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When attempting to make accurate Radar Cross Section (RCS) measurements, it is vital to understand the background levels of both the range and the target support fixture. Typically these support fixtures are either foam columns or metal pylons. Determining the RCS levels of the metal pylons requires the installation of a termination device to hide the rotator which has a significantly lower RCS than the pylon being measured. Quite often this is an impossible task, especially at lower frequencies. An algorithm that accurately determines the pylon background levels independent of the RCS contribution of the pylon terminator is presented. This algorithm requires translating the terminator linearly and isolating the background from the resulting interference pattern. Data is included that validates the implementing computer code.
Clutter returns can seriously limit the performance of high sensitivity Radar Cross-Section (RCS) measurement ranges. Within the direct sample space of the target, clutter is controlled by: minimizing the antenna response outside of the angle subtended by the target and by careful transmit pulse control. However, clutter returns are also produced from areas outside the sample space of the target. This paper discusses the application of pseudo random phase coding techniques to suppress this type of clutter. It defines the nature of this type of clutter, identifies a method to suppress it, describes the hardware used for online suppression, and presents experimental results to demonstrate the effectiveness of the technique. The technique is important for both outdoor and indoor ranges (particularly in unprepared, echoic, environments); experimental data is present for both cases.
Classical radar target imaging uses an inverse synthetic aperture radar (ISAR) algorithm based on the two dimensional Fourier transform. The technique has resolution limitations in the time-domain (or down-range) dimension somewhat larger that the inverse of the band-width of the interrogating radar system (depending on the frequency domain windowing function utilized). The resolution in the cross-range domain (or doppler-domain) is related to the inverse of the aspect angle sector over which the target is observed. This paper will present radar target imaging techniques based on modern autoregressive (AR) spectral estimation algorithms (superresolution) which overcome these limitations. Techniques are shown for the generation of ISAR images even with severly [sic] limited frequency or angle domain data. Images will be shown where the quality of the image does not degrade even when the bandwidth of the original data is reduced by a factor of 16. Thus clear images are produced using these techniques with data where the classical Fourier-based techniques produce only “fuzzy blobs”
Superresolution (SR) processing techniques have been used for many years in direction finding applications. These techniques have proved valuable in extracting more information from a limited data set than conventional Fourier analysis would yield. SR techniques have recently proven to be an extremely powerful radar cross section (RCS) analysis tool. Typical resolution improvements of 2 to 30 times may be achieved over conventional Fourier-based range domain data in both the one-dimensional and two-dimensional image domains. Typical measurement scenarios which can most benefit from SP processing are presented. These include: VHF/UHF RCS measurements, measurement of resonant targets, and performing detailed scattering analysis on complex bodies. Measurement examples are presented illustrating the use of SR processing in a variety of test conditions. When the advantages of SR processing are combined with the accuracy of Fourier techniques, a new window is opened through which target scattering characteristics can be seen more clearly than ever.
This paper addresses measurement and data processing techniques for dynamic helicopter radar signatures. Data products are presented and interpreted to highlight the utility of instrumentation radar systems as a means for determining radar scattering characteristics of objects with rotating components. Investigation of rotor-body multipath phenomena in helicopter imagery cannot sufficiently resolve ambiguities regarding ray traces that contribute to observed scattering events. The diagnostic insights gained from concurrent doppler spectral data aid in resolving these ambiguities. Unique spectral signatures resulting from rotor-body interactions are investigated, and a methodology is developed for diagnosis of the responsible scattering mechanisms. The results provide valuable insights into the radar spectral signatures o conventional helicopters.
Range resolution of a radar image can be obtained by use of wide-band signal (linear FM or chirp waveform) and cross-range resolution by object rotation which synthesized a large antenna aperture (the so called ISAR method, refer [1]). Although both cross-range profiles can be resolved by rotation of the abject about two mutually orthogonal axes, however, the data manipulation would be quite cumbersome and the measurement implementation would require a mechanical support system by which the objet [sic] can be independently tilted and rotated relative to the radar axis. In this paper, the algebraic reconstruction technique (ART)[2] for tomography is used to resolve the vertical cross-range profile (along the axis normal to the ground) while the horizontal cross-range profile still resolved by ISAR method. Applications of the ART to a simple circular pattern and a complicated emblem pattern of the CSIST show that ART is a suitable approach and easier than ISAR method to obtain the second cross-range resolution.
This paper describes new developments in broadband Microwave power amplifiers for compact RADAR Cross Section (RCS) Ranges. The RF Power level of transmitters used in compact RCS ranges for the most part has been limited to a watt or two. This is due to the limitations of the power available from solid state RF amplifiers and the power handling capabilities of PIN diode switches, used to pulse modulate the RF amplifier output. Inherent impedance mismatches of the PIN diode switch, RF amplifier and RF output circuits produce reflections of RF energy. The reflected RF energy reverberates between the output circuits of the RF amplifier and the antenna. Reverberation of RF energy between mismatches continues until circuit losses reduce the energy to zero. These reverberations manifest as deterioration of the RF output pulse fall time waveshape. The radiated pulse fall time is extended and damped rather than abrupt. This deterioration of pulse waveshape, due to reverberations, is ring down time. RF pulse ring down deteriorates the resulting RCS measurements. New broadband microwave Traveling Wave Tube (TWT) technology, combined with extremely quiet power supplies and modulator, provide increased power, low noise floor and reduced ring down time resulting in improved RCS measurements.
The new HP 8530A microwave receiver has been designed specifically for antenna and radar cross section (RCS) measurement applications. With its capabilities and features, high-speed single parameter and multiple parameter measurements are possible. High-Speed measurements are a necessity for certain applications but oftentimes other factors will determine the actual test time. Measurement speed for various applications will be discussed and, more specifically, multiple parameter measurements using the HP 8530A’s internal multiplexer or external PIN switching.
As new military aircraft with low radar signatures pass from the design stage to production and deployment, the techniques for measuring and confirming their low signatures must move from the laboratory to the flight line. Measuring the RCS characteristics of carrier-based aircraft is particularly difficult because it must be done either while the aircraft is in flight or while it is one a crowded flight deck or hanger deck. This paper describes an approach to Navy flight-line RCS measurements that minimizes space, yet still provides enough information to identify a degradation in low signature performance and to pinpoint the source of the problem. It uses a small reflector on a positioner combined with a stepped frequency gated CW radar at 8-12 GHz to sweep a spot illumination over the aircraft while producing downrange profiles at each spot. The primary advantage of this configuration is that it restricts the RF radiation in all three spatial dimensions, thereby minimizing the scattering from other objects in the crowded environment. A secondary advantage is that the data can be processed to yield resolution of scatterers on the aircraft under test to within two or three feet. Adding an automatic focusing ability to the reflector antenna can improve the resolution to about one foot.
Martin Marietta designed and brought on-line an indoor far-field chamber used for radar cross section (RCS) evaluation. The range has conductive walls on all sides except for the pyramidal absorber covered back wall. The chamber was designed such that wall/floor/ceiling interactions occur with a distance (time) delay allowing for their isolation from the test region. Software gating techniques are used to remove these unwanted signals. This paper presents an analysis of the conductive chamber using Geometrical Optics (GO). The objective was to analyze and evaluate the plane wave quality in the chamber test region. The evaluation of the plane wave was performed using the angle transform technique. The measured results were compared to analytical results and measured antenna patterns.
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.