AMTA Paper Archive

To determine the radar cross section of full-scale objects in their operational environment, and for doing countermeasure evaluations, a radar measurement system has been developed. The system is mobile and flexible and can hence be placed in different surroundings. Its main objective is to make trustworthy and accurate measurements of the RCS of ground-, seaand air targets. This is achieved by a calibration procedure that is performed in connection to all measurements. The measurement system is well suited for RCS measurements in dynamic scenarios. The system can transmit radar signals that resemble the signals of existing threat systems. This property together with the fact that the system at the same time measures both the RCS of the target and the effects of ECM make the system well suited for ECM evaluation. Measurements have been made of many different types of targets on land, at sea and in the air. Different types of ECM, e.g. chaff, has also been evaluated.

In this paper, the use of a low cost compact RCS measurement system is described, aimed at the characterisation of superstructure details. This system has been installed in a large room available within a shipyard, so that the measurement process is quite simple and efficient, even though under near-field conditions. Results are relevant to radar images and RCS, and can be used for the selection of details, for the optimisation of their backscattering and/or their installation process, and for the improvement of simulation codes. Comparison with simulations is also reported.

The ability to perform radar cross section (RCS) measurements, where background subtraction is applied, requires a measurement system that is very stable throughout the measurement time span. Background subtraction allows the measurement of low RCS components mounted in high RCS test bodies by permitting the scattering from the test body to be removed by coherently subtracting the test body (background) RCS from the target RCS measurement. Amplitude and phase variation of the illumination signal between the time that the target and background measurements are performed will limit the quality of subtraction achievable. Modern instrumentation radars can maintain extraordinary stability when exposed to controlled temperature environments, but controlling the temperature of a large compact range can be difficult. Other components of the measurement system, such as the reflector, can also be influenced by temperature fluctuations. Methods of controlling the thermal environment can have significant consequences. Lessons learned in the Advanced Compact Range at the Air Force Research Laboratory will be described.

SOLANGE is a large RCS indoor measurement facility operated at SHF and V/UHF frequencies. In the V/UHF band, couplings between the target and the walls can be exhibited. These perturbations due to non-directive transmitting/receiving antenna, and non-absorbing walls must be eliminated to derive the intrinsic response of the target. To reduce their levels CELAR introduced smart methods («SAV »: Site Altitude Variable and « EAV »: Environnement Altitude Variable): the transmitting/receiving antenna (and also the target in the EAV method) is translated along the elevation axis, and the acquired data are averaged. CELAR and CEA collaborated to qualify the chamber in the U/VHF band. The aim of the study is to identify and quantify the error sources, and to suggest some improvements. The analysis, based on RCS measurements of canonical targets, includes data processing (clutter reduction) and evaluation of the effects of SAV and EAV on the couplings. A theoretical algorithm is used to assess the performances of the processing, and to optimize measurement altitudes. It introduces an analytical model for the antenna and its images with respect to the walls, and calculates the scattered near field. This study enabled us to suggest improvements in the parameters of the processing, as well as in the RCS facility configuration.

This paper describes an evaluation of RCS measurements using a free-space focused beam system. Issues including effects from the Gaussian beam width and uncertainties associated with the system have been considered. Measurements and predictions of a generic embedded structure show close correlation over the frequency range of interest and indicates that this technique is ideal for rapid, accurate RCS measurements of physically small features.

A new antenna and RCS measurements facility consisting of four anechoic chambers has recently been constructed at MIT Lincoln Laboratory. The facility was designed with a rapid prototyping focus. The four chambers include a tapered chamber covering the 225 MHz to 18 GHz band, a millimeter wave rectangular chamber covering 4 to 100 GHz, a large rectangular anechoic chamber covering 150 MHz to 20 GHz, and a large compact range covering 400 MHz to 100 GHz. The compact range will be highlighted.

Antennas that are used aboard next generation airborne, maritime and ground vehicles are increasingly required to satisfy both conventional radiation pattern and gain requirements as well as new radar cross section (RCS) requirements. In response to these requirements, Nurad and ORBIT/FR recently completed design, installation, and verification of a high performance, multi-purpose antenna and RCS measurement facility at the Nurad site in Baltimore, Maryland. This compact range facility features a 60x36x26 foot shielded anechoic chamber and a precision machined, serrated edge, offset-fed reflector system that produces a 5.3’H x 8’W x 8’L quiet zone over the 2-50 GHz frequency range. The facility includes a unique feed room structure that positions the primary radar components close to the feed mount for RCS measurements, and allows for easy change of compact range feed antennas. A removable pylon assembly is used for test body support during RCS testing, and a unique add on section to the pylon rotator allows for inclusion of a roll axis that enables measurement of small and medium size antenna assemblies without removing the pylon. Measurements performed on low RCS standard targets and antennas made in the chamber demonstrate that the chamber provides a high performance measurement environment while providing ease of use and rapid configuration and target changeover.

