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It is possible to build a very inexpensive radar which transmits wide band radio noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. In this paper we will discuss the design and operation of a UWB noise radar which was installed in the OSU compact RCS measurement range. Scattering measurements were made for a number of targets over 360 degrees of aspect angle. Calibration was performed, and then the data converted to ISAR images. Example ISAR images will be shown.
The U.S. Air Force is currently building deployable Diagnostic Imaging Radar (DIR) systems to perform quality control (QC) low-observable (LO) measurements of the F-117 fighter. Each system is a stepped-pulse frequency synthetic aperture radar (SAR) built by System Planning Corporation (SPC) combined with analytical software developed by MIT Lincoln Laboratory for generating radar images that will be interpreted to ensure LO integrity. The DIR systems will be used at fixed operating sites such as the F-117A main operating base, the F-117A maintenance depot, and any sites worldwide to which the aircraft may deploy. The F-117A DIR is the first field-level deployable radar cross section (RCS) measurement system for an operational weapon platform that is designed for use by the maintenance squadron. This paper discusses the critical issues of QC measurements for LO systems. It also describes the test requirements that are driving the development of DIR, and highlights the radar and SAR positioner requirements. Also presented is an overview of the diagnostic software and the algorithms used for detecting RCS anomalies and predicting maintenance actions for problem correction by flight-line crews.
Millimeter Wave (MMW) Radar Cross Section (RCS) measurements of full scale ground vehicles are used to develop and validate scattering models for smart weapons applications (target detection, discrimination and classification algorithms) and Hardware-in-the-Loop (HITL) missile simulations. This paper describes a series of MMW RCS measurements performed at Range C-52, Eglin AFB FL on a T-72M in a field environment using an exiting instrumentation radar (with slight modifications to allow for accurate height adjustment) and in-scene phase reference. The test methodology, instrumentation systems, 3-D Imaging Algorithm and sample data sets at 35 and 95 GHz will be presented as well as a detailed sensitivity analysis and discussion of error effects.
Radar images obtained using an adaptive finite impulse response (FIR) filter are compared with the radar images obtained using extrapolated scattered field data. The scattered fields of an experimental target and an airborne target are used in radar imaging. In adaptive FIR filtering, instead of fixed weights, variable weights are used in radar imaging. In this work, adaptive sidelobe reduction (ASR) technique is used to obtain the variable weights. Also, scattered field data extrapolation is carried out using forward backward linear prediction. It is shown that is the data extrapolation is successfully carried out by a factor of two or more, than the radar images obtained using the extrapolated scattered field data have better resolution than the radar images obtained using the adaptive FIR filters.
This paper describes a technique for determining the magnitude of a radiating field from measurements taken using an Infrared (IR) camera. A thin resistive screen of low conductivity is positioned in the radiating field. The resistive screen absorbs energy from the radiating field and converts this energy into heat within the screen. An IR thermal picture is then taken of the heat distribution on the screen. The resulting 2D image is called an IR thermogram, i.e. and iso-temperature contour map of the data which is a representation of the electric field. The thermogram is then processed to determine the intensity (magnitude) of the radiating field at each pixel location in the thermal image. We describe the technique and show comparisons made between standard probe measurements and results from measured IR thermographs.
This paper describes the application of the plane-to-plane (PTP) iterative Fourier processing technique to infrared (IR) thermographic images of microwave fields for the purpose of determining the near-field and far-field patterns of radiating antennas. The PTP technique allows recovery of the phase by combining magnitude-only measurements made on two planes, both in the radiating near field of the antenna under test. We describe the PTP technique and show excellent comparisons between the predicted results and results from measured IR thermograms of the field of a 36 element patch array antenna operating at 4 GHz.
An examination of mutual coupling effects in a linear phased array is presented. The approach derives mutual coupling coefficients from array element patterns measured in the Fresnel region, at R/D=3. The technique allows edge diffraction effects and mutual coupling effects to be identified and separated. The results are compared with conventional mutual coupling measurements and mutual coupling coefficients determined by numerical integration. The technique is used for far-field pattern reconstruction, and for pattern optimization which corrects mutual coupling effects to the maximum extend possible.
Accurate EIRP measurements are possible to make on a near-field range but require great care and attention to detail. NSI has recently implemented a near-field test range for the Globalstar satellite program which makes automated EIRP and gain measurements. Automation for this program is extremely important since the production cycle requires testing many antenna systems per month, each of which has two antennas with 16 separate beams per antenna. Among the various range measurements, EIRP is the key parameter of the Transmit antenna’s performance. This paper reviews the measurement theory of EIRP measurements and presents some of the results of this automated activity.
