AMTA Paper Archive

In 1993, we presented the newly completed compact range and tapered chamber facility [1]. As part of this presentation, the issue of “range certification” was presented. This paper will discuss the work that we have done with the compact range for radar cross section (RCS) measurement acceptance. For customer acceptance, we had to “prove” that the compact range made acceptable measurements for the fixtures and apertures involved. Schedule and funding did not permit the full exploitation of the uncertainty analysis of the chambers, not was it felt to be necessary [2]. The determination of our range capabilities and accuracy was based on system parameters and target measurements. Targets that were calculable either in closed form solutions (spheres) or by numerical methods (cylinders and rods) were used. Finally, range to range comparisons with the Rye Canyon Facility [3] of a standard target was used. The range to range comparison proved especially difficult due to customer exceptions, feed differences, and target mounting. This paper will discuss the “success” criteria applied, the procedures used, and the results. The paper will close with a discuss of RCS standards and the range certification process.

For the past seven years, ERIM has been studying RCS measurement error sources and processing methods by which these errors can be reduced. Image editing is an extension of range-gating where scattering measurements are improved by removing undesired scattering phenomena in the range-crossrange image domain. Conventional image editing methods rely on a user-supplied polygon to segment an image into desired and undesired scattering regions. However, the polygon method suffers from variability due to user and display characteristics, provides little hope for automation, and cannot be easily extended beyond two dimensions. An alterative approach based on peak region segmentation minimizes or eliminates these limitations and adds an element of optimally that can also improve the performance of image editing techniques. In this paper, we will discuss the application of peak region segmentation to the image editing problem and show examples that demonstrate some of the advantages of this approach.

ERIM has developed techniques, based on parametric spectral estimators, for removing additive target support contamination from narrowband RCS measurements [1]. These techniques allow target and support returns to be extracted from frequency sweep data with much greater accuracy and resolution than that afforded by conventional Fourier techniques. These algorithms have recently been enhanced to incorporate scattering mechanism frequency dependence in the underlying signal model. Specifically, damped exponential and power-of-frequency sweep data with much greater accuracy and resolution than that afforded by conventional Fourier techniques. These algorithms have recently been enhanced to incorporate scattering mechanism frequency dependence in the underlying signal model. Specifically, damped exponential and power-of-frequency signal models have been used. The modification substantially improves algorithm performance in measurement situations where there is small absolute bandwidth, but relatively large fractional bandwidth, which can lead to appreciable variation in scattering mechanism amplitude. The paper will demonstrate the technique’s ability to remove target support contamination using numerical simulations and compact range measurements of canonical targets mounted on pylon supports. It will be shown that the algorithm can remove the additive pylon contamination even for situations where the pylon return dominates the target return and cannot be resolved from the target in conventional Fourier range profiles.

Compact Payload Test Ranges (CPTR) for test zones of 5 meters or larger can be used for both payload and advanced antenna testing. In both cases accurate calibration, including amplitude and phase characteristics across the test zone, is required. Accurate data analysis is needed in order to establish corresponding error budgets. In addition, boresight determination will be required in both measurement types for most applications. Since it may be difficult or even impossible to scan the test zone field using a (planar) scanner, application of a large reference target (a rectangular or circular flat plate) can be seen as in interesting alternative. RCS measurements are then performed and test-zone field characteristics are determined in both amplitude and phase. Time- and spectral domain techniques can provide valuable information as to the location of possible disturbances. The evaluations is complemented with the measurement of a VAlidation STandard (VAST) antenna in combinations with an advanced APC technique. These techniques have been demonstrated at the CPTR at ESTEC, Noordwijk, the Netherlands. Results and practical considerations are presented in this paper.

To accurately measure static or dynamic Radar Cross Section (RCS), one must use precise measurement equipment and test procedures. Recently, several DoD RCS ranges, including the Advanced Compact RCS Measurement Range at Wright-Patterson AFB, established procedures to estimate measurement error. Working cooperatively with the National Institute of Standards and Technology (NIST), Wright Laboratory established a baseline error budget methodology in 1994. As insight was gained from the error budget process, we noted that many common RCS measurement calibration techniques are subject to a wide variety of potential error sources. This paper examines two common so-polarized calibration devices (sphere and squat cylinder), and discussed techniques for evaluating calibration induced errors. A rigorous “double calibration” methodology is offered to track calibration measurement error. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.

