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A variety of unique instrumentation radars have been developed by the RF & MMW Systems Division at Eglin Air Force Base in order to support both static and dynam ic Radar Cross Section (RCS) measurements for Smart Weapons Applications. These systems include an airborne multispectral instrumentation suite that was used to collect target signatures in various terrain and environmental conditions (95 GHz Radar Mapping System - 95RMS), a look-down tower based radar designed to perform RCS measurements on ground vehicles (MMW Instrumentation, High Resolution Imaging Radar System MIHRIRS), two high power (35 & 95 GHz) systems capable of mapping/measuring both attenuation and backscatter properties of Obscurants and Chaff (MMW Radar Obscurant Characterization System MROCS: 1&2), and a Materials Measurement System (MMS) which provides complex free space, bistatic attenuation and reflectivity data on Radar Absorbing Materials (RAM), paints, nets and specialized coatings/materials. This paper will describe the instrumentation systems, calibration procedures and measurement techniques used for data collection as well as several applications which support modelina and simulation activities in the Smart Weapon community.
The increase of output power for telecommunication satellites give constraints on design and manufacturing of antenna reflectors. Any non-linearity, such as a junction between two conductive materials, is a potential generator of inter-modulation products (PIM's). ALCATEL SPACE INDUSTRIES implemented a PIM test bench for reflectors. The objective is to validate, as early as possible in the satellite program, the reflector design, with regards PIM specification. The test principle consists in two separate transmit channels, each one having a single carrier at a well selected frequency. This configuration avoids the generation of PIM's by the bench itself. A basic conditions relevant to the output power for the test is that the flight conditions must be covered, in terms of Power Flux Density (PFD), on the reflector surface. The post-processing of the test results is based on a model whose parameters allow the following correlation : - variation of a given PIM order versus transmit power - variation between two consecutive PIM odd orders for a given transmit power. The model allows to correlate the reflector performance in terms of PIM to the flight conditions and to the customer's specification.
For many years the Navy has been using Anechoic Chambers and RF absorbing material. Recent events have brought into question the safety of RF absorbing material and the chambers which are covered with this material. Little, if any, information has been presented in the past to provide a solid picture of the actual danger that exists in these environments. A series of tests and inspections were conducted by the Navy on RF anechoic chambers and the materials inside. The materials were tested for fire susceptibility and chemically analyzed for salt compounds. Salt compounds have been used to make materials fire-safe. Results will be presented which show the susceptibility of various materials to fire from flames, electrical current and heat. A series of recommendations will be presented for using these materials in chambers to maintain safe working conditions.
Because the incident wave on an anisotropic material is likely to be depolarized, a complete characterization of such a media requires to measure its whole scattering matrix, which afterwards complicates the calibration process. A suitable technic is the Wiesbeck calibration method [1]. In this paper, we apply this method to two configurations, the reflection configuration and the transmission configuration, and obtain very good agreements between theoretical and experimental results.
The new EMC-standards in Europe have strengthened the requirements for test facilities. In this paper examinations are concentrated on anechoic chambers, which are mostly used for measuring radionoise emissions. To become accredited a chamber have to own excellent performance, which is only possible by excellent absorbers and a careful choice of the measurement axis. A program for the evaluation of anechoic chambers has been developed and recently extended to permeable materials. This allows the calculation of chambers equipped with ferrite tiles or even a combination of ferrite and foam absorbers. Furthermore the numerical code is a very helpful tool during the planning phase of a chamber and offers the possibility to find the best way to improve the performance of older chambers. To estimate the performance the results are compared to the field distribution in an ideal Open Area Test Site (OATS).
This paper presents a measurement system used for W-band complex permittivity measurements performed in NASA Langley Research Center's Electromagnetics Research Branch. The system was used to characterize candidate radome materials for the passive millimeter wave (PMMW) camera experiment. The PMMW camera is a new technology sensor, with goals of all-weather landings of civilian and military aircraft. The sensor was developed by TRW as part of a cooperative agreement for the Defense Advanced Research Projects Agency (DARPA) and the dual-use technology program. NASA Langley manages the program on behalf of DARPA and also supports the technology development and flight test operations. Other members of the consortium include McDonnell Douglas, Honeywell, and Composite Optics, Inc. The experiment is scheduled to be flight tested on the Air Force's "Speckled Trout" aircraft in late 1997. This paper details the design, set-up, calibration and operation of a free space measurement system developed and used to characterize the candidate radome materials for this program.
