The ambiguity problem of targets in Doppler beam sharpening (DBS) with forward-looking radar is considered. While DBS is proposed earlier to improve the angular resolution of the radar while keeping the antenna aperture size limited, such a solution suffers from ambiguities in the case of targets positioned symmetrically with respect to the platform movement. To address this problem, an approach named unambiguous Doppler-based forward-looking multiple-input multiple-output (MIMO) radar beam sharpening scan (UDFMBSC) is proposed, based on the combination of MIMO processing and DBS. The performance of the proposed method is compared to existing approaches using simulated data with point-like and extended targets. The method is successfully verified using experimental data.

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In this paper, a novel multi-channel waveform agile radar testbed - ASTAP (Advanced Space-Time Adaptive Processing) with eight transmit channels and a single receive channel to form a co-located Multiple-Input Single-Output (MISO) radar is presented. Based on current configuration, the ASTAP radar system possesses agility for synthesizing and transmitting independent waveforms via each transmit channel simultaneously via multi-channel Arbitrary Waveform Generator (AWG). Hardware imperfections influence on digitally created waveforms including limited-bits DAC (Digital-to-Analog Converter) quantization, inter-channel time- and phase-skew (and/or jitter), non-linearities introduced by high-frequency devices (RF amplifiers and mixers etc) and antenna dispersion effects has been studied experimentally.An end-to-end system-level digital calibration using Over-the-Air (OTA) channel measurements to minimize system hardware imperfections prior to transmission is proposed. Calibration accuracy for waveform transmit ambiguity functions, orthogonality in MISO transmissions, received signals separation and beamforming process for the synthesis of target azimuth distributions is examined and quantified. Front-end performance with different waveforms types for colored transmission and simultaneous MIMO is demonstrated and analysed. Extraction of the signals corresponding to each transmit channel from the composite received signal in a single receive channel exploiting waveforms orthogonality is studied experimentally.@en

3-D imaging with irregular planar multiple-input-multiple-output (MIMO) arrays is discussed. Due to signal acquisition on irregular spatial sampling grids by using these antenna arrays, the fast Fourier transform (FFT)-based imaging algorithms cannot readily be used for image formation. To avoid the application of computationally intensive coherent summation algorithms such as filtered backprojection or Kirchhoff migration, we propose a nonuniform FFT (NUFFT)-based MIMO Range Migration Algorithm (i.e., NUFFT-based MIMO-RMA) for efficient microwave imaging. The algorithm exploits NUFFT to reconstruct the wavenumber-domain spectra related to each Fourier frequency. It is generic and applicable to 3-D imaging with irregular planar MIMO arrays. The effects of irregular spatial sampling and signal bandwidth on the imaging performance and computational efficiency of the proposed algorithm are analyzed. Finally, some numerical simulations and experimental results are presented to demonstrate the performance of the proposed imaging algorithm.

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Automotive radars suffer from low angular estimation accuracy due to the limited number of antennas and insufficient onboard processing power. This paper addresses the single target tracking problem in automotive radar by using recently proposed MIMO-monopulse approach which blends the monopulse approach with the collocated MIMO radar. We demonstrate and compare the localization performance of the MIMO-monopulse at an automotive radar on a real-world scenario. Experiments show that MIMO-monopulse results in a better angular accuracy comparing to the conventional digital
beamforming even though the low-level processing power.@en

A noncontact measurement of the heart rate of a specific person using an X-band radar system with a four-element antenna array is presented. The system comprises a two-dimensional planar wide-beam antenna array with a four-channel network analyzer. The direction of arrival is estimated using the Capon method, and the directionally constrained minimization-of-power algorithm is then applied to the received signals. Signals from the four channels are adaptively combined to enhance the accuracy in estimating the instantaneous heart rate in a multi-person scenario. Measurement results indicate that the proposed system with the adaptive array processing technique is effective for the noncontact measurement of the heart rate of a specific person when there are more than one person in the scene.@en

In this paper, a wavenumber-domain matrix-pencil-based multiband signal fusion approach was proposed for multiband microwave imaging. The approach proposed is based on the Born approximation of the field scattered from a target resulting in the fact that in a given scattering direction, the scattered field can be represented over the whole frequency band as a sum of the same number of contributions. Exploiting the measured multiband data and taking advantage of the parametric modeling for the signals in a radial direction, a unified signal model can be estimated for a large bandwidth in the wavenumber domain. It can be used to fuse the signals at different subbands by extrapolating the missing data in the frequency gaps between them or coherently integrating the overlaps between the adjacent subbands, thus synthesizing an equivalent wideband signal spectrum. Taking an inverse Fourier transform, the synthesized spectrum results in a focused image with improved resolution. Compared with the space-time domain fusion methods, the proposed approach is applicable for radar imaging with the signals collected by either collocated or noncollocated arrays in different frequency bands. Its effectiveness and accuracy are demonstrated through both numerical simulations and experimental imaging results.

