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An accurate and efficient method to compute the scattered fields in the target zone of a compact range main reflector is presented in this paper. This method is valid for reflectors of arbitrary rim shape with convex rolled edge terminations. The method is based on the uniform geometrical theory of diffraction (UTD) where the diffraction coefficients are obtained numerically using a procedure involving a physical optics line integration. Results obtained using the numerical UTD (NUTD) are compared to those obtained using UTD and corrected physical optics surface integration solutions for reflectors with both unblended and blended rolled edges. It is shown that the results are in good agreement. In addition, the NUTD is much more efficient than the traditional physical optics surface integration and provides diagnostic information on the effects of individual scattering mechanisms.
A new type of rolled-edge compact range reflector was designed and built by McDonnell Douglas Corporation. To minimize diffraction, the reflector contour was designed such that the surface radius of curvature and all its derivatives are continuous everywhere. This was accomplished by summing a parabolic function and two hyperbolic functions which have appreciable magnitude only in the edge-roll region. The bottom edge was treated using serrations.
A Physical optics (PO) analysis of serrated edge reflectors is presented. It is shown that to obtain the true scattered fields in the target zone, one should use PTD (physical theory of diffraction) along with the PO solution. Using PTD, scattered fields of various serrated edge reflectors are presented. From these scattered fields, one can see that by proper design of the serrations, the edge diffracted fields can be reduced in the target zone. The edge diffracted fields, however, still may be too large for certain applications.
A novel technique is presented for the determination of the quiet zone field distribution of compact range antennas with serrated edges. The main reflector has a linearly serrated rim, so that the rim projection onto the reflector aperture plane is an arbitrary polygon. Additionally, the reflector aperture field is uniform in both amplitude and phase, and can therefore be expressed as a window function. The plane wave spectrum of the aperture field can then be obtained in closed form. Next, the spectrum is expressed at a plane in the quiet zone and the field is obtained by implementing an inverse fast Fourier transform (FFT) algorithm. Quiet zone field distributions are computed for various serrated rim configurations.
Limitations of current compact range reflector systems are discussed with an emphasis on the diffracted energy and its effect on quiet zone size and quality. A new prime focus reflector design which minimizes the edge diffraction is presented. Computer predicted performance of this design is contrasted with measured field probe data.
This paper evaluates the RCS errors associated with measuring a large flat plate which is illuminated by a compact range reflector with significant edge diffraction stray signals. This is done by evaluating the true fields incident on the plate and then using a physical optics technique to predict the backscattered fields. Results are compared with and without the edge diffracted fields present. A simple analytic expression is developed which can approximate the size of this potential error.
In this paper, a general methodology for data reduction and analysis of wide-band RCS data is discussed. This methodology encompasses normal image processing, clutter removal, and noise filtering. Examples of the usefulness of the approach are presented.
Image gating or editing is often used to determine the effect of an isolated scatterer on the RCS in the frequency and aspect-angle domain. In this paper, theoretical computations indicating limits in the image-gating procedure will be presented. The process provides the image-gating capability in combination with phase-corrected (focused) imaging. Targets consisting of two-point scatterers with well-known RCS response have been used. One of the scatterers is gated out and the resulting RCS versus frequency or aspect angle is determined and compared with its theoretical value. Limits in terms of minimum bandwidth or minimum distance in resolution-cell sizes are defined. The influence of several gate shapes and windows have also been examined. Experimental investigation has been carried out in order to verify the theory.
Under many circumstances it is necessary to experimentally estimate the radar cross section of targets in a cluttered environment. A significant reduction in the clutter can be obtained when cross range filtering can be done. In this experimental RC measurement concept, scattering measurements are performed using a moving radar antenna. Thus scattering as a function of target plus clutter versus aspect angle in the near field can be measured. Next, a back projection algorithm can be used to estimate the scattering as a function of position in the neighborhood of the target. The known range to which the signal is to be focussed is used to project back to the target area. An estimate of the RCS at points along a line in the plane of the target is computed. The clutter responses can then be removed from the data, and the remaining target-only values projected forward again (possibly to the far field) to estimate the RCS of the target alone.
This paper describes two signal processing techniques that have been used to overcome specific problems in a Lockheed Aeronautical Systems Corporation (LASC) indoor compact RCS measurement range. Both techniques are post processing techniques used to enhance the accuracy of RCS vs. Aspect measurements. These two techniques can speed up measurement time, increase measurement accuracy, and increase target sizes on a compact range.
An RCS measurement error model, calibration procedure and correction algorithm are discussed. A distinction between frequency response reflections and range-target reflections is made. Special emphasis is placed on the selection of the gate span with time gating used with the calibration and test target measurements. Mathematical simulations and actual measurements illustrate the discussion. It is concluded that frequency response related reflections must and range-target reflections must not be included in the gate for the frequency response calibration measurement.
