Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
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.
The effects of non-systematic receiver instrumentation errors on precision antenna measurements are investigated. A simple uncertainty model relating dynamic range to random perturbation effects on amplitude measurements is proposed. Examples of measurement uncertainty versus both input level and measurement speed are presented using data taken on modern measurement receivers. Dara are compared with the model to estimate measurement uncertainty at various pattern levels and acquisition speeds. Equivalent dynamic range specifications are deduced from the measures data.
This paper reports on a study undertaken to assess the effects of range amplitude tapers on the measurement of low and ultra-low sidelobe levels and gain. It has been shown that low test zone phase tapers are required for the measurement of low and ultra-low sidelobe levels. A few papers have addressed the effect of amplitude errors but not for the measurement of low sidelobe levels. These papers have concluded that amplitude errors have much less effect than phase errors. This paper addresses antenna measurement ranges such as compact ranges where phase taper has been significantly reduced, but amplitude errors remain. The amplitude taper on some modern compact range configurations has not only, not significantly improved, it has often taken on a more complicated “double hump” shape. The effects of these modern amplitude tapers are demonstrated.
Compact range systems have been widely used for high quality RCS measurements. However the taper and cross-polarization effects can lead to significant measurement errors especially as the target approaches the border of the target zone. The taper error is mainly caused by the feed’s finite beamwidth, and the cross-polarization error by the feed’s cross-polarized radiation and the offset configuration of the reflector. A method to correct these errors is presented. In order to perform taper and cross-polarization error corrections, one has to be able to predict the target zone fields and determine the locations and complex strengths of the various scattering centers associated with the target. The correction can then be done by compensating for the taper and cross-polarization effects for each localized scattering center. Several measurements have been taken, corrected and then compared with the theoretically expected results to validate this technique.
The reflective properties of a flat circular plate and a long thin wire are discussed in connection with the quality and calibration of the quiet zone (QZ) of a compact antenna test range. (CATR). The flat plate has several applications in the CATR. The first is simple pattern analysis, which indicated errors as function of angle in the QZ, the second uses the plate as a standard gain device. The third application makes use of the narrow reflected beam of the plate to determine the direction of the incident field. The vertical wire has been used to calibrate the direction of the polarization vector. The setup of an optical reference with a theodolite and a porro prism in relation to the propagation direction of the incident field is presented as well.
The near field technique has grown from experimental systems of the early 1960s to sophisticated accepted means of testing antennas. Several schemes have been employed, namely planar, cylindrical and spherical scanning. The spherical scanning system chosen for one of the near field ranges at GE Aerospace is different from most near field systems in that the test antenna remains stationary while the probe is made to scan over a surface of an imaginary sphere surrounding it. The sampled field is corrected for positional, phase and amplitude errors and transformed to the far field. Radiation patterns, gain, EIRP, group delay and amplitude response were measured for a shaped beam communications antenna.
Effects of probe position errors in planar near-field measurements have been significantly reduced at NIST by accurate alignment of the scanner and an analytic error correction. Currently, the near-field range has probe position errors greater than 0.01cm only at the edges of the 4 x 4 m2 area, and less than that everywhere else. The position errors can be further removed by a theoretical procedure, which requires only the error-contaminated near-field and the probe position errors at the points of measurements. All necessary computations can be efficiently performed using FFTs. An explicit nth-order approximation to the ideal near field of the antenna can be shown to converge to the error-free near fied. Computer simulations with eriodic error functions show that this error-correction technique is highly successful even if the errors are as large as 0.2wavelength, thereby making near-field measurements at frequencies will abobe 60 GHz more practicable.
Antenna engineers recognize that the planar near-field method for calibrating antennas provide accurate pattern and gain measurements. Bothe the pattern and gain measurements require some degree of probe position accuracy in order to achieve accurate results. This degree of accuracy increases for antennas that have structured near-field patterns. These are antennas in which the amplitude and phase change rapidly over a very small position change in the near-field scan plane. The National Institute of Standards and Technology (NIST) has recently measured an antenna with a very structured near-field pattern. This measurement was performed using a new probe positioning system developed at NIST. This measurement will be discussed and results will be presented showing how slight probe position errors alter the antenna pattern and gain.
This paper describes the results of electrical boresight measurement comparisons between one far-field and two near-field ranges. Details are given about the near-field alignment procedures and the near-field error analysis. Details of the far-field measurements and its associated errors are not described here, since the near-field technique is of primary interest. The coordinate systems of the antenna under test and the measurement ranges were carefully defined, and extreme care was taken in the angular alignment of each. The electrical boresight direction of the main beam was determined at a number of frequencies for two antenna ports with orthogonal polarizations. Results demonstrated a maximum uncertainty between the different ranges of 0.018 deg. An analytical error analysis that predicted a similar level of uncertainty was also performed. This error analysis can serve as the basis for estimating uncertainty in other near-field measurements of antenna boresight.
