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A microwave diversity imagery based on parametric modeling of back scattered signal versus frequency and aspect is presented. Forward-backward linear prediction is used to compute the model parameters. After stabilizing the corresponding transfer function, data are extrapolated to adjacent frequencies or aspects. Superior range- and/or cross-range resolution can be obtained by using frequency- and/or aspect-extrapolated data. Cross-range resolution can also be enhanced by extrapolating the frequency data and using data at a higher center frequency. For severly (sic) restricted viewing angles, or for small radar bandwidth, the new imagery can significantly improve the image resolution.
This paper describes an effort to evaluate the effect on RCS of base closures on a metallic frustrum at various depths with conducting and electrically isolated plugs. The tests were conducted at Sandia National Laboratories using System Planning Corporation's (SPC) Mark IV radar from 8 to 18 GHz, in the step chirped Inverse Synthetic Aperture Radar (ISAR) mode. Data reduction was performed on Information Systems and Research's workstation using the KNOWBELL software package. The workstation allowed the study of the imagery data in many different modes, which assisted in determining ways to evaluate RCS matching.
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.
Traditional techniques for evaluating the performance of anechoic chambers, compact ranges, and far-field ranges involve scanning a field probe through the quiet zone area. Plotting the amplitude and phase ripple yields a measure of the range performance which can be used in uncertainty estimates for future antenna tests. This technique, however, provides very little insight into the causes of the quiet-zone ripple. NSI's portable near-field scanners and diagnostic software can perform quiet-zone measurements which will provide angular image maps of the chamber reflections. This data can be used by engineers to actually improve the chamber performance by identifying and suppressing the sources of high reflections which cause quiet-zone ripple. This paper will describe the technique and show typical results which can be expected.
In recent years, a need has arisen to measure the patterns of high gain antennas having ultra low sidelobe levels (ULSA), usually installed on aircraft platforms. The objective is to measure antenna patterns which include the platform multipath, generated by reflections off the aircraft, but which exclude the effects of range multipath. These advanced antennas require testing capabilities to about 60 dB below the maximum level. Moreover, the range multipath is at times expected to be stronger than the direct path if the mainbeam antenna is pointed below the horizon. Aggravating the problem is the fact that usually CW antenna patterns are desired. ATEK developed an innovative technique which accomplishes the required tasks using the following concept; A 20 MHz signal is used to generate desired weight taps needed to cancel the received signal. Since part of these taps are used to cancel the desired signal, it has been shown that they can be used separately, following the adaptive process to measure the antenna patterns with the multipath suppressed. The technique can be used to measure very accurately antenna patterns in response to either CW or non-CW signals.
Near field range design includes the placement of RF absorber in the test area. Absorber placement depends highly on the antennas being tested. A common approach is to design an expensive low-reflection chamber around the near-field scanner. The chamber and the additional floor space can sometimes cost more than the near-field scanning system itself. Another approach seeks to identify multi-path reflection to minimize cost by optimally placing absorberto meet specific antenna test requirements. The results is a lower cost range using less floor space. This paper describes a technique of evaluating near-field range multi-path.
Spherical probing is a technique for detecting extraneous field sources in an antenna range equipped with spherical scanners. In spherical probing the extraneous field sources are detected by taking a plane wave spectrum of the measured range field. The angular location of sources in the range is given by the location of the amplitude peaks in the spectrum. The peak created by the range antenna will obscure some of the extraneous field source peaks. In this paper, a new technique for removing part of the range-antenna peak and improving extraneous source detection will be presented.
Linear probing is used to evaluate test zone quality and detect extraneous field sources on fixed-line-of-sight far-field and compact antenna ranges. Field probing along a line allows the measurement and meaningful display of range field amplitude and phase taper. Since positioners used with far-field and compact ranges are spherical, linear probing requires extra equipment, namely a linear scanner. This paper will present a new technique for generating linear probing data from measurements made with the existing spherical positioners. The steps necessary for implementing this new technique will be presented and demonstrated using measured data.
This paper reports on Far-Field Spherical Microwave Holography (FFSMH), currently being researched at Georgia Tech. Microwave Holography is a technique for evaluating complex electric fields near the field sources. Planewave Microwave Holography involves the use of the planewave spectrum and is the most common technique in use. Spherical Microwave Holography involves the use of a spherical expansion of Maxwell's equations and is the topic of this paper. Spherical Near-Field Microwave Holography (SNFMH) has been successfully used to locate and identify defects in radome walls, and to determine antenna aperture distributions. FFSMH differs from SNFMH only in the location of the measurement surface. FFSMH uses a far-field measurement surface and SNFMH used a near-field measurement surface. Progress in the definition of resolution limits for Spherical Microwave Holography is reported. FFSMH is demonstrated and results are compared to SNFMH and Planewave Microwave Holography
The Antenna Metrology group of the National Institute of Standards and Technology (NIST), working in cooperation with McClellan Air Force Base (MAFB), Sacramento, CA, have examined-measurement techniques to test a large phased-array antenna using planar near-field (PNF) scanning. It was necessary to find methods that would be useful in both field and production testing and could provide gain and diagnostic information in a simple and timely manner. This paper will discuss several aspects of the PNF measurement cycle that impact effective testing of the antenna array. These aspects include the use of a polarization-matched probe, the effect of scan truncation both on the transform to the far field and the transform to the aperture plane, and use of gain prediction curves as a diagnostic tool.
