Stepped-frequency Radar System Research Articles

Rotating Target Size and Rotation Vector Estimation in a Distributed Stepped Frequency Radar System

Radar echoes produced by a target contain the target motion information and physical characteristics. In a stepped frequency (SF) signal, a range-Doppler coupling occurs where the movement of the target affects the target range profile. Using this characteristic, in this paper, an algorithm for estimating the size and rotation vector of the rotating target is proposed. For this, we consider a distributed radar system to utilize target-sensor geometry. By executing signature extraction from the high resolution range profiles (HRRPs) on time-range images and two nonlinear least squares (NLS) estimations, the desired parameters are successfully estimated. Through several simulations, the performance of the proposed method is analyzed and the favorable results are obtained.

Range and Velocity Estimation Using Kernel Maximum Correntropy Based Nonlinear Estimators in Non-Gaussian Clutter

In this article, we propose kernel maximum correntropy based nonlinear estimators for range and velocity estimation in non-Gaussian clutter and system nonlinearity. The proposed estimators are analyzed for linear frequency modulated and stepped frequency radar systems. Additionally, an adaptive update equation is derived for optimization of the kernel width, which further lowers the dictionary size and the variance of the proposed estimators. For performance evaluation of the proposed estimators, an expression is derived for the Cramer-Rao lower bound using a modified Fisher information matrix.

Near-Field Full-Waveform Inversion of Ground-Penetrating Radar Data to Monitor the Water Front in Limestone

Limestone has been a widely used building material for many years. Its degradation process has been strongly linked to its water content, to which the radar waves are sensitive. In this paper, the closed-form, near-field full-wave radar model of Lambot et al. is utilized for monitoring water ingress in limestone. This model is based on planar layered media Green's functions combined with an intrinsic representation of the antenna through global reflection and transmission functions. The Jonscher model is used to describe the frequency dependence of the limestone electromagnetic properties. Then, an inversion method based on a genetic algorithm is developed and optimized. The method is numerically validated by simulating electromagnetic wave propagation in medias, using a three-dimensional finite element software, namely high-frequency electromagnetic field simulation. Finally, the method is experimentally validated, with a stepped-frequency radar system and an ultrawide band antenna, during an imbibition process of a limestone slab. Results are compared to gammadensimetry measurements, a nuclear and semidestructive approach, performed as the reference. Analyses of the results show that the average relative difference between the estimated and the reference water front is 2.5%, thereby demonstrating the performance of the proposed nondestructive method.

Compactness/density assessment of newly-paved highway containing recycled asphalt pavement by means of non-nuclear method

In this paper, the compactness of a newly-paved road is assessed by means of non-destructive Step Frequency Radar (SFR) as an alternative to cores testing and nuclear gauges usually performed in laboratory or on site. Moreover, our approach allows the estimation of compactness whereas the road contained 38% in weight of recycled component with unknown physical properties. Usually Hot Mix Asphalt (HMA) properties have to be well known to apply electromagnetic mixing models which are required to evaluate the density/compactness from the measured permittivity. Two electromagnetic mixing model families, Power Law (PL) models and Unified Mixing Rules (UMR), are investigated here for 5-phase material, including aggregates, recyclates, binder, filler and air. In order to assess accurately the compactness, three methods are developed. First, without any laboratory experiment, compactness is estimated directly from SFR measurements from a best-fitting approach constrained by manufacturer compactness specification. Second, performing a permittivity measurement of aggregates by means of resonant cylindrical cavities in laboratory, compactness is assessed accurately. Finally, in order to evaluate the second approach according to standard methods, the HMA compactness is obtained by back-calculating the permittivity of recyclates based on reference compactness results (cores and nuclear gamma bench controls) performed in laboratory. Additionally, the thickness of the top layer of the base course is estimated simultaneously with the compactness for both static (SFR measurements at given location) and dynamic modes (SFR system embedded in vehicle). This point helps to best-understanding the deviation observed on the results. In conclusion, this study shows that CRIM (PL family), Rayleigh and Böttcher models (UMR family) are well-adapted to assessed HMA compactness in the microwave band. Moreover, the second approach proves that HMA compactness of newly-paved road can be assessed both in static and dynamic modes with an error compared to the standard measurements weaker than 1%, by means of permittivity measurement in laboratory, without the need of nuclear gauges or destructive techniques.

