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In the past, most radar-cross-section imaging has been done after data has been taken. At best, this off-line processing generates images that are returned to a customer the next day. Many projects can benefit significantly by having concurrent imaging and data acquisition. This allows for real-time cause and effect type diagnostics without rescheduling range time. As RCS range time becomes increasingly more expensive and difficult to schedule, real-time imaging provides the project engineer with a valuable tool to optimally use his range time. A technique has been developed to render real-time radar cross section images while acquiring data. All image processing is performed to achieve a fully enhanced image. Focussing, interpolation, and windowing are all used to give a detailed image. The system uses a Hewlett Packard 8510B for data taking and Hewlett Packard computers for data acquisition and image processing.
An image processing method which uses the bistatic scattered fields of a target obtained in a compact range is presented in this paper. The transmitting and receiving antennas can be either two compact ranges or one compact range and a horn antenna. The compact range reflector can be either focussed or defocussed so that a near field situation can be simulated. The bistatic scattered fields are collected as a function of frequency and the angle of rotation of the target. Then they are processed coherently to determine the cross-range and down-range scattering centers of the target. Experimental results are presented to validate this image processing technique.
RCS instrumentation systems capable of combining wide-band and ISAR techniques to obtain two-dimensional images are widely used to perform RCS diagnostic and measurement functions. Objects involving rotating structures, such as blades of propulsion systems complicate the diagnostic task. The paper address the utilization of diagnostic RCS systems to meaningfully determine the radar signatures of objects with rotating components and presents results obtained from a generic data set, typically available from wide-band RCS instrumentation systems. The results provide valuable insight to the signature of objects with rotating components.
The MUSIC (Multiple Signal Characterization) algorithm uses an eigenvector decomposition of measured data to classify signals in the presence of noise. It has been used for the angular classification of multiple radar signal emitters and ISAR imaging. Interest has grown in stray signal analysis in anechoic chambers. This paper will discuss the modification and use of the MUSIC algorithm for the decomposition of field probe data to angular spectrum. A brief discussion of the MUSIC algorithm theory will be presented. Modifications required for use in compact range angular spectrum analysis will be discussed in detail. Requirements on field probe measurements will be presented as well as their effects on the implementation of the algorithm. Both one way and two way measurements are considered for their relationship to the array manifold. Finally, some experimental validation generated on the Boeing range will be presented.
Image gating or editing is often used to determine the effect of an isolated scatterer on the RCS in the frequency and aspect-angle domain. In this paper, theoretical computations indicating limits in the image-gating procedure will be presented. The process provides the image-gating capability in combination with phase-corrected (focused) imaging. Targets consisting of two-point scatterers with well-known RCS response have been used. One of the scatterers is gated out and the resulting RCS versus frequency or aspect angle is determined and compared with its theoretical value. Limits in terms of minimum bandwidth or minimum distance in resolution-cell sizes are defined. The influence of several gate shapes and windows have also been examined. Experimental investigation has been carried out in order to verify the theory.
Images of an open box, closed box, and open and closed box on a ground plane were taken at the Hughes/Motorola Compact Range. Comparison of these images show the effect of multiple reflections in the image of an open box. A simple analytic/computer model was developed to interpret these multiple images. Data and analysis are presented on the various mechanisms that come into play in scattering from the open/closed box and the ISAR images generated as a function of the viewing angle for the box.
In the theory of inverse synthetic aperture imaging of radar targets the measuring distance is ordinarily supposed to be very much larger than the dimensions of the target. If this is not the case errors are introduced. We study these errors and means to decrease their influence by computation. The result is that the maximum tolerable target dimension can be substantially increased in a plane perpendicular to the axis of rotation if error correction is used.
In the past, models suspended indoors for radar cross-section measurements have weighed up to several hundred pounds, suspended on the order of 20' or less from the ground, and measured statically or rotated for great circle cuts. Under these circumstances it has been acceptable to choose the best string configuration from a signature point of view and simply wait for the model to reach a visually calm state before testing. However, indoor ranges are now requiring suspension of models weighing several thousand pounds 40' or more above the floor. In addition, the demand for imaging data during model conics requires both precise dynamic control and model stability. This work discusses techniques developed at Boeing's 9-77 Range in Seattle, to achieve model stability during suspension and manipulation. In addition, techniques to determine spring and damping constants of suspension systems for individual models are addressed.
The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited. This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence. Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.
Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.
The objectives of RCS imaging are to spatially isolate and quantitatively measure the strength of scattering mechanisms on complex objects. Although some isolation can be provided directly by using radars with high spatial resolution, most current RCS systems achieve the required resolution by synthesizing the image from measurements of the object response to variations in frequency and rotation angle.
