Developments in target micro-Doppler signatures analysis: radar imaging, ultrasound and through-the-wall radar

Clemente et al. EURASIP Journal on Advances in Signal Processing 2013, 2013:47 http://asp.eurasipjournals.com/content/2013/1/47 R EVIEW Open Access Developments in target micro-Doppler signatures analysis: radar imaging, ultrasound and through-the-wall radar Carmine Clemente1* , Alessio Balleri2 , Karl Woodbridge3 and John J Soraghan1 Abstract Target motions, other than the main bulk translation of the target, induce Doppler modulations around the main Doppler shift that form what is commonly called a target micro-Doppler signature. Radar micro-Doppler signatures are generally both target and action specific and hence can be used to classify and recognise targets as well as to identify possible threats. In recent years, research into the use of micro-Doppler signatures for target classification to address many defence and security challenges has been of increasing interest. In this article, we present a review of the work published in the last 10 years on emerging applications of radar target analysis using micro-Doppler signatures. Specifically we review micro-Doppler target signatures in bistatic SAR and ISAR, through-the-wall radar and ultrasound radar. This article has been compiled to provide radar practitioners with a unique reference source covering the latest developments in micro-Doppler analysis, extraction and mitigation techniques. The article shows that this research area is highly active and fast moving and demonstrates that micro-Doppler techniques can provide important solutions to many radar target classification challenges. 1 Introduction Moving targets illuminated by a radar system contain frequency modulations caused by the time-varying delay occurring between the target and the sensor. The main bulk translation of the target towards or away from the sensor induces a frequency or Doppler shift of the echo as a result of the well-known Doppler effect. However, the target can contain parts that can have additional movements. These can contribute frequency modulations around the main Doppler shift that are commonly referred to as micro-Doppler (m-D) modulations. Chen [1,2] modelled the radar m-D phenomenon and simulated m-D signatures for various targets, such as rotating cylinders, vibrating scatterers and personnel targets. The authors also showed an effective tool in extracting the m-D signature is the time-frequency analysis of the received signal, leading to additional information on the target that can be used for classification and recognition. Micro- *Correspondence: 1 CeSIP, Department of Electronic and Electrical Engineering, University Of Strathclyde, Glasgow, G1 1XW, UK Full list of author information is available at the end of the article Doppler can be regarded as a unique signature of the target that provides additional information about the target that is complementary to existing methods for target recognition. Specific applications include the recognition of space, air and ground targets. For example, the m-D effect can be used to identify specific types of vehicles and determine their movement and the speed of their engines. Vibrations generated by a vehicle engine can be detected by radar signals returned from the surface of the vehicle. For example, from m-D modulations in the engine vibration signal, one can distinguish whether it is a gas turbine engine of a tank or the diesel engine of a bus. Another application is the use of m-D signature for human identification making possible the identification of humans on different weather or light conditions. In particular, specific components of m-D gait signature can be related to parts of the body for identification purposes [3]. Recently effective signature extraction techniques have been developed and tested on real data [4-11] providing features leading to classification results with a high level of confidence. These results could probably be improved if a multistatic m-D signature is used [12] where the self occlusion problem of the target is avoided. © 2013 Clemente et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Clemente et al. EURASIP Journal on Advances in Signal Processing 2013, 2013:47 http://asp.eurasipjournals.com/content/2013/1/47 In this article, we review recent advances in radar based m-D analysis over the last decade from radar imaging systems and emerging radar techniques. Our attention will focus on four fields that provide interesting results exploiting the m-D features: synthetic aperture radar, inverse synthetic aperture radar, ultrasound and through wall radar. The remainder of the article is organised as follows. In Section 2, the basic concepts of micro-Doppler from a radar system are introduced. Section 3 is the review Section. Subsection 3.1 introduces the m-D effect studies for radar imaging platform, monostatic SAR, bistatic SAR and Inverse SAR. The exploitations of m-D signatures proposed for ultrasound radar systems are described in Subsection 3.2 while Subsection 3.3 describes the ongoing research made in the field of the through the wall radar opening to the opportunity to extract m-D also in the presence of obstacles. 2 Micro-Doppler effect in radar The mathematics of the micro-Doppler effect from radar can be derived from introducing micro-motion to the conventional Doppler analysis. In this section the basics of the micro-Doppler effect are introduced. This is important for the understanding and the derivation of the microDoppler effects in more complex and realistic cases. In Figure 1 the geometry used to analyse the micro-Doppler induced by a vibrating target is shown [2]. The target located in P vibrates with frequency fv and displacement Dv , thus having a displacement function of the kind D(t) = Dv sin(2πfv t) cos β cos αp (assuming α = 0 and βp = 0) [2]. Letting R0 be the distance between the radar and the target initial position O then the range function varies with time due to the target micro-motion R(t) = R0 + D(t). The radar received signal becomes Figure 1 Geometry for the radar and a vibrating point target [2]. Page 2 of 18    R(t) s(t) = ρ exp j 2πf0 t + 4π λ = ρ exp {j[ 2πf0 t + (t)] }, (1) where ρ is the backscattering coefficient f0 is the carrier frequency and λ is the carrier wavelength. Substituting the R(t) in (1) the received signal can be expressed as:   4πR0 s(t) = ρ exp j λ   exp j2πf0 t + Dv sin(wv t) cos β cos αp 4π/λ , (2) where wv = 2πfv . From (2), the derivative of the second phase term leads to the expression of the micro-Doppler shift. wv Dv cos β cos αp cos(wv t) (3) fmD = πλ Figure 2a,b show the simulation results for a 10 GHz radar with a PRF of 2000 Hz. Given Dv = 0.01 m, fv = 2 Hz, αp = 30 ◦ , βp = 30 ◦ , and the center of the vibration at (...truncated)