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A novel low-cost automatic system is described to measure both the complex permittivity and permeability of solid materials at 2 to 18 GHz. It is particularly useful for evaluating the frequency dependence of radar absorbing materials (RAM). The RF and the mechanical setups are described, including the computer algorithm and the measurement procedure. The results and the experimental errors of three materials are presented, which agree with results that were obtained by other methods, while the cost of putting up the system is considerably lower than any comparable alternative.
Measuring the radar cross section of low-observable (LO) vehicles require an RCS quality control (QC) program that will last throughout the life cycle of the vehicle, from component production to operational deployment and depot maintenance. Testing must be done at regular intervals to ensure that surface or sub-surface damage has not degraded the RCS characteristics of the vehicle beyond acceptable limits. In the past, these measurements were complicated by the requirement for and expensive, well-prepared RF test environment. The test range—usually a fixed site—is often remotely controlled. System Planning Corporation (SPC) has developed an RCS QC measurement technique that requires little or no facility improvements while offering high sensitivity inverse synthetic aperture radar (ISAR) images. The instrumentation radar system can be located at the production, maintenance, or operational site of the vehicle or component. As a result, the QC program is both economical and reliable.
To achieve optimum measurement accuracy and range throughput in antenna and radar cross-section (RCS) measurement applications requires a careful and thorough design of the measurement system. Measurement accuracy requirements, test time objectives, system flexibility, and system costs must all be balanced to achieve an optimum system design. Considering these issues independently will result in unwanted and/or unexpected system performance tradeoffs. This paper examines these issues in some detail and suggests a system design approach which balances microwave performance and measurement speed with system cost.
Lockheed’s Advanced Development Company (LADC), located in Burbank, California, has been evaluating the capability of indoor anechoic chambers to measure VHF/UHF RCS. Two chambers were available for evaluation. A 155 feet long, 50 feet high by 50 feet wide tapered horn chamber and a compact range having dimensions of 97 feet long, 64 feet high by 64 feet wide, featuring a 46 feet wide collimator. For comparison purposes, a common instrumentation radar was used in each chamber. This radar was based on a network analyzer using a Lockheed designed pulse-gate unit to increase transmit/receive isolation. Various antenna feed system were tried in both chambers to ascertain their characteristics. Theoretical and experimental data on system performance will be presented emphasizing practical implementation and inherent limitations.
Radar cross section (RCS) measurements were performed in the 0.1-1 GHz band in an anechoic chamber optimized for microwave frequencies. Selection of proper instrumentation, antennas, measurement techniques and processing software are discussed. Experimental results, showing the accuracy and sensibility of the system are presented.
This paper presents an overview of the integrated Radar Measurement System (IRMS) installed at the Air Force Radar Target Measurement Facility (RATSCAT) for AFSC/6585 TG/RX Holloman AFB, New Mexico.
This paper will describe the production version of the Model 200 TRACKSAR radar, which provides high-resolution imaging in downrange and crossrange using wideband waveforms and both synthetic aperture radar (SAR) and inverse synthetic aperture radar (ISAR) processing. Several other novel features of the system and technical aspects of performing such measurements will be addressed, and sample data outputs will be presented.
Several electromagnetics laboratories are now using “string” or line to support their test bodies. There is no standard line material used and often this material is chosen fairly arbitrarily. This paper compare electrical and mechanical characteristics for various types of line. The line types to be tested include Spectra 1000, Kevlar, nylon, Teflon, and wire rope. Each characteristic will be tested for 0.04” and 0.10” nominal diameters. Radar cross-section tests will be run for each string, both in a vertical position and at an angle of approximately 15 degrees. Each measurement will be run with a frequency sweep from 2-18 GHz. Dielectric constants for each of the line types will also be compared. Mechanical attributes such as tensile strength, creep, and yield, if any, are compared for each of the various line types and sizes. Both vendor data and laboratory results will be presented. The electrical and mechanical characteristics will then be used to discuss which line material is optimum for use during electromagnetic testing.
A 45 GHz instrumentation radar system unique in several respects has been developed for inverse synthetic aperture radar (ISAR) and tracking angle scintillation (glint) studies. The system, based on a Hewlett-Packard HP-8510B network analyzer, is fully polarimetric and operates on a 1000-m outdoor far-field range. An amplitude monopulse receiver provides a measure of the instantaneous apparent-center-of-scattering of the target. Successful glint and ISAR measurements have been made on targets as large as 8 m.
A laser tracker using a computer controlled feedback loop has been designed and tested. The tracker follows a small retroreflector embedded in a radar calibration sphere. Angle encoders coupled to two orthogonal scanning mirrors give azimuth and elevation pointing angles to the target. Phase measurements of an intensity modulated laser beam give change in distance to the target, while absolute range is determined by knowing the initial 2p ambiguity interval of the target position. The crossrange accuracy of the system is limited by the scanning mirror encoders to =.063 inches rms at 105 feet (50 microradians). The downrange accuracy of the system is ˜.015 inches rms. This versatile system can be used for: a) contour measurements of models with the aid of a retroreflector moving over the surface, b) accurate determination of the coordinates of a single moving target, and c) determination of the orientation of a large extended target. Anticipated modifications of the system, with their potential precision measurement capabilities and applications, are discussed.
