SAR system based on an OFDM chirp

Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) waveform diversity design has drawn extensive attention since it has a great potential in remote sensing. Waveform designing becomes one of the most significant tasks in the MIMO SAR system. In this study, a novel scheme has been proposed to generate four waveforms using an orthogonal frequency division multiplexing (OFDM) chirp waveform. The four waveforms have good auto-correlation properties, ambiguity performance, and a good peak sidelobe ratio. This scheme makes full use of the bandwidth. The orthogonal waveform is demodulated by padding zeros and under sampling in the frequency domain. Simulation results are obtained to validate that more orthogonal waveforms can be produced by our proposal scheme with a comparative performance.

Perfect orthogonal waveform scheme used for high-resolution multiple-input multiple-output synthetic aperture radar system

We investigate multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) scheme with an ultra-low side-lobe ratio performance for using complete complementary sequences (CCS) waveform. It is shown that the CCS can be applied in a MIMO SAR system to obtain high resolution in range direction. In addition, the azimuth channel scheme with the multiple subsequences of CCS is also considered. First, we introduce the concept of CCS and establish the transmitter model based on CCS in MIMO SAR system. Then, we propose the corresponding imaging algorithm for employing the CCS to accurately focus the raw data of the MIMO SAR system and derive the range cell migration correction. Considering the signal complexity, we make use of the CCS pairs to analyze the MIMO SAR imaging algorithm. Finally, simulation results demonstrate the validity and feasibility of the proposed MIMO SAR scheme and the potential advantage of CCS.

A processing scheme for LFM-based waveform MIMO SAR with digital beam-forming in elevation

Multiple-input multiple-output synthetic aperture radar (MIMO-SAR) is of great potential for future high-resolution and wide swath imaging. However, its development is limited as the condition of orthogonality among the multiple waveforms received simultaneously is hard to satisfy. In this paper, a class of linear frequency modulation-based MIMO waveforms is introduced to settle this problem and correspondingly an effective processing scheme is proposed based on digital beam forming in elevation. In this scheme, for separating the mixed signals, null steering technique is employed to extract one waveform from the others. The main advantages of the proposed scheme include transmitting simultaneously, good inter-orthogonality between different waveforms with identical frequency range, separating them with mutual interference cancellation and good imaging quality. The main procedure of this scheme is derived and described in detail in the paper. Furthermore, an example MIMO-SAR system is provided for numerical simulation and the simulation results are obtained to validate the proposed scheme.

Implementation of the OFDM Chirp Waveform on MIMO SAR Systems

Attention has been devoted to multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) systems in recent years. The applications of MIMO SAR systems, which involve high-resolution wide-swath remote sensing, 3-D imaging, and multibaseline interferometry, are seriously limited by the available sets of orthogonal waveforms. Although orthogonal frequency-division multiplexing (OFDM) chirp waveforms are proposed to avoid intrapulse interferences, this waveform scheme has not been investigated for practical implementation. In this paper, challenges in implementing the OFDM chirp waveforms on practical systems are analyzed and solved. First, the small extra carrier frequency between the mutually orthogonal waveforms, which renders the OFDM chirp waveforms not strictly on common spectral support, is avoided by improving the modulation of the OFDM chirp waveform. Second, the tedious demodulation, which is realized by circular-shift addition in the time domain and subcarrier extraction in the frequency domain, is improved. Third, the radar systematic error and the Doppler shift, which introduce bandwidth leakage and degrade the waveform orthogonality significantly, are compensated. Finally, taking all these challenges into consideration, a novel signal processing algorithm along with a MIMO SAR system model is proposed. Theoretical analysis is validated by simulations and systematic calibration measurements based on a C-band system.

Multiple-input multiple-output synthetic aperture radar antenna arrangement for wide-swath remote sensing

Simultaneous high-resolution and wide-swath (HRWS) remote sensing imaging is of great importance in future microwave remote sensing, but it is limited by the minimum antenna area constraint. Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) offers new opportunities due to its increased degrees of freedom and system flexibility. We investigate four elevation MIMO SAR antenna arrangements, namely narrow-beam transmission narrow-beam reception, narrow-beam transmission wide-beam reception, wide-beam transmission narrow-beam reception, and wide-beam transmission wide-beam reception (WTWR) in HRWS remote sensing. Their signal models, system gains and range ambiguity-to-signal ratio performances are comparatively investigated. Three waveforms that have a large time-bandwidth product and constant modulus are designed for the MIMO SAR system and the impacts of their mutual interferences due to imperfect waveform orthogonality are also simulated. Simulation results show that MIMO SAR is superior to conventional single-antenna SAR and existing multichannel SAR techniques in wide-swath remote sensing.

