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This paper considers hardware and software issues associated with accurate RCS, antenna, and near field antenna measurements. In particular we examine methods for making accurate measurements at high speed using existing network analysis equipment, such as the HP8510B. Techniques which allow for fundamental mixing are examined from the viewpoint of enhanced dynamic range and speed. Harmonic mixing techniques are also discussed and limitations related to IF bandwidth and harmonic locking are presented. The realtime requirements of software systems for these applications are presented and operating system considerations are analyzed. Interface attributes are examined with a view toward use with multi-tasking operating systems and the real-time requirements of high speed measurement systems.
The MUSIC (Multiple Signal Characterization) algorithm uses an eigenvector decomposition of measured data to classify signals in the presence of noise. It has been used for the angular classification of multiple radar signal emitters and ISAR imaging. Interest has grown in stray signal analysis in anechoic chambers. This paper will discuss the modification and use of the MUSIC algorithm for the decomposition of field probe data to angular spectrum. A brief discussion of the MUSIC algorithm theory will be presented. Modifications required for use in compact range angular spectrum analysis will be discussed in detail. Requirements on field probe measurements will be presented as well as their effects on the implementation of the algorithm. Both one way and two way measurements are considered for their relationship to the array manifold. Finally, some experimental validation generated on the Boeing range will be presented.
Accurate calibration methods are of essential importance in RCS measurements. First, absolute RCS determination (in dBsm) can be carried out accurately provided a correct algorithm is used describing the RCS dependence of some reference target at all frequencies. Unfortunately, this technique gives error-free calibrated data at one position only. In this paper a new technique for qualifying of RCS ranges will be described. A reference target with well-known RCS response is used during the calibration measurement. The amplitude and phase distributions are then computed for all required positions within the test zone. Finally, an error estimate in measured RCS responses can be made by using two other application programs.
In an industrial park, a clear area sufficient for antenna measurements is very hard to find and even more difficult to justify. The solution to this problem was to use the roof of a large industrial building. To avoid the reflections from industrial stacks used for air conditioning and spray booths and to provide a flat surface sufficient for good ground reflection operation, an elevated ground reflection antenna range was constructed. The range consists of a ground screen 40 feet by 110 feet mounted on a metal framework 14 feet high. A telescopic source antenna tower is located at one end of the range, and an azimuth-over-elevation antenna positioner with a model tower is located on a raised platform at the other end of the range. The range was evaluated using a lightweight field probe and the experimental data compared to calculated data derived from the NEC-BSC2 computer code. An analysis was made of the probe data defining the sources of extraneous energy and their possible reduction. Pattern comparison data is given to illustrate the correlation between the field probe data and the actual uncertainty experienced in making UHF antenna pattern measurements on the elevated ground reflection range. Finally, planned physical improvements to the range are discussed.
At Southwest Research Institute, an automated antenna performance evaluation system has been developed for evaluation of mast-mounted direction finder antennas. This system utilizes a dual-channel receiving system and IF processor with off-line antenna pattern analysis software. Antennas are mounted on a test range which includes computer-controlled antenna positioners, test frequency transmitters, and a data acquisition equipment group. Amplitude and phase data is digitized and recorded for automated off-line antenna performance evaluation. The evaluation software provides a Fourier analysis of the antenna patterns which characterize distortion, alignment, relative phase relationships, amplitude mismatch, and bearing deviations (from theoretical values) for each antenna array.
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 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.
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.
For transforming a Fresnel region pattern to a far-field pattern, we present here two methods, the "discrete beam sampling" method (DBSM) and the "displaced beam" method (DBM), which allow an accurate characterization for both linear as well as circular antenna apertures. Both methods assume a simple Fourier transform relationship between the aperture field distribution and the far-field of the antenna. The Fresnel region field is then essentially perturbed by an aperture quadratic phase error assumed to exist because of the finite distance at which the field pattern is characterized. Numerical simulation and its results are presented to show the accuracy of the reconstructed far-field data. Finally, an error analysis is performed to show the sensitivity of the above two methods.
A flexible near-field antenna test-facility is presented. This system gathers all that is necessary to design, to debug and to validate the high performance antennas which are made by ESD. ARAMIS has been operational since January 1988. Its applications are: - Near-field measurements (for diagrams): * planar, * cylindrical. - High speed field mapping (for default analysis): * planar radiating surface, * cylindrical radiating surface. - Generation of element excitation (active phased array testing): * planar antennas, * cylindrical antennas. - Direct far-field measurements (probes, small antennas), - Circuit measurement (S parameter). The facility features a specially designed scanner. Thanks to its six degrees of freedom, this positionner allows the differents types of measurements to be made. The instrumentation is based upon the HP 8510 B network analyzer. A single computer performs the measurements, transforms the data and presents the graphics (linear diagrams, color maps, three-dimensional colored projections). In order to grant a high scan speed, the system uses the FAST CW mode of the HP 8510 B. An external trigger is provided during the motion process of the probe. A rate of 500 measurements/sec. has been proved. This on-the-fly process is clearly depicted. Experimental results are presented which include: - Low sidelobe (-38 dB) antenna diagrams. - Default analysis through: * Amplitude mapping (leakage short-circuit in a microstrip antenna). * Phase mapping (out-of band comparison between two radiating element technologies). * Measurement of excitation laws. * 3-D transformation. - Simultaneous on-the-fly acquisition of up to three antenna outputs.
