Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
METRATEK has completed a highly successful program to prove the feasibility of high-resolution, air-to-air diagnostic radar cross section imaging of large aircraft in flight. Experience with the system has proven that large aircraft can indeed be imaged in flight with the same quality and calibration accuracy that can be achieved with indoor and outdoor ranges. This paper addresses the results of those measurements and the Model 100 AIRSAR radar and processing system that were used on this program.
A portable measurement system has been designed and implemented to produce focused three dimensional RCS images. The Synthetic Aperture Radar (SAR) system was especially designed to operate in harsh physical and cluttered electromagnetic environments. The acquisition system, signal processing and 3D visualization capabilities are discussed and representative data ranging from simple canonical objects to production hardware are presented. The technique meets its design goal in effectively discriminating undesired clutter.
ISAR images and RCS signatures of aircract-in-flight using a ground based and an airborne radar system are presented. The ground-based measurements were at X-band and were of a Mooney 231 aircraft, which flew in a controlled path in both clockwise and counterclockwise orbits, and successiely with gear down, flaps in the take-off position and with the speed brakes up. The air-to-air measurements were made by a radar installed in the nose of the TA-3B aircraft which followed a KC 135 airplane at a range of approximately 450 ft. and traversed a cross-range angle component of (plus or minus) 30(degrees). The data indicates that these systems are useful tools for RCS signature diagnostics of aircraft in flight.
Recently, super resolution algorithm have been used in radar target imaging to increase the down range and/or the cross range resolution. In the open literature, however, the super resolution algorithms have been applied to simulated targets or very simple targets measured in a test range. In this paper, the super resolution algorithms, namely the hybrid algorithm and the 2-D linear prediction, are applied to more realistic targets. One of the targets is a flat plate model of the F-117 aircraft. The back-scattered fields of the flat plate model were measured in a compact range. The other target is a Mooney 231 aircraft. The aircraft was flown in a circular pattern approximately 10 miles from the radar. It is shown that the super resolution algorithm can be successfully applied to these targets.
A microwave diversity imagery based on parametric modeling of back scattered signal versus frequency and aspect is presented. Forward-backward linear prediction is used to compute the model parameters. After stabilizing the corresponding transfer function, data are extrapolated to adjacent frequencies or aspects. Superior range- and/or cross-range resolution can be obtained by using frequency- and/or aspect-extrapolated data. Cross-range resolution can also be enhanced by extrapolating the frequency data and using data at a higher center frequency. For severly (sic) restricted viewing angles, or for small radar bandwidth, the new imagery can significantly improve the image resolution.
This paper describes an effort to evaluate the effect on RCS of base closures on a metallic frustrum at various depths with conducting and electrically isolated plugs. The tests were conducted at Sandia National Laboratories using System Planning Corporation's (SPC) Mark IV radar from 8 to 18 GHz, in the step chirped Inverse Synthetic Aperture Radar (ISAR) mode. Data reduction was performed on Information Systems and Research's workstation using the KNOWBELL software package. The workstation allowed the study of the imagery data in many different modes, which assisted in determining ways to evaluate RCS matching.
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.
Despite their high cost, phased array antennas are becoming popular for radar applications because of their ability to provide reliable information even in a hostile environment. Evaluation of these antennas requires parameters like gain, radiation pattern, beam width, sidelobe (both near and far off) azimuth and elevation null depth, etc. to be tested over the entire range of frequency spots and scan angles. Typically, if the number of frequency spots are 24 and the number of beam positions for which the measurement has to be done are about 100, then the total number of measurements needed to generate the required data are 7200. In addition, phased arrays with a space feed have to be initially collimated at all the spot frequencies. The outdoor testing of these many parameters may not be convenient, and at times it may even be impossible. The planar near field measurement technique provides a systematic and accurate method of measuring large array antennas for all the required parameters.
