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A variety of unique instrumentation radars have been developed by the RF & MMW Systems Division at Eglin Air Force Base in order to support both static and dynam ic Radar Cross Section (RCS) measurements for Smart Weapons Applications. These systems include an airborne multispectral instrumentation suite that was used to collect target signatures in various terrain and environmental conditions (95 GHz Radar Mapping System - 95RMS), a look-down tower based radar designed to perform RCS measurements on ground vehicles (MMW Instrumentation, High Resolution Imaging Radar System MIHRIRS), two high power (35 & 95 GHz) systems capable of mapping/measuring both attenuation and backscatter properties of Obscurants and Chaff (MMW Radar Obscurant Characterization System MROCS: 1&2), and a Materials Measurement System (MMS) which provides complex free space, bistatic attenuation and reflectivity data on Radar Absorbing Materials (RAM), paints, nets and specialized coatings/materials. This paper will describe the instrumentation systems, calibration procedures and measurement techniques used for data collection as well as several applications which support modelina and simulation activities in the Smart Weapon community.
ARCS acquisition software for antenna and RCS measurements has been modified such that it is now based on LabWindows/CVI of National Instruments. With open system architecture, industry-standard tools and platform flexibility, new ARCS software delivers all components which are required for an advanced antenna and RCS measurement system. This means tht the portability and modularity of the software is increased considerably. Such a concept has the major advantage of simple adaptation/modification by the user, for instance by adding new menu pages. The virtual instrument concept of CVI guarantees easy adaptation of the newest interface technology, such as USB and firewire. Furthermore, there is a large base of instrument drivers which can be readily used to extend the measurement capabilities of ARCS in a minimum of time Special care is taken in the design of the user interface. This is to avoid complex procedu res for entering measurement parameters. Even less experienced operators must be comfortable with the software and be able to perform complex calibration and data acquisition procedures. Finally, a large number of application programs is written for advanced antenna and RCS calibration, microwave holography, ISAR imaging and frequency extrapolation techniques.
Electromagnetic field measurement systems based on the Advanced Modulated Scattering Technique (A MST) permit fast and accurate diagnostic testing to be performed in the near-field (NF) or the far-field (FF) of antennas and scattering objects. A-MST probe arrays are particularly effective for rapid diagnostic testing applications where it is desired to obtain overall measurement duration reductions of 80% to 98% compared to conventional single-probe measurement times.
The collection of radar scattering data necessary for imaging targets with three-dimensional resolution requires frequency diversity, combined with angular diversity over two orthogonal axes fixed to the target. Although the necessary data can easily be collected using modern instrumentation systems when the target is outfitted with an embedded two-axis rotator, some applications preclude the intrusion of the rotator. This paper describes an alternative method for obtaining the required data which uses conventional target rotation in the azimuth plane, combined with a linear displacement of the compact range feed along the vertical axis of the collimator's focal plane. Frequency diversity is provided by a stepped-frequency radar and angular diversities in the horizontal and vertical directions are provided by the target rotation and vertical feed displacement, respectively. The data-collection scheme samples a wedge-shaped volume of the target spatial spectrum (k-space) with radial and angular extents de terminated by the bandwidth and target rotation relative to the radar axis. A three-dimensional image is formed by processing a three-dimensional array of data, typically consisting of 128xll8x128 data samples. The paper describes the experimental set-up used to collect Ku-band data and presents two- and three dimensional images obtained from the data. Considerations of the following issues are addressed in the paper. 1. Aberrations resulting from displacing the feed from the collimator focal point. 2. Control of the linear feed displacement, target rotation, and radar operation to automate the data collection. 3. Methods for calibrating and aligning the data. 4. Signal processing methods which combine wideband, ISAR and spotlight SAR processing for three-dimensional applications. 5. Clutter suppression using zero-Doppler filtering.
This paper presents an efficient three dimensional (3-D) SAR imaging algorithm us ing range migration techniques. The algorithm is used to form 3-D radar reflectivity images of targets measured in anechoic chambers. As an input, the algorithm requires frequency domain backscatter data which have been acquired us ing a stepped frequency system equipped with an antenna that synthesizes a 2-D planar aperture. Resolution in the vertical and horizontal cross-range directions is given by the dimensions of the synthesized aperture, whereas resolution in ground-range is provided by the synthesized frequency bandwidth. The presented formulation has been justified by using the stationary phase method. Results both with syn thetic and measured backscatter data show the high efficiency of the technique. The extension of the algorithm when the antenna synthesizes a 2-D spherical aperture has been addressed. Re sults with this aperture geometry show that the technique is still highly efficient.
