AMTA Paper Archive

In order to better understand the target-background interaction, we present new observations on the azimuthal and frequency dependences of the backgrounds, with the upper turntable (UTT) either kept stationary or in a constant rotation. In the stationary case, vector subtraction of backgrounds measured within seconds yields the lowest achievable residual levels between -50 and -60 dBsm. For the rotating UTT, the hot spots (regions of high background) exhibit a 4-fold symmetry in the azimuth, in frequency from 0.5to 4.0 GHz, and are positively identified as due to Bragg diffraction from the periodic 2-D structure pf absorbers with a 12"-square unit cell. Subtraction of backgrounds by azimuth yields a characteristic residual which mimic the structure of the hot spots. Aluminum rods (of small ka, supported by strings from the UTT in a horizontal position) provide an opportunity for studying the background interference with the echoes in HH, VH and VV, in order of decreasing signal. The results suggest that knowledge about the hot spots is essential for choosing the low background regions for measurements on low RCS objects.

Radar cross section (RCS) is a primary determinant of ship susceptibility to attack by antiship cruise missiles. RCS management benefits from the clear association of individual scatterers on a ship with measured ship RCS data, which is the scatterer identification problem. It is an. inverse scattering problem in which the scattering object is extremely complex, and environmental effects such as multipath and ducting corrupt the measurement channel. This paper describes a new method of solution to this important problem. The approach uses high­ fidelity models of ship RCS, of the radar signal processing, and of the environment in a constrained optimization framework. In so doing, advances are made in the areas of scatterer identification and predictive RCS model validation. Promising experimental results are presented that directly relate scatterers in a predictive RCS model of a ship to measurements of the ship taken in a maritime environment.

The NRTF and MRC have recently completed the first bistatic RCS test utilizing the Bistatic Coherent Measurement System (BICOMS). BICOMS is the first true far-field, phase coherent, bistatic RCS measurement system in the world and is installed at the NRTF Mainsite facility. The test objects include a 10 foot long ogive and a 1/3 scale C-29 aircraft model. Full pol rimetric, 2-18 GHz monostatic and bistatic RCS measurements were performed on both targets at 17 degree and 90 degree bistatic angles. BICOMS data demonstrates excellent agreement to method-of­ moments RCS predictions (ogive) and indoor RCS chamber measurements (monostatic, ogive). This paper describes the BICOMS system and the test process, highlights some process improvements discovered during testing, assesses the quality of the collected data set, and analyzes the accuracy of the bistatic equivalence theorem.

The Radar Signature Management Group of Racal Defence Electronics Limited specializes in the measurement, prediction and analysis of radar signatures. Types of measurement ranges used by the Group fall into three categories: • Indoor instrumented ranges • Outdoor measurement ranges • Full-scale trials, in which dynamic measurements are made of the target in its normal operational environment This paper describes a methodology used for characterizing the uncertainties within data from one of the outdoor RCS measurement ranges, at frequencies from 8 to 12 GHz. The results are summarized and uncertainties arising from the following sources are quantified: • Linearity • Absolute Accuracy • Stability and Repeatability • Polar Diagram The effects of background and target-to-pylon support interface are also discussed. The individual uncertainties are combined in a simple manner in order to obtain an overall uncertainty bound for the range, and recom mendations are made for reducing uncertainties against the difficulty and cost of implementation.

CAMELIA is a large RCS measurement facility (45m.12m. 13m in dimensions) whose compact range is optimized in the SHF band (1-18 GHz). Exploiting it at lower frequencies requires the modification of the absorbers and the use of huge broad band horns as RF sources (since the compact range is now not well adapted). To help understanding the radioelectric behavior of the large scale facility, we have developed a 1:10 small scale model as well as 1:10 scale horns, that are operated in the SHF band. It enables the experimental simulation of RCS measurements in the V/UHF band. Thus, all dimensions and frequencies are homothetic, only electromagnetic properties of materials are not. RCS measurements of several canonical targets have been performed in both facilities and compared. Due to non directive transmitting/receiving antenna, coupling between the targets and the wans has been exhibited. A simple ray tracing model, taking into account the measured reflection coefficient of the walls and the bistactic RCS of the target, shows good agreement with the measurements.