In this paper, we present an analysis of the impact of positioning errors on the performance of the GDAIS circular image-based near field-to-far field RCS trans­formation (CNFFFT). The analysis is part of our con­tinuing investigation into the application of near field­to-far field transformations to ground-based signature diagnostics. In particular, the analysis focuses on the errors associated with ground-to-ground, near-field, whole-body measurements where the radar moves on a nominally circular path around the target. Two types of positioning errors are considered: slowly-varying, long term drift and rapidly-varying, random perturbations about the nominal circular path. The analyses are con­ducted using simulated data from a target comprised of an array of generalized point scatterers which model both single and multiple interactions on the target. The performance of the CNFFFT was evaluated in terms of the angle sector cumulative RCS statistics. The analyses were performed as a function of frequency for varying amounts of position error, both with and without (ap­proximate) motion compensation. As expected, the re­sults show that the CNFFFT is significantly more sensi­tive to rapidly-varying position errors, but that accept­able performance can be achieved with motion compen­sation provided an accurate estimate of the errors is available.

The time-dependent spectrum of rotating structures presents many significant challenges to radar cross section (RCS) test design, instrumentation parameter selection, signal processing methodology, data analysis, and data interpretation. This paper presents a multi-dimensional signal processing tool and a suite of associated data products, based on an efficiently scripted test design and execution strategy, that are responsive to the high throughput, high data volume requirements and real time data analysis demands associated with rotorcraft testing. We specifically address the NRTF’s realization of a suite of spectral, cepstral and statistical signal processing tools supported by animation that facilitate near-real time parametric data analysis and interpretation.

This paper presents a Radar Crossed Section (RCS) time-domain near-field measurement and its Inverse Synthetic Aperture Radar (ISAR) imaging. The target includes a pyramidal horn and a metallic aircraft scale model. A pulse generator excites the transmit antenna and a digital sampling unit collects the data at the receiving side. A time gating window is subsequently applied to reject the multiple reflections. An efficient 3-D algorithm for ISAR based on time-domain near-field data is presented. The test results for six cases demonstrate excellent ISAR images. In particular the geometry of 3-D reconstructed target can be displayed in perspective manner. The advantage of using time-domain near-field measurements is three-fold. First, it reduces measurement time in the order of one-tenth compared to frequency-domain measurements. Second, it mitigates the multiple reflection effects via time gating. Third, near-field measurements require relatively little real estate which reduces the cost tremendously since a compact range is not needed.

This paper is designed to illustrate the technical advances in Network Analyzers and how they can be effectively utilized in an RCS test range. The Hewlett-Packard 8530A [1 - 4] has been utilized in antenna test ranges since the 1980’s and will be used as a reference comparison. Advances in network analyzer hardware and software provide increased functionality, speed and accuracy for RCS measurements. A typical RCS full polarization matrix imaging measurement will be used to illustrate these advances in technology. Range gating, digital and down-range resolution and alias-free range topics will be discussed illustrating the technical advances that can be utilized in an RCS test range. Flexibility of network analyzer hardware will also illustrate the effectiveness of reducing measurement hardware complexity resulting in an increase in measurement speed and accuracy.

Co-polarized and cross-polarized radar cross sections (RCS) are required to completely characterize a complex target. However, it is common for a RCS range to measure only the co-polarized RCS. This practice is primarily due to the inability to produce accurate cross-polarization analysis data for the calibration targets. The most commonly used calibration targets, spheres and cylinders, cannot be used to calibrate cross-polarized RCS due to lack of cross-polarized returns. In this paper, we consider objects that can potentially be used as calibration targets for cross-polarization measurements. Specifically, we numerically study the cross-polarized responses of the Tungsten rod, the grooved cylinder, and triangular dihedrals. Co-polarized measurement data are also included in this initial assessment. From this initial study, we find the counter-balanced dihedral to be a suitable calibration target for cross-polarized measurements.