A stressed-skin inflatable target support provides an improvement over a foam column for radar cross section (RCS) measurements in an anechoic chamber. Theoretical analysis indicates that backscatter from the support is minimized because its mass is reduced below that of a foam column and is distributed to favor incoherent scattering. Compared with a foam column, a pressurized thin shell has superior mechanical stability under both axial and transverse loads. Experimental observations using Mylar -- a low dielectric constant, high tensile strength film -- confirm these results. Spurious reflections from rotational machinery located below an inflatable column are reduced by a layer of absorber within the base of the inflatable support.
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited. This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence. Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.
In practical ISAR applications the quality of the image obtained depends upon the distortions in the wavefront illuminating the target, effects introduced by the radar-target path, the accuracy of the angle and frequency steps used in obtaining the data, vibration, and multiple reflections from neighboring objects. Results of analysis, simulation and data obtained in an RCS compact range are presented to quantify the relationships of the image degradation introduced by these effects.
A simple computer program which performs a direct calculation of the Discrete Fourier Transform (DFT) was written to generate the plane wave spectrum of a compact range from sampled field probe data. This program was used to analyze idealized quiet zone fields, computer predicted quiet zone fields and measured field probe data. Data from this analysis is presented along with suggestions for the correct interpretation of the results.
The use of serrated edge treatment in the design of a compact range collimating reflector is one method of mitigating the effects of edge diffraction on quiet zone performance. In this note a physical optics analysis is applied to the serrated reflector. The computational procedure is described and several results are presented. In particular, computed results are presented for the S-A Model 5755 compact range reflector and compared with experiment.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
NRL arches have been used to measure the reflection properties of material samples. The transmit and receive horns in the arch fixture are oriented to obtain the "specular" reflected field from the material sample. In actuality, the measured scattered field also has components which emanate from the edges of the material sample due to diffraction. This behavior is confirmed with moment method and geometrical theory of diffraction (GTD) calculations. Although edge diffraction has a different scattering behavior than does specular reflection, the GTD diffraction coefficients simulate specular scattering near the reflection shadow boundaries. These diffraction coefficients contain the material's specular properties by incorporating this information through the specular reflection coefficients for the material and geometry. Hence proper sample mounting is required to insure an adequate measurement of the material properties, since the edges dominate the measured scattered field.
The ISAR image is a domain that possesses many of the spatial physical characteristics of the target. Certain procedures can be performed in the image domain that are equivalent to physical operations on the target. These operations include the ability to modify the amplitude of the scatterers that are represented in the image and, after performing these modifications, subjecting the image to an inverse transformation that recovers RCS data of the whole body as a function of frequency and aspect angle. The RCS plots obtained by transforming the edited image are representative of similar modifications made to the physical body and are of value in eliminating the need for many model modifications and retests in low-observable model development. This paper describes, using simulated and actual target data, some of the procedures that can be fruitfully applied in this type of analysis.
Modern analysis techniques of radar scattering data or radar cross section (RCS) data often include transformation to the time domain for the purpose of understanding the specific scattering mechanisms involved or to isolate or identify specific scattering points. The classic technique is to transform from the frequency domain to the time domain using an inverse (Fast) Fourier Transform (IFFT). Often, however, the scattering centers are too close together to resolve or the requirement for accuracy in the measurement of the differential time delay is too high given the IFFT inverse bandwidth. This paper presents a technique for determining the time domain response of a radar target by processing the data using modern autoregressive (AR) spectral analysis. In this technique, the scattering from a radar target in the high frequency regime is shown to be autoregressive. This paper will show examples using the maximum entropy method (MEM) of Burg.
The weakest link in antenna metrology is the antenna range itself. Unknown reflections can cause large errors in antenna measurements and can change unpredictably. Conventional range probing methods typically provide a go/no go test with very little information about the location of the range scatterers. A number of techniques show promise for locating antenna range scattering centers. This paper describes the theory of a probe analysis method being implemented at Lockheed Missiles and Space Company. The method is based on planewave spectral analysis. A specialized probe system to test the planewave spectral theory will be described.