Radar Cross Section measurements require the target to be in the far field of the illuminating and receiving antennas. Such requirements are met in a compact range in the SHF band, but problems arise when trying to measure at lower frequencies. Typically, below 500 MHz, compact ranges are no more efficient, and one should only rely upon direct illumination. In this case, the wavefront is spherical and the field in the quiet zone is not homogeneous. Furthermore, unwanted reflections from the walls are strong due to the poor efficiency of absorbing materials at these frequencies, so the measurement that can be made have no longer something to see with RCS, especially with large targets. We first propose a specific array antenna to minimize errors caused by wall reflections in the V-UHF band for small and medium size targets. Then an original method based upon the same array technology is proposed that allows to precisely measure the RCS of large targets. The basic idea is to generate an electromagnetic field such that the response of the target illuminated with this field is the actual RCS of the target. This is achieved by combining data collected when selecting successively each element of the array as a transmitter, and successively each other element of the array as a receiver. Simulations with a MoM code and measurements proving the validity of the method are presented.

The utility of range gating in reducing the effects of clutter on RCS measurements is well known. While the range-gating process is a form of time- delay filtering, the time-delay/range equivalence allows the process to be viewed as spatial filtering in the range domain. Responses of features separated on the basis of range and cross range have been processed by two­ dimensional filters to extract the RCS of the feature; this is a extension of the gating concept which relies on the spatial separation of one feature from all others. This viewpoint can be carried to its final extension of three dimensions, thus providing a unified framework suitable to establish fundamental capabilities and limits of these processes. The three­ dimensional gating function is achieved by processing data obtained from three-fold diversity in frequency and two angles. The possibility of spatial gating in the direction of the target rotational axis offers the potential of reducing effects of clutter from target support structures which cannot be separated from target features on the basis of range or horizontal cross range. The effectiveness of the spatial gating methods is enhanced by knowledge or estimates of the target scattering characteristics. The paper addresses various schemes applicable to SAR and ISAR systems suitable to reduce effects of noise and clutter on the measurement of RCS. Examples derived from experimental data are presented to support the assertions.

One problem in a RCS ground bounce range is that the direct signal can be interfered with by the ground reflected signal. The undesired ground bounce signal will cause errors in the RCS measurement. This paper presents a study of ground bounce reduction using a tapered and stepped resistive sheet fence. In order to show that the proposed R-card fence technique can reduce the ground reflected signal significantly, both experimental and theoretical studies are performed. The resistance of the R-card varies based on a Kaiser-Bessel taper function. The experimentall results with and without the R-card fence show that the ground reflected signal can be attenuated by about 20dB. Both vertical and horizontal polarization cases are considered. This paper also the results of a simulation using NEC-BSC (Numerical Electromagnetic Code - Basic Scattering Code, developed at The Ohio State University ElectroScience Laboratory). Comparison of the results between measurements and simulations will be shown in this paper.

A radar transceiver operating at 1.56 THz has recently been developed to obtain coherent, fully polarimetric W-band (98 GHz) RCS images of 1:16 scale model targets. The associated optical system operates by a scanning a small focused beam of swept­ frequency radiation across a scale model to resolve individual scattering centers and obtain the scaled RCS values for the centers. Output from a tunable microwave source (10 - 17 GHz) is mixed with narrow band submillimeter-wave radiation in a Schottky diode mixer to produce the chirped transmit signal. Two high-frequency Schottky diode mixers are used for reception of the V-pol and H-pol receive states, with a fourth mixer providing a system phase reference. The full 2x2 polarization scattering matrix (PSM) for each resolved center is obtained following off-line data processing. Measurement examples of five simple calibration objects and a tank are presented.

