Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
Spacecraft payload testing with full power necessitates to prevent radiation over areas reserved to personnel. Moreover, the increase of spacecraft Radio Frequency power needs to install high power absorbers in front of the RF flux. The acceptable limit temperature of such absorbers is provided by the manufacturers. This so being, the temperature behavior of such absorbers have to be correlated with RF Power Flux Density in order to define the spacecraft test set up. Using a well known flight spacecraft antenna, Alcatel Space Industries have performed a non destructive characterization of the temperature behavior of a high power absorber versus PFD. First, the total transmitted power by the antenna was computed using TICRA Grasp8 [1] software. Then the predicted PFD was correlated to the measured one and the temperature of the wall was recorded with an infrared camera, and related to this measured PFD. Such a result gave a simple relation between PFD and temperature. The relation is now used for temperature predictions on the absorbers during high power spacecraft tests, and helps to manage test set up and air cooling installation. The available results are limited to Alcatel Space Industries needs, but could be extended to any type of absorber, in a wide temperature and frequency range.
Undesired antenna motion can significantly degrade SAR and ISAR image quality on an instrumentation radar operating in an outdoor or uncontrolled environment. Antenna vibration on the order of only a few hundredths of an inch at X-band frequencies can degrade performance to the point that one cannot reliably differentiate between the true and false peaks in the radar image. This paper describes a motion compensation technique that utilizes the measurements from an auxiliary antenna pointing at a stationary target. This "Phase Monitoring Subsystem" accurately records the linear antenna motion profile, which can then be used for compensation. Data collected at the US Naval Undersea Warfare Center (NUWC) Fisher Island Test Facility on a calibration target demonstrate that this compensation technique can reduce image artifacts by more than 20 dB.
Inverse synthetic aperture radar (ISAR) imaging on a turntable-tower test range permits convenient generation of high resolution two- and three dimensional of radar targets under controlled conditions, typically for characterization of the radar cross section of targets or to provide data for testing SAR image processing and automatic target recognition algorithms. However, turntable ISAR images suffer zero-Doppler clutter (ZDC) artifacts and near-field errors not found in the airborne SAR images they seek to emulate. In this paper, we begin by reviewing a technique to suppress ZDC while minimizing effects on the target signature. Next, turntable ISAR images of a vehicle formed at Georgia Tech's Electromagnetic Test Facility are used to demonstrate a computationally-efficient implementation of a backprojection (BP) image former. BP-formed ISAR images are free of all first order near-field errors. Finally, images generated using these techniques are compared to images obtained using electromagnetic prediction codes.
Analyzing very large ISAR RCS data files using traditional processing software is often a cumbersome experience. The user is often forced to print out hundreds of images manually to get an overview. We propose a solution to this problem. A generalized ISAR algorithm is utilized to automatically generate a series of complex images, creating a "movie" of images with all the information in every pixel. Regions of interest can be zoomed in or scaled to the desired range. Regions can be gated out and the corresponding RCS. presented. The time to perform analysis tasks can be reduced by factors of 10-100. The implementation, which also contains modules for filtering and statistics, has been named Columbus. The use of Matlab and C provides portable code and a flexible platform for further development.
This paper presents the development and testing of a small low-cost (COTS) 3.5 GHz Ground Vehicle Detection Radar. In this frequency band, the signal can penetrate light brush and foliage. However, the detection and tracking of radar targets where both the radar and the target are close to the ground is particularly difficult because of ground wave attenuation and foliage dynamics. The radar uses all surface mount components for small size and low cost. A VCO is used to cover the frequency band from 3.1 to 3.6 GHz. A power splitter and a quadrature mixer follow the VCO. Thus the radar operates as a base-band system for direct down-conversion. We will show the design procedure for this radar as well as test results confirming the design. We will also show detection and tracking results for vehicle targets in this foliage/brush penetration close-to-the-ground environment.
The measurement of the frequency response of complex targets of interest for the purpose of radar cross section (RCS) analysis has become a common task for modern radar ranges. When carefully done to avoid transients, the stepped frequency continuous wave (CW) method directly measures the frequency response of the target. On the other hand, dechirp-on-receive processing utilized by linear frequency modulated (LFM) radars introduces certain distortions to the measurement that are rarely fully considered. In this paper, we derive the relationship between the true frequency response of a target and what is measured with an LFM radar utilizing dechirp-on-receive. One can use this relationship to analyze the effects of the LFM processing as a function of the target geometry or scattering mechanisms and radar parameters. Radar parameters may then be selected so as to minimize the differences between the LFM measured response and the true frequency response of the target.
