Research on extended micro-motion target echo simulation and characteristic extraction

Early predator detection is a key component of the predator-prey arms race and has driven the evolution of multiple animal hearing systems. Katydids (Insecta) have sophisticated ears, each consisting of paired tympana on each foreleg that receive sound both externally, through the air, and internally via a narrowing ear canal running through the leg from an acoustic spiracle on the thorax. These ears are pressure-time difference receivers capable of sensitive and accurate directional hearing across a wide frequency range. Many katydid species have cuticular pinnae which form cavities around the outer tympanal surfaces, but their function is unknown. We investigated pinnal function in the katydid Copiphora gorgonensis by combining experimental biophysics and numerical modelling using 3D ear geometries. We found that the pinnae in C. gorgonensis do not assist in directional hearing for conspecific call frequencies, but instead act as ultrasound detectors. Pinnae induced large sound pressure gains (20–30 dB) that enhanced sound detection at high ultrasonic frequencies (>60 kHz), matching the echolocation range of co-occurring insectivorous gleaning bats. These findings were supported by behavioural and neural audiograms and pinnal cavity resonances from live specimens, and comparisons with the pinnal mechanics of sympatric katydid species, which together suggest that katydid pinnae primarily evolved for the enhanced detection of predatory bats.

This article presents an efficient and effective design approach of the phase distribution calculation across metasurface for significant radar cross section (RCS) reduction of a circular polarization (CP) and liner polarization (LP) radar waves. The RCS reduction using the proposed design approach is achieved by imposing a novel cusp phase mask (which is usually used to generate 3D self-accelerating and self-healing cusp beams) at each geometric phased anisotropic unit cell composing the proposed cusp phased metasurface. By solving the cusp phase formula using MATLAB, it is found that the cusp phase mask required to achieve more than 10 dB of RCS reduction over a wide frequency band can be calculated without the need of significant lengthy optimizations or huge computer resources. The ability of such cusp phase mask metasurfaces to achieve significant backward scattering and RCS reduction has been rigorously investigated by means of simulations and measurements. When illuminated by a far-field radar CP or LP plane wave, the proposed cusp phased metasurface realizes more than 10 dB of RCS reduction from 10.9 to 26 GHz, corresponding to a fractional bandwidth of FBW = 81.8%. The 10 dB RCS reduction bandwidth of the cusp metasurface is maintained under both normal and wide angular incidence up to 75∘. The proposed cusp metasurfaces have potential applications to make objects stealthy where the incidence radar signal has an unknown frequency, polarization, or angle of incidence.

As an important passive sensing technology, infrared sensor has been used in satellite remote sensing systems to discriminate exo-atmosphere targets. At a long range, even relatively large objects are represented as single pixels on the IR image. The infrared signature, i.e., the time sequence of the target point's gray level serves as potential discriminants and has been used in many systems to classify the objects. Studying the generative process of the infrared signature is the premise of this research. In this paper, finite element analysis is used to simulate the infrared signature of exo-atmosphere micro-motion object. Modeling the dynamic of surface temperature field of the targets is a critical step. The main contribution of this paper is to analyze the influence of the micro-motion to the modeling of temperature field. Micro-motion is an important characteristic of the ballistic target. The way of motion causes the radiation received by the surface element changes dramatically, which may affect the simulation process. However, to the best of our knowledge, this problem has not been addressed in other papers. Two temperature field modeling techniques, i.e., the explicit and hidden schematic, are assessed at different sampling time steps. The results show that the explicit method is easy to cause oscillating for the micro-motion object. Although the hidden method is able to avoid the oscillating problem, sufficient small sampling time step is still needed to generated stable surface temperature field. The simulated temperature field dynamic shows that the temperatures in the field approach to be the same and do not change much in a short time period. After the temperature field is generated, micro-motion target's infrared signature is also generated.