A method to generate time and direction of arrival (TADOA) spectra of the quiet zone fields of a RCS/ antenna range is presented. The TADOA spectra is useful for locating the stray signal sources in the RCS/ antenna range. To generate the TADOA spectra, quiet zone fields along a linear scan over the desired frequency band are probed. The probed data are calibrated to remove the magnitude and non-linear phase variation versus frequency. A calibration technique is also proposed in the paper. The TADOA spectra for simulated probed data as well as experimental probed data are shown.

Dynamic ground-to-air measurement of aircraft RCS has several advantages over static measurements. The target may be measured in flight configuration and the support pylon is eliminated. Although dynamic RCS imagery has been performed since the late 1970s, the cost and complexity of such measurements have limited their utility for routine testing. In this paper, an easily deployable ground-to-air radar imaging system developed by Aeroflex Lintek is presented. This system forms images of aircraft in straight flight, requiring no on-board instrumentation or special pilot training. The radar system, flight profiles, and processing tools required for generating images of aircraft in flight are presented, along with examples of measured target data.

Calibrated radar images are often quantified as radar cross section (RCS). This interpretation, which is not strictly correct, can lead to misunderstanding of test target scattering properties. To avoid confusion, we recommend that a term such as "scattering brightness" (defined below) be adopted as a standard label for image-domain data.

This paper describes the current status of the present cylinder family, and introduces theoretical and experimental RCS data for a modified "bicone" calibration standard. These standards, when used appropriately, greatly improve the quality and efficiency of primary RCS calibration measured within indoor or outdoor ranges. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.

The Joint Strike Fighter (JSF) office sponsored a Navy directed limited technical demonstration of diagnostic Radar Cross Section (RCS) imaging on-board an aircraft carrier at sea. The overall objective was to obtain experience and data sufficient to assist the Navy in defining any future shipboard diagnostic imaging measurement system requirements. Measurements were conducted in the hangar bay to assess the challenges posed by the carrier environment. A technique for making diagnostic imaging measurements in spatially confined areas was developed.

Range characterization is becoming a very important topic for the operators of RCS measurement ranges. Techniques for characterization can be expensive and time consuming. We present a top down approach that recognizes that the range construction and optimization is the responsibility of the range operators. Once the range is operating satisfactorily from the point of view of the range operator, then characterization of t he range performance as achieved can be done. Measurements are proposed that perform this characterization rapidly and inexpensively.

The Wright Laboratory at WPAFB, OH, operates an advanced compact range facility (ACRF) for RCS measurements. The ACRF employs a dual chamber compact range system to generate a plane wave in the target zone. The main reflector, which is a blended rolled edge paraboloid, is housed in the main chamber; whereas, the feed assembly and the subreflector, which is a serrated edge ellipsoid, is housed in the sub­ chamber. The two chambers are electromagnetically coupled through a small opening near the focal point of the main reflector. The compact range system was originally designed to perform RCS measurements at frequencies above 1 GHz. Recently, there has been some interest in us­ ing the ACRF to perform RCS measurements at lower frequencies, from 100-1000 MHz. In fact, the ACRF facility has been successfully used to measure small targets at these lower frequencies, but one would like the target zone to be as large as possible. In order to accommodate a larger target zone, the first step was to evaluate the performance of the ACRF at lower frequencies. The performance evaluation revealed that the subreflector edge diffraction was leaking through the coupling aperture into the target zone. Some feed spillover was also observed in the target zone. To control these stray signals in the target zone, an absorber fence was designed for the ACRF. The absorber fence sits near the focal point of the main reflector. A prototype absorber fence has been built and installed in the ACRF. The performance of this absorber fence is discussed in terms of the improvement in the target zone fields.

Sanders, A Lockheed Martin Company, measures radar cross section (RCS) and antenna performance from 2 to 18 GHz at the Com­ pany's Compact Range. Twelve feed horns are used to maintain a constant beam width and stationary phase centers, with proper gain. However, calibration with each movement of the feed tower is required and the feed tower is a source of range clutter. To Improve data quality and quantity, Sanders and The Ohio State University ElectroScience Laboratory designed, fabricated, and tested a new wide band feed. The design requirement for the feed was to maintain a constant beam width and phase taper across the 2 - 18 GHz band. The approach taken was to modify the design of the Ohio State University's wide band feed [1]. This feed provides a much cleaner range which reduces the dependence on subtraction and other data manipulation techniques. The new feed allows for wide band images with increased resolution and a six fold increase in range productivity (or reduction in range costs). This paper discusses this new feed and design details with the unique fabrication techniques developed by Ohio State and its suppliers. Analysis and patterns measured from the feed characterization are presented as well. This paper closes with a discussion of options for further improvements in the feed.

