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As new military aircraft with low radar signatures pass from the design stage to production and deployment, the techniques for measuring and confirming their low signatures must move from the laboratory to the flight line. Measuring the RCS characteristics of carrier-based aircraft is particularly difficult because it must be done either while the aircraft is in flight or while it is one a crowded flight deck or hanger deck. This paper describes an approach to Navy flight-line RCS measurements that minimizes space, yet still provides enough information to identify a degradation in low signature performance and to pinpoint the source of the problem. It uses a small reflector on a positioner combined with a stepped frequency gated CW radar at 8-12 GHz to sweep a spot illumination over the aircraft while producing downrange profiles at each spot. The primary advantage of this configuration is that it restricts the RF radiation in all three spatial dimensions, thereby minimizing the scattering from other objects in the crowded environment. A secondary advantage is that the data can be processed to yield resolution of scatterers on the aircraft under test to within two or three feet. Adding an automatic focusing ability to the reflector antenna can improve the resolution to about one foot.
This paper demonstrates the performance of the McDonnell Douglas Technologies Incorporated (MDTI) Compact Range A. This HP8510B network analyzer based system utilizes a R-card treated prime focus main reflector in a tandem with broadband 2-18 GHz feeds. A six foot quiet zone can be maintained over the 2-18 GHz bandwidth with no feed or hardware changes, allowing targets to be measured over the full bandwidth in one continuous sweep. Measured data will be presented demonstrating performance features such as quiet zone quality, dynamic range, sensitivity, and image resolution.
Martin Marietta designed and brought on-line an indoor far-field chamber used for radar cross section (RCS) evaluation. The range has conductive walls on all sides except for the pyramidal absorber covered back wall. The chamber was designed such that wall/floor/ceiling interactions occur with a distance (time) delay allowing for their isolation from the test region. Software gating techniques are used to remove these unwanted signals. This paper presents an analysis of the conductive chamber using Geometrical Optics (GO). The objective was to analyze and evaluate the plane wave quality in the chamber test region. The evaluation of the plane wave was performed using the angle transform technique. The measured results were compared to analytical results and measured antenna patterns.
A novel low-cost automatic system is described to measure both the complex permittivity and permeability of solid materials at 2 to 18 GHz. It is particularly useful for evaluating the frequency dependence of radar absorbing materials (RAM). The RF and the mechanical setups are described, including the computer algorithm and the measurement procedure. The results and the experimental errors of three materials are presented, which agree with results that were obtained by other methods, while the cost of putting up the system is considerably lower than any comparable alternative.
Measuring the radar cross section of low-observable (LO) vehicles require an RCS quality control (QC) program that will last throughout the life cycle of the vehicle, from component production to operational deployment and depot maintenance. Testing must be done at regular intervals to ensure that surface or sub-surface damage has not degraded the RCS characteristics of the vehicle beyond acceptable limits. In the past, these measurements were complicated by the requirement for and expensive, well-prepared RF test environment. The test range—usually a fixed site—is often remotely controlled. System Planning Corporation (SPC) has developed an RCS QC measurement technique that requires little or no facility improvements while offering high sensitivity inverse synthetic aperture radar (ISAR) images. The instrumentation radar system can be located at the production, maintenance, or operational site of the vehicle or component. As a result, the QC program is both economical and reliable.
To achieve optimum measurement accuracy and range throughput in antenna and radar cross-section (RCS) measurement applications requires a careful and thorough design of the measurement system. Measurement accuracy requirements, test time objectives, system flexibility, and system costs must all be balanced to achieve an optimum system design. Considering these issues independently will result in unwanted and/or unexpected system performance tradeoffs. This paper examines these issues in some detail and suggests a system design approach which balances microwave performance and measurement speed with system cost.
