Victor C. Chen

Micro-Doppler effect in radar: phenomenon, model, and simulation study

When, in addition to the constant Doppler frequency shift induced by the bulk motion of a radar target, the target or any structure on the target undergoes micro-motion dynamics, such as mechanical vibrations or rotations, the micro-motion dynamics induce Doppler modulations on the returned signal, referred to as the micro-Doppler effect. We introduce the micro-Doppler phenomenon in radar, develop a model of Doppler modulations, derive formulas of micro-Doppler induced by targets with vibration, rotation, tumbling and coning motions, and verify them by simulation studies, analyze time-varying micro-Doppler features using high-resolution time-frequency transforms, and demonstrate the micro-Doppler effect observed in real radar data.

Joint time-frequency transform for radar range-Doppler imaging

Conventional radar imaging uses the Fourier transform to retrieve Doppler information. However, due to the complex motion of a target, the Doppler frequency shifts are actually time-varying. By using the Fourier transform, the Doppler spectrum becomes smeared and the image is blurred. Without resorting to sophisticated motion compensation algorithms, the image blurring problem can be resolved with the joint time-frequency transform. High-resolution time-frequency transforms are investigated, and examples of applications to radar imaging of single and multiple targets with complex motion are given.

ISAR motion compensation via adaptive joint time-frequency technique

A novel approach for inverse synthetic aperture radar (ISAR) imaging is presented for both target translational motion and rotational motion nonuniformity compensation. The basic idea is to perform Doppler tracking to individual scatterers via an adaptive joint time-frequency (AJTF) projection technique. After maximizing the projection of the phase function to a set of basis functions in time-frequency plane, the Doppler frequency drift of the strongest scatterer in the range bin is automatically tracked out and the multiple prominent point processing (PPP) scheme is implemented to eliminate both the translational motion error and rotational motion nonuniformity. Further the azimuth spacing can be estimated, which permits polar reformatting of the original collected data.

Three-dimensional ISAR imaging of maneuvering targets using three receivers

The conventional ISAR image is a two-dimensional (2-D) projection of a three-dimensional (3-D) object surface. The image (projection) plane is related to the motion of a target with respect to the line of radar sight (LOS). In general, the image plane and the image scale in the cross range direction can not be determined by the traditional ISAR system with one receiver unless the target motion knowledge is known. In this paper, we propose a new ISAR system with three receivers. Using the three-receiver ISAR system, 3-D images of maneuvering targets can be generated, where the knowledge of the target motion is not required.

Analysis of radar micro-Doppler with time-frequency transform

Micro-Doppler induced by mechanical vibration or rotation of structures in a radar target is potentially useful for target detection, classification and recognition. While the Doppler frequency induced by the target body is constant, the micro-Doppler due to vibrating or rotating structures of the target is a function of dwell time. Analysis of the time-varying Doppler signature in the joint time-frequency domain can provide useful information for target detection, classification and recognition.

Joint time-frequency analysis for radar signal and image processing

The Fourier transform has been widely used in radar signal and image processing. When the radar signals exhibit time- or frequency-varying behavior, an analysis that can represent the intensity or energy distribution of signals in the joint time-frequency (JTF) domain is most desirable. In this article, we showed that JTF analysis is a useful tool for improving radar signal and image processing for time- and frequency-varying cases. We applied JTF analysis to radar backscattering and feature extraction; we also examined its application to radar imaging of moving targets. Most methods of JTF analysis are non-parametric. However, parametric or model-based methods of time-frequency analysis, such as adaptive Gaussian and chirplets, are more suitable for radar signals and images.

Spatial and temporal independent component analysis of micro-Doppler features

Micro-Doppler features can be regarded as a unique signature of an object with movements and provide additional information for classification, recognition and identification of the object. Independent component analysis (ICA) can decompose micro-Doppler features into independent basis functions that represent salient physical movement attributes of the object. To study ICA of micro-Doppler features, we used a dataset generated by simulation of radar returned signals from rotating objects and tumbling objects. Fast ICA algorithm was used in our study to decompose micro-Doppler features into a set of spatial and temporal independent components. Spatial characteristics of the independent components combined with the corresponding temporal characteristics can be used to improve performance of classification, recognition and identification.

Quantitative SNR analysis for ISAR imaging using joint time-frequency analysis-Short time Fourier transform

V.C. Chen recently presented an inverse synthetic aperture radar (ISAR) imaging technique using the joint time-frequency analysis (JTFA), which has been shown having a better performance for maneuvering targets over the conventional Fourier transform method. The main reason is because the frequencies of the radar returns of the maneuvering targets are time varying and a JTFA is a technique that is suitable for such signals, in particular a JTFA may concentrate a wideband signal, such as a chirp, while it spreads noise. We quantitatively study the signal-to-noise ratio (SNR) in the ISAR imaging using one of the typical JTFA techniques, namely the short time Fourier transform (STFT). We show that the SNR increases in the joint time-frequency (TF) domain over the one in the time or the frequency domain alone both theoretically and numerically. This quantitatively shows the advantage of the JTFA technique for the ISAR imaging.

Micro-Doppler effect of micromotion dynamics: a review

In this paper, we review the micro-Doppler effect induced by micro-motion dynamics and provide mathematics of the micro-Doppler effect by introducing micro motions to the conventional Doppler effect. Micro-Doppler effect was originally introduced in laser systems, but it can also be observed in microwave radar systems. We introduce some interesting results on observing micro-Doppler phenomenon using microwave radar systems and potential applications to target’s feature extraction.

Reconstruction of inverse synthetic aperture radar image using adaptive time-frequency wavelet transform

Inverse synthetic aperture radar (ISAR) uses targets motion to generate images on the range- Doppler plane. The conventional ISAR uses Fourier transform to compute Doppler spectrum for each range cell. Due to the target irregular translational and rotational motion, the Doppler frequency in fact is time-varying. By using Fourier transform, the reconstructed image becomes blurred. To represent time-varying Doppler spectrum, time-frequency transform should be utilized. Adaptive time-frequency wavelet transform is a very useful tool in analysis of signals with time-varying spectrum. We applied adaptive time-frequency wavelet transform to ISAR image reconstruction and developed a simulation procedure to describe the characteristics of the algorithm. By replacing the conventional Fourier processor with the adaptive wavelet processor, a 2-D range-Doppler Fourier ISAR frame becomes a 3-D time- range-Doppler wavelet ISAR cube. By sampling in time, a time sequence of 2-D range- Doppler images can be viewed. Each individual wavelet ISAR image provides not only superior resolution but also the temporal information within each frame time. Both simulated and real ISAR data have been tested. The result from simulated ISAR data is illustrated in this paper.