The Seeker Test and Evaluation Facility (STEF) located on Range C-52A at Eglin AFB FL. is used to perform high-resolution multispectral (EO-IR-RF-MMW) signature measurements of US and foreign ground vehicles primarily to support the Research, Development, Test and Evaluation (RDT&E) of smart weapons (seekers, sensors and Countermeasure techniques). In order to support two major DOD signature measurement programs in 1997 this facility required significant range upgrades and enhancements to realize reduced background levels, increase measurement accuracy and improve radar system reliability. These modifications include the addition of a 350'X 120' asphalt ground plane, a new secure target support facility, a redesigned low RCS shroud for the target turntable and a new core radar system (Lintek elan) and data acquisition/analysis capability for the existing radars Millimeter-Wave Instrumentation, High Resolution, Imaging Radar System - MIHRIRS). This paper describes the performance increase gained as a result of this effort and provides information on site characterization and radar instrumentation improvements as well as examples of measured RCS of typical ground vehicle signatures and ISAR imagery
Two normalized pattern functions appropriate to Ultra-Wideband (UWB) antennas were discussed at the 1992 AMTA meeting [1]. The normalized pattern spectrum is an image showing radiated signal intensity as a function of azimuth or elevation angle and frequency. The spectrum is complex, and thus requires both an amplitude spectrum image and a phase spectrum image to be complete. It is normalized by dividing by the complex radiated signal at the defined boresite angle for the designated antenna. Therefore, on boresite, the normalized pattern spectrum is unity. The normalized impulse response pattern function is the Fourier Transform of the normalized pattern spectrum. This image plots intensity(and polarity) of the real impulse response of the antenna vs time and angle. On boresite, it is a band-limited impulse corresponding to the normalized pattern spectrum. This paper will discuss measurements of seven UWB antennas, and present normalized pattern results of these antennas. The antennas include both off-the-shelf products and experimental prototypes. Included are antennas which have been used for wide-angle UWB SAR imaging, a coherent UWB application where both signal attenuation and dispersion vs angle are important. The results show how pattern behavior can be separated from boresite transfer function information, and how antennas compare in this compact image format.
It is known that electromagnetic signalls can penetrate through non-metallic barriers such as building walls. A hand-held Sythetic Aperture Radar (SAR) unit capable of transmitting and receiving such signals is desirable in various military and civilian applications. Theoretical and experimental issues associated with through-wall Ultra Wide Band (UWB) SAR imaging of buildings are studied here. It may be inconvenient and impractical for a hand-held unit to collect data at uniformly spaced positions. A backprojection algorithm is developed for the case where spatial sampling is not uniform. In addition, a spherical wavefront (as opposed to a uniformly planar wave front) is assumed in the algorithm to account for the proximity of a radar unit relative to a target scene. Images of simulations using point targets and measurements of canonical targets such as a corner reflector and a cylinder are generated. Images of a standing human in free-field and through-wall are compared.
The image-based near-field to far-field transformation is based on a reflectivity approximation that is commonly used in ISAR imaging. It is a limited but computationally efficient transform whose accuracy, for appropriate targets, rivals that of computationally more intense transforms. Previous results include applications of the transform to lOA. long wire and lOA. long conesphere numerical data. Here, 1-D and 2-D versions of the transform are applied to conesphere near-field measurements data and the results are compared to corresponding far-field measurements data. Transform errors obtained for these data are compared to corresponding results obtained using newly generated near-field and far-field numerical data. The image-based transform is believed to be especially applicable to the far-field correction of near-field measurements of complicated targets like aircraft or vehicles that are too large or too poorly defined to be simulated numerically.