Lockheed’s Advanced Development Company (LADC), located in Burbank, California, has been evaluating the capability of indoor anechoic chambers to measure VHF/UHF RCS. Two chambers were available for evaluation. A 155 feet long, 50 feet high by 50 feet wide tapered horn chamber and a compact range having dimensions of 97 feet long, 64 feet high by 64 feet wide, featuring a 46 feet wide collimator. For comparison purposes, a common instrumentation radar was used in each chamber. This radar was based on a network analyzer using a Lockheed designed pulse-gate unit to increase transmit/receive isolation. Various antenna feed system were tried in both chambers to ascertain their characteristics. Theoretical and experimental data on system performance will be presented emphasizing practical implementation and inherent limitations.
The need to calibrate large antennas and radio stars is driven by needs in satellite communications systems, deep space communications systems and navigation systems. NIST presently is able to calibrate standard gain antennas up to 10 feet in diameter using their planar near-field facility and has sought means to extend their calibration services to larger antennas. During the last ten years, NIST developed an ETMS (Earth Terminal Measurement System) to measure the gain of large antennas using both radio sources and noise sources calibrated by NIST. This ETMS, however, requires that the flux density of the radio sources be accurately known. This often is not the case. NIST is currently involved in two measurement efforts using calibrated standard gain antennas, calibrated noise sources and the gain comparison method to accurately determine the absolute gains of large antennas and accurately determine flux densities of radio stars and planets. Recent progress on these efforts will be discussed.
Recently an antenna pattern measurement technique has been developed which eliminates the effects of the finite ground plane on which the test antenna is mounted. The scattered fields from the edge of the ground plane can often cause perturbations in the total fields, and thus, result in significant differences in the measured patterns as compared the theoretical predictions. This technique consists of the measurement of the edge diffracted fields and their subsequent subtraction from the original pattern. A simple theoretical model is developed to introduce the subtraction technique, and comparisons are made which show the excellent agreement between theoretical (obtained assuming an infinite ground plane) and “corrected” experimental antenna patterns. Experimental results are given from an open-ended waveguide opening into both circular and square ground planes.
The end-to-end G/T performance of an antenna system has typically been measured using celestial bodies. The technique requires high G/T performance ( 10 dB/K) to obtain accuracies of 0.5 dB. In addition, the measurement is dependent on several atmospheric and environmental conditions. This paper describes a technique for measuring the G/T performance of a low directivity, wide beamwidth antenna. The discussion details the measurements, extrapolation technique to demonstrate performance at specified atmospheric conditions, measurement uncertainties, and test results. This new G/T measurement technique offers advantages over the existing technologies. Measurements are limited to the output port of the antenna so as to include all interactions between components within the system. This allows for accurate characterization of phased array performances. In addition, testing can be performed on indoor antenna ranges under environmentally controlled conditions.
Adaptive antenna systems will expand the test requirements for conventional antenna testing. The specific example of adaptive uplink antennas for satellite communications illustrates this required expansion. Test facilities will require additional capabilities to generate both desired and interference test signals with differing arrival directions. A novel extension of compact range technology is described for testing spaceborne designs. Instrumentation likewise will require further development for testing wide bandwidth adaptive cancellation designs used with spread spectrum modems.
A novel differential phase measurement method is developed. No flexible cables or rotary joints are needed in this method. Phase center positions and phase patterns of two corrugated horns are measured at 105-115 GHz and 176-190 GHz by using this method. Good agreement between the measured values and theoretical values, calculated with the modal matching technique, is obtained. Also a new phase error correction method is introduced. This method makes possible to measure the phase error in the cable and then to remove the error numerically from the results. The accuracy of the phase error correction is limited by the phase measurement device in the system. Experimentally this method is verified at 10 GHz.
Traditional range requirements are evaluated for spherical and compact range measurement systems. Based on the chamber cost to meet these requirements, it is shown that a compact range is more appropriate for targets as small as 3 (wavelengths). The commercially-available compact range systems are, however, normally restricted to 10 or larger targets. This is due to the excessive diffraction levels associated with it presently-available reflectors. It is shown that one can overcome this limitation by using a blended rolled edge reflector. For example, a 9 x 9 blended rolled edge reflector can be used to measure 3 targets at its lowest frequency of operation. As the frequency of operation increases, the test zone of the reflector approaches one half of its linear dimension.