A new measurement technique that is used to measure the polarization properties of dual port, circularly polarized antennas is described. A three antenna technique is used, and high accuracy results are obtained for all three antennas without assuming ideal or identical properties. This technique eliminates the need for a rotating linear antenna, reduces the setup time when gain measurements are also performed, and reduces errors for antennas with low axial ratios.
The weakest link in antenna metrology is the antenna range itself. Unknown reflections can cause large errors in antenna measurements and can change unpredictably. The planewave spectral (PWS) probe technique is one proposed method for identifying the location and magnitude of range scattering. This paper presents the results of a PWS probe of a compact range. The interpretation of the PWS plots is discussed in comparison with the range geometry. Nine separate scattering centers are identified. The meaningfulness of the PWS picture was tested by introducing a known dipole source.
The purpose of this paper is to discuss the accuracy requirement of a generic measurement system for in-flight antenna pattern evaluations. Elements of the measurement technique will be described. An attempt is made to distinguish the measurement requirement for a narrow beam radar antenna in contrast to that for broad beam communication antennas. Major elements of the measurement technique discussed include the flight path geometry, the multipath propagation problem, and the measurement errors. Instrumentation requirements consist of the ground segment, the receive and the tracking subsystems, and the airborne equipment, the radar components and the navigation and attitude sensors. Considering the in-flight antenna pattern testing as a generalized antenna range measurement problem, various sources of measurement errors are identified. An error budget assumption is made on each error component to estimate the overall expected accuracy of the in-flight antenna pattern measurement.
This paper evaluates the RCS errors associated with measuring a large flat plate which is illuminated by a compact range reflector with significant edge diffraction stray signals. This is done by evaluating the true fields incident on the plate and then using a physical optics technique to predict the backscattered fields. Results are compared with and without the edge diffracted fields present. A simple analytic expression is developed which can approximate the size of this potential error.
In the theory of inverse synthetic aperture imaging of radar targets the measuring distance is ordinarily supposed to be very much larger than the dimensions of the target. If this is not the case errors are introduced. We study these errors and means to decrease their influence by computation. The result is that the maximum tolerable target dimension can be substantially increased in a plane perpendicular to the axis of rotation if error correction is used.
A 400 element phased array antenna has been constructed at the GEC-Marconi Research Centre. Each radiating element is fed from its own phase shifter. The radiation patterns of this array have been measured using a recently constructed Cylindrical Near Field Test Facility. The radiation pattern is obtained on a two dimensional grid and contains both amplitude and phase information. It is therefore possible to transform these data back to the array aperture to obtain the array excitation amplitudes and phases. The spatial resolution obtained in the aperture is a function of the angular coverage of the radiation pattern used. The effect of deliberately introduced phase errors on the calculated aperture data is shown.
This paper describes techniques for coherently suppressing multipath and other error sources in planar near-field measurements. Of special interest is a simple, yet effective technique of suppressing axial multipath and mutual coupling between the nearfield probe and an antenna. This is of particular value in the testing of low sidelobe antennas. Traditionally, self comparison tests with different separations between the probe and the antenna under test are used to identify the magnitude of multipath errors. What is not generally realized is that these tests can be used to produce a coherent estimate of the induced error, which can often be suppressed. A series of tests was performed with a small X-band phased array antenna, resulting in a reduction of the sidelobe noise background from a 25 dB level to better than 50 dB.
A study of RF testing methods was conducted for the Radarsat SAR antenna. The implementation tolerances of a planar and a cylindrical near-field facility were computed, by simulation of the effects of different types of measurement errors on the reconstructed far field. The results are presented and the two types of near-field facility are compared.
This paper first presents an overview of the types of errors present in phase length measurements. Next, a minimum-error test method is formulated. The first test results using the test method are included, demonstrating accuracies on the order of plus-or-minus 3 electrical degrees at 18 GHz. Measurement time using currently available ANAs is accomplished in less than 1/2 minute when run in the reflective, time-domain mode.
A unique RCS field probe system is described which determines: 1) the two way phase and amplitude field taper, and 2) the RCS measurement error within the quiet zone. The RCS of a suspended target is measured by the radar at selected locations or while moving in the quiet zone. The field taper is obtained from a time gated target return. The quiet zone RCS error for a target is obtained by comparing RCS measurements from anywhere in the quiet zone with the target RCS measured at the center of the quiet zone. A quiet zone containing a high quality illumination field was measured and found to have more than a 5 dB quiet zone RCS error. The RCS error magnitude is dependent upon the radar variables which are determined by the target size. There is a significant difference between the implied RCS error based on the illumination field quality and RCS measurement error caused by the additional contributions of multipath and target dependent clutter that are peculiar to each facility. Accurate RCS measurements require detailed knowledge of the test facility's multipath, target dependent clutter characteristics, and the target's bistatic signature.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.