CSVE radar: high‐range‐resolution radar using compressive sensing and virtual expanding technique

Range estimation is an important issue of radar system, and its performance is mainly limited by the transmitting bandwidth. To improve the range resolution, the existing approaches often either occupied wider bandwidth or employed some non-linear techniques which cause high complexity. The wider the transmitting bandwidth has not only the more the network resource takes but also the easier the radio frequency interference (RFI) is introduced. The authors propose a new radar system by utilising jointly the compressive sensing (CS) and the proposed virtual expanding technique, referred to as the compressing sensing and virtual expanding (CSVE) radar system. The proposed CSVE radar system not only provides high-range resolution with low cost by saving transmitting bandwidth but also is robust to the bad effect of the RFI by utilising a pre-detection phase at the transmitter and a detection phase at the receiver. Simulation results validate the superiority of the proposed CSVE radar system over both the high-resolution techniques such as Capon, multiple signal classification and matched filter, and the CS step-frequency radar system.

CFAR Detection of Moving Range-Spread Target in White Gaussian Noise Using Waveform Contrast

In wideband stepped-frequency radar systems, relative motions between radar and target can induce high distortions of high-resolution range profiles (HRRPs), such as range migration, shape deformation, and signal-to-noise-ratio (SNR) loss. These distortions, if ignored, can lead to unacceptable performance deterioration in detection. To solve this problem, a new algorithm for detecting moving range-spread targets (RSTs) is proposed in this letter. The proposed detector utilizes the waveform contrast of the HRRP to perform both motion compensation and constant false-alarm rate target detection, and it is simple and robust even for low-SNR scenarios. Simulated experiments are carried out to verify the effectiveness and advantages of the proposed detector.

A 3-D Electromagnetic-Model-Based Algorithm for Absolute Attitude Measurement Using Wideband Radar

In this letter, a novel algorithm for attitude measurement based on a 3-D electromagnetic model (3-D em-model) is proposed. The 3-D em-model is established offline based on the geometric structure of the target, and it can be used to predict the scattering features at different target attitudes. In order to measure the attitude of the air target, we design a bistatic step frequency radar system. The directions of the two radars' lines of sight (LOSs) relative to the target are acquired by matching the high-resolution range profiles (HRRPs) from the target echoes to the HRRPs generated from the 3-D em-model. Since the directions of two radars' LOSs relative to the Earth are already known, the absolute attitude of the target can be acquired. The innovative contributions of this letter are as follows: 1) A comprehensive theoretical analysis of air target attitude measurement based on its own 3-D em-model is proposed; 2) the method can be applied to different kinds of air targets such as aircraft, satellite, missile, etc.; 3) the proposed attitude measurement method does not require target motion model in advance; and 4) the proposed algorithm can be applied to any kind of step frequency waveforms. Experiments using both data predicted by a high-frequency electromagnetic code and data measured in the chamber verify the validity of the method.

Performance analysis of forward-looking GPR ultra-wideband antennas for buried object detection

We are currently developing a Stepped-Frequency Radar (SFR) which utilizes a custom-made uniform linear array of 16 Vivaldi notch receive antennas and two Transverse Electromagnetic (TEM) horn transmit antennas. The SFR has an operating band of 300–2000 MHz, and a minimum frequency step-size of 1 MHz. The custom-made TEM horn antennas are used for the transmission of the SFR’s ultra-wideband (UWB) spectrum. This paper discusses a comparison analysis between a commercially available UWB antenna and the currently used TEM horns. Gain, Voltage Standing Wave Ratio (VSWR), and antenna pattern measurements for each antenna are presented. The antennas were also tested for their ability to detect buried targets in a simple stepped-frequency radar system using a network analyser as a transmitter and receiver. An analysis of the gain, VSWR, beamwidth, and measured data from radar test of each antenna was performed, providing insights into each antenna’s performance on the SFR’s ability to detect buried targets. The information provided in this paper will be useful to the radar community in exploring developmental standoff detection solutions for military applications such as obscured target detection of obstacles and explosive hazards.