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In this communication, a novel approach to rotated antenna array topology design is proposed for fully polarimetric short-range imaging. The rotated antenna array proposed acquires two co-pol and one cross-pol signals in terms of the “local” polarization basis by means of three antenna pairs. Then, the fully polarimetric signals in a global polarization basis are retrieved via simple polarization basis transformation, which overcomes the spatially varied polarizations of the signals acquired at different positions and makes valid the assumption of traditional imaging algorithms that the polarization is constant within the aperture. The proposed rotated antenna array takes advantage of the synthetic aperture technique to synthesize a 2-D array for 3-D full-pol imaging. It utilizes a significantly smaller number of antennas in comparison to traditional fully polarimetric imaging arrays and also provides sufficiently accurate estimation to the full-pol electromagnetic signals scattered from targets. Both numerical simulations and experimental measurements have been performed and the results show the tolerance of the antenna array topology proposed to the quasi-monostatic measurements, its effectiveness, and accuracy for full-pol short-range microwave imaging.@en

Ground penetrating radar imaging from the data acquired with arbitrarily oriented dipole-like antennas is considered. To take into account variations of antenna orientations resulting in spatial rotation of antenna radiation patterns and polarizations of transmitted fields, the full-wave method that accounts for the near-, intermediate-, and far-field contributions to the radiation patterns is applied for image reconstruction, which is formulated as a linear inversion problem. Two approaches, namely, an interpolation-based method and a nonuniform fast Fourier transform-based method, are suggested to efficiently implement the full-wave method by computing exact Green's functions. The effectiveness and accuracy of the method proposed have been verified via both numerical simulations and experimental measurements, and significant improvement of the reconstructed image quality compared with the traditional scalar-wave-based migration algorithms is demonstrated. The results can be directly utilized by forward-looking microwave imaging sensors such as installed at tunnel boring machine or can be used for the observation matrix computation in regularization-based inversion algorithms.

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A matrix-pencil based wavenumber-domain multiband fusion approach is proposed for improved microwave imaging. The suggested fusion approach is based on the fact from the Born approximation that the wavenumber-domain signal in a given scattering direction can be represented as a sum of the same number of contributions over the whole bandwidth. Utilizing this fact, the wavenumber-domain signal in each radial direction can be modeled as a superposition of damped/undamped exponential functions, where measured multiband data are part of the observations of the whole-band signal. Then the multiband signal fusion is addressed by estimating a unified signal model over the while bandwidth with the matrix pencil approach in the wavenumber domain. By using the estimated signal model, the missing data in the frequency gap can be extrapolated, thus synthesizing an equivalent wideband signal spectrum. After an inverse Fourier transform, the synthesize spectrum leads to a focused image with enhanced resolution. Compared to the existing frequency-domain method, the proposed fusion approach can be used for radar imaging with the signals acquired by either collocated or non-collocated arrays in different frequency bands. The effectiveness and accuracy of the proposed fusion approach are demonstrated through some numerical simulations.@en

In this study, the authors propose a novel direction of arrival (DoA) estimation algorithm called ‘multiple-input–multiple-output (MIMO)–monopulse’ by combining the monopulse approach with MIMO radar. Monopulse is fast and accurate angle estimation algorithm, which has been well developed for tracking radar. The application of the monopulse technique on MIMO radar is not much considered before, especially for automotive-radar application, and will be discussed in this study. Conventional methods of monopulse DoA estimation include amplitude and phase comparison monopulse. In this study, to improve the performance of monopulse, they utilise Chebyshev and Zolotarev weighting to synthesise sum and difference patterns. A new visualisation method for monopulse ratio is discussed. Finally, they demonstrate the success of the proposed algorithm by processing real data from a 79 GHz frequency-modulated continuous-wave automotive radar.@en

