Range Of Aspect Angles Research Articles

Radar target classification of commercial aircraft

With the increased availability of coherent wideband radars there has been a renewed interest in radar target recognition. A large bandwidth gives high resolution in range which means target discrimination may be possible. Coherence makes cross-range resolution and radar images possible. Some of the problems of classifying high resolution range profiles (HRRPs) are examined and simple preprocessing techniques which may aid subsequent target classification are investigated. These techniques are applied to HRRP data acquired at a local airport using the Microwave Radar Division (MRD) mobile radar facility It is found that Boeing 727 and Boeing 737 aircraft can be reliably distinguished over a range of aspect angles. This augers well for future target classification studies using HRRPs.

The BEAN experiment - An EISCAT study of ion temperature anisotropies

Abstract. Results are presented from a novel EISCAT special programme, SP-UK-BEAN, intended for the direct measurement of the ion temperature anisotropy during ion frictional heating events in the high-latitude F-region. The experiment employs a geometry which provides three simultaneous estimates of the ion temperature in a single F-region observing volume at a range of aspect angles from 0° to 36°. In contrast to most previous EISCAT experiments to study ion temperature anisotropies, field-aligned observations are made using the Sodankylä radar, while the Kiruna radar measures at an aspect angle of the order of 30°. Anisotropic effects can thus be studied within a small common volume whose size and altitude range is limited by the radar beamwidth, rather than in volumes which overlap but cover different altitudes. The derivation of line-of-sight ion temperature is made more complex by the presence of an unknown percentage of atomic and molecular ions at the observing altitude and the possibility of non-Maxwellian distortion of the ion thermal velocity distribution. The first problem has been partly accounted for by insisting that a constant value of electron temperature be maintained. This enables an estimate of the ion composition to be made, and facilitates the derivation of more realistic line-of-sight ion temperatures and temperature anisotropies. The latter problem has been addressed by assuming that the thermal velocity distribution remains bi-Maxwellian. The limitations of these approaches are discussed. The ion temperature anisotropies and temperature partition coefficients during two ion heating events give values intermediate between those expected for atomic and for molecular species. This result is consistent with an analysis which indicates that significant proportions of molecular ions (up to 50%) were present at the times of greatest heating.

A Simple Correction to Physical Optics Solution for the Estimation of the Scattering Components of a Finite Metallic Cone

This paper describes the contributions of tip scattering, first-order rim diffraction and tip-rim interaction to the high-frequency scattering from a finite perfectly conducting right-circular cone. Empirical correction factors are introduced to the classical physical optics (PO) approximation. It is shown that the contribution due to tip scattering as obtained from physical optics approximation, with a correction factor included, closely matches with Felsen's result for small angle cones and for a wide range of aspect angle. It is also shown for axial incidence that the firstorder diffraction from the rim is expressible in terms of the PO integral after incorporating a correction factor. Axial radar cross section measurement results on narrow and large angle cones are presented and compared with the results of the modified PO result thus obtained in this paper and geometrical theory of diffraction (GTD). The contribution of the proposed tip-rim interaction is subsequently discussed and it is shown that this interaction is significant only for narrow angle cones. Backscattering measurement results on two cones over a wide range of aspect angle are also compared with the classical results of PO and GTD. From this comparison, it is inferred that for large angle cones, second order diffraction mechanisms contribute insignificantly when the incident wave is vertically polarized. This shows that PO can be a reasonably accurate analytical tool for large angle cones and for vertical polarization, except at axial incidence. At axial incidence, the correction factor introduced in this paper has to be used in the PO solution to make it match with experiment.

A modified strip grating with dual periodicity for RCS reduction

Elimination of specular reflection over a wide range of aspect angles for TE polarization by a strip grating surface of two-dimensional periodicity is reported. This overcomes the main disadvantage of conventional strip grating, which eliminates specular reflection for a single angle of incidence only. The structure reported here may find wide application in reduction of the radar cross section of targets. © 1994 John Wiley & Sons, Inc.

An alternative look at the aspect sensitivity of auroral radar backscatter: A forward scatter effect

A simple model for diffuse forward scatter is presented in order to account for the aspect angle dependence of auroral radar backscatter over a wide range of aspect angles as can be observed by UHF radars. The basic assumption is that the backscatter itself is highly anisotropic but that the incident rays are partially scattered by quasi‐isotropic small to medium scale irregularities in the bottom/top part of the E/D regions. The model is developed to provide a theoretical forward scatter/aspect angle relationship, which is compared with the observations. The model exhibits reasonable agreement with recent 440‐MHz Millstone Hill radar observations over aspect angles ranging between 0 and 10 deg. Other supporting evidence is also presented.