The purpose of millimeter wave RCS measurements is often to evaluate the performance of scale model aircraft. To representative ISAR it is important that also the resolution cell size is scaled in proportion to the frequency. A typical bandwidth used for full scale aircraft measurements at 10 GHz is 2 GHz. This means that for at a 1:10 scale model measured at 100 a bandwidth of 20 GHz should be used. By modifications of a HP83558A W-Band antenna measu rement equipment, a powerful RCS measurement equipment covering 75 - 103 GHz with high receiver have been achieved. The hardware modifications and the radar and turntable performance are presented. This paper also shows the W-Band requirements for the SAAB indoor RCS measu rement facility in Linkoping, Sweden, and how these requirements are fulfilled. RCS measurements have been performed on 1:50 and 1:10 model aircraft. These measurements are discussed and ISAR images with resolution cell sizes down to 10 mm x 10 mm are presented.

As a result of Government and Industry RCS Teaming, initial RCS range certification exercises are underway. One critical element of certification exercises is the modeling and characterization of error terms according to the unique properties and requirements of individual RCS ranges, and the development of a method for propagating these errors into overall RCS measurement uncertainty. Previously, we presented the statistical model for the case where errors are grouped into multiplicative and additive classes, as well as a robust methodology for the propagation of errors in both the signal space and RCS (dBsm) domains [1-3]. Initial data at the National RCS Test Facility (NRTF) RAMS site located in the White Sands Missile Range near Holloman AFB, NM, have been collected for range certification exercises. Preliminary analysis has been accomplished on certain dominant error terms for calibration uncertainty characterization only. A general approach [7] has been followed here, with the exception that multiplicative and additive error terms are treated separately. In addition, only variance effects are treated (not bias). This paper is a status of work in progress. The ultimate goal of this work is the full implementation of previously described concepts [1-3]. We plan to demonstrate an improved ability to capture the effects of both error bias and variance (as has been demonstrated mathematically to date) using a more complete set of data collections.

(U) Precise radar cross-section (RCS) calibration are needed for all RCS measurement facilities. In 1996, AFRL began to advocate the use of a series of precision, short cylinder RCS calibration standards, demonstrating consistently greater accuracy than traditional sphere targets. Previous AMTA publications [1,2,3,4] demonstrated the overall measurement fidelity of these targets. However, questions regarding the accuracy and stability of the numerical RCS solutions to these cylinders continue to be raised. This paper will strictly and thoroughly examine the accuracy of several numerical techniques used to predict the AFRL calibration cylinder RCS, and will examine such "real world" issues as gridding sensitivity, conductivity vanat1ons, frequency bandwidth, and practical manufacturing tolerances.

In order to have definitive measurement traceability according to, ANSI-Z-540, a radar cross section measurement facility must have solid traceability to a known and accepted measurement standard. The Air Force Research Laboratory choose short right circular cylinders as calibration standards for their facilities. We describe a general RCS uncertainty analysis technique, and apply the method to our calibration standards to establish absolute traceability to a known standard. Though applied to cylinders in the current paper, the uncertainty method is general enough for any arbitrary target

Boeing is currently pursuing certification of their 9-77 indoor compact range facility as a voluntary industrial participant in the ongoing DoD/NIST RCS certification demonstration program. In support of that process, V­ EI has applied a novel statistical method for analysis to VHF measurements of a canonical target from the Boeing 9-77 range. The dominant error sources in the range were identified and categorized according to their dependence on frequency, aspect angle, and the target under test. Range characterization data were collected on canonical targets and then used to estimate the statistical parameters of each of the errors. Finally, these were incorporated into expressions for the combined RCS measurement uncertainty for a test body whose RCS exhibits many of the characteristics of modern, high-value targets. The results clearly demonstrate the importance of accounting for the target-dependence of the errors and the bias they introduce into the overall uncertainty.

This paper describes how ANSI standard Z-540 [l,2,3] was applied in a DoD demonstration project to organize radar cross section (RCS) range documentation for the Air Force Research Laboratory Advanced Compact Range (ACR) and Patu:xent River Atlantic Test Range (ATR) Dynamic RCS measurement facility. Both parts of this paper represent a follow-up report on the DoD demonstration program introduced at AMTA 97 [4]. In June 2000, the DoD Range Commanders Council Signature Measurement and Standards Group (RCC/SMSG) certified that these two facilities met the ANSI-Z-540 documentation standards established by the DoD demonstration project. Since AFRL plans to require mandatory ANSI-Z-540 compliance for DoD contractors performing RCS measurements with AFRL after January 1, 2004, the review process described in this paper will be the likely model for industrial compliance. After a brief review of the ANSI-Z-540 standard, Part 1 of this paper will outline the certification review process and discuss the outcomes, results, and lessons learned from the DoD demonstration program from the perspective of the AFRL range and volunteer range reviewers.

