AMTA Paper Archive
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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.
Extended Imaging Technique for the Investigation of Higher-Order Diffraction Centers
This paper presents an approach to experimental identification and investigation of the higher-order diffraction effects. The proposed technique allows one to determine parameters (particularly coordinates of the attachment and launching points) of the higher-order diffraction centers and can be considered as an extension of the Inverse Synthetic-Aperture Radar (ISAR) imaging technique.
Re-Qualification of the Optical Alignment of the Advanced Compact Range (ACR) Using Coherent Laser Radar Metrology
Originally installed in 1992, the Advanced Compact Range (ACR) at Wright-Patterson Air Force Base was completely aligned using a Leica multi-theodolite measurement system. The Coherent Laser Radar (CLR) System provides an automated precision measurement capability which can gather significantly more data permitting a more complete characterization of the range in a relatively unobtrusive manner. This paper presents the process and results of applying Laser Radar Metrology as an optical range re-qualification tool within the Air Force Research Laboratory’s ACR.
Enhanced Frequency Selective Absorber
In some antenna applications, it is desirable to introduce an interior surface that is absorptive at one frequency, and reflective at an adjacent frequency. Even a narrow band absorber, such as iron loaded Magnetic RAM, has absorption qualities far outside its optimal absorption band. The concept is to use a conductive-backed Radar Absorber Material (RAM) covered by a band pass Frequency Selective Surface. The FSS allows the frequencies to be absorbed to pass through to the absorber while reflecting frequencies away from the pass band. The example shown in this paper was designed to absorb energy in the 2-4 GHz band, and to be reflective below 500 MHz. Design considerations include: Overall thickness; Coupling between the FSS and RAM, and Size of the FSS elements relative to the internal antenna structure. Potential applications include: broad band antennas, scatter control, and cosite interference mitigation.
A Combined Measrurements and Simulation Based Design of a Novel Polarimetric Array for De-Mining Applications
Recently, remarkable efforts have been spent to develop GPR (Ground Penetrating Radar) systems able to detect shallow anti-personnel mines. In order to achieve high resolutions, large bandwidths are necessary; furthermore antennas must operate detached from ground. The paper describes how an existing surface based antenna, developed for high resolution inspection of man-made structures, has been optimized following a combined measurementssimulation approach. The novel antenna is the basic element of a polarimetric array, composed of 35 elements, that will be part of a multi-sensors demining system under development in the frame of a European Union funded project (DEMAND). Measurements have been carried out in the frequency domain, by the means of an S-parameters modal decomposition. Results concerning bandwidth, leakage, impulse response of array channels and input impedance of the basic element are reported in the paper. Comparison between measurements results and simulations are presented.
Algorithms and Mechanics Employed for Successful Portable Imaging Via the SCI-Xe Microwave Imaging System
Sensor Concepts, Inc. has developed the SCI-Xe Portable Microwave Imaging System prototype for use in the assessment of the low observable (LO) characteristics of fielded military platforms in their native environments. The SCI-Xe is a single man deployable suitcase-size system that employs a small linear rail in order to acquire Linear Synthetic Aperture Radar (LSAR) data in the 8-18 GHz frequency range. Data collections are performed via a single button push and the data is stored on a removable harddrive for comparison to an existing database for analysis. Recent deployment of the SCI-Xe prototype has provided excellent feedback on the viability of performing repeatable field measurements using alignment techniques that do not significantly affect the overall system size and weight. The SCI-Xe employs a video camera and uses video image algorithms such as edge detection, thresholding, and overlay masks to provide a simple coarse alignment to a stored baseline position. Once positioned, a single LSAR collection is performed to provide the radar data necessary for analysis, which includes a robust image registration algorithm to first, perform a quantitative assessment of the positioning accuracy and second, align the data for further image filtering and statistical processing.
VHF Capability and RCS Measurements from Long Cylinders
In order to better understand the capability and limitation of the radar in the VHF band, we present the results from RCS measurements on simple calibration objects of sizes from small to large. Though the uncertainty for measuring a small object is usually well behaved to within +0.2 dB, the greatest difficulty for a large object is the lack of knowledge on the distribution of the incident field. Some qualitative ideas may be obtained from fieldprobes along a few directions. Yet, a thorough investigation of the field in 3-D as a function of the frequency and polarization is generally beyond time and budget constraints. For the special cases of long and thin cylinders at broadside, we find that the difference in HH-VV is very sensitive to ka, which allows us to distinguish them apart.
Design and Analysis of a New Angularly Insensitive RCS Calibration Device
The accurate measurement of static Radar Cross Section (RCS) requires precise calibration. Conventional RCS calibration objects like plates and cylinders are subject to errors associated with their angular alignment. Although cylinders work well under controlled alignment conditions, and have very low targetsupport interaction, these devices may not always suitable for routine outdoor ground-plane RCS measurements. We seek a design which captures the low interaction mechanisms of a cylinder, yet can be easily aligned in the field due to its excellent angular insensitivity. In a sense, this target has the best characteristics of both the cylinder and the sphere. This paper will describe the design of a "hypergeoid", a new calibration device based on a unique body of revolution. Calculations and measurements of some elementary hypergeoids are presented.