Experimental evaluation of time domain features of the backscattered field of finite cylindrical shells

The mid-frequency scattering properties of three different cylindrical shell models have been evaluated experimentally to study the influence of resiliently mounted internal structures and typical structural discontinuities such as endcaps and transverse stiffeners. The models consist of an empty shell, and unequally spaced ring-stiffened shell, and a ring-stiffened shell with resiliently mounted, wave-bearing internal structural elements. The measurements were conducted over a frequency range of 2.75<ka<10.5 corresponding to 3/4 to 3 times the ring frequency of the empty shell. The measurements were all conducted with the use of wide-band pulses yielding good time resolution of propagating wave effects. Various time domain representations of the scattered field are presented to illustrate the evolution of observed backscattering processes. The influence of induced helical membrane wave propagation on the shell, scattering at the ring stiffeners, and interaction of the shell and internal structures is readily observed in the time domain. Monostatic and bistatic measurements are compared to contrast the fields observed in the specular and back directions. Emphasis is placed on evaluation of the scattered field properties observed over a range of aspect angles within 30 deg of beam aspect where phase-matched excitation of membrane waves can be accommodated over the cylindrical portions of the shells. [Research sponsored by ONR.]

Height and aspect sensitivity of large aspect angle coherent backscatter at 933 MHz

During a special experiment in May 1988, coherent echoes were obtained with the UHF EISCAT radar system while the Tromsö antenna was directed to the north at elevation angles between 10° and 13°, with corresponding magnetic aspect angles between 5.8° and 7.6°. The echoes were found to be consistent with scattering from a layer having approximately a Gaussian height profile centered on about 103 km and with an average thickness of about 6 km. This height is considerably lower than most other reports of auroral backscatter height, and the thickness is also smaller than most other reported values. These unusually low heights were confirmed with observations at the remote sites Kiruna and Sodankyla. The heights seem to be independent of aspect angle over the range of aspect angles observed. By modeling the scattering layer and comparing echo intensities at Tromsö and the remote sites, estimates of aspect sensitivity of about 6 dB/deg were obtained, with an indication that the aspect sensitivity decreases with increasing aspect angle.

Inclusion of filter modulation in synthetic-discriminant-function construction

A technique in which the filter modulation is included in the synthesis of a synthetic-discriminant-function matched spatial filter (SDF MSF) is presented. In the filter synthesis, a system of simultaneous nonlinear equations is solved with an iteration procedure. A computer simulation of the new method using thresholded images of the Space Shuttle over a range of aspect angles was performed for phase-only filters (POFs) and binary-phase-only filters (BPOFs). The filters constructed are capable of obtaining the specified peak-correlation response to within 1% with a high signal-to-clutter-ratio for the one-class problem, the two-class problem, and the multilevel problem. In contrast, conventional projection SDF POFs and BPOFs are unable to produce the desired peak-correlation response.

Monostatic angular distributions and their relation to resonances in acoustical scattering from fluid loaded solid elastic objects

It is possible to detect resonances from backscattered echoes (form functions) by varying the nondimensional frequency kL/2 in small increments. This gives rise to a familiar pattern of nulls followed by a sharp rise for the spherical case and similar results for the spheroidal case. This approach, however, limits one to one aspect angle for three‐dimensional nonspherical problems and often entails tedious and time consuming calculations to find the often narrow resonance regions. Another approach suitable for three‐dimensional problems is to examine closely spaced monostatic angular distributions enabling one to look for possible resonances over a continuous range of aspect angles. This method is used to examine scattering from solid steel spheroidal shells. The results indicate that Rayleigh type resonances are aspect angle dependent in contrast to our recent results on spheroidal shells. The results are consistent with the standing wave interpretation of resonances proposed by use earlier for shells an...