This paper will describe new developments in a gated-CW radar that has been designed to improve the productivity and sensitivity of RCS measurements. Improvements in data acquisition speeds result from the design of a fast synthesizer, a data acquisition co-processor and a pulse modulator. Each of these new products have been specifically designed to take advantage of the high speed capabilities of Scientific-Atlanta’s Model 1795 Microwave Receiver. The RF sub-system has also been designed to permit continuous 2-18 GHz, full polarization data acquisitions. Critical RF components are now mounted at the feed in the chamber, improving the sensitivity and ringdown of the system. Productivity in analysis activities has been improved by the use of a multi-tasking system controller which permits simultaneous use of the system for acquisitions, analysis and plotting.
This paper describes results of extensive polarimetric Radar Cross Section (RCS) measurements on canonical targets. Amplitude and phase of both co- and cross- polar returns are measured for horizontally and vertically polarized transmit signals in order to determine the complete complex scattering matrix. Measurements have been carried out on a variety of targets. Results presented with this summary show data for a metallic and a dielectric disk. Details of measurement and calibration procedure, hardware, and software are also presented.
This paper is concerned with the measurement of RCS in a room with conducting walls. The experimental measurement system uses a moving antenna to produce a scan of the target and the clutter. The scattered signal as a function of frequency and position is recorded. New field crossrange processing is then used to map the target zone. Example images will be shown for both two-dimensional and one-dimensional scans. Images from point targets and distributed targets will be presented.
Accurate radar measurement of complex marine targets from a shore-based radar are difficult to achieve because of the effects of a multipath environment. This paper summarizes multipath effects at low and very low grazing angles. The investigation of scattering from the sea surface and from marine targets is important for radars operating at low elevation angles over a sea surface because severe fading and distortions in measured target radar cross section can occur due to multipath propagation. It can be shown that the lobing structure due to the interference of the direct and reflected signal is still a problem for very low elevation angles even in high sea states, suggesting a limited usefulness for low elevation radar cross section measurement sites. Further it has been observed that in the microwave region at elevation angles smaller than 0.5 degrees, scattering centers located at certain heights above the sea surface may be masked. At higher elevation angles, however, multipath interference is reduced, thus giving a more stable basis for measurement and evaluation.
In-phase and quadrature (I/Q) aberrations in radar receiver data create problems in radars used for radar cross section (RCS) measurements. I/Q errors cause incorrect representations of the target under test. A method for correcting I/Q error and calibrating the measured amplitude to a scattering standard provides a means of obtaining a more accurate representation of the target under test. The RCS measurement instrumentation addressed here uses a wide band receiver with a single quadrature mixer for conversion of radio frequency (RF) to base band (also referred to as video) frequency. In the one-step down conversion, distortions in the I/Q constellation occur, causing I/Q errors. This method quantifies the extent of the I/Q problem by estimating the actual I/Q error from a series of calibration measurements. An algorithm is presented which quantifies parameters of the I/Q distortion, then uses the distortion parameters to remove the I/Q aberrations from the target measurement.
Pulse-to-pulse amplitude and phase noise can affect the overall measurement accuracy of RCS instrumentation radars. Depending upon the measurement requirements, such noise can limit the overall performance whenever pulse-to-pulse repeatability is required in the signal processing. Radar systems using pulsed TWTAs are subject to high noise due to limitations in the performance of the TWTA modulators and power supplies. A characterization of this additive noise is important to understand the limitations in system performance. Measurements have been made on kilowatt power TWTAs at L and X band as well as 20 watt pulsed TWTAs at S, C, and X/Ku band at various duty cycles and PRFs.
In coherent measurements, one measures the interference of signals from test and reference paths. These techniques are widely used in RF image measurements of antennas and radar cross sections. The success of a coherent measurement depends highly upon the stability of the path length difference between test and reference signals as well as the quality of the reference signal. The stability and quality are hampered when the experiment has to be conducted with a long reference path length, particularly at outdoor ranges. A new measurement scheme, based on the scattering process initiated by two coherent beams, will be presented here are has the advantage over others in reducing the problems associated with the path length difference instability.
This paper presents an overview of the test and set-up requirements of active Synthetic Aperture Radar (SAR) antennas. The specific antennas under consideration are those that are intended to be used in the next generation of spaceborne SAR C-Band satellites. These antennas are typically 1m to 2m wide and 10m to 20m long, possessing between 3000 to 12000 radiating elements. The paper considers each unit of the active antenna in turn and identifies which tests are to be carried out where. In considering the test of the whole antenna some initial result of focussing techniques, to allow the antenna to be tested in real time at reduced distance, are presented.
A novel and very powerful measurement technique is presented which allows the determination of antenna radiation and scattering by radar-cross-section (RCS-_ measurements. The antenna under test is treated as a loaded scatterer using a polarization dependent network model that allows a complete antenna description in terms of scattered, radiated and absorbed waves. A load variation principle is used to determine the network model parameters and all commonly used antenna parameters like gain, antenna polarization, axial ratio, polarization decoupling, input impedance and also structural scattering can be derived from the backscatter measurement without using any additional standard antenna. With the antenna network description it is furthermore possible to examine the antenna behavior for arbitrary excitation or loading on their waveguide or radiation port.
A major objective in the design of an RCS measurement facility is to obtain the greatest possible productivity (overall measurement efficiency) while maintaining the accuracy and sensitivity necessary for low radar cross section targets. This paper will present parameters affecting the total throughput rates of an indoor facility including instrumentation, target handling, and band changes-one of the most time consuming activities in the measurement process. Sensitivity and accuracy issues to be discussed include radar capabilities, feeds and feed clustering, compact range, background levels, and diffraction control.