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The RATSCAT Radar Cross Section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and program office desires for certifies RCS data. The first step is to characterize the Low Frequency portion of the RATSCAT Mainsite Integrated Radar Measurement System (IRMS). This step is critical to identifying error budgets, background levels, and calibration procedures to support various test programs with certified data. This paper addresses characterization results in the 150 – 250 MHz frequency range. System noise, clutter, background and generic target measurements are presented and discussed. The use of background subtraction on an outdoor range is reviewed and results are presented. Computer predictions of generic targets are used to help determine measurement accuracy.
In 1993, we presented the newly completed compact range and tapered chamber facility [1]. As part of this presentation, the issue of “range certification” was presented. This paper will discuss the work that we have done with the compact range for radar cross section (RCS) measurement acceptance. For customer acceptance, we had to “prove” that the compact range made acceptable measurements for the fixtures and apertures involved. Schedule and funding did not permit the full exploitation of the uncertainty analysis of the chambers, not was it felt to be necessary [2]. The determination of our range capabilities and accuracy was based on system parameters and target measurements. Targets that were calculable either in closed form solutions (spheres) or by numerical methods (cylinders and rods) were used. Finally, range to range comparisons with the Rye Canyon Facility [3] of a standard target was used. The range to range comparison proved especially difficult due to customer exceptions, feed differences, and target mounting. This paper will discuss the “success” criteria applied, the procedures used, and the results. The paper will close with a discuss of RCS standards and the range certification process.
It is shown that for dispersive scatterers, one cannot relate the intensity of the spatial response in the radar image to a particular location on the scatterer. For such cases, an imagery capable of characterizing both the spatial (dominant scattering centers) as well as spectral (resonant modes) wave objects. In the new image reconstruction, first a focusing point is selected for the image. The section of the image close to this focusing point provides good spatial resolution. As one moves away from the focusing point, the utilized bandwidth is reduced to provide good spectral resolution. The capabilities of the proposed imagery are illustrated using several examples.
In this paper we present a recently-developed adaptive method for decomposing radar signals into a wide range of physically meaningful mechanisms. The signal decomposition we will pursue is based on a set of functions referred to as “atoms” in the signal processing literature. These atoms can be derived from disparate mechanisms such as scattering center responses given by the Geometric Theory of Diffraction (GTD), which are localized in range, and resonance phenomena, which are localized in frequency. The atoms populate a “dictionary” of waveforms which are used to decompose radar signals. Traditional Fourier methods have restricted the class of atoms solely to harmonic exponentials. In this paper, we consider a number of signal decomposition methods. We chose a method based on the Basis Pursuit procedure of Chen and Donoho [1],which uses an L1 norm as opposed to a least squares approach to search the dictionary. We chose this technique because of its sparsity of representation and convergence properties. We will show results of this technique applied to numerically simulated data and show that disparate scattering mechanisms can be isolated and identified in the radar data.
To accurately measure static or dynamic Radar Cross Section (RCS), one must use precise measurement equipment and test procedures. Recently, several DoD RCS ranges, including the Advanced Compact RCS Measurement Range at Wright-Patterson AFB, established procedures to estimate measurement error. Working cooperatively with the National Institute of Standards and Technology (NIST), Wright Laboratory established a baseline error budget methodology in 1994. As insight was gained from the error budget process, we noted that many common RCS measurement calibration techniques are subject to a wide variety of potential error sources. This paper examines two common so-polarized calibration devices (sphere and squat cylinder), and discussed techniques for evaluating calibration induced errors. A rigorous “double calibration” methodology is offered to track calibration measurement error. These techniques should offer range owners fairly simple methods to monitor the quality of their primary calibration standards at all times.
The calibration of nonreciprocal radars has been studied extensively. A brief review of known calibration techniques points to the desirability of a simplified calibration procedure. Fourier analysis of scattering data from a rotating dihedral allows rejection of noise and background contributions. Here we derive a simple set of nonlinear equations in terms of the Fourier coefficients of the data that can be solved analytically without approximations or simplifying assumptions. We find that independent scattering data from an additional target such as a sphere is needed to accomplish this. We also derive mathematical conditions that allow us to check calibration data integrity and the correctness of the mathematical model of the scattering matrix of the target.