Determination of the fluctuation level of ionospheric irregularities from radar backscatter measurements

The expression for the radar cross section for backscatter from elongated, field‐aligned ionospheric irregularities is used to find a general formula for the backscattered power received by any radar for a given rms electron density fluctuation level. The formula involves an integral of the three‐dimensional irregularity power spectrum over a volume defined by the beam width of the antenna and the pulse length. The three cases considered are where: (1) the irregularities fill the beam but the beam width measured parallel to the earth's magnetic field is so large that a wide range of aspect angles falls within the beam, (2) the irregularities fill the beam and the beam width is so narrow that the aspect angle is approximately constant, and (3) the irregularities fill only a small portion of the beam, over which the aspect angle is constant. The three cases are typified by the Goose Bay HF radar, the Jicamarca radar, and STARE‐type radars, respectively. An empirical formula for the power spectrum is used to obtain explicit formulas for the three types of radar. The Goose Bay radar works at a low enough frequency so that ionospheric refraction can be used to achieve good aspect sensitivity. The effect of ionospheric refraction and focusing is therefore included in the formulas for backscatter. The results of numerical computations are presented in order to compare the sensitivity of the Goose Bay radar with other types of radar and to estimate the levels of fluctuation needed to produce backscatter. Curves of signal‐to‐noise ratio versus range are plotted for the radar.

High Resolution Doppler Spectrum Measurements of 3-meter Auroral Irregularities at Large Magnetic Aspect Angles

Doppler spectral properties of 50 MHz auroral echoes observed at several magnetic aspect angles are presented. The measurements were made with high resolution CW Doppler systems at 10 different aspect angles ranging between 1 and 14 degrees, during a series of experiments carried out the summers of 1981, 1982 and 1983 in western Canada. The extensive data set was representative of several backscatter periods and different geomagnetic conditions. We found that three basic spectral types associated with different irregularity groups and classified as type 1, 2 and 3, existed throughout the aspect angle range. Type 1 and 2 spectra are reminiscent of those observed in both the equatorial and auroral backscatter at small aspect angles, whereas type 3 is a newly established spectral signature possibly related to electrostatic ion cyclotron waves. The evidence suggests that 3 m primary and secondary plasma waves can exist in aurora with k||/k⊥ wavenumber ratios perhaps larger than 0.25. Statistical analysis of the data showed the magnetic aspect angle effect upon the Doppler spectrum properties to be weak for all types of irregularities at 50 MHz. It is now clear that equatorial E region instability theories, which had been applied directly to aurora, need important modifications, and possibly the inclusion of field aligned current effects, to account for the magnetic aspect angle observations.

Empirical models for the plasma convection at high latitudes from Millstone Hill observations

Since 1978, radar observations of F region electric fields within the region 55° < Λ < 75° have been made from Millstone Hill (42.6°N, 71.5°W). Averge convection patterns have been calculated from the ion drift data gathered in 109 of these experiments conducted between January 1978 and August 1981. Most of the experiments lasted between 24 and 48 hours, and over 3,700,000 values of the line‐of‐sight velocity were determined and included in the averages. The observed velocities were sorted into “bins” of ½‐hour intervals of magnetic local time and 2° intervals of apex latitude. Each of these cells had been viewed by the radar over a wide range of aspect angles in the course of the 109 experiments, allowing the average vector velocity to be determined. The data were separated further into three levels of Kp, according to whether the interplanetary magnetic field (IMF) was “toward” or “away” from the sun, and by season. The average patterns are discussed and compared with earlier models based on satellite and incoherent scatter data. There is an expansion and intensification of the pattern with Kp for all seasons and IMF orientations. The polar cap entry and exit points of the plasma, the center of the cells, and the polar cap boundary all depend on the IMF. The Harang discontinuity, which can be seen clearly on individual days, is largely lost in the averages.

On ion acoustic plasma waves at large magnetic aspect angles in the high‐latitude E region of the ionosphere

A series of 50 MHz Doppler experiments has been performed during 3 consecutive summers, to investigate auroral backscatter properties at different magnetic aspect angles ranging from about 1° to 14°. In this paper the large data base is used to study a particular class of echoes, namely the ion acoustic‐type echoes. In the present understanding of radio aurora, this category of scatter, with characteristics reminiscent of equatorial type 1 echoes, is attributed to electrostatic plasma waves generated by the Farley‐Buneman instability. Linear theory predicts that the propagation vectors of such waves are limited to a range of angles within 0 to 1.5 degrees of perpendicularity to the magnetic field. Our observations, however, show that type 1 auroral echoes at 50 MHz occur regularly in the entire aspect angle range from about 0° to 14°. Therefore, the corresponding 3‐m irregularities could have finite wavevector components along the magnetic field, with ratios k∥/k⊥ as large as 0.25. On the average, the echoes appear to be weakly aspect sensitive at a rate near −1.5 dB/degree. The observed phase velocities decrease slowly with aspect angle at a rate of about 7 m/s per degree and, in general, have values in the nominal ion acoustic speed range of the plasma. The observations might be explained within the framework of the two‐stream instability, by invoking a very high anomalous electron collision frequency, since that would have a strong destabilizing effect at the larger aspect angles. At present, however, there is no theoretical justification for this interpretation and the aspect sensitivity of radio aurora remains an open and challenging problem.