In this paper, Ground Penetrating Radar (GPR) imaging with arbitrarily oriented antennas within the data acquisition aperture is studied. The variations of antenna orientations change both the polarization of the radiated electromagnetic fields and spatial radiation patterns, which are assumed to be constant in traditional imaging algorithms. To overcome these effects, a linear inversion scheme is proposed by considering the exact radiation patterns of antennas. In terms of implementation, spatially varied radiation patterns are needed for arbitrarily orientated antennas. To efficiently acquire the radiation patterns, we suggest an interpolation-based method to compute the accurate Green's functions. Moreover, thanks to the frequency domain modeling, the proposed linear inversion approach also facilitates the signal fusion of multi-band GPR measurements for improved imaging, which is necessary for some practical applications. Finally, the imaging performance of the linear inversion approach is demonstrated through a numerical experiment.@en

Ground penetrating radar (GPR) using a 3D antenna array is employed for data acquisition over an area contaminated with landmine simulants. The performance of different processing methods applied in 1D, 2D and 3D radiograms are evaluated on real data obtained from two different test sites. Preliminary results are presented on the effectiveness of 3D GPR and on the applicability and limitations of widely used processing schemes.@en

This study presents a simple yet ingenious transmit diversity technique (so-called hybrid codes) for phased array radar. Its main property is the capability of digital beamforming on transmit just like with a set of orthogonal signals employed by coherent multiple-input–multiple-output radar and simplicity (only one waveform has to be generated in time). The performance of the hybrid codes is examined by analysing the transmit ambiguity functions containing beampatterns and range profile for all angular directions of interest.@en

Recently, the authors proposed a fast high-resolution imaging algorithm, called frequency-wavenumber (F-K) Kirchhoff migration. It was developed by combining three algorithms: F-K migration, Kirchhoff migration and inverse boundary scattering transform. The F-K Kirchhoff migration is considered promising in improving the image resolution of images taken with airport security body scanners for which costs can be mitigated by introducing sparse arrays. We investigate the imaging performance of the F-K Kirchhoff migration using ultra-wideband radar with such sparse arrays.@en

We propose a fast method of ultrawideband (UWB) radar imaging that can be applied to a moving target, having in mind such application as concealed weapon detection. We demonstrate the performance of the proposed method using simulations and measurements with static and moving targets. We also compare the computational complexity of the proposed method with that of a conventional method to clarify the feasibility of applying the proposed method to the intended real-time systems.@en

A novel approach to full-polarimetric short-range imaging with rotating arrays is proposed. Instead of taking advantage of all four possible combinations of the polarizations of transmit and receive antennas as in the typical full-polarimetric imaging systems, the suggested antenna array acquires three different co-polarized measurements at each spatial sampling point with its rotation. A simple algebraic operation is derived to accurately retrieve the full-polarimetric signals in the H/V polarization basis from the co-polarized measurements. After the retrieval of the full-polarimetric signals, the traditional imaging algorithms can be applied to reconstruct the polarimetric images. The effectiveness and accuracy of the suggested approach to fullpolarimetric imaging are validated on the signal and image levels through both numerical simulations and experiments. The results show that the proposed approach can accurately retrieve the fullpolarimetric signals and provides an alternative method to signal acquisition for full-polarimetric imaging.@en

We present frequency-domain Kirchhoff migration, which is a fast and high-resolution imaging algorithm for ultra-wideband radar. The algorithm is developed by combining three conventional methods: Kirchhoff migration, frequency-wavenumber (F-K) migration, and inverse boundary scattering transform (IBST). Frequency-domain Kirchhoff migration is compared with conventional methods in terms of the computational load and resolution by applying the methods to measurements made for a handgun, which is an important target in various security-related applications including airport security.

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The authors have developed a target speed estimation method for an ultra-wideband radar imaging system. The method is able to generate a focused image even if the target is moving at an unknown speed during the measurement, where many other techniques produce blurred images. Our technique uses the cross-range blurriness as a metric to measure the image focusing, and select the best-focused image among many generated for various candidate speeds. The target speed is estimated by finding the minimum cross-range blurriness, and the method compensates for the estimated speed to generate a focused three-dimensional image automatically. By exploiting the fact that the head size differs relatively little among individuals, we detected the head position in a radar image, and calculated the blurriness around the region to evaluate the focuse of the image. The speed estimation method is applied to the measurement of a mannequin on a moving stage, and it is shown that the target speed is estimated with an accuracy of 5%. Application results show that the developed technique can estimate target speeds accurately, and produce an image with sufficient resolution for the intended application.@en