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The use of electronically scanned phased array antennas in demanding rolls such as satellite communications and radar systems has led to an increasing desire to analyze the sources of error present in the boresight alignment of such systems. Not the least among these errors are those introduced by thermal effects on the various components which comprise the array structure. In an effort to understand this mechanism, this paper will discuss a technique which uses an infrared camera system to analyze the beam deflection errors caused by the effects of temperature gradients present in the antenna system.
This paper will discuss one method of characterizing the scan plane for planar near-field measurements. The method uses a theodolite auto-collimator, a laser interferometer, an electronic level and an optical square. The data obtained using these techniques are first used to make alignment corrections to the scan plane; then new data are used to determine the best fit for the realigned scan plane. The normal to this place is referenced using a permanently placed mirror. In addition, the final data obtained can be used in probe position-correction techniques, developed for planar near-field measurements.
Unique instrumentation is required for dynamic (in-flight) measurements of aircraft radar cross section (RCS), jammer-to-signal (J/S), or chaff signature. The resulting scintillation of the radar echo of a dynamic target requires special data collection and processing techniques to ensure the integrity of RCS measurements. Sufficient data in each resolution aspect cell is required for an accurate representation of the target's signature. Dynamic RCS instrumentation location, flight profiles, data sampling rates, and number of simultaneous measurements at different frequencies are important factors in determining flight time. The Chesapeake Test Range (CTR), NAVAIRWARCENACDIV, Patuxent River, Maryland, is a leader in quality dynamic in-flight RCS, J/S ratio, and chaff measurements of air vehicles. The facility is comprised of several integrated range facilities including range control, radar tracking, telemetry, data acquisition, and real-time data processing and display.
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.
The National Institute of Standards and Technology (NIST) has written a certification plan to ensure that a proposed planar near-field facility is capable of measuring high-performance phased arrays. Generally for a complete plan, one must evaluate many aspects including scanner alignment, near-field probe alignment, alignment of the antenna under test, RF crosstalk, probe position errors RF path variations, the receiver's dynamic range and linearity, leakage, probe-antenna multiple reflections, truncation effects, aliasing, system drift, room multipath, insertion loss measurements, noise, and software verification. In this paper, we discuss some of the important aspects of the certification plan specifically written for the measurement of high-performance phased-array antennas. Further, we show how the requirements of each aspect depend on the measurement accuracies needed to verify the performance array under test.
A Precision Optical Measurement System (POMS) has been designed, constructed and tested for tracking the position (x,y,z) and orientation (roll, pitch, yaw) of models in Boeing's 9-77 Compact Radar Range. A stereo triangulation technique is implemented using two remote sensor units separated by a known baseline. Each unit measures pointing angles (azimuth and elevation) to optical targets on a model. Four different reference systems are used for calibration and alignment of the system's components and two platforms. Pointing angle data and calibration corrections are processed at high rates to give near real-time feedback to the mechanical positioning system of the model. The positional accuracy of the system is (plus minus) .010 inches at a distance of 85 feet while using low RCS reflective tape targets. The precision measurement capabilities and applications of the system are discussed.
Microwave holography has proven to be a powerful technique for various evaluations, diagnostics, and RF performance improvements for large reflector antennas. The technique utilizes the Fourier Transform relation between the complex far-field radiation pattern of an antenna and the complex aperture field distribution. Resulting aperture phase and amplitude distribution data can be used to precisely characterize various crucial performance parameters, including panel alignment, subreflector position, antenna aperture illumination, directivity at various frequencies, and gravity deformation effects. The methodology of the data processing presented in this paper was developed at JPL and has been successfully applied to the NASA/JPL Deep Space Network (DSN) 34m beam waveguide antennas. The performance improvement of the antenna was verified by efficiency measurements and additional holographic measurements. The antenna performance was improved at all operating frequencies of the antenna (wide bandwidth improvement) by reducing the main reflector “mechanical surface” rms error to 0.43 mm. At Ka-band (32-GHz) the estimated improvement is 4.1 dB, resulting in aperture efficiency of 52%.
A panelized 56 by 50 foot compact range reflector with a wrap-around rolled edge treatment was installed in an anechoic chamber. Good quiet zone performance required that the as-built surface precisely follow the theoretical cosine blended contour. Commercially available CAD/CAM software served as the design platform for development of the overall system layout, rolled edge panel designs and the CNC milling machine source code for contour machining the rolled edge panels. Formed aluminum and machined composite panel fabrication techniques are described, and resulting aggregate surface accuracies as good as 1.0mil rms are presented. The use of multiple triangulating theodolites, photogrammetric measurements with peak accuracies of 0.5 mils, and custom bestfitting software used in surface alignment are described.