Mirror antenna dual-band lightweight mirror design

The mirror antenna, sometimes called an inverse Cassegrain antenna, is used in modern radar systems where efficient wide angle scan is required in combination with light weight and small size. Although previous applications have been single band, an additional advantage of the mirror antenna is that it is practical to implement dual-band configurations which can greatly enhance the capability of radar or other radiating systems. The most difficult part of a dual-band mirror antenna design is the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90\deg</tex> polarization rotating mirror (half-wave plate). The challenge is to achieve effective performance at two bands and maintain that performance over a wide range of aspect angle. Basic mirror antenna operation and design and fabrication techniques for a dual-band half-wave plate mirror are described. The solution for design parameters is derived for perfect <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90\deg</tex> polarization rotation at a given aspect angle for each of the two bands. The design approach includes a process for optimizing the design for the full desired range of aspect angles. Practical fabrication considerations are discussed along with inclusion of the effects of capacitive susceptance of a protective mirror coating in the design theory. Theoretical performance is compared with measurements of mirror samples.

A CW Radar Cross-Section Measurement Facility in X-Band

A monostatic CW radar cross-section measurement facility in the X-band (8.2–12.4 GHz) at the Radar and Communication Centre, IIT Kharagpur, is described. This set-up is capable of automatically measuring the CW monostatic radar cross-section over the range of aspect angle 0° to ± 180° for both parallel and perpendicular polarizations. The transmitting/receiving antenna and the rotating target is housed on the roof-top of the centre and the microwave circuit with the recording arrangement is in one of the airconditioned rooms. The set-up is capable of handling a target of arbitrary shape of maximum size equal to 70 cm and uses the two-stage background cancellation technique. A typical value of effective isolation between transmitted and received signals is of the order 60 dB and a dynamic range of 35 dB. The set-up is described in considerable details and measurements made for different types of targets are compared with theoretical results obtained by analytical investigations either by the authors or tho...

High‐Frequency Backscattering From a Finite Cone

The high‐frequency backscattered field produced by a plane electromagnetic wave at oblique incidence on a perfectly conducting, right circular cone with a flat base is considered. The first two terms of the asymptotic expansion are obtained by applying the geometrical theory of diffraction; these terms reduce to results derived earlier in the particular case of a circular disk. Owing to the axial caustic, functions must be introduced to match the wide‐angle formulas to the known results for nose‐on incidence. This matching is effected by employing Bessel functions and Fresnel integrals for the first‐ and second‐order terms, respectively. The resulting expressions are valid for all cone angles and for a wide range of aspect angles about nose‐on; they are also found to be in good agreement with experimental data.

High-frequency backscattering from a metallic disc

The paper treats the backscattered field produced by a plane electromagnetic wave at oblique incidence on a perfectly conducting circular disc. The geometric theory of diffraction is used to obtain the first two terms of the asymptotic expansion at high frequencies. The first term is attributable to direct scatter from the edge of the disc and the second term arises from rays that cross the disc once. It has been found that the ray geometry is fixed for one polarisation but is aspect-dependent for the other. Owing to the axial caustic, functions must be introduced to match the wide-angle formulas to the known results for axial incidence. Bessel functions are used for the 1st-order terms and Fresnel integrals for the 2nd-order terms and evidence is presented to support this choice of function. The resulting 1st-order expression is in reasonable agreement with experimental data, and may be adequate for many purposes. However, the expressions incorporating the 2nd-order terms reveal the polarisation dependence of the scattering, and are in excellent agreement with measured data over a widt range of aspect angles.

Transient radiative cooling of a semi-infinite solid with parallel-walled cavities

A semi-infinite solid of blackbody material is separated into slabs by periodically spaced ininfinitely deep cavities bounded by plane parallel walls. The solid material is brought to uniform initial temperature and subsequently is allowed to cool by conduction in the interior and by radiation at the surfaces. The temperature history of the solid with cavities was calculated numerically. Similar calculations were performed to obtain the rate of cooling of the same material without cavities. Comparison is made between the apparent brightness temperatures measured at a great distance by a thermal radiation detector with a resolving power insufficient to discriminate between a smooth surface and one with cavities. It is shown that over a wide range of aspect angles the solid with cavities may have a higher apparent temperature even when the top surfaces of the slabs are cooler than the surface of a homogeneous solid. Phenomena such as the one described may be useful in the interpretation of measurements of infrared radiation from the lunar surface.