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RCS measurements with frequency and angle diversity offer the benefit of spatial resolution obtained by synthesizing the equivalent of short pulses and large apertures. Recent research in several specialized fields has been directed to spectral estimation techniques which seek to maximize the achievable resolution beyond limits imposed by traditional Fourier methods. These techniques, known as: autoregressive modeling, linear predictive modeling, super-resolution, or maximum entropy, offer the possibility of enhanced resolution and band-limited signal extrapolation. The methods apply to situations in which an estimate of a required feature is derived from an incomplete set of measured data linked to the required feature by a known relation. In RCS applications, spatial distributions of reflectivity are linked to measured band-limited frequency responses by a Fourier transform. Maximum entropy methods, therefore, apply directly to the objectives of increasing spatial resolution or extrapolating band-limited frequency responses.
This paper addresses measurement requirements for space communication antennas and identifies antenna parameters most influenced by indoor compact range quiet zone quality. These parameters include sidelobe level, beam pointing, and gain. The compact range mechanisms limiting measurement accuracy are identified and discussed. Proven methods for characterizing quiet zone performance are described and demonstrated through illustration and example. Analysis is presented which related quiet zone quality characteristics to antenna measurement accuracy. The paper summarizes typical measurement results and error levels achievable for modern compact range systems. Methods for improving compact range performance for satellite antenna testing are also presented.
The Slotline/Bowtie Hybrid (SBH) antenna concept has been applied to develop an ultra wide bandwidth feed for compact range applications. The initial design requirements were to develop a feed with a 30 degree 1dB beamwidth from 1 to 18 GHz. It was felt that one could sacrifice the beamwidth at the lower frequencies somewhat because that would reduce the feed spill over which is normally the worst at lower frequencies. The resulting antenna has an 18" by 18" aperture and basically meets the bandwidth requirements. In the worst case, it has 2 dB variation across the desired 30" beamwidth. The phase center is relatively constant, and VSWR is basically less than 2:1 from 1 to 18 GHz. Measured and calculated results are shown to illustrate the performance of this new feed antenna. In addition, the measured amplitude and phase patterns have been input to a reflector analysis code to predict the field probe data in the simulated quiet zone. These results clearly show that this new feed performs very well from 1 to 18 GHz.
Considerable attention has been paid in recent years to the interpretation of measured field probe data in order to locate and quantify error sources present in the quiet zone of a compact range. This paper describes a new general purpose software package for that analysis. This software has been written to analyze data acquired in a plane-polar configuration. Analysis options include raw data analysis, near-field focusing of single or multiple line cuts, and plane wave spectrum propagation. A graphical user interface gives the operator extensive control over analysis and display parameters. The analysis algorithms used for multiple-cut processing can function with as few as two radial line cuts.
The simultaneous illumination of the Quiet Zone by number of beams is helpful and cost-effective for broadband antenna and RCS measurements. For an application such as, for instance, Electronic Warfare development, the use of scanning beam or multiple beams gives more extensive opportunities for designers. When the antenna-under-test is small in size, the lightweight and small single reflector Compact Range is very well suited for the above applications. Such a Compact Range being moved within the test facility (anechoic chamber or outdoor range) provides additional flexibility for the tests. This paper describes the development of a small Compact Range with a rolled edge reflector and a two-foot diameter Quiet Zone. Analysis of the Compact Range is performed for different feed positions, providing the beam scan in elevation and azimuth with respect to on-axis beam.
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.
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.
Implementation of planar near-field (PNF) technology has become more practical in recent years due to the availability of turn-key measurement systems. McClellan AFT (SM-ALC) has developed an automated PNF measurement system by re-configuring a sonar immersion tank positioner. Modifications to the hardware and software have produced an integrated PNF scanner capable of accurate gain and diagnostic measurements. This paper describes the evolution of the SM-ALC near-field measurement system from proof-of-concept diagnostic scanner to a production tool capable of repeatable gain measurements. Analysis of accuracies, limitations, and processing capabilities is provided. Comparative analysis of data for a transfer standard antenna measured on the SM-ALC measurement system and the PNF measurement system at the National Institute of Standards and Technology is also included.
For the last few years, the Periodic Moment Method (PMM) has been used to analyze the scattered fields from an infinite absorber wall. Using this approach the absorbe4r can have different periodicities in the x and y directions, as well as arbitrary shapes and any dielectic (sic) distribution. This makes this analysis method very general such that it can treat any conventional wedge or pyramidal designs. Plus, it has been used to develop new ones, which is the subject of this paper. Traditionally there have been chamber uses for both wedge and pyramidal absorber (sic). In a normal RCS range, one uses pyramidal material in forward sector around the feed, wedge absorber through the target zone and pyramids on the backwall. Using this approach, one takes advantage of the basic features of the two types of absorber. To improve wedge material, one is interested in reducing its normal incidence reflection coefficient because the long straight edge is a rather large scatterer. Through the use of the PMM analysis, curved and serrated wedge absorber designs have been developed and tested. Both show significant improvement relative to conventional material. As for the pyramidal model, one would hope to improve its size requirements especially for lower frequencies. Recall that two wavelengths at 100 MHz is twenty feet. By placing twenty foot material throughout a chamber, one greatly restricts the size of the room. Again, the PMM analysis has been used to develop a new curved pyramid design which can perform as well as a conventional pyramid twice its size. Thus, one could use curved pyramids that are ten feet at 100 MHz and achieve the same performance as the commercially available twenty-foot material.
A comparative analysis of different geometries of dual compact antenna test ranges is done looking at the cross-polarization level and the scanning capability of the system. The analysis is based on a very simple and quick computation of the fields over the main refector [sic] projected aperture.