Non‐destructive assessment of hot mix asphalt compaction/density with a step‐frequency radar: case study on a newly paved road

ABSTRACTA Step‐Frequency Radar (SFR) is used for assessing the compaction of Hot Mix Asphalt (HMA) layers. The system is composed of a network analyser and an Ultra Wide Band (UWB) antenna placed above the road surface. The measurements are carried out on a newly paved road in Cagny (Normandy, France). The SFR system provides the permittivity of the first overlay. The data is corrected from vehicle vibrations and calibrated at fixed locations. Then, the HMA compaction is deduced with a Lichtenecker–Rother (LR) model. The results are compared with standard tests (gamma bench testing on cores and in‐place nuclear gauge). We show that the SFR system allows the nondestructive assessment of HMA overlay with a high density of points and with accuracy close to the compaction provided by standard tests.

Investigations on the sensitivity of a stepped-frequency radar utilizing a vector network analyzer for Ground Penetrating Radar

Employing a continuous-wave radar system, with the stepped-frequency radar being one type of this class, all reflections from the environment are present continuously and simultaneously at the receiver. Utilizing such a radar system for Ground Penetrating Radar purposes, antenna cross-talk and ground bounce reflection form an overall dominant signal contribution while reflections from objects buried in the ground are of quite weak amplitude due to attenuation in the ground. This requires a large dynamic range of the receiver which in turn requires high sensitivity of the radar system.In this paper we analyze the sensitivity of our vector network analyzer utilized as stepped-frequency radar system for GPR pipe detection. We furthermore investigate the performance of increasing the sensitivity of the radar by means of appropriate averaging and low-noise pre-amplification of the received signal. It turns out that the improvement in sensitivity actually achievable may differ significantly from theoretical expectations.In addition, we give a descriptive explanation why our appropriate experiments demonstrate that the sensitivity of the receiver is independent of the distance between the target object and the source of dominant signal contribution. Finally, our investigations presented in this paper lead to a preferred setting of operation for our vector network analyzer in order to achieve best detection capability for weak reflection amplitudes, hence making the radar system applicable for Ground Penetrating Radar purposes.

Improvement in the range resolution of THz radar using phase compensation algorithm

The paper describes the principle of 0.2 THz stepped-frequency radar system which is utilized to achieve a one-dimensional range profile and range resolution. Terahertz (THz) stepped frequency radar is more susceptible to the phase error which will cause the spread and shift of range profile, thus affecting the quality of the high resolution range profile and signal-to-noise ratio. Therefore, a method of phase compensation is proposed to improve the range resolution. After phase compensation, the resolution and signal-to-noise ratio are improved remarkably. The range resolution can reach centimeter scale. Experimental and simulation results indicate that THz stepped frequency radar can reach high resolution range profile with the phase compensation method, which provides a foundation for further research on two-dimensional and three-dimensional image in the THz band.

High Range Resolution Profiling of Moving Targets via Compressive Sensing in Stepped Frequency Radar

With respect to the problems of position mismatch, oversampling redundancies, distortion and dispersion in range profile caused by radical motion between radar and the interested targets in stepped-frequency radar system, a combined range-velocity estimation method is presented based on the compressive sensing (CS) theory. Considering the sparsity of the interested targets in the illuminated scene by the radar beam, a sparse presence model of the targets is established, and then the range and velocity of the targets are co-estimated directly in the range-velocity plane employing multi-snapshots data, thus the redundancies and mismatch problems existing in traditional method are avoided, besides the output signal-noise-ratio improves dramatically in this case. To reduce the complexity of the computing process, a dynamic sensing matrix is constructed using information acquired from pre-estimated range and velocity, following with which a singular value decomposition (SVD) of the reduced echo data is employed to further reduce the computation load. The effectiveness of the proposed method and robust performance to noise are validated by simulation and experimental results.

Novel imaging methods of stepped frequency radar based on compressed sensing

The theory of compressed sensing (CS) provides a new chance to reduce the data acquisition time and improve the data usage factor of the stepped frequency radar system. In light of the sparsity of radar target reflectivity, two imaging methods based on CS, termed the CS-based 2D joint imaging algorithm and the CS-based 2D decoupled imaging algorithm, are proposed. These methods incorporate the coherent mixing operation into the sparse dictionary, and take random measurements in both range and azimuth directions to get high resolution radar images, thus can remarkably reduce the data rate and simplify the hardware design of the radar system while maintaining imaging quality. Experiments from both simulated data and measured data in the anechoic chamber show that the proposed imaging methods can get more focused images than the traditional fast Fourier transform method. Wherein the joint algorithm has stronger robustness and can provide clearer inverse synthetic aperture radar images, while the decoupled algorithm is computationally more efficient but has slightly degraded imaging quality, which can be improved by increasing measurements or using a robuster recovery algorithm nevertheless.