Dynamic ground-to-air measurement of aircraft RCS has several advantages over static measurements. The target may be measured in flight configuration and the support pylon is eliminated. Although dynamic RCS imagery has been performed since the late 1970s, the cost and complexity of such measurements have limited their utility for routine testing. In this paper, an easily deployable ground-to-air radar imaging system developed by Aeroflex Lintek is presented. This system forms images of aircraft in straight flight, requiring no on-board instrumentation or special pilot training. The radar system, flight profiles, and processing tools required for generating images of aircraft in flight are presented, along with examples of measured target data.
The Hughes Space and Communications Company (HSC) has recently undertaken the task to modify a RCS range once operated by Hughes Radar and Communications Systems, to accommodate the testing of Satellite Antennas. This measurement facility's configuration, design and current status will be discussed herein. This RCS range is located in El Segundo, California.
A portable system for measuring the RF environment at remote sites is described. A frequency range between 500 MHz and 18 GHz is covered by this system. The design, calibration and use of this system are discussed.
The imaging of radar targets is typically accom plished by measuring the radar cross section (RCS) of the target as a function of frequency and az imuth angle. We measure a third dimension of the RCS by tilting the target and collecting data for conical cuts of the RCS pattern. This third dimension of data provides the ability to estimate the three-dimensional location of scattering centers on the target. Three algorithms are developed in order to process the three-dimensional RCS data.
New windows which allow the user to select the level of sidelobe suppression near the DFT resolution limit are reported. By a parametric study, we identify the truncated Lorentzian and Gaussian functions as better choices compared with the popular Hann windows.
The Hughes Space and Communications Company (HSC) has recently undertaken the task to modify a RCS range once operated by Hughes Radar and Communications Systems, to accommodate the testing of Satellite Antennas. This measurement facility's configuration, design and current status will be discussed herein. This RCS range is located in El Segundo, California.
RCS data measured under near-field conditions is corrected to the far-field. The algorithm uses the HUYGEN's principle approach. The processing technique is describes and validates using anechoic chamber data and simulations taken on flat plate target at a distance from the radar R << 2D2/A, where D is the target cross range extend and A the wavelength. Good agreement with the theoretically predicted far-field RCS patterns is obtained.
At high frequencies, the scattered fields from a radar target can be modeled as a sum of contri butions from a finite number of scattering centers. We use a parametric model based on the Geometric Theory of Diffraction (GTD) to estimate the location and type of scattering centers present in a frequency domain data set. The parameters of the model are estimated using a modified MUSIC algorithm that incorporates the GTD model. A new spatial smoothing algorithm is also introduced.
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.
A novel test article, the Transformable Scale Aircraft-Like Model (TSAM), which holds great promise for validating complex computational electromagnetic (CEM) codes more effectively is described. The novelty of TSAM is in the use of removable/replaceable canonical shaped structural components. The complexity in TSAM can be tailored to the modeling capabilities of the CEM code under test allowing discrepancies between measurement and simulation to be more explainable. A set of preliminary measurements on TSAM have been made and the results compared to calculations from the General Electromagnetic Model for the Analysis of Complex Systems (GEMACS) program (1), a standard CEM code.
Boresight and gain determination play an important role in antenna measurements. Traditionally, on outdoor ranges, optical methods are used to determine the boresight. Accuracy requirements better than 0.001 degrees are difficult if not impossible to obtain on outdoor ranges using these method since the effect of incident electromagnetic fields are not taken into account. On indoor ranges no technique is available at present that achieves the desired accuracy demands. In this paper, an improved method for boresighting will be presented. It will be shown that using this technique, desired accuracy demands on both outdoor and indoor can be obtained. Furthermore, the method can also be combined with accurate gain calibration. Advantages and disadvantages of this technique will be discussed.
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.
Planar near-field measurements are the usual choice when testing phased array antennas. NSI recently delivered a large state-of-the-art near field measurement system for testing a multi beam, solid state phased-array antenna. The critical sidelobe and beam pointing accuracy specifications for the antenna required that special attention be paid to near-field system design. The RF path to the moving probe was implemented using a multiple rotary joint system to minimize phase errors. Additional techniques used to minimize system errors were an optical probe position correction system and a Motion Tracking Interferometer (MTI) for thermal drift correction.
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.