In this paper we present an improved theoretical modelling for the free space technique for measuring the complex permittivity of materials at microwave frequencies. The theory was developed for a transmission set-up with two identical pyramidal horn antennas. By performing a spectral decomposition of the aperture fields, the new model takes the effect of the non plane wave character into account when the sample is not placed in the far field of the transmitting antenna. With the use of the new theoretical model it becomes possible to place the sample much closer to the antennas without infringing the theoretical assumptions since no plane wave incidence is needed. In this way the transversal dimensions of the sample can be reduced significantly. The validity of the new theoretical model was verified by measurements on many dielectric (Plexiglas, polystyrene,…) and lossy materials. A comparison was made with the values obtained when the usual plane wave theory is used.
Possible mechanisms and structures for realising a dynamically adaptive radar absorbing material (DARAM) are discussed and their potential evaluated through computer simulation. Some pointers towards practical implementation are outlined and measured results for large-area DARAM panels operating over I and J bands are shown.
Techniques for the X-band inverse synthetic aperture radar (ISAR) imaging of a naval ship at sea are presented. We show that the longer the observation time (and thus the angle span), the better the image until a limit based on the pitch roll and yaw motion of the ship is reached. A Fourier transformation ISAR algorithm will be shown and a modified hybrid algorithm will be demonstrated using autoregressive spectral estimation. A hybrid algorithm based on data extrapolation obtained using FBLP coefficients will be demonstrated. Specific motion compensation tradeoffs will also be discussed.
The Naval Command, Control and Ocean Surveillance Center RDT&E Division (NRaD) has been using a 500 MHz Linear Frequency Modulated (LFM) radar to collect measurements of flying aircraft. These data have been used to generate high resolution Inverse Synthetic Aperture Radar (ISAR) images of the targets [l]. Digital Signal Processing (DSP) hardware had been added to the radar and algorithms have been implemented to perform ISAR processing on the data in real time. A VME bus architecture has been developed to provide a scaleable, flexible platform to test and develop real-time processing software. Algorithms have been developed from a system model, and processing software has been implemented to perform pulse compression, motion compensation, polar reformatting, image formation, and target motion estimation.
Due to inherent cost, safety and logistical advan tages over dynamic measurements, Ground-to-Ground (G2G, aircraft and radar on tarmac) diagnostic radar measurements may be the preferred method of assessing aircraft RCS for signature maintenance. However, some challenging complications can occur when interpreting SAR imagery from these systems. For example, the effect of ground induced multi-path often results in the measurement of a significantly different image based RCS than would have been obtained by a comparable Ground-to-Air (G2A) or Air-to-Air (A2A) system. Although conventional 2-D SAR images are useful in determining the physical source (down-range/cross range) of scatterers, it is difficult at best to deduce whether an image pixel is a result of direct (desired) or ground induced multi-path (undesired) scattering. ERIM and MRC recently completed an experiment testing the utility of collecting and processing interfero metric (2-antenna) SAR radar data. This effort produced not only high resolution SAR imagery, but also a com panion data set, derived from interferometric phase, which helps to isolate the source (direct or multi-path) of all scattering within the SAR image. Additionally, the data set gives a measure of the physical height of direct scatterers on the target. This paper outlines the experiment performed on a RCS enhanced F-4 aircraft using a van mounted radar. Conventional high resolution imagery (down-range/ cross-range/intensity) will be shown along with down range/height/intensity and cross-range/height/intensity images. The paper will also describe the processing pro cedure and present analysis on the interferometric results. The unique motion compensation processing technique combining prominent point and motion mea surement instrumentation data, eliminates the need for a tightly controlled collection path (e.g. bulky rail sys tems). This allows data to be collected with the van driven somewhat arbitrarily around the target with side mounted antennas taking measurements at desired aspects.
In order to measure the performance of microwave absorbing materials a broadband free- space measurement system constructed in INTA. The is a kind of N RL Arch that gives us the possibility of measurements in d ifferent configurations. It comprises a set of dielect ric loaded rectangular waveguide antennas, coaxial vector analyzer, sample support and a computer. A TRL calibration technique in the plate near field is developed taking advantage of the calibration functions implemented in the network analyzer and the time domain gating. We introduce the use of typical RCS calibration standards as the calibration reflect standards. It gives us the possibility of performing the near field free space calibration in the same way that it is usually done in waveguide, but for directions di fferent to the normal. This calibration allows us to check the edge diffraction behaviour of the samples in the measurement and is thought to be adecuated for thin materials.