This paper considers the radar cross-section (RCS) simulation, measurement, and analysis of rotating structures found in today’s modern airframes. Addressed will be scattering characteristics from helicopter main and tail rotor systems; how these characteristics can be simulated, measured, and reduced to identify the individual scatterers withing the helicopter. The effect of radar system parameters on the scattered signal will also be discussed. Finally, actual RCS measurements from helicopters in flight wil be resented and analyzed using the above discussed techniques.
The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.
An assessment of instrumentation error sources and their respective contributions to overall accuracy is essential for optimizing an electromagnetic field measurement system. This study quantifies the effects of measurement receiver signal processing and the relationship to its transient response when performing measurements on rapidly varying input signals. These signals can be encountered from electronically steered phased arrays, from switched front end receive RF multiplexers, from rapid mechanical scanning, or from dual polarization switched source antennas. Numerical error models are presented with examples of accuracy degradation versus input signal dynamics and the type of receiver IF processing system that is used. Simulations of far field data show the effects on amplitude patterns for differing rate of change input conditions. Criteria are suggested which can establish a figure of merit for receivers measuring input signals with large time rates of change.
A performance simulation for analyzing the measurements of target RCS in a compact radar range has been applied to a small indoor range which will be installed at RRI. A dual reflector collimator has been examined with respect to both quiet-zone quality and the amount of stray energy in the chamber which eventually end up as clutter or multipath interference. The complicated ray geometries, beyond the reach of hand calculation, are discovered by complete tracing of all the rays from the feed source. The ray pats which interfere with target measurements are shown convincingly by graphical display. Vector clutter subtraction is widely used in compact ranges in order to reduce the background clutter to an acceptable level. Some of the effects which limit the effectiveness of clutter subtraction are also addressed in the paper. The sources of measurement errors which are obtained by this simulation are used in the measurement-error budget analysis, which is the subject of the follow-on paper.
The RAFAEL general purpose radome measurement range has been modernized and refurbished, maintaining its capability to accommodate all range of radome sizes up to 1.2 meters in diameter. It is based on a 3-axis positioner placed in an open anechoic chamber with a null seeker placed 20 meters away and about 10 meters above the ground. All the positioner’s axes are controlled by an automatic positioner controller. The receiver and source are based on a HP-8510B system. The X-Y null seeker serves for boresight error measurements. It has a 0.7m x 0.7m total motion span, which is about 2º. It is controlled by a dual-motor controller, so that the scanning antenna can be moved in any kind of motion. Instrumentation control and data acquisition and analysis is performed using a HP-330 UNIX controller. Present software handles monopulse antennas with or without a comparator, and can implement the comparator in software. There are two major measurement modes: One for BSE measurements and the other for radiation patterns.
This paper describes measurement system performance parameters that were considered during the design phase of a planar near-field measurement range for Spar Aerospace Limited. All aspects of the planar near-field measurement system are addressed. These include; instrument selection, scanner interface hardware, system controller/computer hardware, software for data collection, near-field to far-field transformation, data analysis, networking and system configuration. The Scientific-Atlanta Model 2095 Microwave Measurement System with its near-field options is used as the basis for meeting the Spar requirement. The various data collection parameters of the Model 2095 are described with special emphasis on how the factors relate to near-field requirements such as fixed grid sampling. Examples of typical test scenarios are presented as an aid in exploring detailed data collection system timing.
A novel bi-polar planar near-field measurement range is described. This range is mechanically simple and has a reduced implementation cost compared to other planar techniques. The particular physical implementation and comparison with the plane-polar range is presented. Development aspects of the customized bi-polar range at UCLA are summarized. An optimal near-field interpolation is used to enable the near-field to far-field (NF-FF) processing via fast Fourier transform (FFT). Computer simulated near-field and far-field results are given.
Most radomes are tested by measuring the antenna parameters without the radome, repeating the measurement after the radome is put into position and noting the changes introduced by its presence. In many cases this method is inapplicable to large ground based radomes. This paper presents a testing method based on a combination of individual panels (or rather, pairs of panels) measurements followed by analysis. Each procedure was separately established long ago. Most of the radome scattering which degrades antenna performance originates at the joints connecting the panels. Kay has shown how to predict the radome performance using the concept of induced field ratio (IFR) of the joints. Rusch et.al developed a method to measure this IFR. The combination approach came into use about five years ago and recently gained some general recognition.
We are carrying out research and development work on 23/26 GHz 5m-diameter solid-reflector deployable antennas for future satellites or platforms. For such large antenna systems, the thermal distortion arising from the severe space environment are the determining factors in their performance. To measure the thermal distortion we fabricated an antenna surface measurement system (ASMS) (1) that makes use of a laser beam. Using this measurement system we measured the thermal distortion of a partial model of the antenna placed in a constant-temperature chamber, and we also did a thermal distortion analysis in the test configuration and compared the two results. As a result, we verified the appropriateness of the mathematical model. Next we devised a mathematical model of the entire antenna in orbit and did a thermal distortion analysis. We found that using ultra-high-modulus CFRP material, the reflector-surface precision error meats the target precision.
A code based n Geometric Optics, but applicable to diffuse surface scattering, it is evaluated for prediction of downrange high range resolution (HRR) plots of signatures generated in a compact range. A description of the technique is given, including physical justification, underlying assumptions, and flexibility of implementation. Data collected at the Hughes Compact Range will be presented in support of the analysis. Usefulness of this code in generating tradeoffs for compact range designs is demonstrated. Variations in the performance of the compact ranges are shown as a function of various range design parameters, including horn performance, chamber length, and target/wall interaction. Results are analyzed and presented in space and time domains.