A metamaterial approach to wideband radar cross section reduction that is thin, low-cost and doesn't dictate object shaping is presented in work from Iran. The approach uses principles potentially applicable to infrared and visible light as well as radar. The wideband RCS reducing surface is produced on a commercial high-frequency circuit substrate using conventional printed circuit manufacturing techniques Reducing an object's detectability to radar systems is now a well established priority for military applications, most famously in aircraft applications. ‘Stealth’ aircraft that were once well kept secrets are now a familiar concept to the general public and even fly in air shows. The radar cross section (RCS) of an object is essentially a measure of how detectable an object is to radar systems and so reducing an object's RCS is the basic goal of radar focused ‘stealth’ technologies. There are three main approaches to reducing RCS; radar absorbing material (RAM), object shaping and reflected wave cancelling by object coating. RAM materials, as the name suggests, are designed to reduce radar reflections back to the detector by absorbing the electromagnetic energy transmitted by the radar system, converting it into heat. However, they can be bulky and high maintenance. Object shaping works by shaping the surface of the object to minimise reflections of the radar waves back to the radar system, instead reflecting them in other directions. The obvious disadvantage to this approach is that it dictates the shape of the object, which can compromise the performance of the object for its primary purpose. For example, above all an aircraft must be shaped to enable it to fly, an aircraft that is very hard to detect but cannot take off is of no use to anyone. The work reported in this issue, by researchers at Iran University of Science and Technology (IUST) uses the third approach, in which a surface coating reflects the radar waves in such a way as to reduce the amount of radar energy that returns to the detector. This approach is low profile, low bulk and doesn't dictate the shape of the object. In the IUST team's work it can also be cheaply and simply manufactured using printed circuit board manufacturing techniques. Their proposed structure consist of a chessboard arrangement of two different artificial magnetic conductor (AMC) unit cell patterns, each of which reflects the radar waves differently. The key difference in the reflections is a 180° phase shift, which causes the reflections from the two different AMC cell types to interfere destructively and so significantly reduces the useful energy returned to the radar system's detector. “Since radar systems usually send out a short pulse signal in the time domain, wide bandwidth of the RCS reducer is necessary. The proposed AMC structure achieves a RCS reduction over a very wide bandwidth, about 85%,” said Dr Seid Hasan Sedighy, one of the IUST researchers on this project. The wide bandwidth achieved is possible because the critical 180° phase difference in the reflections is produced by the AMC unit cells over a wide frequency range, from 9.4 GHz to 23.28 GHz, providing 10 dB RCS reduction in that range. The peak RCS reduction is in excess of 40 dB, at 15 GHz. Printed circuit manufacturing techniques were used to create the AMC patterns on a commercial high-frequency circuit substrate, providing a low-cost manufacturing method. As flexible circuit board technologies are now available and continue to develop, this AMC-based, effective and relatively cheap approach to RCS reductions could be applied to arbitrary shapes. AMCs are metamaterial structures that behave as perfect magnetic conductors in a limited range of frequencies. The two unit cell AMC patterns used in this work are an arrangement of ‘E’ shapes and saltire arrows. The patterns were optimised to achieve the desired 180° phase difference in their reflections by simulation before being confirmed by experiment. The RCS reducer consists of a chessboard pattern of AMC patches using two different AMC unit cell designs, which produce reflections 180° out of phase with each other. The resulting destructive interference provides the intended RCS reduction The principles of AMCs and using destructive interference to reduce reflected energy are of course applicable to other frequencies of the EM spectrum, not just the band used for radar. This means that similar effects could be achievable in very different frequencies if the structures can be manufactured. Dr Sedighy expects this to lead to developments closer to photonic frequencies “It seems that this design approach will develop in the other frequency bands such as visible and infrared frequencies, which have many civil and military applications.”

A novel method for measuring small displacements based on the total-internal-reflection-heterodyne interferometry is presented. In this method, a common-path heterodyne interferometer is applied to measure the phase differences between s and p polarizations at total-internal reflection. The phase differences depend on the incident angles which are the function of displacements by using the image formula. Hence, small displacements can be evaluated only by measuring the phase differences. In our optical setup, a heterodyne light source with a frequency difference 800 Hz between s and p polarizations is used. A light beam passing through a lens is reflected from a mirror located near the focal plane of the objective lens and is driven by a piezo- electric transducer. If the mirror shifts from the focal plane of the lens, the reflected beam deflects with a small angular deviation to a phase difference measurement system. From Fresnel's equations, the phase difference between s and p polarizations is the function of incident angle and the numbers of TIR at total-internal reflection condition. To compare this test signal with a reference signal using a phase meter, the phase difference can be measured in real- time. Consequently, the displacement also can be calculated.