A phased array antenna has been considered the favorite candidate and been developed for land mobile satellite communications. However, in communication experiments, a noise level in a receiving frequency band is found to be increased when a signal is transmitted. The amount of noise increase is found to depend on a scanning angle in azimuth directions to track the satellite, and the value is up to 20 dB in maximum and 5 dB in minimum. The noise increase was found to be caused by an nonlinear (sic) effect of a PIN diode, which are essentially used in phased array antennas.
Despite their high cost, phased array antennas are becoming popular for radar applications because of their ability to provide reliable information even in a hostile environment. Evaluation of these antennas requires parameters like gain, radiation pattern, beam width, sidelobe (both near and far off) azimuth and elevation null depth, etc. to be tested over the entire range of frequency spots and scan angles. Typically, if the number of frequency spots are 24 and the number of beam positions for which the measurement has to be done are about 100, then the total number of measurements needed to generate the required data are 7200. In addition, phased arrays with a space feed have to be initially collimated at all the spot frequencies. The outdoor testing of these many parameters may not be convenient, and at times it may even be impossible. The planar near field measurement technique provides a systematic and accurate method of measuring large array antennas for all the required parameters.
The National Institute of Standards and Technology (NIST) has written a certification plan to ensure that a proposed planar near-field facility is capable of measuring high-performance phased arrays. Generally for a complete plan, one must evaluate many aspects including scanner alignment, near-field probe alignment, alignment of the antenna under test, RF crosstalk, probe position errors RF path variations, the receiver's dynamic range and linearity, leakage, probe-antenna multiple reflections, truncation effects, aliasing, system drift, room multipath, insertion loss measurements, noise, and software verification. In this paper, we discuss some of the important aspects of the certification plan specifically written for the measurement of high-performance phased-array antennas. Further, we show how the requirements of each aspect depend on the measurement accuracies needed to verify the performance array under test.
A Precision Optical Measurement System (POMS) has been designed, constructed and tested for tracking the position (x,y,z) and orientation (roll, pitch, yaw) of models in Boeing's 9-77 Compact Radar Range. A stereo triangulation technique is implemented using two remote sensor units separated by a known baseline. Each unit measures pointing angles (azimuth and elevation) to optical targets on a model. Four different reference systems are used for calibration and alignment of the system's components and two platforms. Pointing angle data and calibration corrections are processed at high rates to give near real-time feedback to the mechanical positioning system of the model. The positional accuracy of the system is (plus minus) .010 inches at a distance of 85 feet while using low RCS reflective tape targets. The precision measurement capabilities and applications of the system are discussed.
The Georgia Tech Research Institute has designed and built a field probe for the U.S. Army Electronic Proving Ground Compact Range. The field probe is an R-0 scanner covering a 59-foot diameter area. It includes a laser-referenced Z-axis correction servomechanism, a polarization positioner, and a cable handling system for one-way data acquisition.
This paper presents a feasibility study of the productivity improvements that are possible for the production test of the E2C antenna, using multiple-parameter, multiple-frequency measurement techniques. The measurement requirement for the antenna are presented along with the current measurement times. A multiple-channel, multiple-frequency measurement technique is described which will greatly reduce the measurement times. The new measurement times are calculated, and used to determine if the productivity improvements are justified financially. An economic analysis is include also (sic), which examines the financial impact of the improved productivity, and compares this to the cost of implementing the new measurement system. The financial analysis calculates the payback period, return on investment, net present value, and internal rate of return.
Data collection is increasingly becoming the limiting factor in overall antenna and RCS measurement time. An equation for data collection time for multiple parameter measurements is presented along with and ordering function for determining the optimum nesting order for parameters. An example is used to demonstrate measurement speed enhancement techniques, reducing data collection time by 65 percent. Changing from stepped to linear near-field scanning reduced collection time by 75 percent.
Many current, and near future, antenna and Radar cross section measurement requirements dictate improvement in microwave power amplifier performance and capabilities. Maturing of Traveling Wave Tube (TWT) technology and breakthroughs in modulator and power supply design now enable exploitation of the maximum possible RF performance from TWTs.
An alternate method is presented for computing far-field antenna patterns from planar near-field measurements. The method utilizes near-field data to determine equivalent magnetic current sources over a fictitious planar surface which encompasses the antenna, and these currents are used to ascertain the far-fields. An electric field integral equation (EFIE) is developed to relate the near-fields to the equivalent magnetic currents. Method of moments (MOM) procedure is used to transform the integral equation into a matrix one. The matrix equation is solved with the conjugate gradient method (CGM), and in the case of a rectangular matrix, a least squares solution for the currents is found without explicitly computing the normal form of the operation. Near-field to far-field transformation for planar scanning may be efficiently performed under certain conditions by exploiting the block Toeplitz structure of the matrix and using CGM and Fast Fourier Transform (CGFFT) thereby drastically reducing comparison and storage requirements. Numerical results are presented by extrapolating the far-fields using experimental near-field data.
A theoretical comparison for the application and derivation of modal expansion and integral equation methods is presented. It is shown that one formulation can be transformed into the other one using Fourier transform. From this point of view it can be stated that both method solves the same integral equation but for the modal expansion approach the integral equation is solved in the spectral domain while for the integral equation method the same equation is solved in the space domain. It is shown that for most of the practical antenna types the integral equation method gives more accurate far-field estimation than the modal expansion method, particularly in the planar scanning case.