This paper presents a conceptual overview of the instrumentation system and signal processing involved in dynamic RCS and Imaging measurement systems.
To construct an image of a complex target, increasingly smaller range cells are desired. To decrease range cell size and improve resolution the bandwidth must be increased. The bandwidth of RCS measurements utilizing an HP8510 based collection system is limited to a maximum of 801 frequency points. This paper will present a technique to extend the bandwidth by using off-line processing to overcome this hardware limitation. Fully focused ISAR images formed at millimeter wave frequencies, with the addition of eternal mixers, will be demonstrated. Bandwidths of 5, 10, and 14 GHz, measured from 7-17 GHz and 26-40 GHz will be shown. The comparison of these focused images, with 3.2 cm, 1.6 cm, and 1.1 cm resolution will illustrate a powerful engineering tool to analyze closely spaced scatterers.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
Conventional radar imaging requires large amounts of data over large bandwidths and angular sectors to produce the location of the dominant scattering centers. A new approach is presented here which utilizes only two swept frequency scans at two different look angles for two-dimensional images or three swept frequency scans at three different look angles for three-dimensional images. Each swept frequency scan is the backscattered response of a target. A different plane wave illumination angle can be conveniently obtained by offsetting the feed horn from the focus of a compact range reflector without rotating the target. The two- and three-dimensional target information for the location of the dominant scattering centers is then obtained from the band limited impulse responses of these swept frequency scans.
It is common practice to window RCS data prior to inverse Fourier transformation into an image. Windowing reduces image sidelobes at the expense of some loss of resolution. When the window shape is adjusted to give the best resolution-sidelobe tradeoff for the given application, however, the apparent RCS of features in the image varies unless the correct calibration of normalization is applied. This paper discusses the proper calibration and normalization techniques to use with RCS imaging. These techniques permit efficient generation of images that accurately depict the RCS of significant target features, independent of the data window shape.
The characteristics of a unique indoor RCS modeling facility are described. The David Taylor Research Center (DTRC) has implemented an indoor, over-water radar cross section measurement facility. Major components of the facility are the DTRC Seakeeping Basin, an imaging radar, an underwater target mount and rotator, a calibration system, and video monitoring equipment. Initial operational capabilities include dynamic pulse-to-pulse polarization-agile measurements at X and Ku bands, elevation angles from grazing to 7 degrees, maximum target length of 50 feet, and simulated sea states adjustable between state 0 and state 3. Several data products are available, including high-resolution inverse synthetic aperture radar images. Eventual capabilities will include extended elevation angles up to 30 degrees, frequencies to beyond 100 GHz, and SAR imagery.
This paper discusses an anechoic chamber absorber evaluation which was conducted for the purpose of improving anechoic chamber and compact range performance through better absorber characterization. This study shows that performance of conventional absorber materials is dependent on selection of the material's shape, size and orientation with respect to the incident energy direction. This, demonstrates the importance of better characterization of the material. Nonhomogeneities in the material composition and physical structure were also found to significantly modify performance; in some cases even improving it. Also shown, is the need for improved evaluation techniques and procedures over conventionally used methods. An evaluation procedure using modern imaging techniques is presented. Several measured results for various absorber types and sizes are presented which show the usefulness of the evaluation technique and demonstrate relative performance characteristics for these materials. Measured reflectivity data on various absorber types, which consistently show better performance than levels specified by the vendors, are also presented.
Imaging by microwave holography was initially envisaged as a two dimensional diagnostic technique applicable to a wide variety of objects and environments [1], [2], being particularly relevant to reflector antenna metrology [3]. For electrically large paraboloidal reflectors the radiation is well collimated and can be assumed to arise from an effective aperture field at a specified plane within the antenna volume. Fresnel or far field measurements are then restricted to a small angular range around boresight so as not to violate the assumptions made for reconstruction of the aperture field. The processed image represents the aperture illumination function whose phase can be accurately related to feed position and profile error by comparison with 'a priori' knowledge of the ideal reflector shape [4]. Since the aperture field approximation imposes severe restrictions on the data window size the intrinsic depth resolution of the image is characteristically poor, and wide angle scattering from feed support struts for example is not recorded causing the struts to appear as geometric shadows on the image. Regions of the reflector surface lying beneath these blockages cannot therefore be reconstructed. Moreover, the narrow data recording bandwidth also produces inferior transverse resolution of profile perturbations on the reflector surface.
High resolution radar imaging is becoming an increasingly important component of RCS measurement systems. The primary purpose of radar imaging as applied to RCS measurements is to locate and quantify the various scattering components that contribute to the total RCS of a model under test. The technique when properly applied by trained personnel can greatly improve the productivity of measurement programs by reducing the number of measurements needed to find defects in a model, and by rapid improvement in the understanding of the scattering phenomena itself.