AMTA Paper Archive
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Planar Near-Field Scan Plane Truncation Applied to the Measurement of Large Phased Array Antennas
An empirical study on Planar Near-Field Scan Plane Truncation applied to the measurement of a large phased array radar antenna saves test time per antenna. Lockheed Martin has been manufacturing, aligning, and verifying the AEGIS SPY-1B/D phased array radar antenna for the past 17 yrs . A custom built planar nearfield scanner system (ANFAST II) was designed and built specifically for this purpose. Existing raw near-field measured data sets were cropped in both the X and Y scan planes, processed to the far field, and compared with the un-truncated data to determine the error sensitivity vs near-field amplitude level truncated. Near-field measurements were then acquired at the truncated scan plane dimensions and compared. It was demonstrated that 100 hrs of test time could be saved by applying this technique without adversely effecting the antenna measurement uncertainty. This paper discusses the application of the truncation technique, results of the experiments, and practical limitations.
Broadband Radar Cross Section Predictions and Measurements for a Canonical 3-Meter Ogive Body of Revolution
In previous AMTA Symposia, the Air Force Research Laboratory reported on a successful effort to fabricate, measure, and predict the precise radar cross section (RCS) for various cylindrical calibration targets . In this paper, we apply what we have learned about calibration cylinders to the study of a 3.048 meter ogive body of revolution. Recall that an ogive is simply the arc of a circle spun on its axis. The radar signature of this shape is extremely small in the direction of the "point", even at low frequencies. A few years ago, AFRL had the subject ogive built for an RCS inter-range comparison between AFRL and the NRTF bistatic RCS measurement system . In this paper, we utilize this ogive body to assess both the quality and accuracy of VHF RCS measurements and predictions performed using multiple calculation schemes. In the end, reconciling the ogive measurements and predictions led us to reassess how composite objects are "conductively coated" to simulate a perfect electric conductor. This insight resulted in refinements in the process for measuring and predicting the ogive at low frequencies where electrical size and electromagnetic skin depth considerations are important.
Aircraft Tracking and Dominant Scatterer Imaging With Integrated X-Band Radar Systems
Radar return data from various types of aircraft were collected and analyzed during varying flight profiles to determine the presence of consistent, dominant radar returns of point scatterers on the aircraft. These measurements were performed by integrating two separate X-band radars into one system with the ability to simultaneously track and image aircraft. Selected processed data from both radar systems were analyzed and are presented as a function of time, azimuth and elevation angle, and range. I/Q data, high-range resolution (HRR) profile data and inverse synthetic aperture range (ISAR) data are presented for selected flight profiles of helicopters, propeller aircraft, and jet aircraft.
Compact RCS Imaging System
The implementation of low observable (LO) materials and the fielding of aircraft with controlled signatures creates a new degree of difficulty for maintaining, executing prompt accurate inspections and achieving meaningful evaluations. To address this problem, Sensor Concepts, Inc (SCI) has prototyped a new radar system, (the SCI-Xe) to provide a test bed for a lighter, smaller RCS measurement and imaging system. The hardware consists of a suitcase containing RF hardware, computer and display and a hand-held or rail-mounted unit containing two X/Ku band antennas. In the rail-mounted application, imaging is followed by registration and image differencing, which allows an operator reproduce a baseline measurement geometry and evaluate RCS changes. The hand-held application forms a synthetic aperture by moving the antennas by hand. This can be used to quickly investigate an object under test.
On the Use of Wavenumber Migration for Linear SAR Image Formation and Near-Field to Far-Field RCS Transformation
This paper presents a first-principles algorithm for estimating a target’s far-field radar cross section (RCS) and/or far-field image from extreme near-field linear (1- D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. Wavenumber migration (WM) is an approach that was first developed for the problem of geophysical imaging and was later applied to airborne SAR imagery , where it is often referred to as the “Range Migration Algorithm (RMA)”. It is based on rigorous inversion of the integral equation used to model SAR/ISAR imagery, and is closely related to processing techniques for near-field antenna measurements. A derivation of WM and examples of approximate farfield RCS and image reconstructions are presented for the one-dimensional (1D) case, along with a discussion of the angular extent over which the far-field estimates are valid as a function of target size, measurement standoff distance, and near-field aperture dimensions.
Uncertainty Analysis on the RCS Measurements from Calibration Objects
In order to better estimate the uncertainties in measured RCS for the Boeing 9-77 Compact Range, we study the responses from three high-quality objects, i.e., two ultraspheres of 14” and 8” in dia., plus the 4.5" squat-cylinder, each supported by strings. When calibrated against each other in pairs, the differences between measured RCS and predicted values are taken as the uncertainties for either object. Two standard-deviations from the target, reference, and background, as computed from repetitive sweeps, are taken as the respective uncertainties for the signals. Using the root-sum-squares (RSS) method, the error bars are found to be between + 0.1 to 0.2 dB for most of the frequency F, from 2 to 17.5 GHz. We also analyze the responses from a thin steel wire (dia. 0.020"), supported by fine fishing strings (dia. 0.012"), at broadside to the radar. When the ‘wire and string’ assembly is oriented vertically, the HH echo from the 3-ft metal wire alone happens to be comparable to the HH from the 30-ft dielectric strings. Varying with F4, the combined RCS in HH for the assembly spans a wide range of 38 dB from 2 to 18 GHz. The error bounds are found to bracket the measured traces even when the signals are barely above the noise floor.