RCS data from 8 to 18 GHz on an ensemble of aluminum spheres (dia. 14", 8", 6". and 3.22x) and stainless steel ball bearings (dia. 1.25", 1.0", and 0.75"), as supported by strings in the 9-77 Range, have been collected. For inter-range comparison, the same spheres as supported separately by strings and by a foam tower have been measured in the Millimeter Wave Range (MMWR). By taking selected dual calibration pairs, the uncertainty analyses on the three sets of data show general consistency between the two Ranges, as well as between the two methods of support. In addition, the results allow us to sort out the good spheres for calibration from the bad ones.

There is a trend within the RCS community to use squatty cylinders in place of spheres for calibration. A higher degree of accuracy can be achieved; however, cylinder calibrations require much more precision in the alignment procedures. This effort is doubled when the dual calibration target is also a cylinder. The dual calibration test article could be a sphere thus reducing calibration efforts as long as good correlation exists between theory and measurement sphere data. A series of measurements were collected at the NASA Langley Research Center Compact Range Pilot Facility to study measurement errors of spheres atop foam columns to determine their feasibility for dual calibration use.

In recent years the need for higher quality RCS calibrations has lead to several different calibration technique investigations, such as squat cylinders, bi-cones and hybrids of both. A desirable calibration technique requires: easy implementation, a known theoretical or calculable solution and minimal interaction. The sphere as a calibration target satisfies two of the three requirements. It has no alignment issues and can be easily calculated, but the sphere-holder interaction introduces several dB of error. To reduce this interaction error, a 3D string-reel support system has been developed and demonstrated that significantly improves sphere calibration accuracy. The string-reel sphere positioning system utilizes low dielectric and highly swept strings to achieve minimal calibration error. An additional benefit of this technique allows for field probing and quick quiet zone evaluations.

The paper deals with the Range Book review process, and describes the evaluation of the National RCS Test Facility (NRTF) RATSCAT Advanced Measurement System (RAMS) Central Measurement System (RCMS) Range Book against the criteria approved by the Range Commander’s Council Signatures Measurement Standards Group (RCC/SMSG). Three RCC/SMSG approved reviewers and one observer were charged with reviewing the processes and procedures documented in the RCMS Range Book against published criteria based on the ANSI-Z540 standard. The paper will concentrate on the processed used by the evaluators to perform their task, the training opportunities afforded the observer, the lessons learned by the evaluation team, and the benefits of the process to both the RCMS site and measurement community at large.

This paper discusses the status of the RCS Measurement Standard, IEEE Standards Project P1502. This standard (actually a “recommended practice”) is sponsored by the Antenna Standards Committee of the IEEE Antennas and Propagation Society (Mike Francis, 2004 Chair). The title is “Recommended Practice for Radar Cross Section Test Procedures”. The standard is being generated by the Radar Cross Section Subcommittee of the IEEE AP-S Antenna Standards Society (Dr. Eric Walton, 2004 Chair). The RCS Measurement Practice Standard is being written for the personnel responsible for the operation of a test range, and not for the design of such a range. The purpose of this presentation is to give the community an update on our progress. The briefing will also review the contents and direction the document is heading. We solicit input from members of the community with a goal of getting the document released for general review within the IEEE and publication within the next year.

As antennas become more complex, their test requirements are also becoming more complex, requiring more data to fully evaluate the performance of today’s modern antennas. At the same time, competition and time-to-market concerns are driving the need to reduce the cost of test for most antenna test facilities. This places stringent demands on our test facilities, personnel, and resources. To be competitive, new and creative ways are needed to meet these new demands. Fortunately, technology is changing, and these advances in technology if properly applied, can provide a way to reduce total test times and increase the productivity of test ranges. This paper will look at this new technology and examine how it can be applied to antenna measurements to significantly reduce measurement times. This paper will describe new technology features applicable to antenna/RCS measurements, configuration diagrams, typical antenna/RCS measurement scenarios, and measurement time comparisons for the different measurement scenarios. This will allow antenna test professionals to determine the measurement time reductions and productivity gains that can be achieved for their specific measurement ranges and test scenarios.

Field level RCS measurements are difficult to perform in rugged, unimproved environments, even under the best of conditions. Recently, NASA tasked AFRL to measure the scattering characteristics of a Solid Rocket Booster (SRB) Booster Separation Motor (BSM) Plume at China Lake's "Skytop" Measurement Facility, as part of characterizations needed to return the Shuttle to safe flight. AFRL's Mobile Diagnostic Laboratory (MDL) was used to measure the RCS of six sequential BSM plume firings, a major technical challenge since each burn lasts only 0.8 seconds. The residual smoke plume RCS was also measured during the post firing period. The experimental set up and scattering results are described.