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To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.
There has been a great deal of interest in microstrip antennas and arrays in the past decade or so due to their low cost, light weight, and conformability. Most research to date on microstrip antennas has been focused on developing techniques for characterizing their radiation properties. However, interest in evaluating the scattering properties of such antennas is increasing. The RCS of three configurations of circular patch antennas have been measured versus frequency and are compared to Moment Method predictions; a single open-circuited element, a single element terminated in a 50 ohm load, and a 3 x 3 array of open-circuited elements. In most cases, the measurements and predictions are in good agreement.
The design of many modern RCS instrumentation systems is driven by the time required to complete a measurement which establishes the throughput rate of the RCS facility and therefore impacts the operating cost and efficiency. Time considerations are of particular importance when wideband systems are used to measure large targets with low RCS because multiple observations are required to span the frequency band or to increase sensitivity by coherent integration. Although significant improvements have been made to minimize inefficiencies in instrumentation systems, the fundamental limit of measurement time is governed by physical considerations of power, energy, noise, target dimension, and RCS. Evaluating the performance of a particular radar design can be facilitated by comparing the predicted measurement time with a theoretical optimum. The purpose of this paper is to develop estimates of the minimum measurement time under optimum conditions. Although likely precluded by practical considerations, the theoretical limits provide estimates of the maximum degree of radar performance and measures of optimality in practical systems.
A recently completed Hughes program successfully demonstrated an airborne multi-spectral (VHF through X-Band) Synthetic Aperture Radar (SAR) measurement of the radar cross section (RCS) of an aircraft in flight, producing two-dimensional (2-D) diagnostic RCS images of the test aircraft. Ground-to-air imaging of full-scale aircraft was demonstrated by Hughes in 1990. In early 1992, a Hughes A-3 aircraft made air-to-air radar images of a test aircraft in flight. To date, Hughes has collected imagery on nine aircraft from VHF through X-Band, including nose, side and tail aspects at several elevation angles. Reference (2) describes the VHF/UHF capability of the imaging system and this paper will describe the image processing steps developed and will display S- and X-Band radar images with resolution as fine as 6 x 4 inches. The images presented in this paper are dominated by a few very large cavity-type scatterers and do not show the ultimate sensitivity and fidelity of the system. The air-to-air images do demonstrate the spectacular diagnostic utility of this technology.
A recently completed Hughes program successfully demonstrated an airborne multi-spectral (VHF through X-Band) Synthetic Aperture Radar (SAR) measurement of the radar cross section (RCS) of an aircraft in flight, producing two-dimensional (2-D) diagnostic RCS images of the test aircraft. The Air-to-Air Radar Imaging Program was a multi-phase program to develop, demonstrate and exploit this new technology for the design and evaluation of advanced technology aircraft. Radar images with resolution as fine as 6 x 4 inches were produced. To date, Hughes has collected imagery on nine aircraft from VHE through X-Band, including nose, side and tail aspects at several elevation angles. The ability to generate a radar image while in flight is a significant technical achievement. The VHF images presented demonstrate the utility of the system but the images do not show the ultimate sensitivity and fidelity of the system because the aircraft presented in this paper are dominated by a few very large cavity-type scatterers. The ability to measure the VHF/UHS RCS of an aircraft in flight and to make high resolution images is one of the major accomplishments of this program. VHF/UHF in-flight images, never achieved before this program, are a powerful diagnostic tool for use in aircraft development.
In ISAR applications data is acquired on a circular grid. In further processing, data on a rectangular grid is obtained by interpolation. This causes the loss of data outside the interpolated area. The latter can be corrected by extrapolation, but this can give incorrect information. A new technique s proposed which uses a larger rectangular area than in the above mentioned case. Some parts of this rectangle are calculated by extrapolation. Because most of the data in the larger rectangular area consists of original data, only minor parts are extrapolated. Consequently, this method is expected to be more reliable than traditional extrapolation techniques. Simulations have shown that the data obtained by the new interpolation - extrapolation scheme provide a considerable improvement to the amplitude - and phase accuracy across the enlarged rectangular grid.