A Truck-Based Multifrequency SAR Platform
The growing need for a mobile radar system able to conduct measurements away from fixed radar ranges has prompted System Planning Corporation (SPC) to develop a mobile MkV radar system. Planned helicopter-based SAR measurements generated a requirement for a ground-based platform to verify functionality of X-band and VHF/UHF data collection and processing systems. Accordingly, SPC developed TruckSAR, a DGPS-equipped mobile testbed to collect side-looking and normal-incidence SAR data. Interleaved step chirp data were collected at 9.0-9.3 GHz (HH polarization) and 150-450 MHz (HH, VV, HV, and VH polarization). The system is self-contained and is proving useful for applications beyond ground and foliage penetration SAR investigations. This paper describes the TruckSAR hardware and data analysis systems. Results of measurements are presented, along with observations of challenges in data interpretation. Promising extensions of this mobile ground-based radar are also discussed.
Thermographic/Holographic Calibration of Phased Array Antennas
A thermal technique for the remote calibration of phased array radar antennas is proposed in this paper. The technique is based on infrared (IR) measurements of the heat patterns produced in a thin planar detector screen placed near the antenna. The magnitude of the field can be measured by capturing an isothermal image (IR thermogram) of the field with an IR imagining camera. The phase of the field can be measured by creating a thermal interference pattern (IR/microwave hologram) between the phased array antenna and a known reference source. This thermal imaging technique has the advantages of speed and portability over existing hard-wired probe methods and can be used in-the-field to remotely measure the magnitude and the phase of the field radiated by the antenna. This information can be used to calibrate the individual elements controlling the radiation pattern of the array.
A Simple Analysis of Near-Field Boresight Error Requirements
The need to measure the boresight pointing direction of radar antennas to a high degree of accuracy yields a requirement for excellent positioning accuracy on near-field antenna ranges. Evaluation of this requirement can be accomplished by a full and complete sensitivity analysis. Alternatively, to gain an understanding of the effects of errors more simply, one can approach the question of accuracy required in the setup, by use of a physical model and straightforward physical reasoning. The approach starts with the assumptions of a collimated wave with planar phase fronts and the premise that the boresight direction of such a sum beam is along the normal to the phase fronts. A sensitivity analysis of the simple trigonometric boresight relationship between mechanical boresight and phase front normal, shows how accurate the receiver and the positioner must be to achieve a given boresight determination. Such an approach has been known for many years as it regards planar scanning; and, the results are known to be applicable. In this paper this consideration is extended to spherical scanners to arrive at estimates of the mechanical positioner accuracies and electrical receiver accuracies needed to make boresight measurements of radar antennas with spherical near-field ranges.
Estimating the Contribution to RCS Uncertainty From Non-Planar Illumination in a Compact Range
Compact RCS measurement ranges all suffer from some level of non-ideal field illumination. Stray fields from interactions with the chamber wall and diffraction effects are major contributors to the non-uniformity of the incident field at the target. This non-uniformity gives rise to unavoidable errors in RCS measurements. We present a detailed analysis of how non-uniform illumination manifests itself into RCS measurement errors. The analysis approach is based on the plane wave spectral decomposition of the illumination. We compute the energy scattered by the planar components of the illumination and determine how much of this energy is coupled backi nto the radar antenna. We model the target as a diffuse scatterer by using a collection of point scatterers distributed within a specified volume. We present uncertainty results based on a simulation as well as field probe data collected from AFRL’s Advanced Compact Range (ACR).
Performance of an Experimental Outdoor RCS Range With R-Card Fences
A 30-meter experimental outdoor RCS range designed to operate from 6-18 GHz is described. In the range, the radar antenna height is 60 cm; whereas the center of the quiet zone is 3 meters above ground. The test range, therefore, has features of many real world outdoor RCS ranges. The test range uses six R-card fences with edge taper to eliminate the ground bounce term. Using the quiet zone field probe data and backscatter measurements, it is demonstrated that the R-card fences are very effective in eliminating the ground bounce term.
Three-Dimensional RCS Imaging of Ground Vehicles at an Outdoor Static Test Range
Static RCS ranges typically generate RCS imagery using ISAR imaging techniques. This provides a twodimensional image of amplitude plotted within some down-range and cross-range extent. The down-range resolution is a function of the bandwidth of the radar system while the cross-range resolution is a function of the target motion between consecutive measurements. A radar look down angle of 0-degrees provides the maximum cross-range information because the target’s movement is normal to the transmitted wave front. As the radar look down angle is changed from 0-degrees to 90-degrees less cross-range information is gathered as the target movement becomes more coplanar to the transmitted wave front. At a radar look down angle of 90 degrees no cross-range information can be discerned. To collect 3-dimensional data for imagery at a look down angle of 90-degrees a raster scan type process can be used. In this implementation the beamwidth of the radar antenna was changed to produce a 6-inch spot on the target rather than fully illuminating the target as is typical with ISAR imaging. A rail was built over the target to support a linearly scanned reflecting plate to direct the transmitted pulse down onto the target to simulate a radar look down angle of 90-degrees. The target was rotated 370-degrees (10-degree overlap) beneath the stationary reflecting plate providing a circumferencial scan i.e. a ring. After each rotation, the reflecting plate was moved a fixed interval radially and another ‘ring’ of data was collected. This procedure was repeated until the entire target was measured. This method of scanning provided two-dimensional information of the target’s length and width with height information obtained by using a 256-stepped-frequency waveform over a bandwidth of 1.6 GHz providing complete three-dimensional imagery.