SAR Imaging with Noise Radar

Two ground-based coherent imaging systems designed on the basis of noise radar technology (NRT) using continuous waveform with a linear synthetic aperture and pulse coherent noise waveform with a circular synthetic aperture are presented. A short description of noise waveform synthetic aperture radar (SAR) theory and data processing algorithms are given. Experimental results obtained show that the quality of the imagery is comparable to that of frequency-modulation continuous-wave (FMCW) or stepped frequency radar systems. Experimental validation of the technique at different locations provides confidence in the ability of noise waveform SAR to achieve high-resolution imaging at low cost. The exploitation of the interferometric phase combining pairs of images spaced in time is shown to allow the detection of displacements in the scene with a submillimeter accuracy.

HPRF pulse Doppler stepped frequency radar

Stepped frequency radar is a well known scheme to generate high range resolution profile (HRRP) of targets. Through appropriate radar parameter design, the radar enables both unambiguous velocity measurement and high resolution ranging within a single dwell in a high pulse repetition frequency (HPRF) mode. This paper analyzes in detail the design principle of the HPRF stepped frequency radar system, the solution to its ambiguity issue, as well as its signal processing method. Both theoretical analysis and simulation results demonstrate that the proposed radar scheme can work independently to solve the problem of motion compensation, and is therefore highly applicable to many new types of radar.

Application of Subarray Averaging and Entropy Minimization Algorithm to Stepped-Frequency ISAR Autofocus

Subarray averaging and entropy minimization (SAEM) algorithm is applied to stepped-frequency ISAR autofocus to compensate for the phase error along the down-range due to high target speed and low pulse repetition frequency (PRF) of radar systems. In stepped-frequency radar systems, the phase error due to target motion in each burst make range profiles blur. Thus, the stepped-frequency ISAR often adopts a high PRF radar system in order to suppress the effect of target motion in each burst. However, it inevitably leads to loss of many advantages by adopting low PRF radars in stepped-frequency ISAR imaging. In order to take advantage of a low PRF radar in stepped-frequency ISAR imaging, this paper makes use of a method called SAEM (subarray averaging and entropy minimization) that uses a subarray averaging concept in conjunction with the entropy cost function in order to find the target's motion parameters. Well-focused ISAR images also can be provided from the combination of the SAEM and a traditional autofocus algorithm, even when low PRF stepped-frequency waveform is used. The effectiveness of this method is illustrated and analyzed with simulated targets comprised of point scatters and measured Boeing-737 data.

A hyperbolic summation method to focus B‐scan ground penetrating radar images: An experimental study with a stepped frequency system

AbstractThere are many techniques to focus raw B‐scan ground penetrating radar (GPR) images mainly distorted from well‐known hyperbolic dispersion. In this study, we apply a hyperbolic summation (HS)‐type focusing technique to the simulated and measured GPR data at C‐band frequencies. The formulation of the proposed HS method is presented. Numerical GPR images for different metal targets are generated by the help of a physical optics simulation code. An experimental set‐up is constructed with the help of a stepped frequency radar system and corresponding B‐scan images are generated. Proposed HS method is applied to the measured B‐scan GPR data. Focused GPR images of different objects were obtained for both the simulated and the experimental data. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 671–676, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22230

Selective inverse synthetic aperture radar imaging of moving targets

An approach is presented for separating and compensating wideband signatures of radar targets. The signatures thus modified are processed into focused radar imagery. In this approach, the time- frequency representation of superimposed signatures is utilized to define selective binary filters that are enhanced and labeled using computer vision techniques. On filtering, the signature of each target is synthe- sized from its short-time Fourier transform representation. Following syn- thesis, the filtered signature is motion compensated in the frequency domain. Radar images can then be generated for scenarios in which multiple moving targets cannot be imaged otherwise. Data collected with an actual stepped frequency radar system is processed to demonstrate that combined signatures of aircraft are effectively separated, compen- sated and transformed into individual inverse synthetic aperture radar (ISAR) images. © 2001 Society of Photo-Optical Instrumentation Engineers.

Recovery of surface parameters from stepped-frequency radar returns

This paper discusses a method for using the reflected signal from a stepped-frequency radar system to obtain the electrical permittivity and conductivity in a thin layer at the flat surface of a (possibly inhomogeneous) body. The method is based on a band-limited version of geometrical optics.