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The antennas used in an anechoic chamber illuminate not only the target but also the walls, thus generating spurious signals. This problem is particularly significant at low frequencies. This study describes improvements to a standard, rectangular horn. Several solutions are tested, such as lenses, prolongation of the horn face, metal boxes with absorbers surrounding the horn, etc. The best solution appears to be the prolongation of the horn faces, with the fitting of a metal plate with absorbers at the rear. However, as the dimension of the horn increases with the ogival plates, the horn/target interaction also has to be taken into account.
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.
Over the past few years several flightline RCS measurement systems have been developed. Some of these systems use a linear rail to collect aspect angle data and other systems use no rails or "free path" circular collections. A comparison of these two different collection methods have never been publicly presented. In this paper, a discussion of the differences between the Linear-SAR and Circular-SAR collection methodologies will be presented. Specifically, issues associated with field quality, nearfield effects, and processing requirements will be covered in the discussion. Linear-SAR has the advantage of being more easily controlled and therefore not requiring motion compensation. Linear-SAR systems generally do not have mechanisms to point the antenna toward the target, thus limiting the angle extent of the collection aperture. In contrast, the Circular-SAR can theoretically collect data 360 degrees around the target at a constant range. However, the free-path CircularSAR requires some form of motion compensation of the data for image formation processing.
The collection of radar scattering data necessary for imaging targets with three-dimensional resolution requires frequency diversity, combined with angular diversity over two orthogonal axes fixed to the target. Although the necessary data can easily be collected using modern instrumentation systems when the target is outfitted with an embedded two-axis rotator, some applications preclude the intrusion of the rotator. This paper describes an alternative method for obtaining the required data which uses conventional target rotation in the azimuth plane, combined with a linear displacement of the compact range feed along the vertical axis of the collimator's focal plane. Frequency diversity is provided by a stepped-frequency radar and angular diversities in the horizontal and vertical directions are provided by the target rotation and vertical feed displacement, respectively. The data-collection scheme samples a wedge-shaped volume of the target spatial spectrum (k-space) with radial and angular extents de terminated by the bandwidth and target rotation relative to the radar axis. A three-dimensional image is formed by processing a three-dimensional array of data, typically consisting of 128xll8x128 data samples. The paper describes the experimental set-up used to collect Ku-band data and presents two- and three dimensional images obtained from the data. Considerations of the following issues are addressed in the paper. 1. Aberrations resulting from displacing the feed from the collimator focal point. 2. Control of the linear feed displacement, target rotation, and radar operation to automate the data collection. 3. Methods for calibrating and aligning the data. 4. Signal processing methods which combine wideband, ISAR and spotlight SAR processing for three-dimensional applications. 5. Clutter suppression using zero-Doppler filtering.
Target support and clutter contamination can be a limiting factor in radar cross section (RCS) measurements of signature controlled targets. Conventional ISAR image editing methods can be used to remove contamination, but their performance degrades rapidly when the available resolution is insufficient to identify and separate the support returns from those of the target. ERIM International, Inc. (EI) has developed and successfully demonstrated data post-processing techniques based on 1-D parametric spectral estimators for removing additive contamination from low resolution swept frequency measurements [1, 2]. To further enhance performance and take advantage of the cross-range resolution afforded by target aspect information, EI has investigated the use of coherent 2-D spectral estimation techniques for improved identification and mitigation of measurement contamination in frequency and angle diverse data. In particular, parametric signal history editing (PSHE) algorithms based on 2-D TLS-Prony [3] and 2-D MEMP [4] have been developed and exercised on numerical simulations and measured data. The paper demonstrates 2-D spectral estimation in representative measurement situations, identifies strengths and limitations, and quantifies mitigation algorithm performance. In addition, automated filtering of spectral representations using energy level ordering, Cramer Rao Bounds (CRBs), and spatial filtering are discussed.
This paper presents an efficient three dimensional (3-D) SAR imaging algorithm us ing range migration techniques. The algorithm is used to form 3-D radar reflectivity images of targets measured in anechoic chambers. As an input, the algorithm requires frequency domain backscatter data which have been acquired us ing a stepped frequency system equipped with an antenna that synthesizes a 2-D planar aperture. Resolution in the vertical and horizontal cross-range directions is given by the dimensions of the synthesized aperture, whereas resolution in ground-range is provided by the synthesized frequency bandwidth. The presented formulation has been justified by using the stationary phase method. Results both with syn thetic and measured backscatter data show the high efficiency of the technique. The extension of the algorithm when the antenna synthesizes a 2-D spherical aperture has been addressed. Re sults with this aperture geometry show that the technique is still highly efficient.