Design of Target Support Columns Using EPS Foam
When making RCS measurements on a ground bounce range, EPS foam columns are frequently used as target supports for testbodies and air vehicles. Since background subtraction is rarely used to suppress foam column scattering in large scale RCS measurements, the columns must be structurally sound while maintaining a minimized RCS signature over the aspect angles and radar frequency band of interest. The goal is to devise a column that is unnoticeable in the measured data yet strong enough to support a specified weight. The major factor that contributes to EPS foam column scattering is shaping, and finding the optimal shape for a particular test is not trivial. This paper describes methods in the design and construction of EPS foam columns. Subjects include determination of EPS material properties, mathematical specification of column geometries, accurate and efficient computation of column mechanics and scattering, and effective optimization of column parameters.
Positioning System Upgrade of an Existing Measurement System
An accurate and reliable target positioning system is mandatory for a good antenna and/or radar cross section (RCS) measurement facility. Most measurements involve characterizing the radiation or scattering of the unit under test as a function of angle and frequency. Accuracy and repeatability become increasingly important in RCS measurements where background subtraction is utilized. Any error in target position will reduce the subtraction effectiveness. Wear and tear of existing equipment coupled with improvements in motion control technology may compel some measurement facilities to upgrade their positioning system. Doing so, while keeping the rest of the measurement system intact, poses integration challenges that cannot be over emphasized. Problems will inevitably be encountered. Their source could be the new positioning system, the old measurement system, or the communication between the two. Subtleties of how the motion control system works can be overlooked during the requirements definition phase of the project. Further idiosyncrasies can be missed during acceptance testing of the system. The Air Force Research Lab compact range has recently upgraded their target positioning system and will share the lessons learned as a result.
3-D Radar Cross Section Imaging Using Interferometric ISAR Technique
In this paper, we present an interferometric in verse synthetic aperture radar (IF-ISAR) image processing technique for three-dimensional (3-D) radar cross section (RCS) imaging of complex radar targets. A general bistatic 3-D imaging geomet ry and the corresponding 3-D image pro cessing algorithm which relates the interferomet ric phase to the target altitude are developed. The impact of multiple scattering centers on al tit ude image formation is discussed. 3-D RCS image formation examples from both indoor and outdoor test range data are demonstrated for complex radar targets.
Assessment of the NIST DoD RCS Demonstration Project, An
During the last 6 years scientists at NIST have been focusing on radar cross section (RCS) measurements to improve RCS uncertainty analysis, and to develop new measurement and calibration artifacts and procedures. In addition, NIST has been asked to provide technical support to the DoD RCS self-certification effort. In this talk I review the technical accomplishments of the program, and will make suggestions for future research to improve RCS calibration and measurement technology. I will also present the structure of the certi fication process, and discuss NIST's role in the ongoing certification activities.
Measurement of a Large Active Planar Array for Spaceborne Radar Using Near-Field Scanning Techniques
The requirement to calibrate and test large active pulsed planar array RADAR antennas, such as the one developed for the advanced synthetic aperture radar (ASAR), places certain requirements on the measurement facility and analysis software that are perhaps not encountered in other areas of application. This paper gives a brief overview of ASAR and an introduction to some of the difficulties encountered during the test and measurement campaign. Results are presented that compare measurement with theoretical prediction. Good agreement has been obtained for both far and near field data.
Projection of Near-Field Data to Far-Field
Near-field ground-to-ground imaging systems are widely used to discover damage that could degrade the radar signature of low observable vehicles. However, these systems cannot presently assess the impact of this damage on the far-field signature of these vehicles. We describe progress made on a method to accurately project the near-field data from these to the far field. Near-field data for the algorithm development is provided by the hybrid finite element/integral equation RCS computer code SWITCH. The near-field data is processed to extract the near-field scattering centers using imaging. The imaging algorithm used differs from the usual far-field imaging formulation in that it incorporates some near-field physics. The processing algorithm, which incorporates a modified version of the CLEAN technique, verifies that the scattering centers that were extracted reproduce the original data when illuminated in the near-field. These near-field scattering centers are then illuminated by a plane wave to produce far-field data. This procedure was tested using VHF band scattering data for a full size treated planform. The near field data was projected to the far-field and then compared to data from a far-field SWITCH computation.
Motion Compensation in ISAR Imaging Using a Phase-Monitoring Subsystem
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.
Algorithms for High-Precision Two-Dimensional ISAR Imaging on an Outdoor Turntable Range
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.
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