Spaceborne MIMO Synthetic Aperture Radar for Multimodal Operation

In this paper, we introduce a novel multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) concept for multimodal operation. The proposed system employs waveforms based on the orthogonal frequency division multiplexing (OFDM) technique and digital beamforming (DBF) on receive. Thereby, it becomes feasible to maximize spatial degrees of freedom, which are necessary for the multimodal operation. The proposed MIMO SAR system produces multiple high-resolution wide-swath SAR imageries that are used for coherent postprocessing. Through this paper, we aim to open up a new perspective of using MIMO concept for a wide-swath SAR imaging with high resolution in interferometric and polarimetric modes, based on OFDM and DBF techniques. Therefore, this paper encompasses broad theoretical backgrounds and general system design issues as well as specific signal processing techniques and aspects.

A Novel Motion Compensating Method for MIMO-SAR Imaging Based on Compressed Sensing

Multiple-input multiple-output-synthetic aperture radar (MIMO-SAR) can realize high-resolution imaging by its predominance of parallel sampling. However, the motion compensation and large data amount are the inevitable problems to be solved. In this paper, a novel motion compensating method for MIMO-SAR imaging with the under-sampled echo signal is put forward. First, the echo signal model of MIMO-SAR system with motion error is analyzed and a compensating method with the Nyquist-sampled echo data is proposed. Using the technique of two-step compensation, the motion error is compensated and the imaging result is obtained. Second, to compensate the motion error with the under-sampled echo signal, based on the above compensating method, the transform operator and the CS-based imaging scheme are constructed, which could implement the first-step compensation, range compression, and range cell migration correction simultaneously. At last, the imaging result is obtained by the second-step compensation and azimuth compression. Using the proposed method, just a small amount of imaging data is required for MIMO-SAR imaging. Finally, the effectiveness of the proposed method is proved by the imaging simulations.

Large-Area Remote Sensing in High-Altitude High-Speed Platform Using MIMO SAR

Large-area mapping is of great valuable in microwave remote sensing, but it is a contradiction between swath width and azimuth resolution due to the minimum antenna area constraint. In this paper, we consider a specific wide-area mapping technique with high-altitude high-speed platform, where range ambiguity suppression is a technical challenge. To resolve this problem, we present a multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) with multiple antennas placed in the cross-track direction. The increased degrees-of-freedom (DOFs) provide a potential to resolve possible range and/or azimuth ambiguities. After formalizing the system scheme and signal model, an iterative matched filtering algorithm is presented, which can efficiently suppress the cross-correlation interferences in the multichannel data separation. Furthermore, a range-Doppler based image formation algorithm is derived. The MIMO SAR system performance is evaluated by the range-ambiguity-to-signal ratio (RASR) performance. Numerical simulation results validate the effectiveness of the proposed MIMO SAR in high-altitude high-speed platform SAR for large-area remote sensing.

Shifting MIMO SAR System for High-Resolution Wide-Swath Imaging

A multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) system is presented for high-resolution wide-swath imaging. For a system with N transmit and N receive antennas, the proposed method using slope-varying chirp signals, azimuth beamwidth extension, and transmit antenna shift can give an increased azimuth resolution or a decreased pulse repetition frequency (PRF), which results in wider swath width, by a factor of N compared to a SAR system based on a displaced phase center antenna. To verify the proposed shifting MIMO SAR system, we carried out the point target simulations and the timing analysis which showed enhanced resolution and widened swath width performances.

APPLICATION OF COMPLETE COMPLEMENTARY SEQUENCE IN ORTHOGONAL MIMO SAR SYSTEM

The Complete Complementary Sequence (CC-S) consists of several complementary orthogonal sequences and has optimal sidelobe level performance, which is satisfled with the requirement of the orthogonal Multiple Input Multiple Output (MIMO) radar signals. Aimed at the di-culty of high sidelobe level in Synthetic Aperture Radar (SAR) imaging processing in range dimension, an approach of depressing sidelobe level based on CC-S in MIMO SAR system was proposed. The transmitter model for orthogonal MIMO SAR system using CC-S and the corresponding matched fllter (MF) were established in this paper. In addition, the MIMO SAR imaging results were simulated. The simulation results demonstrate that the performance of CC-S employed in orthogonal MIMO radar system is much better than that with traditional Linear Frequency Modulation (LFM) signal, by which the feasibility and validity of CC-S applied in orthogonal MIMO SAR system are justifled.