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In the search for an ideal test-object support for simulate free-space radar-cross-section (RCS) measurements, low-density polystyrene foam has achieved considerable popularity. However, significant error can be introduced into a measurement by the use of an inappropriately designed support. Although low back-scatter radar cross section (RCS) can be obtained with this material, interactions can occur between the test object and the mount which will cause measurement errors in excess of several dB. We present results of measurements performed on a simple test object supported on a low-density foam column which demonstrate this effect. As we discuss, this error can be incorrectly interpreted to be caused by poor alignment of the test object with the radar-range coordinate system. Finally, we show that the errors can be explained by differential propagation effects. In addition, this simple theory provides the insight necessary to devise appropriate measures to minimize the errors cause by the presence of the support.
Clutter rejection is designed to remove range clutter and repeatable radar parameters from the measurement. The technique to be used is one that gains the customer acceptance, does not significantly increase range time, and produces good results. Techniques which require significant data processing have not been accepted by our customers. Fixture subtraction requires very accurate target positioning and is too slow. Only background subtraction met all the requirements. This paper will discuss a new background subtraction method. In this technique the pylon is effectively removed from the measurement area, but not from the chamber. This is done with a small pole termination target and the antenna measurement slide. Thus a true background is measured. The technique has been highly successful, gaining the acceptance of our customers and users alike. Range measurements will show how well the technique works.
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.
Errors due to the interaction between test body and the Device Under Test are often overlooked in test body design. Interactions which cannot be gated or subtracted can be present even in low RCS test bodies. This paper presents an approach to evaluate the edge interaction errors of a component RCS test body. In order to quantify the interactions, small cylinders were attached to the face of the test body and measured from grazing to 50 degrees. The scattering of the cylinders illuminated the edges so that the interactions could be measured. This data is presented along with the results of several computer models which were used to determine the interactions involved. A method of moments model of the cylinders on an infinite ground plane gave the theoretical level of the cylinders. A pattern of a monopole antenna on a test body shaped ground plane was used to determine the contribution of each edge; and a point source model was used to locate the points on the edge where the diffraction occurred. This technique allows the dominant source of error signals to be identified.
General guidelines for using software gating are presented. Examples which demonstrate both proper and improper use of gating are presented. The effects of RAM materials on the time domain signature and the selection of the gate parameters are discussed. A brief review of the general theory of high resolution RCS measurements is presented.
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”
Based on the complexity of the scattering mechanisms associated with a real-world target, it is obvious that measurement diagnostic tools are extremely helpful. On technique that has found great success in this regard is the conventional ISAR or down range/cross range image. However, the results are basically two-dimensional, which limits the usefulness of the data in that most real-world targets have significant three-dimensional features. A very efficient class of 3D image algorithms has been developed which are based on various time domain look angles relative to the target [1]. It has been shown that one can use multiple feed antennas in a compact range to collect this data and then process it directly to obtain a 3D image of the target. This can be done very rapidly, say every 10 seconds, using an approximate solution, or in 10 minutes using a 3D ISAR approach. The system design and techniques used to implement this system are presented in this paper.
Two-dimensional RCS imaging systems utilize wide-band, ISAR processing to spatially isolate scattering sources on complex objects. Although the measured data consist of the frequency and angle responses of the entire object, the image process allows the possibility of extracting the responses of scattering components which comprise the total signature. These methods of image editing generally involve the application of spatial filters to the image, followed by a reconstruction of the angle and frequencies responses associated with the filtered image. The objective of these procedures is to determine the responses of localized scattering sources or to delete the contributions of scattering sources on the overall signature of a complex object.
A number of spectral analysis techniques which offer significantly higher resolution than the FFT technique have been developed in recent years. The application of these super-resolution techniques to scattering analysis is of interest. With these techniques it is possible to identify the closely spaced scattering centres even with RCS data over relatively small bandwidths. This can be of significant importance in applications where data over large bandwidths are not available. The use of Autoregressive and Eigen analysis based super-resolution techniques in the scattering analysis of two basic targets, a sphere and a cube, is investigated and the results of the study are presented in this paper.
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.
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 target handling system that was developed for use in the compact range facility at the University of Pretoria. The system was locally designed and manufactured and comprises of a lift platform, RCS pylon and utility trolley. The pylon utilises a unique design approach resulting in a structure with very high stiffness and surface finish.
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.
The wave constraints typically placed on high-gain microwave antennas in a space environment, such as light weight construction and unfurlable deployment, preclude the rigid construction necessary to accurately maintain a required surface configuration over extended time periods. Present designs are limited by conventional, passive fabrication techniques. The ability to measure and control the antenna figure permits operation at tens of GHz, key to presently contemplated applications. The electro-optical figure sensor monitors the phase error of an antenna surface (parabolic or planar) by viewing optical fibers attached to the antenna, thereby providing feedback for active control of the antenna to a specified shape. Least-squares fitting of measurement data permits less stressful active control to the homologically-equivalent best-fit, or even the simpler tilt-alignment. Optical analytical techniques appear applicable to large, high-frequency antennas, offering new configuration designs and simpler analysis.
The purpose of this paper is to present an overview of a turnkey mobile dynamic R.C.S. system, presently under design and development. The test system includes no less than 16 antennas, installed on two heavy duty tracking positioners, trailer mounted. The RF instrumentation is split over racks located on the positioners and in the mobile shelter housing the control equipment and operators and includes 14 receivers and 7 high power transmitters. The paper describes the antenna system, RF instrumentation, control and processing software as wek as operational and modularity aspects of this dynamic RCS facility.