AMTA Paper Archive
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Study of a corner reflector of finite thickness
New measurements on the complete polarimetric responses from a 4" dihedral corner reflector from 4 to 18 GHz are presented. As a function of the azimuth, the vertically suspended object may present itself to the radar as a dihedral, a flat plate, an edge, a wedge, and combinations of these. A two dimensional method-of-moment (2-D MOM) code is used to model the perfectly electrical conducting (PEC) body, which allows us to closely simulate the radar responses and to provide insight for the data interpretation. Of particular interest are the frequency and angular dependences of the responses which yield information about the downrange separation of the dominant scattering centers, as well as their respective odd- or even-bounce nature. Use of the corner as a calibration target is discussed.
High resolution filtering of RCS measurements
This paper deals with High Resolution (HR) Filtering. Extracting the frequency dependence of radar scatterers is a common task in Radar Cross Section Analysis (RCS). This is usually achieved with signal processing tools like finite impulse response filters allowing filtering in the range domain. However, when range resolution is poor, it becomes impossible to extract the exact feature since it is not deconvolved in the range domain. We thus propose to use HR methods to overcome these difficulties. These methods are applied to estimate the frequency response of the creeping wave of a small sphere. The results show good agreement with the theoretical response.
Implementing an expert system to analyze phase-array antenna range and diagnostic data
The Antenna Repair Facility at McClellan AFB, which has been responsible for the repair and maintenance of Air Force antennas and radars over several decades, now faces a new challenge: transferring their many years of skill in antenna maintenance from their closing base to acquiring bases or contractors. This must be done while maintaining their in-house expertise and production levels, while manpower decreases. A possible solution to these problems is the implementation of expert systems. This is where human knowledge and expertise is transferred to computerized systems. Currently, the repair shop is testing such a system for analyzing diagnostic data on an electronically steered, phased-array satellite ground station antenna. This paper will examine the design options considered, such as using a commercial software package versus building in a higher order language. It will also discuss the design process for such an expert system, and cover the key issues of knowledge acquisition and selection. The paper will include details of the current design, such as structure and control, and discuss plans for future enhancements.
Analysis of radar measurement system stability factors, A
Instrumentation Radar systems evolution includes improved stability. Metrologists know frequency within Hertz. Amplitude and Phase variations are low. Ranges check drift with reference systems. Still, with increased capability, expectations of accuracy have increased. Todays instrumentation makes analysis of stability factors practical. This study analyzes Radar Cross Section (RCS) return of a stable target under controlled conditions. Methodology will be an analysis of a constant RCS target return. The target is a stable object at a typical measurement site. Data points are at several discrete frequencies in bands between S and Ku. This study sample is a set of data taken over a 87 hour span with several duty factors. Duty factors will range from minimal 0.1% to 1.5%, near the 2% maximum for the output amplifiers. Acquisition times for data sets are chosen for outdoor temperatures ranging from hot -- desert afternoon -- through cool in the early morning. This data will be analyzed statistically. If statistical correlations exist, analysis will quantify factor contributions with multiple linear regression. Hypothesis: Drift does not correlate to variables such as duty factor, & temperature.
Integrated antenna/RCS/EMI compact range based measurement facility, An
ORBIT/FR has recently delivered an integrated facility capable of being used for Antenna, Radar Cross Section (RCS), and EMI measurements to the Naval Underwater Warfare Center in Newport, RI. The facility includes a shielded anechoic chamber, a compact range system capable of producing a 6 foot diameter quiet zone, multi-axis positioning equipment, and a complete complement of Antenna, RCS, and EMI measurement instrumentation and data collection hardware/software. The facility is capable of operation over a frequency range of 100 MHz to 50 GHz, with compact range operation feasible above 2 GHz. The facility can be reconfigured to go between antenna and RCS measurements in any band using both frequency band and antenna/RCS mode switching. In addition, automatic positioning of the appropriate compact range feed to the reflector focal point is available. EMI measurements require minimal relocation of absorber in an isolated area of the chamber floor. Performance of the system is optimized by location of critical RF equipment on the compact range feed carousel or on the positioning system rail carriage. This system offers a unique combination of performance and convenience for making all three types of measurements.
Indoor RCS measurement capability at VHF in the Boeing 9-77 range
This paper discusses Radar Cross Section (RCS) measurement capability at Very High Frequencies (VHF) in the Boeing 9-77 Range in Seattle, Washington. This indoor facility provides a unique asset to the RCS measurement community. Initially operational in 1989, the 9-77 Range was upgraded in 1995 to include a VHF measurement capability. This was achieved using a 56 foot square array of 256 elements, for RCS measurements at frequencies from approximately 140 to 220 MHz, with a 40 foot quiet zone. In this paper, we discuss results from the characterization process used to verify the initial capability and ongoing operation of the RCS measurement system at VHF. We include data demonstrating the sensitivity, stability and dynamic range of the system. We also present samples of recent field probes, and background subtraction and stability measurements. A comparison is made between calculated and measured canonical target signatures.
Technique for collecting and procesing flight-line RCS data, A
Recently, several deployable, ground-to-ground col lection systems have been developed for the assessment of aircraft RCS on the flight-line. The majority of these systems require bulky rail or scanning hardware in order to collect diagnostic imaging data. The measurement technique described in this paper, while not a "cure-all", does eliminate the need for bulky hardware by allowing the collection system to move freely around the target while collecting radar backscattering data. In addition, a nearfield-to-farfield transformation (NFFFT) algorithm is incorporated in the process to allow the collection of scattering data collected in the near field to be processed and evaluated in the far field. The techniques described in this paper are a part of a data conditioning process which improves the data quality and utility for subsequent analysis by an automated diagnostic system described elsewhere in this proceedings . The techniques are described and demonstrated on numerically simulated and experimentally measured data.