Accurate mechanical-to-electrical axis alignment (boresighting), gain, and pattern testing of radar antennae requires specialized tooling/fixturing. This requirement is often taken for granted and seldom discussed in the EE community. Particularly in a production environment, where rapid change of test configurations to accommodate multiple radar platforms are required, a convenient mounting scheme is mandatory. This paper describes and illustrates a method implemented at the Warner Robins Air Logistics Center to satisfy this demand. Drawings and/or photos of a three-point Universal Adapter fixture and several UUT Specific radar mounting fixtures are discussed. The paper discusses tolerances, materials, manufacturing processes, alignment, and antenna boresight methodologies.
The design of wide-band, multi-layer radar absorbing materials involves the solution of what is essentially an N-dimensional optimization problem. Genetic algorithms appear to offer significant advantages over conventional optimization techniques for this type of problem due to their robustness and independence of performance function derivatives. To illustrate their use, the paper considers the optimum design of wideband, multi-layer, Jaumann radar absorbers for normal and oblique incidence.
A simple change to the HP8510C or HP8720C vector network analyzer block diagram coupled with the TRM (Thru Reflect Match) calibration leads to accurate measurements of the material properties of flat samples. Algorithms developed for transmission line measurements can also be used in free space measurements. A description of recent improvements in the transmission/reflection algorithms is reviewed. Free space measurement results based on the transmission/reflection algorithms found in the HP85071B materials measurement software package are presented.
A 585 GHz compact range has been developed for obtaining full scale RCS measurements on scale model targets. The transceiver consists of two CW submillimeter-wave gas lasers along with two colled-InSb heterodyne mixers. Coherent detection has been implemented to maximize sensitivity and allow for a vector measurement capability. In addition, the target can be rapidly translated in range to generate a doppler modulation which is used to reject background signals during low-RCS measurements. Although most scaling has evolved to develop non-metallic materials with scaled dielectric properties as well as validation and test program, RCS measurements are made on scaled simple and complex shapes and compared with full-scale measurements and computer predictions. A description of the 585 GHz compact range along with measurement examples are presented in this paper.
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
As many institutions and companies have constructed anechoic chambers in the past few years, there has been little work done to codify the specification requirements. Often chambers have been constructed from woefully inadequate specifications resulting in chambers that may be too costly, unable to meet the performance criteria, and in some cases, be unsafe. This paper shall present various model specifications and guidelines to properly specify a chamber complex. Compact ranges, tapered chambers, as well as traditional rectangular chambers will all be examined. How to specify absorbing materials and quiet zone sizes, as well as tradeoffs associated with them, will be discussed. Finally, a guide for coping with facility concerns such as civil, structural, RF shielding, HVAC, electrical, and fire protection will be presented. Examples of good specifications and inadequate specifications will be demonstrated and reviewed.
To characterize a material's electrical performance or to understand a material's affects (sic) on electromagnetic systems, the constitutive parameters (e, u) of the material must be determined accurately. Materials with high dielectric constants, high loss tangents, or which are layered or complex (e.g., frequency selective surfaces, radomes, radar absorbing material, etc.) are difficult to measure and analyze. For example, germanium is an infrared window substrate in high performance aircraft. The germanium is doped to 1 - 4 ()-cm to raise the maximum operating temperature and to provide electromagnetic shielding. The material is very brittle. The standard methods (coaxial, waveguide, and cavity) are difficult to use. The brittle germanium pieces cannot be made thin enough or have a center conductor hole inserted making coaxial donuts are nearly impossible to fabricate and use. Usable waveguide samples absorb the transmitted energy needed in standard waveguide tests. The brittle sample cannot be made thin enough for X-band measurements or above. The sample, having a high dielectric constant, and having a high conductivity, reduces the Q of resonance techniques difficult and not repeatable. This paper discusses our methodology and shows comparisons with calculations. The technique is based on reflection measurements against a ground plane standard. This technique requires more measurements than other techniques, but the results are numerically more stable.
Many aerospace applications specify materials with precise surface resistivities to satisfy electromagnetic requirements. Frequently, composite materials are physically protected by a layer of dielectric (such as Mylar) or by virtue of being imbedded in a dielectric (such as fiberglass). Conventional resistivity measurement techniques, such as four point probes and busbars, fail in measurement of surface resistivity due to the inability to establish direct electrical contact with the composite structure. This paper describes a new methodology for measuring resistivity of aerospace materials. Capacitive coupling is used in a VHF surface resistivity test system. This system measures ohms per square in a non-destructive, non-intrusive fashion using a capacitive coupled probe technique. Several probe designs have been fabricated and tested. Baseline measurements are presented that illustrate performance against surface resistance which vary from ten to two thousand ohms per square. The effects of different types and layers of resistive materials are also discussed.