A Precision Optical Measurement System (POMS) has been designed, constructed and tested for tracking the position (x,y,z) and orientation (roll, pitch, yaw) of models in Boeing's 9-77 Compact Radar Range. A stereo triangulation technique is implemented using two remote sensor units separated by a known baseline. Each unit measures pointing angles (azimuth and elevation) to optical targets on a model. Four different reference systems are used for calibration and alignment of the system's components and two platforms. Pointing angle data and calibration corrections are processed at high rates to give near real-time feedback to the mechanical positioning system of the model. The positional accuracy of the system is (plus minus) .010 inches at a distance of 85 feet while using low RCS reflective tape targets. The precision measurement capabilities and applications of the system are discussed.
The target support system at Boeing Defense & Space Group's 9-77 Compact Range includes a new string support system. The string support system consists of twelve string reels, six each of the High Capacity String Reels (HCSR). The string reel system is used to suspend and manipulate a target for radar cross section (RCS) measurements, primarily at frequencies below 1.5 GHz. The string reel system is capable of supporting targets up to 10,000 pounds and 40' in length and width. The manipulation and handling of targets, is a major consideration in a RCS measurement test plan. The following paper discusses the newly installed string reel system, enhancements to the 9-77 Range equipment which directly affect the use of the string support system, and future developments planned for the system.
To characterize a material's electrical performance or to understand a material's affects (sic) on electromagnetic systems, the constitutive parameters (e, u) of the material must be determined accurately. Materials with high dielectric constants, high loss tangents, or which are layered or complex (e.g., frequency selective surfaces, radomes, radar absorbing material, etc.) are difficult to measure and analyze. For example, germanium is an infrared window substrate in high performance aircraft. The germanium is doped to 1 - 4 ()-cm to raise the maximum operating temperature and to provide electromagnetic shielding. The material is very brittle. The standard methods (coaxial, waveguide, and cavity) are difficult to use. The brittle germanium pieces cannot be made thin enough or have a center conductor hole inserted making coaxial donuts are nearly impossible to fabricate and use. Usable waveguide samples absorb the transmitted energy needed in standard waveguide tests. The brittle sample cannot be made thin enough for X-band measurements or above. The sample, having a high dielectric constant, and having a high conductivity, reduces the Q of resonance techniques difficult and not repeatable. This paper discusses our methodology and shows comparisons with calculations. The technique is based on reflection measurements against a ground plane standard. This technique requires more measurements than other techniques, but the results are numerically more stable.
Performance trade-offs are investigated between the use of clustered waveguide bandwidth feeds and the use of one multi-octave bandwidth single aperture feed in a prime focus compact range for dual linear polarization. The results show that feed structure may be used for advantage for the particular test requirements of compact range systems for Radar Cross Section Measurement.
The frequency accuracy of the HP 8530A receiver and HP 8360 Series synthesizers in ramp sweep is measured using a delay line discriminator. The effect of the frequency error on measurement accuracy is derived for radar cross section (RCS) measurements of one and two point constant-amplitude, scatterers and for background subtraction. The results of swept and synthesized frequency measurements are compared, showing that the errors due to ramp sweep are negligibly small for practical RCS measurements.
This paper describes the technique and advancement of diagnostic radar imaging technology by comparing past SAR and ISAR techniques to the more recent advancement of Autofocus SAR techniques. This recent advancement has meant the relaxation of the stringent mechanical stability requirements needed to produce high quality, high dynamic range, calibrated RCS images.
Advanced airborne radar antennas will consist of ultra low sidelobe arrays of thousands of T/R modules and radiating elements. The detrimental effects of the aircraft structure on the antenna performance becomes increasingly important for ultra low sidelobe antennas will require large aperture, high fidelity antenna test facilities. In this paper, the major errors associated with measurement of an ultra low sidelobe antenna on the far field range are isolated and demonstrated by computer simulation. Data from measurements of a T/R module array on a scale model aircraft is provided to demonstrate typical sircraft effects on antenna performance.