NRTF's 14 Foot Pylon
The National RCS Test Facility (NRTF) has a variety of unique test capabilities. Looking to further expand our testing options at the Mainsite test facility, the NRTF began developing a pit/pylon and rotator shroud test bed capability that would allow for radar cross section (RCS) measurement of test articles that are physically too small to accept a rotator. To reach the desired background RCS levels, the use of an expanded polystyrene foam column was not a viable option. In order to maintain the integrity of the calibrated system and enable the measurements of test articles with and without rotator bays on the same pit/pylon, a pit/pylon and shroud combination was required. Other important considerations that influenced the viability of a pylon system include cost effective mounting/dismounting of test articles, safety of the test articles and personnel, and the effective determination of backgrounds due to a stable and low observable pylon system. Our primary goal was to design and fabricate an inhouse system that met the needs of potential customers while satisfying our own clutter and background criteria. This paper documents the fabrication of the pylon and rotator shroud test bed. The results of an RCS characterization are also presented demonstrating the system’s ability to meet the desired RCS background goals.
Phase-Dependent RCS Measurements
Free space, coherent radar cross section measurements on a moving target trace a circle centered on the origin of the complex (I,Q) plane. Noise introduces only small random variations in the radius of the circle. In real measurement configurations, additional signals are present due to background, clutter, targetmount interaction, instrumentation and the average of the time-dependent system drift. Such signals are important contributors to the uncertainty in radar cross section measurements. These time-independent complex signals will translate the origin of the circle to a complex point (I0,Q0). Such data are then defined by the three parameters (I0,Q0), the center of the circle, and st, the radar cross section of the target. Data obtained when a target is moved relative to its support pylon can be separated into phasedependent and phase-independent components using the techniques of (1) three-parameter numerical optimization, (2) least-median-squares fit, (3) adaptive forward-backward finite-impulse response procedure, and (4) orthogonal distance regression applied to a circle fit. We determine three parameters with known and acceptable uncertainties. However, the contribution of systematic errors due to unwanted in-phase electric signals must still be carefully evaluated.
The Design of Broadband Foam Columns
We present a methodology for the design of foam columns useful for the support of targets during static outdoor radar cross section (RCS) measurements. The methodology uses modal solutions along with genetic algorithms to optimize the design of a homogeneous column with resistive layers that provides minimal scattering over the design bandwidth. The methodology widens the design space, allowing for better design trades between electromagnetic and structural column performance. Results are presented for two representative design cases (broadband and spot-frequency narrowband), and the performance of the optimized column design is shown to be significantly better than that of the baseline foam column. Further design improvements are also suggested, including the use of the Born approximation for non-axisymmetric columns.
Outdoor Low Frequency Bistatic Far Field Radar Cross Section Measurements
The bistatic radar signature of military systems is of interest for various applications including performance evaluation of semi-active missile systems, surveillance systems, and survivability assessment. While bistatic radar cross section (RCS) measurements have been made for high frequencies at several U.S facilities, there has been little reported work in low frequency bistatic RCS measurements. This paper presents the results of recent low frequency coherent bistatic RCS measurements from 210 MHz to 1.99 GHz at bistatic receiver angles of 0°, 35°, 70°, 120° and 145°. These measurements were successfully completed at the Naval Air Systems Command Weapons Division Etcheron Valley Range (EVR), formerly known as Junction Ranch (JR), China Lake, California This paper describes the process and provides results of low frequency bistatic RCS measurements on a hemisphere-capped cylinder target. Comparisons are presented of measured data to predicted results from moment method models of the calibration object and the cylinder target. Methodologies used in optimizing RCS data quality are also provided.
The Effects of Target Motion on ISAR Imagery
There is a conflict between the requirement of a very low RCS target support system, and the need for high stability and accurate target setting. To meet the ideal of measuring targets in free space, multiple string suspension systems from overhead gantries have been devised. Despite measures to the contrary, it was found air turbulence and mechanical vibration could produce complex perturbations of the target during ISAR imaging. Over the frequency range of interest (1-100GHz), even sub-millimetre disturbances can produce significant and unwanted image artefacts. Model code was written to provide representative parametric dynamic models for the oscillatory motion of the targets. Modelling results over a wide range of motion patterns, acquisition configurations, and radar parameters allows a quantitative assessment of the limitations and validity of ISAR imagery. Image degradation is affected not only by the amplitude of the target’s motion, but also by its direction, and relationships between the radar frequency sweep rate and characteristic period of oscillation. The benefits to image recovery of data averaging and frequency sweep randomisation are examined. A motion-correction system is discussed, based around a video photogrammetry system that provides a record of a target’s 3-dimensional motion during data acquisition. This work was carried out under the UK Ministry of Defence’s Corporate Research Programme.
Selection of the Optimal Image Formation Algorithm for a Ground Based Synthetic Aperture Radar
A Ground-Based Synthetic Aperture Radar (GB-SAR) interferometer system operating at 17 GHz is used to monitor the movement of an active landslide. The selection of the optimal image formation technique for such an imaging system is addressed. The algorithms considered in this study are those previously developed for spaceborne and airborne SAR. A near-field algorithm that forms the image in the time domain is selected as the optimal solution. Furthermore, example results obtained in a measurement campaign in Schawz (Austria) are shown.
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