Motorola Cellular Subscriber Research Laboratory has developed and installed new hardware and software for measuring the performance of cellular and satellite phones. The hardware facilities consist of twin 16' cubical, near-field, anechoic chambers. Each has a spherical-scan system custom designed for cellular phone testing. The software consists of data collection, data presentation and database management software running under Windows NT 4.0™. Accurate testing of cellular phones requires that the measurements be made with a human phantom consisting of a human-shaped, liquid-filled fiberglass shell. These phantoms are fragile and must remain vertical. This required that an arm be implemented for the theta axis while a typical azimuth only positioner is used for the phi axis. The Theta axis arm is shaped like a "U" and is mechanically driven from both ends allowing the cross-piece of the "U" to be of a lightweight dielectric material so as to have minimum scattering.
Large deployable reflectors were developed by Harris to support the implementation of regional mobile satellite communication systems. High antenna gain as well as very high transmitted power are required to enable communications with hand-held telephones from geostationary orbit. The resulting high repeater gain and the relative proximity of the frequency bands for mobile satellite communications lead to very stringent PIM requirements for the large deployable antennas. Harris has developed deployable reflectors which meet the challenging mechanical and PIM requirements. The PIM verification of such antennas is rather complex due to the significant scattering and interaction between the bus and the very large antennas. This paper presents the large state-of-the-art antenna test facility designed and built by Harris to allow reliable, accurate and straightforward PIM measurements of antennas up to 30 m in diameter. The test facility can also accommodate a full scale mock up of the satellite for the antenna level PIM verification or the satellite itself for an overall system level PIM performance evaluation.
This paper will show a solution to the problem of isolating defogger power connections from an automotive antenna using a tri-filer wound transformer with a ferrite core where the power current flow is organized so that the magnetic field induced by the heater power wires is canceled. The transformer coupling remains effective at low frequencies. A capacitive coupling link is used to permit the system to work at FM frequencies.
This paper describes an antenna measurement system which combines three types of measurements into one integrated range operating from 45 MHz through 18 GHz. A vehicle can be measured at short ranges inside a protective enclosure at various elevation angles for both low frequency Far Field (FF) measurements and higher frequency Near Field (NF) measurements. The vehicle can also be measured on an extended FF 120m range by radiating through the transparent enclosure. The vehicle enters a 12m radius radome and is mounted on a 6m diameter turntable which enables continuous rotation of the vehicle at a maximum speed of 3 rpm. An elevation positioner moves a lOm arm equipped with linear and roll axes at the top, which provide the NF probe movement. Azimuth rotation of the vehicle and elevation movement of the arm provide a complete hemispherical scan. During FF measurements from outside of the radome, the arm is stored below ground level and is covered.
Antenna measurement techniques historically have been dominated by an assumption that an antenna is a discrete component of the overall electronic system into which it is built. Under this assumption, the measurement technique is to remove the antenna from its host electronic system and place it in a generic test system to measure the gain, pattern, etc. Although this technique still applies to many antenna measurements, it does not work well in cellular/PCS handset measurement applications because cellular/PCS handsets exhibit significant electromagnetic coupling to the human holding the phone. Therefore, the antenna should be measured in situ with a person holding the phone or, for practical reasons, with a mannequin arranged such that it can hold the phone. The mannequin is placed on an azimuth positioner and a near-field probe is moved on a very accurate circular arch from zenith to a significant angle below the mobile phone horizon plane. A description of the chamber and system, and measured results are provided.
In this paper, a near-field time domain scattering measurement technique is described. Near-field measurements are typically performed for radiation applications but not scattering applications. This time domain measurement approach borrows from many of the principles developed in the frequency domain and is ideally suited for broadband scattering characterization. The goal of determining the scattered far-fields of a structure is accomplished by the transformation of near-field data collected over a planar sampling surface. The scattered near-fields were generated with a probe excited by a fast rise time step. In particular, the near-fields were sampled with a second probe and digitized using a digital sampling oscilloscope. The bandwidth of the excitation pulse was approximately 15 GHz. The overall accuracy of this approach is examined through a comparison of the transformed far-field pattern to a numerical calculation.
The close proximity of the ground to the radar antenna and the target under test is often hard to avoid at an outdoor RCS measurement range. Ground reflection of energy from the antenna leads to target illumination errors, and target-ground interactions lead to multipath errors. By proper positioning of the antenna and target, ground reflections of the antenna illumination can be exploited to increase overall system sensitivity by concentrating more energy on the target; however, this is only effectivefor narrowband measurements over a limited target region [1]. Reducing target-ground interactions by increasing the target height above the ground generally has limits due to mechanical restrictions on both the radar antennas and the target. This paper will present a model-based data post-processing technique to mitigate illumination errors and target-ground interactions in ground plane range RCS measurements. The algorithm is an extension of the network model multipath mitigation technique previously developed for indoor RCS measurement ranges [2,3,4]. The technique will be described and demonstrated using a numerical simulation of the RCS measurement of a canonical target over a ground plane.