This paper describes how ANSI/NCSL standard Z- 540 [1, 2] was applied in a DoD demonstration project to organize radar cross section (RCS) range documentation for the Air Force Research Laboratory (AFRL) Advanced Compact Range (ACR) and the Naval Ai:r Warfare Center - Aircraft Division (NAWC-AD) Atlantic Test Range (ATR) Dynamic RCS measurement facility. Both parts of this paper represent a follow-up report on the DoD demonstration program introduced at AMTA 97 [3]. In June 2000, the DoD Range Commanders Council Signature Measurement and Standards Group (RCC/SMSG) certified that these two facilities met the ANSI/NCSL Z-540 documentation standards established by the DoD demonstration project. Since AFRL plans to require mandatory ANSI/NCSL Z- 540 compliance for DoD contractors performing RCS measurements with AFRL after January 1, 2004, the review process described in this paper will be the likely model for industrial compliance. Part I of this paper contained a brief summary of the ANSI/NCSL Z-540 standard, outlined the certification review process and discussed the outcomes, results, and lessons learned from the DoD demonstration program from the perspective of the AFRL range and volunteer range reviewers. Part II will discuss the review process as it applied to ATR, as well as the outcomes, results, and lessons learned.

Nowadays, highly accurate antenna pattern and RCS measurements are performed in compensated compact range test facilities, which fulfil the stringent space requirements for measurements up to 500 GHz and more. As the suppression of diffracted fields from the reflectors mainly determine the quiet zone field performance, the reflector edge treatment is an important design parameter for this type of test facilities. Within the present paper a novel serration design wm be shown. The analyses as well as measurement results exhibit a clear improvement of the quiet zone field performance when compared to previous solutions. The new serration design was implemented and proved with the CCR 20/17 of Astrium GmbH at the Munich University of Applied Sciences.

Test results of the compact range facility in the National University of Singapore are presented in this paper. The tests were performed for antenna and RCS measurements from L-band to Ka-band. Errors of experimental measurements are compared to errors in measurements calculated by results of field measuring in the quiet zone.

A system has been developed for acquiring an antenna’s complete (3D) radiation pattern and radar cross-section (RCS) measurements. The system consists of a motion controller, a network analyser and tower assembly. The tower assembly is in an anechoic chamber. The tower has a novel design. It uses three motors in a special configuration, thereby allowing 2 ½ degrees of freedom. This freedom gives the ability to run complete antenna or RCS measurements automatically. Another advantage stemming from the degrees of freedom is expansion of the range of measurements. This is enabled by a variety of possible positions inside the chamber. Tests have also been carried out on system performance. The data acquisition rate becomes crucial when dealing with 3D pattern measurements. The performance of an HP 8720 or 8753 network analyser series can be dramatically increased by using the power sweep mode for data acquisition. Together with the “external trigger-on-point” mode, this gives the best positioning accuracy. The six-month experience has demonstrated the flexibility and reliability of the set up and ideas.

In June 2001, the DoD Range Commanders Council Signature Measurement and Standards Group (RCC/SMSG) certified that the Helendale Measurement Facility (HMF) outdoor radar cross section (RCS) measurement Range Book met the ANSI-Z-540 documentation standards established by the DoD demonstration project. This paper describes how Lockheed Martin Aeronautics (LM Aero) applied the ANSI Z-540 [1,2,3] standard to obtain National Certification of the HMF RCS range. The dual calibration results for Pit #1 and Pit #3 are presented showing upper and lower uncertainty error bounds established by this process. Schedule, cost, range book format, and “lessons learned” from the LM Aero experience are also discussed.