This paper contrasts indoor and outdoor implementation of efforts during upgrades of VHR RCS measurement capabilities. Sites studied are two McDonnell Douglas Technologies Incorporated, Range Measurements Services facilities. Indoor. Radar Measurement Center (San Diego, CA) is a large compact range. Equipment-Harris Corporation Model 1630 Collimator System, Scientific Atlanta Model 2090 radar. Outdoor. Microwave test facility (Victorville, CA), large ground plane facility. Equipment-Steerable dipole feed dish, System Planning Corp, Mark III radar.
The use of electronically scanned phased array antennas in demanding rolls such as satellite communications and radar systems has led to an increasing desire to analyze the sources of error present in the boresight alignment of such systems. Not the least among these errors are those introduced by thermal effects on the various components which comprise the array structure. In an effort to understand this mechanism, this paper will discuss a technique which uses an infrared camera system to analyze the beam deflection errors caused by the effects of temperature gradients present in the antenna system.
This paper describes the significant upgrades to METRATEK's Model 100 AIRSAR Dynamic Imaging System since the earlier version was discussed at last year's conference. This system consists of three wideband radars mounted on a A-3 aircraft. It can generate diagnostic images airborne targets up to 200 feet in length and width. We will present examples and discussions of the solutions found to the many difficulties involved in generating high quality, high resolution, fully-calibrated SAR images of aircraft in flight from aircraft in flight. Data collection and processing hardware and software, as well as lessons learned from over 6 months of flight tests will also be described.
Northrop Corporation's Business and Advanced Systems Development Group has recently completed a very successful Radar Cross Section (RCS) measurements program on the USAF/Northrop B-2 bomber. One of the capabilities spawned from the program is a measurements radar system, comprised largely of off the shelf hardware, which provides high resolution whole body two-dimensional RCS images of large targets on the ground in the near field. Its high gain antennas allow operation in a space limited area and utilizes Synthetic Aperture Radar (SAR) data collection techniques. The system, though designed for use at VHF, has been expanded to operate from 100-2000 MHz in three bands. The hardware, associated signal processing, its applications and limitations are discussed.
Lockheed has recently completed the construction of a Large Compact Range (LCR) for antenna and RCS measurements. The dimensions of the facility are 60' (h) x 100' (w) x 120' (l) with a 20' x 20' cylindrical quiet zone and operational capabilities from 0.1 to 18.0 GHz. The requirement to measure low RCS levels in a room which is smaller that the desired has resulted in a unique system design. Elements of this design include a feed pit, a feed hood, and a rolled edge reflector; special absorber layouts to minimize background scattering, a high performance instrumentation radar, fast ring down feed antennas, and a unique string suspension and positioning system. This paper presents the various sub-systems that make up the LCR along with chamber validation methods and preliminary performance data. The subsystems listed in this paper are LCR's: Reflector, radar system, feed antennas, feed positioner, absorber, target handling equipment, and string positioning system. Initial design requirements are listed for some sub-systems along with range characterization data such as un-subtracted clutter levels, background subtraction performance, and theory vs. measured data for some simple conical shapes.
Traditional Compact Range Antenna (CRA) applications are related to Antenna Pattern and RCS measurements. For these purposes, as a rule, CRA are installed within or outside of an anechoic chamber as stationary equipment. However, for some modern applications, such as Electronic Warfare development, radar tracking system testing, indoor RF environment simulation and others, where dynamic and pointing properties of an AUT are to be tested, the mobile and multi-beam CRA is of great importance, since it provides the designer with powerful simulation and testing capabilities. Such a CRA has been designed, built and tested at ORBIT ADVANCED TECHNOLOGIES, LTD. The design trade-offs, CRA analysis, test set-up and results are discussed in the presented paper.