RCS characterization on a portable pit with a foam column at VHF/UHF
The RATSCAT radar cross section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and customer desires for certified RCS data. This paper discusses the low frequency characterization of the RATSCAT VHF/UHF Measurement System (RVUMS). The characterization was conducted on a portable pit with a 30' foam column at the RAMS site. System noise, clutter, backgrounds and generic target measurements are presented and discussed. Potential error sources are examined. The use of background subtraction and full polarimetric calibration are presented. Potential errors, which can occur from using certain cross-pol calibration techniques, are discussed. The phase relationship between each polarization components of the scattering matrix and cross-pol validation techniques are considered.
UWB noise radar using a variable delay line
The Ohio State University ElectroScience Laboratory (OSU/ESL) has built a series of radars that transmit UWB random noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. When the response of the system is taken as a function of the delay time, the result is proportional to the impulse response of the system. After background subtraction and calibration, the impulse response of the target results. We will present a description of the variable delay line system and show an example ISAR image made from measurements taken in the OSU compact range.
Antenna/RCS range evaluation using a spherical synthetic aperture radar
We describe an imaging technique which allows the isolation of sources of unwanted radiation on an antenna/RCS range. The necessary data may be collected by using a roll-over azimuth mount to scan a probe over a spherical measurement surface.
Inflatable antenna technology with preliminary shuttle experiment results and potential applications
This paper describes the status of NASA’s Inflatable Antenna Experiment (IAE) and a brief discussion on potential future applications. The space experiment of a 14-meter diameter reflector antenna was flown and deployed successfully aboard the Space Shuttle, STS-11, launched May 19, 1996. Since the flight data is still being processed and reduced, only preliminary results can be presented at this time. The development of the IAE will be discussed along with the results of ground test measurements which were conducted to determine the overall mechanical and projected electrical performance characteristics of this inflatable concept. Large, space-deployable antennas are needed for numerous applications which include mobile communications, Earth remote sensing, and space-based radar systems. Due to the traditionally high cost to develop and launch such large antennas, new technology must be developed which is cheaper, faster, and better. Inflatable antenna technology provides the opportunity to accomplish these objectives.
Measurement system considerations for characterizing antennas with integrated sources
Antennas with integrated RF or microwave sources are becoming more prevalent as the wireless explosion continues to evolve into specific programs and products. These types of antenna modules span several different business areas such as communication satellites, radars, and collision warning systems and cellular or wireless systems. In order to evaluate a device’s true performance parameters, it is desirable to test the device in its actual operating environment. There are a number of different tradeoffs that must be considered when configuring an antenna measurement system to test antennas with integrated sources or transceiver based products. This paper will discuss the tradeoffs available in the antenna measurement system design for a test range that can measure antennas with integrated sources. Several antenna test ranges will be presented and the advantages and disadvantages of each configuration will be discussed.
A Beam-steered array for ground penetrating radar
This paper reports on a ground penetrating radar (GPR) antenna with an electronically steered beam, currently being developed at South Dakota Tech. The increased power and directivity that result from beam-steered operation have potential utility in deep/lossy GPR environments. The antenna is a transmitting array of up to eight bow-tie dipoles, each driven by a narrow pulse generator connected directly to the dipole. The beam is steered in real time by controlling the timing of the individual element transmitters using digitally-programmed pulse delay units. Reception is through a conventional GPR receiver using a single bow-tie antenna. Modeling the air-ground interface as a lossy half-space, numerical results indicate that, under certain conditions, time-domain beam-forming is possible in such an environment. Antenna patterns and standard antenna measurement parameters, such as beamwidth and directivity, are presented in support of this finding.
Performance analysis of the image-based near field-to-far field transformation
At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.
Performance comparison of the analog and digital ramps in a linear-FM chirp RCS measurement radar
The designer of a linear-FM homodyne RCS measurement system must consider the nonlinearity present in the chirp waveform. Two basic methods employed in obtaining the chirp waveform are to apply either a digital ramp or an analog ramp to a YIG oscillator source. Nonlinearity can occur as the result of the characteristics of the YIG oscillator and the applied ramp waveform. The point spread functions useful in characterizing the performance of both the digital and analog ramp excited YIG oscillator systems are given. Both range resolution and dynamic range of the measurement system are dependent on the target range and can be adversely effected by the nonlinearity. Theory shows that the point spread function of a digital ramp is suitable for short range RCS measurements. However the analog ramp system has improved performance at extended range. By using the analog ramp, we have been able to improve performance of RCS measurements over the digital ramp. Experimental data from both the digital and analog ramp systems are provided.
Experimental radar cross section reduction of an array of aluminum cubes using a genetic algorithm
We use a genetic algorithm to optimize a linear array of aluminum cubes in order to reduce the relative Radar Cross Section at a specific angle. A genetic algorithm models biological reproduction and natural selection on a computer in order to optimize the output of some function or experiment. We also examine the initial parameter tradeoffs required to achieve an optimal design with a minimal number of unique measurements. We actually discovered two very different designs, each providing more than 44 dB reduction at five degrees off broadside.
ISAR imaging using UWB noise radar
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.
Diagnostic imaging radar system for the F-117A stealth fighter
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.
3-D imaging of a T-72M at 35 and 95 GHz
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.
Comparison of radar imaging using data extrapolation and adaptive FIR filters
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.
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