Recently there has been a large effort to improve RCS range performance. Reducing errors associated with an RCS measurement requires the identification of stray signal sources, highly accurate calibration, and an understanding of the target mount interactions. This paper will illustrate the potential errors resulting from target mount interaction. A complex RCS target of generic shapes was designed to illustrate target support interactions. Target features include a front wedge shape, a rear circular shape and a vertical fin. All the target features are separable in time using a 2-18 Ghz measurement system. The target features were designed to strongly interact with the ogival pylon. Measurements using the metal ogival support show strong interactions resulting from the shadowing effect produced by the metal ogival pylon. The measurements were repeated using a foam column mount. Since the foam column interacts much less strongly than the metal ogive, the foam column results are much more accurate.
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.
Telecommunications are critical in the lives of every individual on Earth and are rapidly with time. To meet the demand and ever decreasing system cost and bandwidth requirements; many communication systems transmit and receive simultanrously through one antenna path. Any interaction andmixing in the signal paths can produce unwanted mixed (passive intermodulation PIM), if occurring within the receiver band could degrade the system performance. It is therefore critical to ensure that PIM products are insignificant within a receiver band. To achieve this, appropriate system design, build techniques and measurement standards must be used. Although much published work is available, known, repeatable, standardized test methods and data are needed to cost effectively designed down PIM emissions to the required values. Internationally agreed standards for PIM measurement and guidance information, are being written by IEC working group 5. Standards, IEC 1580-2 and IEC 62037 have been published and more are in progress.
Good communication quality in wireless telecommunication systems requires maintaining an acceptable Carrier-to-Interference (C/I) ratio. Therefore, the goal is to keep 'I' as low as possible. In the ideal case, 'I' would always be below the receiver noise floor. One source of undesired interference is passive intermodulation (PIM). AH components and subsystems in the basestation that carry two or more RF signals along their transmission path have the potential to generate PIM. Consequently, PIM has become an important performance criterion. To bring uniformity and consistency to this difficult measurement, the International Electrotechnical Commission (IEC) has defined recommended test methods for PIM measurements. Assembling and maintaining a test setup that provides the necessary test capabilities can be a formidable task. The test system described herein provides a convenient means for companies to realize this test capability in both an engineering and production environment at a level of performance that exceeds that required for accurate and repeatable measurements.
The successful PIM measurement of an antenna is usually a rather challenging task which is greatly complicated by the fact that the antenna is, in essence, coupled to its operating or test environment. External PIM sources or extraneous interfering signals can substantially increase the difficulties associated with antenna PIM testing. This paper provides some general recommendations regarding the derivation of the test requirements, the different generic test setups, the selection of a suitable test environment and the actual PIM measurements of antennas.
1. Introduction 2. IM2-a new unknown criteria for dual-band systems 3. Theoretical approach 4. Comparing measurements of IM2 and IM3 5. Devices under test (DUT) 6. Measurement results 7. Discussion of results 8. Basic technologies to minimize generation of intermodulation 9. Silver surface 10. Solid inner and outer conductors 11. Solderjoints to corrugated copper cables 12. Summary
Standard gain horn antennas are commonly used as reference antennas in establishing absolute gain levels of antennas under test. However, their high sidelobes and backlobes can interact with the structure surrounding the horn in the measurement setup. These interactions degrade the accuracy of the gain values. Thus, while the gain of the horn may be carefully calibrated in free space, its gain value in the measurement environment can differ from its free space value. Examples will illustrate this problem and ways are described to reduce the sensitivity to the environment.
The design of hybrid-mode dielectrically-loaded horns [1][2] for antenna test range illumination is described. These horns have a wide operating bandwidth of 5:1 or greater and were designed to replace conventional corrugated- or smooth-walled illumination horns that, typically, have a bandwidth of 2:1or less. Dielectrically-loaded horns have the radiation characteristics desirable for test range illumination: principal plane pattern symmetry, reasonably low cross-polarization and low sidelobes, low reflection coefficient and relatively constant beamwidth. At CSIRO we have developed software and manufacturing techniques to design and make these horns accurately. Measured results, that show close agreement with predicted values, are presented for a horn made for the frequency range, 7 to 40 GHz.