This paper considers an electromagnetic field simulation of an anechoic chamber with experimental verification. The simulation is a Geometric Optics (Ray Tracing) mathematical model of the direct path between two antennas and interfering scattering. There are two separate models due to the frequency dependent nature of the pyramidal radar absorbing material (RAM). The model for the frequency range of 30 to 500 MHz was used to characterize the specular scattering. The specular scattering was modeled as a lossy, tapered, TEM transmission line in an inhomogeneous anisotropic tensor material. The frequency range from 500 MHz to 18 GHz was characterized by dominant tip diffraction of RAM patches and the model made use of a Uniform Theory of Diffraction code for a dielectric corner. The measurements and simulations were based on an azimuthal cylindrical scan. Diagnostic measurements were also performed by a cylindrical scan of a directional horn antenna to locate scattering sources in the chamber. A cylindrical wave, modal expansion of the diagnostic data which included a one dimensional Fast Fourier Transform with Hankel function expansions.
A Ground to Air Imaging Radar system (GAIR) used to perform diagnostic imaging and total RCS measurements on low observable airborne targets has been developed by the Environmental Research Institute of Michigan (ERIM). In order to ensure accurate measurement of the scatterers contributing to a target's radar signature, proper calibration in imperative. The use of external calibrators to measure the end-to-end system transfer function is the ideal way to perform a system calibration. However, this is a more difficult and challenging task with a ground based radar viewing an airborne target, as opposed to a traditional airborne SAR which views an array of ground based trihedral corner reflectors. This paper will discuss the internal and external calibration methods used in performing an end-to-end system calibration of the GAIR. Primary emphasis is placed upon the external calibration of the GAIR and the three independent measurements utilized: a ground based corner reflector, a sphere drop, and an in-scene calibrator. The system calibration results demonstrate that the GAIR is an accurately calibrated radar system capable of providing calibrated images and total RCS data. Moreover, only the ground and internal measurements are required on a daily basis in order to maintain system calibration
A small compact range measurement facility has been installed at the Environmental Research Institute of Michigan (ERIM) for research aimed at improving RCS measurement and radar imaging techniques. This paper describes the facility, which is referred to as the Experimental Range Facility (ERF). The ERF has two instrumentation radars; a Flam & Russell FR959 gated CW radar and a Hughes MMS-300 pulsed radar. The radars are connected to a suite of workstations, which support a variety of internally and externally developed radar imaging and data exploitation software. The ERF is also equipped with sophisticated target positioning control and sensing equipment.
The U.S.Army Redstone Technical Test Center (RTTC), Test and Evaluation Command, has developed a comprehensive antenna metrology and Radar Cross Section (RCS) evaluation facility. This facility features the compact antenna test range technique for millimeter wave measurements and the near-field scanning technique for microwave measurements. This paper described RTTC's use of these measurement techniques, instrumentation with PC Windows based automation software, anechoic chambers, and types of tests performed. Planned future thrust areas are also discussed.
A complete description is given of the unique radar cross-section (RCS) measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. The uniqueness of this chamber comes from its ability to independently move the transmit and receive antennas, which can each be moved to any position within their respective ranges of motion to a resolution of about 0.05 degrees. The transmit antenna is fixed in azimuth, but can be moved in elevation: the receive antenna is free to move in both azimuth and elevation. Additionally, the target can be rotated in azimuth by means of an azimuth positioner. Analysis has been performed to determine the impact of chamber effects on measurement accuracy. The most notable chamber effect comes from the two large aluminum truss structures, which are the mounting supports for the transmit and receive antennas. Fortunately, the scattering from these structures can be readily separated from the desired target return through the use of range (time) gating. Time domain results are presented showing the effects of these structures.
Numerous monostatic radar cross-section (RCS) calibration routines exist in the literature. Many of these routines have been implemented at the RCS measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. Key monostatic results are presented to give an indication of the measurement accuracy achievable with this chamber. Unfortunately, bistatic calibration routines are not nearly as common in the literature. As with the monostatic routines, a number of bistatic routines have been implemented and typical results are presented. Additionally, descriptions are given for some of the reference targets along with their support structures that are used during calibration.
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.