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.
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.
A new implementation of the planar near-field back projection technique for phased array testing and aperture imaging is described. In the alignment of phased arrays, the aperture field is treated as a continuous distribution rather than using idealized array concepts. The continuous field is then sampled to obtain element excitations. In this way, nonrectangular arrays can easily be accommodated. The method also produces highly interpolated images of apertures that can offer much insight into their nature. Also, any polarization of the aperture field may be obtained if the probe pattern has been characterized. The technique uses large FFTs which are computed very quickly by a workstation located in the facility. Results from an iterative phase alignment of a 12x18 phased array are presented, as well as highly interpolated images of apertures and results which demonstrate the polarization selection.
A technique for aligning a phased array is described. Array element attenuation and phase commands are derived from far-field patterns measured without calibrations. The technique is based on iteratively forming mulls in the antenna pattern in the directions specified by a uniform array illumination. It may be applied in situations where array elements are not individually accessible, or where an array contains no build-in calibration capacity. The alignment technique was evaluated on a far-field range with a linear, 32-element array operating at L-band. The array containing transmit/receive modules with 12-bit amplitude and phase control. Insertion attenuation and phase measurements were comparable to those obtained by conventional techniques. However, the alignment procedure tends to compensate for the effects of nonuniform element patterns and range multipath. Thus, when used to implement other excitation functions, the array sidelobe performance with adaptive calibrations was substantially better.
The quiet zone performance of the Harris 1606 compact Range Collimator has been reported in the literature for 2 through 35 GHz 1,2. This paper discussed our achievements in the past year with the 1606 at 95 GHz. We will summarize the improvements in our fabrication and alignment methods that have yielded excellent performance at these frequencies using an intermediate size multi-panel main reflector. Quiet zone performance data will be presented from recent measurements on the Millitech Corporation’s Millimeter Wave Antenna Test Range in South Deerfield, MA and from the Harris 1606 Capital test equipment range.
Accurate near-field cross-polarization measurements on circularly polarized (CP) antennas at millimeter-wave frequencies require well-characterized probes with low axial ratios. We have recently obtained and calibrated dual-port CP horns for use as near-field probes at frequencies of 40-50 GHz. These horns have axial ratios which are 0.3 dB or less over a 10% frequency bandwidth. With these good axial ratios the difference between vector and scalar probe correction is usually small. Additional advantages of the dual-port probes are the need for only a single alignment, more accurate knowledge of the relative phase between two ports of the same probe, and the ability to obtain both main and cross polarized data during one scan. The axial ratios of the dual port CP probes are also better than those of single-port CP Probes. In this paper we present some gain, axial ratio, and pattern measurements for these probes and show that they give accurate cross-polarization measurements.
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.
A new spherical near field facility has been recently implemented at the Electromagnetic Propagation Area of INTA. The facility makes use of an existing big anechoic chamber (12 x 12 x 12 m.) and the near field/fair field transformation software developed by TICRA. This range has been calibrated by measuring an offset reflector antenna and comparing the results with those obtained in previous measurements of this antenna in other European testing facilities of different types. An experimental study has been carried out to check the dependence of the transformation software on the scanning parameters and different misalignments have been produced in order to determine the impact of the mechanical deformations on the accuracy of the system.
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 special purpose 80 element linear phased array antenna was aligned using an iterative phase cycling method. First, the array was aligned to yield maximum main-beam power in the reactive near-field zone and then in the far-field zone. A record of the phase-shifters settings achieved for each zone was kept for use as look-up table during operation. In situ electronic main-beam steering was performed to compare sidelobe performance for the two cases. This report describes the measured results obtained using the phased cycling alignment procedure and compares the measured one-way radiation pattern for the two distance conditions.
At Southwest Research Institute, an automated antenna performance evaluation system has been developed for evaluation of mast-mounted direction finder antennas. This system utilizes a dual-channel receiving system and IF processor with off-line antenna pattern analysis software. Antennas are mounted on a test range which includes computer-controlled antenna positioners, test frequency transmitters, and a data acquisition equipment group. Amplitude and phase data is digitized and recorded for automated off-line antenna performance evaluation. The evaluation software provides a Fourier analysis of the antenna patterns which characterize distortion, alignment, relative phase relationships, amplitude mismatch, and bearing deviations (from theoretical values) for each antenna array.