For indoor antenna measurements, compact ranges or near-field/far-field techniques are most frequently used. One of the major problems is the handling of physically large antennas. Compact ranges will in general provide test-zone sizes up to approximately 5 meters in diameter. Applying the planar NF/FF technique, even larger test-zone sizes can be realized for certain applications. On the other hand, requirement of real-time capability, for instance in production testing, will exclude NF/FF techniques. It has been shown previously that single-plane collimators have a pseudo real-time capability which makes these devices comparable to compact ranges. Furthermore, the physical test-zone sizes which can be realized when compared to compact ranges are approximately 2-3 times larger for the same size of the anechoic chamber. Finally, it will be shown that the accuracy in sidelobe level determination, gain and cross polarization is considerable higher than with other indoor techniques, even at frequencies below 1 GHz.
In recent years there has been much interest in developing low frequency radar cross section (RCS) measurement capability indoors. Some of the principal reasons for an indoor environment are high security, all-weather 24-hour operation, and low cost. This paper describes recent efforts to implement VHF/UHF RCS measurement capability down to 100 MHz using the large compact-range collimator in the Bistatic Anechoic Chamber (BAC) at Point Mugu. The process leading to this capability has given rise to a number of technical insights that govern successful test results. An emphasis is placed on calibration and processing methodology and on measurement validation using long cylindrical targets and comparing the results with method-of-moment computer predictions and with measurements made at other facilities.
A fully-polarimetric compact range operating at 160 GHz has been developed for obtaining X-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, CW, X-band synthesizer driving dual X16 transmit multiplier chains and dual X16 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and a rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 40 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 20” quiet zone. A description of this 160 GHz compact range along with measurement examples are presented in this paper.
A folded compact range configuration has been developed at the Sandia National Laboratories’ compact range antenna and radar-cross-section measurement facility as a means of performing indoor, environmentally-controlled, far-field simulations of synthetic aperture radar (SAR) measurements of distributed target samples (i.e. gravel, sand, etc. ). In particular, the folded compact range configuration has been used to perform both highly sensitive coherent change detection (CCD) measurements and interferometric inverse-synthetic-aperture-radar (IFISAR) measurements, which, in addition to the two-dimensional spatial resolution afforded by typical ISAR processing, provides resolution of the relative height of targets with accuracies on the order of a wavelength. This paper describes the development of the folded compact range, as well as the coherent change detection and interferometric measurements that have been made with the system. The measurements have been very successful, and have demonstrated not only the viability of the folded compact range concept in simulating SAR CCD and interferometric SAR (IFSAR) measurements, but also its usefulness as a tool in the research and development of SAR CCD and IFSAR image generation and measurement methodologies.
This paper discusses the test capabilities of the Benefield Anechoic Facility (BAF) and its mission to support avionics and electronic warfare (EW) test and evaluation (T&E) of current and future generation manned and unmanned aerospace vehicles. Testing at the BAF can provide the dense, complex, and realistic signal environment necessary to evaluate integrated systems/subsystems to meet both Development Test and Evaluation (DT&E) objectives. The BAF, located at the Air Force Flight Test Center (AFFTC), Edwards Air Force Base, California, USA, is part of the Avionics Test and Integration Complex (ATIC). The BAF provides a quiet, secure, and controlled electromagnetic environment to test installed/integrated systems, their associated weapons, avionics and EW systems. This testing is accomplished within a very large anechoic chamber, providing a realistic free-space and controllable radio frequency (RF) environment.
The Hughes Space and Communications Company (HSC) is in the process of completing the construction, installation and validation of two large horizontal near-field antenna measurement ranges. These new measurement systems are located in the existing HSC satellite factory building. These ranges will be used to measure various types of directive satellite antennas both at a unit level and at spacecraft level. The facility will accommodate mechanical integration of the test articles as well. This facility is the result of Hughes committing the time and money to create a state of the art antenna measurement facility that will be highly efficient and accurate. A detailed description of this facility’s configuration, design and current status will be discussed herein.
Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG). We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.
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.
Some precautions necessary for accurate RCS measurements using a short model range are discussed. Sources of error in these measurements such as non ideal range geometry, misalignment of the target and inappropriate time domain gating are discussed. A simple technique to estimate possible errors in RCS measurements due to factors such as bistatic angle due to finite separation of source and receive horns and finite length of the measurement range, is presented. The range of RCS values that can be measured within defined error bonds is identified.
A new approach for certification of RCS ranges is discussed. This new approach is based on evaluating the major expected sources of errors in a RCS range rather than evaluating each and every error source and then defining the error bar for a given RCS measurement. The new approach is, therefore, called a top-down approach. Based on our experience with many indoor RCS ranges, we can say that the main sources of errors in RCS measurements are range related. (stray signals, chamber drift, target/mount interactions etc.) One should, therefore, critically evaluate these errors such that the performance level of the range can be verified. A test approach is defined to characterize the range related errors. Various tests are based on the RCS measurement of specific targets, and thus, can be easily performed using standard RCS measurement procedure. This approach will provide range operators with the needed information to justify the use of their range to measure RCS of a given target. Also, one can spend more effort fixing the error sources which lead to large RCS measurement errors.
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.