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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.
For the past seven years, ERIM has been studying RCS measurement error sources and processing methods by which these errors can be reduced. Image editing is an extension of range-gating where scattering measurements are improved by removing undesired scattering phenomena in the range-crossrange image domain. Conventional image editing methods rely on a user-supplied polygon to segment an image into desired and undesired scattering regions. However, the polygon method suffers from variability due to user and display characteristics, provides little hope for automation, and cannot be easily extended beyond two dimensions. An alterative approach based on peak region segmentation minimizes or eliminates these limitations and adds an element of optimally that can also improve the performance of image editing techniques. In this paper, we will discuss the application of peak region segmentation to the image editing problem and show examples that demonstrate some of the advantages of this approach.
ERIM has developed techniques, based on parametric spectral estimators, for removing additive target support contamination from narrowband RCS measurements [1]. These techniques allow target and support returns to be extracted from frequency sweep data with much greater accuracy and resolution than that afforded by conventional Fourier techniques. These algorithms have recently been enhanced to incorporate scattering mechanism frequency dependence in the underlying signal model. Specifically, damped exponential and power-of-frequency sweep data with much greater accuracy and resolution than that afforded by conventional Fourier techniques. These algorithms have recently been enhanced to incorporate scattering mechanism frequency dependence in the underlying signal model. Specifically, damped exponential and power-of-frequency signal models have been used. The modification substantially improves algorithm performance in measurement situations where there is small absolute bandwidth, but relatively large fractional bandwidth, which can lead to appreciable variation in scattering mechanism amplitude. The paper will demonstrate the technique’s ability to remove target support contamination using numerical simulations and compact range measurements of canonical targets mounted on pylon supports. It will be shown that the algorithm can remove the additive pylon contamination even for situations where the pylon return dominates the target return and cannot be resolved from the target in conventional Fourier range profiles.
Compact Payload Test Ranges (CPTR) for test zones of 5 meters or larger can be used for both payload and advanced antenna testing. In both cases accurate calibration, including amplitude and phase characteristics across the test zone, is required. Accurate data analysis is needed in order to establish corresponding error budgets. In addition, boresight determination will be required in both measurement types for most applications. Since it may be difficult or even impossible to scan the test zone field using a (planar) scanner, application of a large reference target (a rectangular or circular flat plate) can be seen as in interesting alternative. RCS measurements are then performed and test-zone field characteristics are determined in both amplitude and phase. Time- and spectral domain techniques can provide valuable information as to the location of possible disturbances. The evaluations is complemented with the measurement of a VAlidation STandard (VAST) antenna in combinations with an advanced APC technique. These techniques have been demonstrated at the CPTR at ESTEC, Noordwijk, the Netherlands. Results and practical considerations are presented in this paper.
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.
A method to generate time and direction of arrival (TADOA) spectra of the quiet zone fields of a RCS/ antenna range is presented. The TADOA spectra is useful for locating the stray signal sources in the RCS/ antenna range. To generate the TADOA spectra, quiet zone fields along a linear scan over the desired frequency band are probed. The probed data are calibrated to remove the magnitude and non-linear phase variation versus frequency. A calibration technique is also proposed in the paper. The TADOA spectra for simulated probed data as well as experimental probed data are shown.
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.
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.
The application of rapid near-field measurement systems based on the Modulated Scattering Technique (MST) and Spherical Angular Function (SAF) data processing of the measured data to extract far-zone RCS of complex targets is discussed in this paper. A first-generation Spherical near-field measurement system for efficiently determining bistatic RCS is presented.
Antenna positioners using a single pivot joint and two linear actuators are attractive for applications requiring limited two-axis motion. Such applications include antenna and RCS measurement systems, and scanning antennas. Minimum swing clearance is required. Positioners can be light, compact and stiff. Position feedback can be independent and linear for both axes. Design and selection considerations are presented. Two examples are described
We discuss how RCS target depolariza tion enhances cross-polarization contamination, and we present a graphical study of measurement error due to depolarization by an inclined dihedral reflector. Error correction requires complete polarimetric RCS measure ments. We present a simple polarimetric calibration scheme that is applicable to reciprocal antenna radars. This method uses a dihedral calibration target mounted on a rotator. Because the calibration standard can be ro tated, there is no need to mount and align multiple sepa rate standards, and clutter and noise may be rejected by averaging over rotation angle.
Historically, radar imaging sensors have been divided into two categories, SAR and ISAR systems. Even though they are solving the same imaging prob lems the data collection environment is dramatically dif ferent between the two. Consequently, the particular waveforms selected for the two have been different. The primary waveform for ISAR RCS measurement systems is stepped frequency, while the FM-chirp (linear-FM) waveform has been used much more often in SAR applications. However, recently this boundary has been blurred, in that stepped frequency radars are being applied to long range dynamic measurements, long the domain of chirped waveforms, and conversely the chirped waveform has been applied to target RCS mea surements of both static and dynamic targets. This paper will address the system parameter tradeoffs involved in selecting between the two waveforms for two different applications; (i) near range static target imaging, and (ii) far range dynamic target imaging. The system parameter tradeoffs involve RF bandwidth, PRF, scene size, trans mitter power, doppler frequency spread of target, etc. The advantages, disadvantages, and inherent limitations of each waveform will be analyzed to yield a better understanding of the tradeoffs involved, and the data collection examples will further illustrate these tradeoffs for the two specific applications.
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.
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.
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.
The ESAT-TELEMIC division at Katholieke Universiteit Leuven (KUL) has three antenna ranges: an indoor Far-Field range, an indoor planar Near-Field range and an outdoor Far-Field range. The positioning equipment is of a variety of manufacturers. The division launched an effort to modernize the range complex and add automatic measurement capabilities, while still retaining control of all three ranges from one control console and using one positioner controller, one angle readout and a single receiver to save costs. The system upgrade included some electrical refurbish ment of the positioning equipment and the replacement of all the old control and data recording equipment with Orbit Positioner Controller/Programmer, Power Control Unit and combined Near-Field and Far-Field software. Control of all three sites is achieved using a special Orbit Junction Box. With the new configuration all three ranges can be operated in fully automatic mode, one range at a time. The software package controls both Near-Field and Far Field measurements using compatible data formats and human interfaces.