This paper presents a brief overview of ANSI/NCSL standard Z-540 (1). Z-540 offers a straightforward way to organize range documentation. We discuss the major points and sections of Z-540, and how to organize a format-universal "range book". Since Z-540 is the US equivalent of International Standard (ISO) 25, it is especially useful for two reasons; (1) it is applicable to Radar Cross Section (RCS) ranges and (2) its quality control requirements are consistent with the ISO 9002 series of quality standards. Properly applied, Z-540 may greatly improve the quality and consistency of RCS measurements produced, and reported to range customers.

The National Institute of Standards and Technology (NIST) is coordinating a radar cross section (RCS) interlaboratory comparison study using a family of standard cylinders developed at Wright Laboratories. As an important component of measurement assurance and of the proposed RCS certification program, interlaboratory comparisons can be used to establish repeatability (within specified uncertainty limits) of RCS measurements within and between measurement ranges. We discuss the global importance of intercomparisons in standards metrology, examine recently conducted comparison studies at NIST, and give a status report on the first national RCS intercomparison study. We also consider future directions.

Calibration of monostatic radar cross section (RCS) has been studied extensively over many years, leading to many approaches, with varying degrees of success. To this day, there is still significant debate over how it should be done. In the case of bistatic RCS measurements, the lack of information concerning calibration techniques is even greater. This paper will present the results of a preliminary investigation into calibration techniques and their suitability for use in the correction of cross-polarization errors when data is collected in a bistatic configuration. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. Results will be presented for data collected in the C and X bands on potential calibration targets. Recommendations for future efforts will also be presented.

The National Institute of Standards and Technology (NIST) is coordinating a radar cross section (RCS) interlaboratory comparison study using a rotating dihedral. As an important component of measurement assurance and of the proposed RCS certification program, interlaboratory comparisons can be used to establish repeatability (within specified uncertainty limits) of RCS measurements within and among measurement ranges. The global importance of intercomparison studies in standards metrology, recently conducted comparison studies at NIST, and the status of the first national RCS intercomparison study using a set of cylinders are discussed in [1]. In a companion program, we examine full polarimetric calibration data obtained using dihedrals and rods. Polarimetric data is essential for the complete description of scattering phenomena and for the understanding of RCS measurement uncertainty. Our intent is to refine and develop polarimetric calibration techniques and to estimate and minimize the correstponding measurement uncertainties. We apply theoretical results [2] to check on (1) data and (2) scattering model integrity. To reduce noise and clutter, we Fourier transform the scattering data as a function of rotation angle [2], and obtain the radar characteristics using the Fourier coefficients. Calibration integrity is checked by applying a variant of the dual cylinder calibration technique [3]. Future directions of this measurement program are explored.

Ell has been extensively involved in the development of advanced processing techniques (APT) to improve the quality and utility of both indoor and outdoor RCS/ISAR measurements. These include algorithms for removal of clutter, RFI, and target­support contamination (including interactions), prediction of far field RCS from near field measurements, suppression of multipath contamination, and extraction of scattering features/components. These techniques have been implemented in a framework based on ERIM International's IMGMANIP signal/ image processing toolbox and stream input-output (SIO) data flow paradigm. This paper describes a recently-developed Graphical User Interface (GUI) which incorporates the most mature and frequently-used APT algorithms.

Image Editing and Reconstruction (IER) is used to estimate the RCS of component parts of a complex target. We discuss the general areas of controversy that surround the technique, and present a set of practical data processing procedures for assisting in validation of the process. First, we illustrate a simple technique for validating the end-to-end signal processing chain. Second, we present a procedure that compares the original unedited, but fully calibrated, RCS data with the summation of all IER components. For example, if we segregate the image into two components - component of interest, remainder of the target mounting structure plus other clutter - we require that the two patterns coherently sum to the original. This indirectly references the results to the calibration device. In addition, it provides a quantitative means of assessing the relative contribution of the component parts to overall RCS. We demonstrate the procedures using simulated and actual data.

Recently, a new method of wide band radar imaging has been developped within the framework of the two dimensional (2-D) continuous wavelet theory. Based on a model of localized colored and non isotropic reflectors, this method allows to obtain simultaneously information about the location, the frequency and the directi­ vity of the scatterers which contribute to the RCS of a target. We obtain a 4-D data set that we call hyperimage namely a series of images which depend on the frequency and orientation of illumination. In order to exploit efficiently hyperimages an interactive visual display software called i4D has been specifically designed. The purpose of this paper is to present the capabilities of i4D through the analysis of hyperimages constructed from monostatic and bistatic scattering data. The results show that the interactive and dynamic analysis that i4D procures allow to better understand the mechanisms that contribute to the RCS of targets.