Radar Cross Section (RCS) measurements are typically made at linear polarizations (usually horizontal and vertical) and the transmit and receive polarizations are the same (co-polarized). In addition, however, it is sometimes desirable to measure the cross-polarized RCS of a target (i.e., transmit horizontal, receive vertical or vice-versa). A complete set of both co-and cross-polarized RCS of a target is called a scattering matrix. This paper describes the algorithm used for calibrating a scattering matrix measurement in the McDonnell Douglas Technologies Inc. (MDTI), Radar Measurement Center (RMC). Verification data collected at Ka band on various targets is included to validate the algorithm and implementing computer code.
The radar cross section (RCS) of a target depends on nature environment as well as many physical variables. The objective of a compact range is to exclude environmental effects on RCS measurements of a target. It is also true for time gated RCS measurements as well. RCS obtained in above manners is more suitable for a space borne than for a ground based target. The contribution from surrounding environment is an inseparable part of RCS for a ship, truck, bridge, and building. We need a suitable method to characterize RCS of a ground based target and its dependence on the environment. The uncontrollable natural change makes environmentally dependent RCS results difficult to compare for a ground based target measured at different time instants. A way to reduce the uncertainties induced from changes is to exhaust all possible RCS measurements before the change. A measurement of this kind is referred to as a concurrent RCS measurement, which in a sense is equivalent to take an optical picture of a rapidly changing object with a strobe light. The step frequency radar located at Chesapeake Bay Detachment of Naval Research Laboratory is such a radar, which is equivalent to at least 45 single frequency radars operating simultaneously from 2.0-18.0 Ghz. Last year, we briefly mentioned this radar in our presentation. We will make a detail discussion of this radar and its capability on concurrent RCS measurements.
This paper compare some of the features and capabilities of gated CW and pulse radars for RCS and imaging measurements. At the conceptual level, these two types of radars are very similar. The primary conceptual difference is that a pulse radar has a relatively high bandwidth receiver while a gated CW system has a relatively narrow bandwidth receiver. The measures of performance of an RCS and imaging system include sensitivity, measurement time, clutter rejection, dynamic range and accuracy. Other considerations such as inter-pulse modulation may be important in some cases. For some applications, typically where long ranges are involved, a pulse system has significant performance advantages. For many applications, the performance advantage of a pulse system is not significant, particularly when viewed in light of the large difference in cost. This is particularly true of Quality Assurance applications which are normally characterized by both short range and lower budgets. Typically, the price of a gated CW system is in the range of ¼ to ½ the price of a comparable pulse system. This paper discusses general similarities and differences in the fundamental operating characteristics of the two systems. Specific performance measures are discussed including system sensitivity, gate performance, clutter rejection, and measurement times. Other considerations such as pulse modulation are discussed. A summary of the various considerations is presented in order to give the reader an understanding of the applications for which a gated CW system is more appropriate.
A bistatic calibration technique for wide-band, full-polarimetric instrumentation radars is presented in this paper. First general bistatic measurement problems are discussed, as there are the coordinate systems, the definition of polarization and the bistatic scattering behavior of convenient calibration targets. In chapter two the new calibration approach is presented. The general mathematical and physical description of errors introduced in the bistatic system is based on the radiation transfer matrix. The calibration procedure is discussed for the application with a vector network analyzer based instrumentation radar. For verification purposes measurements were performed on several targets.
Computer codes for the computation of scattering are based on physical, mathematical, and numerical assumptions and approximations that impact the accuracy of the results in ways that are not obvious or quantifiable analytically. This paper stresses the usefulness of a concurrent measurement program to provide reliable RCS data for targets of special interest in establishing the range of validity of the various assumptions upon which a specific computer code is based. This in turn assists in developing “modelling guidelines” restricting the design of computer models for input to the code such that reasonable accurate results are likely to be obtained.