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New results of wideband polarimetric radar-cross-section-(RCS-) antenna measurements are presented. A special antenna network description including polarization information and multiport feeding offers new insight in antenna behavior. The procedure omits the utilization of a standard gain antenna for absolute gain determination and no RF-feedline is necessary to the antenna under test. Antenna radiation, scattering and feed characteristics are all obtained with one measurement setup. Theory as well as measurements on different dual-polarized antenna types demonstrate the efficiency and uniqueness of this technique.
The new Compact Test Range at Dornier GmbH, operational since early 1990, is presented. The system is designed for both antenna and RCS measurements, for support of in-house projects as well as for third party measurement needs. Great emphasis has been on improving measurement through put to reduce effective measurement costs. The major system components are evaluated (anechoic chamber, compact range reflector system, RF instrumentation, positioner system, computer system and measurement software). System specifications, and where possible measured performance data are presented. Finally a typical antenna and RCS measurement are described to get an idea of possibilities together with required range time.
The Concept of Compact Test Range has been recently much used for antenna testing facilities, its main characteristic of having far-field conditions in a small and closed place, for a very large frequency band, makes it very attractive. Antenna manufacturers are building them up when the millimetric waves and the spacecraft flight model antennas become part of their activities. The change of the point of view of the antenna characteristics – now, parameters like Gain and Radiation Patterns are replaced by EIRP, Flux Density or Coverage- modifies the classical test philosophy. It makes different the Test Procedures which, in addition, have to take into account the cleanliness and the quality control required for handling flight models, as well. The Compact Payload Test Range (CPTR) in ESTEC shows up a PWZ of 7 x 5 x 5 metres for a frequency range from 1.5 to 40 GHz.; it has been created for testing whole Spacecraft Payloads in space required cleanliness area. The particular properties of the CPTR as such as shielded room, feed scanning, multiaxis test positioner, etc. are used to improve its test possibilities.
The plane-wave spectral range probe technique introduced by Coblin can be used to locate multiple scattering centers on an antenna range. The x-y positioner presented by him is too costly for many applications. A plane-polar implementation of the technique provides a less costly alternative. A preliminary study of such an implementation is presented. The plane-polar positioner presented makes use of the roll-axis of a standard roll-over-azimuth positioner and the instrumentation of the range which was being used for this study.
Recently, super resolution techniques have been applied to image spurious signals in compact range measurement systems. These techniques include parametric modeling of the probe data as well as eigen-space based methods. In these techniques, in incident signals on the probed aperture are assumed to be planar, which may or may not be true. In general, if the separation between a signal source and the probed aperture is more than , where D is the size of the probed aperture, one can assume that the signal incident on the probed aperture is nearly planar. It is shown that this is not necessarily true for super resolution techniques. The signal level also affects the minimum distance requirements. The stronger the signal, the farther its source should be from the probed aperture to achieve the optimum performance.
The spherical probing technique for the angular location of secondary scatterers in antenna measurement ranges is demonstrated for an anechoic chamber far-field range. Techniques currently used for source location use measurements of the range field on a line or plane. A linear motion unit and possible a polarization rotator are necessary to measure the range field in this manner. The spherical range probing technique uses measurements of the range field over a spherical surface enclosing the test zone allowing existing range positioners to be used for the range field measurement. The spherical probing technique is demonstrated on an anechoic chamber far-field range with a known secondary reflection source. The plane wave spectrum of the measured range field is computed and used for source angular location. Source locations in the range correspond to the angular locations of amplitude peaks in the spectrum. The effects of the range field probe on this spherical probing is investigated by performing probe compensation.
Accurate scattering and antenna measurements require excellent plane wave purity in the target zone; however all measurement systems are contaminated by various stray signals which result in measurement errors. In this paper, a technique of evaluating the stray signal sources in a compact range using a diagonal plat plate as a test target is presented. The scattering cross section of the diagonal flat plate as a function of frequency and angle of rotation is first measured. Then the time domain response for each projection angle is processed to obtain a two dimensional ISAR image of the plate as well as the stray signals. From the stray signal images, the location and relative strength of the stray signals can be determined. Experimental results from the OSU/ESL Compact Range Facility are presented to demonstrate this stray signal imaging technique.
Much research has been done recently on the interpretation of measured field probe data in order to locate and quantify error sources present in the quiet zone of a compact range. This paper examines an alternative method of analyzing those data by applying spherical phase offsets to focus the field probe data to near-field distances. This method is applied to simulated field probe data for a large compact range. The technique yield the correct [x,y,z] coordinates of multiple scattering sources deliberately introduced into the simulated data.
Problems exist with the measurement of large aperture antennas due to the far field requirement. This paper discussed a new method to measure a phased array at about 1/10 the normal far field. The basic idea involves focusing the test array at probe antenna a distance R away from the aperture. In the described measurement technique the probe antenna is placed on an arm that rotates 100º on the focal arc given by Rcos(?). This arc minimizes defocusing due to phase aberrations. To minimize the amplitude errors, the pattern of the probe antenna is carefully matched in order to compensate for the 1/R variation induced amplitude error. The application of this technique will enable arrays to be measured in anechoic chambers, allowing convenient classified testing, while avoiding the effects of weather, and will reduce the risks inherent in the high power testing on transmit. The results of a computer simulation is presented that characterizes the validity and limitations of the technique.
Beamspace techniques are usually employed to synthesize phased array antenna patterns of arbitrary shape. In this paper a beamspace method is used to calibrate the pattern of a 32-element linear array with a conventional array taper. By measuring the antenna pattern in specific directions the beamspace technique permits the actually applied excitation function to be determined with little mathematical effort. Iterative corrections can then be made to the excitation function to maintain low sidelobe performance, or to compensate for element failures. Since local corrections to the array pattern result in global changes to the excitation function, explicit knowledge of where an element failure has occurred is not required. The beamspace analysis was carried out using antenna patterns obtained by electronically scanning the array past a far-field source. Such pattern measurements offer the possibility of maintaining phased array performance in an operational environment.
Transmit – receive modules (T/R) utilizing GaAs monolithic microwave integrated circuit (MMIC) technology for amplifiers, attenuators, and phase shifters are becoming integral components for a new generation of radars. These components, when used in the aperture of a low sidelobe electronically steerable antennas, require careful alignment and calibration at multiple stages along the RF signal path. This paper describes the calibration technique used to measure the performance of an active aperture 64 element S-band phased array antenna that employs T/R modules at every element. RF component performance and phased array sidelobe characeristics are presented and discussed.
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.
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.