Over the past six years the Navy has developed a portable measurement capability. As part of the validation of this tool a comparison test was developed to understand the issues involving testing complex targets in a near-field cluttered environment. The test was designed to evaluate not only the effects of near field curvature, but how clutter from ceiling and walls have an effect on the accuracy of the measurement. The test measured all test objects in the far-field as a baseline, then repeated the same measurements at five different near-field configurations. The results of the test will be shown on a simple 15 ft. pole target, along with the metrics for evaluation of the results.
The possibility of using the quasi-optical waveguide modeling (QWM) method in the near millimeter and submillimeter regions of the electromagnetic spectrum for study of backward and forward scattering amplitude-phase characteristics of physical objects and their scale models is grounded in the article. The laboratory measuring quasi-optical installation has been carried out for realizing the method. The microwave part of the installation was built on the basis of a hollow dielectric waveguide and quasi-optical devices and transmission-line elements. The results of measurements of backward scattering characteristics and forward scattering characteristics of a number of standard objects made in the 4-mm wave range are presented.
The Seeker Test and Evaluation Facility (STEF) located on Range C-52A at Eglin AFB FL. is used to perform high-resolution multispectral (EO-IR-RF-MMW) signature measurements of US and foreign ground vehicles primarily to support the Research, Development, Test and Evaluation (RDT&E) of smart weapons (seekers, sensors and Countermeasure techniques). In order to support two major DOD signature measurement programs in 1997 this facility required significant range upgrades and enhancements to realize reduced background levels, increase measurement accuracy and improve radar system reliability. These modifications include the addition of a 350'X 120' asphalt ground plane, a new secure target support facility, a redesigned low RCS shroud for the target turntable and a new core radar system (Lintek elan) and data acquisition/analysis capability for the existing radars Millimeter-Wave Instrumentation, High Resolution, Imaging Radar System - MIHRIRS). This paper describes the performance increase gained as a result of this effort and provides information on site characterization and radar instrumentation improvements as well as examples of measured RCS of typical ground vehicle signatures and ISAR imagery
The scattered field from an arbitrary target may include a variety of scattering mechanisms such as specular and diffraction terms, creeping waves and resonant phenomena. In addition, buried within such data are target-mount interactions and clutter terms associated with the test environment. This research presents a method for decomposing a broadband complex signal into its constituent mechanisms. The method makes use of basis functions (words) which best describe the physics of the scattered fields. The MUSIC algorithm is used to estimate the time delay of each word. A constrained optimization refines the estimate and determines the energy for each. The method is tested using two far-field radar cross section (RCS) measurements. The first example identifies targetmount interactions for a common calibration sphere. The second example applies the method to a low observable (LO) ogive target.
Low dielectric permittivity columns are often used in RCS measurements to support targets. Electromagnetic and mechanical interactions between target and mast occur and subtraction cannot eliminate them. In this paper we will study mechanical phenomena such as bending and compression (up to buckling) under load and we will calculate the dielectric mast backscattering level owing to these two deformations. It appears that compression effects are usually negligible compared to bending effects. Finally, we will propose some rules on mast design. More specifically, a cubic section can perform the support of the targets and so decrease the spurious backscattering.
Full polarimetric scattering measurements are increasingly being required for radar cross-section (RCS) tests. Conventional co-and cross-polarization calibrations fail to take into account the small amount of antenna cross-polarization that will be present for any practical antenna. In contrast, full polarimetric calibrations take into account and compensate for the cross-polarization the calibration process. We present a full polarimetric calibration procedure and a simulation-based performance study quantifying how well the procedure improves measurement accuracy over conventional independent channel calibration.
In-situ pattern measurement of JHU/APL's 60-foot parabolic reflector antenna (S-band), using a low-earth orbit satellite as the source is described. The signal strength and X and Y tracking error voltages are measured as the antenna dish sweeps a matrix of points around the position of the moving satellite. The swept region is approximately ±0.30° from the antenna's boresight. This technique was evaluated during April 1998. This measurement was used to baseline the current performance of the ground station before the feed underwent significant modifications. Before the new feed assembly was installed, the position of the current feed was translated to the new feed assembly. Once installed the performance of the reflector was verified. Misalignment of the feed broadens the main beam and increases the sidelobes. More importantly, the inclusion of new components inside the feed also has the potential to introduce phase errors onto the tracking signals. These phase errors will be translated by the auto-track electronics into pointing errors causing the antenna system to inaccurately follow a target. This paper describes the measurement of the reflector antenna pattern and tracking pattern before the new assembly was installed. Results of pattern measurements with the new assembly will be presented at the conference