Recently, a new method of wide band radar imaging has been developped within the framework of the two dimensional (2-D) continuous wavelet theory. Based on a model of localized colored and non isotropic reflectors, this method allows to obtain simultaneously information about the location, the frequency and the directi vity of the scatterers which contribute to the RCS of a target. We obtain a 4-D data set that we call hyperimage namely a series of images which depend on the frequency and orientation of illumination. In order to exploit efficiently hyperimages an interactive visual display software called i4D has been specifically designed. The purpose of this paper is to present the capabilities of i4D through the analysis of hyperimages constructed from monostatic and bistatic scattering data. The results show that the interactive and dynamic analysis that i4D procures allow to better understand the mechanisms that contribute to the RCS of targets.
One approach to getting near-field ISAR measurement costs down is to dispense with track or turntable, and instead mount the SAR antenna on a vehicle and simply 'drive by' the target of interest, which might be a vehicle or aircraft standing on tarmac. In this situation the antenna path will depart from a perfect straight line or circular arc, and there will also be vibrational wobble at the antenna phase center. These effects can defocus the images obtained. One way to overcome the focus problem is to mount strategically placed corner reflector(s) in front of the target, each in a different range cell. These then act as phase references, used to refocus the image. However it is not strictly necessary to employ reflectors - a good focus can normally be obtained by suitably processing just the target return itself. This paper will describe autofocus procedures which have been sucessfully used in conjunction with chirp radar data, in the 'driveby' situation.
One of the key problems in SAR interferometry is the determination of the absolute phase of a scatterer by unwrapping the calculated phase difference of two SAR images. Since the phase difference is a function modulo 2p software algorithm are used to perform a phase unwrapping to obtain an unambiguous phase and thus the corresponding height information. This paper presents a procedure using two different transmit frequencies to enlarge the unambiguous range for height determination. Measurements performed in an anechoic chamber are used to test the processing. Using this procedure it is not only possible to resolve the height of a surface but also of single scatterers in space or in urban areas with steep slopes.
The recently introduced Fractional Fourier Transform (FRT) operation was shown to be useful for various spatial filtering, optical and signal processing applications. In contrast to the Fourier Transform, the real part of the FRT of a delta function can be "tailored" (by the position of the delta and the transform order) to have many zeros in specific areas of the spectrum and fewer zeros at other areas. This property was exploited to design new spectral windows, which pass through zero many times in the spectral points we define. These windows can be used in case of loss of information or partial information collected in a usual ISAR stepped-frequency data process. Another, more valuable property of the FRT, used here for radar imaging applications, is it's being a projection of a rotated Wigner transform of a signal. This property was used to filter resonant structures in the radar image, which cause the image to be smeared in the range.
This paper discusses the efforts of an on-going research program which has been exploring the use of expert systems (artificial intelligence) techniques to support automated analysis of wideband radar scattering data. The primary objective of this research is to explore and demonstrate the applicability of expert system techniques to the analysis of diagnostic radar images. There are two modes which are being explored. The first is an automated system that would allow lesser skilled (in radar imaging science) individuals to do the work of highly skilled engineers and analysts. The second mode would aid the highly skilled worker with the application and correct implementation of software tools, interpretation of phenomenology, and data quality assessment. In both cases, the expert system should allow for the increase through-put and accuracy of data being analyzed. A software prototype is being developed and tested with real data to demonstrate the feasibility and potential accuracy of such as system.
The Joint Strike Fighter (JSF) office sponsored a Navy directed limited technical demonstration of diagnostic Radar Cross Section (RCS) imaging on-board an aircraft carrier at sea. The overall objective was to obtain experience and data sufficient to assist the Navy in defining any future shipboard diagnostic imaging measurement system requirements. Measurements were conducted in the hangar bay to assess the challenges posed by the carrier environment. A technique for making diagnostic imaging measurements in spatially confined areas was developed.
A method is presented that gives a 3D ISAR image from a 2D measurement. An ordinary 2D image is created. An extra receive channel is used to give height information in every pixel. This channel gets its signal from two extra receive antennas with different elevation lobes. The antennas feed a hybrid which creates a difference signal that goes to the extra receive channel. Height information is derived like in an amplitude comparison monopulse radar. This way a height number is assigned to every pixel in the 2D image. Thus a 3D image is created. It is required that in a 2D resolution cell the reflexes come from only one height. If not, the height information given by the difference signal will be a weighted average of the heights of the reflexes. The method is applied to a ground plane RCS range. No measurements have yet been performed.
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 [1]. The techniques are described and demonstrated on numerically simulated and experimentally measured data.
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.
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.