Based on the heterodyne interferometry and Fresnel equations, an alternative method for measuring the complex refractive index of a turbid medium. A light beam is incident on the boundary between a right-angle prism and a turbid medium. The phase difference between s- and p- polarizations of the reflected light occurs. The phase difference depends on then incident angle and the complex refractive index of a turbid medium; their relation can be derived from Fresnel equations. The phase difference can be measured accurately with the heterodyne interferometry. Because there are two unknown parameters to be estimated, at least the phase differences under two different conditions should be measured. Then, these measured data are substituted into the derived relation, and a set simultaneous equation is obtained. If the simultaneous equation is solved, the complex refractive index can be estimated. Because the reflected light from the boundary is measured, the scattering noises coming from the turbidity of the tested medium can be greatly reduced. In addition, this method has some merits such as simple optical setup, high sensitivity, high stability, and suitability for a little amount of the tested medium in its native state (without dilution).

Aiming at ballistic missile target is hard to be distinguished by the radar seeker when there is a source of the disturbance which has the same scattering character or the same shape. A method to recognize target micro-motion by harmonic wave seeker was brought forward. The target's harmonic wave radiation model was built. The target's vibration caused harmonic wave scattering frequency shift which can deduce the frequency shift target function. The target function parameter was searched by genetic algorithm. The algorithm made the seeker could recognize the target which had the biggest harmonic wave scattering frequency shift. The harmonic wave scattering frequency shift was simulated according different target characters. The output of simulation showed that method can find the biggest harmonic wave scattering frequency shift target and distinguish the difference between real target and fake target. According to the output, the seeker can decide whether to attack the target or not. It proved that harmonic wave radar seeker had practicability in the field target identification which made a foundation in the application of harmonic wave scattering theory in radar seeker design.

Flow separation was investigated in pulsatile flow within a computational fluid dynamics (CFD) model of glottis motion. Two-dimensional time-dependent laminar flow was calculated for given dynamic wall geometries and flow boundary conditions. A Lagrangian–Eulerian method with a multiblock mesh was employed to allow grid points to move in space to conform to the boundaries. The glottal wall was approximated with two sinusoidal curves (one for the glottal entrance and one for glottal exit) connected with a straight line. The entrance and exit glottal diameters were changed as a sinusoidal function of time with a phase difference to generate the desired glottal wall movements. Results will be reported on the mean velocity and pressure distributions for various phase angles, frequencies, oscillation amplitudes, and Reynolds numbers. The locations of the flow separation points were obtained through a shear stress analysis using MATLAB. An increase in the glottal amplitude causes the separation points to move upstream. The study provides a better understanding of the effect of inferior–superior phase differences on the glottal flow. [Work supported by NIDCD Grant No. 1 R01 DC03566.]

Fourier transform processing is applied to digital signals obtained by analog-to-digital conversion of signals supplied by two microphones spaced by a fixed distance to produce two series of discrete data each datum of which represents the energy and phase of a spectral frequency band of the received sound. A dominant angle of incidence representing the angle of incidence of a speech signal component of the received sound signal relative to the two microphones is determined from phase differences between the discrete data in the same frequency bands of the two series and is used to combined the two series of discrete data into a single instantaneous spectrum in which any speech signal component is amplified relative to the noise. A noise spectrum is updated by comparing for each frequency band of the instantaneous spectrum the absolute value of the difference between the dominant angle of incidence and the angle of incidence of the frequency band in question with a tolerance threshold, and setting the energy of the noise spectrum equal to that of the instantaneous spectrum if that difference is above the tolerance threshold. The updated noise spectrum is then subtracted from the instantaneous spectrum to obtain an output spectrum consisting of the speech spectrum.

A fast hybrid method based on physical optics (PO) and the reciprocity theorem is proposed to analyse the Doppler spectrum of a two-dimensional, moving plasma-coated object above a one-dimensional time-evolving lossy dielectric rough sea surface. This hybrid method eliminates the need for numerical solution of the polarization currents on the object and the time-evolving sea surface, thereby greatly improving the computing speed. The method is verified by conventional numerical methods. The characteristics of the Doppler spectrum of a plasma-coated object above a time-evolving sea surface are analysed in detail, including the effects of several parameters such as the incident angle and the velocity of the moving object. The effects of some key parameters on the stealth performance are also examined.