The use of shaped reflector compact range collimators for application to indoor bistatic RCS measurements is discussed including electromagnetic performance and structural design issues. Room sizing and layout are presented for an assumed measurement system configuration. Coupling paths between the two collimators and associated time delays are reviewed for the assumed configuration and a range of bistatic angles. Collimator/chamber interaction issues are discussed. The mechanical design of the moveable collimator in a bistatic range is similar to the design of large steerable antenna structures. The same analytical tools and techniques are applied directly to the panelized reflector system, resulting in a design that will accommodate small deflections between the individual panels without permanent deformation. These conditions are not unlike the requirements for the Harris 40 foot quiet zone compact ranges to withstand Zone 4 earthquakes. The forces resulting from moving the collimator and the unevenness of the track are the input conditions to the finite element model. A real-time characterization of the collimator is provided by a laser measurement system similar to that used on the Harris compact range field probe.
In dual chamber compact range measurement systems, a Gregorian subreflector is used to illuminate the main reflector. Since the subreflector is finite in size, there will be diffracted fields from its edges which degrade the incident field on the main reflector and subsequently lead to undesired stray fields in the target zone. Some treatment of the subreflector edges is therefore required. One way to reduce the subreflector edge diffraction is to use a serrated edge subreflector. In this paper, the performance of a dual chamber compact range system with a serrated edge Gregorian subreflector is discussed. It is shown that by using the serrated edge subreflector, one can reduce the ripple in the target zone due to the subreflector edge diffraction from 3 dB to 0.5 dB. One can further reduce the ripple by separating the two chambers by an absorber fence with a small coupling aperture.
Increased productivity and higher resolution imaging capabilities are becoming of greater concern for RCS ranges. The ideal measurement scenario involves taking data on all desired frequencies for a target combination in a single rotation. This could involve one or more frequencies in several bands, imaging data on more than one band or very high resolution imaging data covering several bands. Placing several feeds in a cluster at the focal point of an offset fed com-pact range can provide these capabilities. The effects of feed clustering such as beam tilt are discussed along with cluster sizes that provide little if any degradation in compact range performance. Experimental data is shown that gives an indication of the quality of data that may be obtained. The concepts are also applicable for outdoor ranges that have an array of antennas offset from range boresight.
Compact range, millimeter wave reflector, serrated edges.
MBB has designed, built, and extensively tested their Compact Antenna Test Range (CATR). Ford Aerospace has entered into an agreement with MBB for the procurement of such a CATR for satellite testing at Space Systems Division in Palo Alto. During the procurement program some interesting testing has been conducted to investigate the generation of scanned quiet zones for custom applications involving satellite testing. Due to the long effective focal length of the optics selected by MBB, excellent scanning performance is predicted for this geometry. This allows multiple CATR feeds to be used in scanned positions in the vicinity of the nominal focus position, which provide multiple simultaneous quiet zones. These quiet zones have demonstrated very good electrical characteristics and can be positioned to concentrate on specific antennas on the satellite, by translation of the CATR feeds. These quiet zones may be of different frequency bands depending upon feed selection, and multiple antenna testing is realized in a single raster scan of the satellite positioner. The satellite need not be translated to center a specific antenna in the nominal axial quiet zone so that convenience, expeditious data collection, and less danger to the satellite are realized.
The aperture opening design of the subreflector chamber for a dual-chamber Gregorian compact range system is presented in this paper. The subreflector is a serrated edge ellipsoidal reflector. The performance of the subreflector chamber and absorber aperture opening has been evaluated in terms of pattern measurements and by cross-range diagnostic techniques. The results of this evaluation have been used to further improve the design of the aperture opening of the subreflector chamber.
A plane wave spectrum method of analysis is employed to examine a hybrid approach to compact range reflector design. In order to reduce edge diffraction, an illumination taper is used in conjunction with a serrated reflector. The optimum illumination taper is determined for several serrated reflector geometries. Maximum quiet zone is the optimality criterion. The aperture illumination functions considered are -symmetric, cosinudoidal in amplitude, and uniform in phase. The reflectors considered are characterized by a circularly periodic aperture boundary. The analysis is restricted to the low frequencies at which diffraction effects are most prominent.