Images of an open box, closed box, and open and closed box on a ground plane were taken at the Hughes/Motorola Compact Range. Comparison of these images show the effect of multiple reflections in the image of an open box. A simple analytic/computer model was developed to interpret these multiple images. Data and analysis are presented on the various mechanisms that come into play in scattering from the open/closed box and the ISAR images generated as a function of the viewing angle for the box.
The reflectivity levels of the three anechoic chambers of TKK and VTT were measured at 110 GHz. The sidewall reflections were measured by the free space voltage standing wave ratio method. Typical values measured with 20 dB pyramidal horns were below -60 dB at azimuth angles less than 20 degrees and about -50 dB at angles larger than 50 degrees. When a waveguide end was used as the transmitting antenna, the reflectivity level was nearly 10 dB higher. The backwall reflections could be measured directly because the reflected field was much larger than the direct field. The maximum backwall reflection varied in the three chambers from -33 dB to -36 dB.
Serrations are used on Compact Antenna Test Range reflectors to reduce the effects of edge diffraction. It has been found that the traditional triangular shape for these serrations is not optimal and that more continuous shapes should perform better. To verify this, RCS measurements were performed on test targets consisting of strip reflectors terminated by end sections of various shapes. The RCS vs. angle data were corrected for the field irregularities caused by the measurement range and then converted to the induced current distributions on the targets, from which the fields in front of the targets were calculated using Physical Optics. These fields are equivalent to the test-zone fields of an actual Compact Range. The results are compared with theoretical data. The agreement is good.
To fulfill the future demand of highly accurate antenna-, RCS- and payload testing, MBB built a new antenna test centre at Ottobrunn (Ref. 1). This paper describes the development and qualification of the large, dual reflector Compact Range (CR) which has a plane wave zone of 5.5 x 5.0 x 6.0 m (w x h x d). It starts with the results of a detailed electrical trade-off study between different CR-concepts, followed by some mechanical/thermal construction aspects of the large, highly accurate reflectors. Finally, some qualification results are shown, covering the frequency range from 3.5 GHz up to 200 GHz (lowest frequency of operation approx. 2 GHz). The achieved plane wave performance (amplitude ripple ±5o, phase ripple ±5o, cross-polarization isolation > 40 dB) verifies the high quality overall system design.
In the theory of inverse synthetic aperture imaging of radar targets the measuring distance is ordinarily supposed to be very much larger than the dimensions of the target. If this is not the case errors are introduced. We study these errors and means to decrease their influence by computation. The result is that the maximum tolerable target dimension can be substantially increased in a plane perpendicular to the axis of rotation if error correction is used.
A technique is presented for the measurement of antenna patterns, in which a long, thin wire is moved past the antenna aperture while the changes in reflection coefficient at the antenna feed are recorded. By suitable processing of these data, the antenna pattern can be calculated.
Conventional receivers for pulsed radar systems employ a wideband final filter that is matched to the pulse width and risetime. However for pulsed RCS measurements on small test ranges, instrumentation receivers with narrow IF bandwidth have proven useful. This paper analytically examines the differences between narrowband and matched filter instrumentation receivers and describes typical conditions under which gated CW measurements are made. Useful relationships between PRF and IF bandwidth are derived.
Currently many new compact range facilities are being constructed for making antenna pattern measurements indoors. Limited suppression of stray signals ~ due to range layout, confined surroundings and residual absorbing material reflectivity ~ represents a limitation on the accuracy of the measurements made in these facilities. Time-gating of the compact range signal appears to be a very attractive technique to reduce unwanted reflections. The authors have carried out an experimental investigation of time gating in a compact range. It is demonstrated that time-gating can improve the uniformity of the aperture field by removing the feed backlobe radiation; and, it is demonstrated that time-gating can remove the effects on a pattern of certain room reflections and of feed backlobes. When compared to conventional methods of reducing reflections based on placement of absorber, time gating appears equivalent. It does not appear however that time gating improves the conventional methods, except for measuring wide beamwidth antennas.
A technique to determine the radiation centers of large reflector antennas in a given direction is presented in this paper. Coherent processing is used to determine various radiation centers based on far zone pattern data of the antennas provided that adjacent centers are separated far enough so that their locations can be resolved. Numerical results for processing of two reflector antennas, a prime focus fed and a Cassegrainian, are presented to validate this technique. The diagnostic value of this technique for reflector antennas is demonstrated by processing the actual measured pattern and identifying some unexpected radiation centers. One can also use this technique to fine tune numerical pattern simulations of reflector antennas.