A semi-empirical polarimetric scattering model for bare soil surfaces is developed. This scattering model is constructed based on the existing theoretical models in conjunction with an extensive experimental data collected with polarimetric scatterometer systems at microwave frequencies. The backscattering coefficients as well as parameters of phase difference statistics, degree of correlation (/spl alpha/) and polarized phase difference (/spl zeta/), are expressed in terms of both surface parameters (rms height, correlation length, and dielectric constant) and radar parameters (frequency and incidence angle). The semi-empirical model is used as a basis for an inversion algorithm to estimate the surface parameters from the polarimetric backscatter response of a surface when the radar parameters are known. By performing a sensitivity analysis, a set of optimum parameters are chosen for the inversion algorithm. It is shown that the co-polarized ratio (/spl sigma//sup 0/ /sub hv///spl sigma//sup 0/ /sub vv/) the cross-polarized ratio (/spl sigma//sup 0/ /sub hv///spl sigma//sup 0/ /sub vv/), and the degree of correlation for co-polarized phase difference are most sensitive to the surface parameters and least affected by the measurement errors. >

A new and simple method of interpreting the of the backscatter radar cross section (RCS) from ocean waves of individual wavelength is presented. Using the Kirchhoff scattering (Physical Optics) theory, the RCS from the ambient waveheight spectrum is first computed as a function of the wavenumber. Differentiating this cumulative RCS yields the distribution function of the RCS from ocean waves of different wavenumbers. The present study shows, as a general feature, that the maximum RCS contribution tends to come from the ocean waves of wavenumbers closer to the Bragg wavenumber as the radar incidence angle increases. Under low wind speeds (/spl sim/2 m/s) and at incidence angles between 200 and 45/spl deg/, the dominant RCS contribution at both C-band and L-band comes from ocean waves of wavenumbers close to the Bragg wavenumber. Under intermediate to high wind speeds (/spl sim/10-15 m/s) and at the small incidence angle of 200, the radar backscatter is dominated by long ocean waves with little contribution from the Bragg waves at both bands. At the intermediate incidence angles (/spl sim/35/spl deg/ to 45/spl deg/), the RCS distribution is centered at wavenumbers near the Bragg wavenumber at all wind speeds from 2 m/s to 15 m/s. These features are more pronounced at C-band than at L-band. The present theory is based on the Kirchhoff scattering model, and as such, its validity may also be limited to a range of small to intermediate incidence angles where the polarization effect is not very significant. The RCS distribution is evaluated for the ambient sea surface and the surface perturbed by varying currents induced by the interaction between the current and bottom topography.

In the free field, making full use of scalar and vector information of acoustic field can bring many advantages for acoustic measurement, but under the conditions of the existence of sound baffle, the sound field should be distorted due to the superposition of diffraction wave. Taking the soft spherical baffle as background, the sphere scattering model is established, the diffraction pressure field and particle velocity field under the condition of different frequency, distance and incident angle is analyzed. The effect of diffraction wave on sound pressure and particle velocity is determined by the amplitude of diffraction wave and the phase difference between the diffraction wave and the incident wave, which is related with the frequency, distance and incident angle. The effect of diffraction wave on sound pressure and the particle velocity components in y axis is strong only on near field and high frequency, but for particle velocity component in x axis, on near field, the effect is always strong both at high and low frequency. The effect of diffraction wave can be ignored only in the far field for particle velocity component in x axis. The description of reference for influences on the directivity of vector sensor by soft ball boundary is not consistent with us.

The focus of this work is to determine at which threshold can the results for both plane and spherical wave incidence assumptions either converge or deviate when performing multiple diffraction attenuation calculations. The analysis has been carried out—for various millimeter-wave frequencies, inter-obstacle spacings, and angles of incidence—by employing a pair of two-dimensional (2D) hybrid formulations based on both the uniform theory of diffraction and physical optics (UTD-PO). This way, we seek to demonstrate under which circumstances each wave incidence assumption can be valid in environments that entail millimeter-wave bands. Based on this, we may ensure the minimum necessary distance from the transmitter to the first diffracting obstacle for the convergence of the spherical wave incidence solution onto that of the plane wave with a relative error below 0.1%. Our results demonstrate that for less than four diffracting elements, the minimum necessary distance engages in quasi-linear behavior under variations in both the angle of incidence and obstacle spacing. Notably, the considered frequencies (60–100 GHz) have almost no bearing on the results. Our findings will facilitate the simplified, more accurate and realistic planning of millimeter-wave radio communication systems, with multiple diffractions across various obstacles.

In this paper, physical optical (PO) method is introduced and applied to calculate the echo data from a missile illuminated by plane waves with arbitrary aspects and different frequencies for inverse synthetic aperture radar (ISAR) imaging. The missile is approximated by component disassembly method (CMD) and concept of scattering matrix is introduced in order to simplify complexity of the numerical calculation. Generated images of several incidence angles are analyzed by signal processing theory and can precisely represent the mirror scattering region and the rotation of the target. The results of the simulation indicate that PO method is simple and effective to calculate the radar echo of a missile for ISAR imaging.