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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.
This paper describes results of extensive polarimetric Radar Cross Section (RCS) measurements on canonical targets. Amplitude and phase of both co- and cross- polar returns are measured for horizontally and vertically polarized transmit signals in order to determine the complete complex scattering matrix. Measurements have been carried out on a variety of targets. Results presented with this summary show data for a metallic and a dielectric disk. Details of measurement and calibration procedure, hardware, and software are also presented.
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.
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.
Calibration is one of the most important activities to be performed during the assembly, integration and testing of a phased-array antenna. Space-based phased-array antennas, conceived for remote sensing applications and for satellite communications, are going to require both on-ground and on-orbit calibration techniques. The paper reviews on-orbit calibration methods being envisaged for an electronically steerable receiving array at S-band, to be embarked onto the forthcoming PSDE/Artemis satellite.
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.
A method for obtaining the individual element excitations of an array antenna from measured radiation patterns is presented. Applications include element failure diagnosis, phased array antenna calibration, and pattern extrapolation. The measured far-field information is restricted to visible space which does not always contain the entire Fourier domain. A typical example is phased array antennas designed for large scan angles. A similar problem arises during near-field testing of planar antennas in which case the significant far-field domain is restricted by the scanning limitations of the near-field test facility. An iterative procedure is then used which is found to converge to the required solution. The validity of the approach has been checked both using the theoretical radiation patterns and real test cases. Good results have been obtained.
The Dassault Electronique flexible near-field antenna test facility, ARAMIS, has been used for test and calibration of state-of-the-art active phased-array antennas which were designed for military SATCOM operation. The 14-month successful program dramatically emphasized the benefits of a flexible antenna test facility such as ARAMIS. These benefits are the following: • Flexibility o Far-field mode (test of radiating elements and modules) o Planar near-field mode (test of sub-arrays and complete antenna) o High-resolution field mapping mode o Array Element testing • Speed: quick mode switching, “on the fly” multiplexed acquisition • Versatility: calibration of a module, a sub-array and the antenna; radiation patterns; gain; faulty element detection • Productivity: a single indoor facility performing different types of measurements, integrated software Test results gathered during this program and showing the ARAMIS contribution are presented.
A technique is presented for performing vector error corrected measurements of reflection coefficient using an antenna measurement receiver, the Scientific-Atlanta Model 1783, in an automated system. The technique uses an open-short-load calibration implemented in software in the system controller. The technique is simple and accurate. The equations for the measurement are derived and results as compared to the HP 8510B network analyzer are presented.
Accurate calibration methods are of essential importance in RCS measurements. First, absolute RCS determination (in dBsm) can be carried out accurately provided a correct algorithm is used describing the RCS dependence of some reference target at all frequencies. Unfortunately, this technique gives error-free calibrated data at one position only. In this paper a new technique for qualifying of RCS ranges will be described. A reference target with well-known RCS response is used during the calibration measurement. The amplitude and phase distributions are then computed for all required positions within the test zone. Finally, an error estimate in measured RCS responses can be made by using two other application programs.
Large arrays require large separations between the transmit antenna and the antenna under test (AUT) to measure pattern parameters in the far field. For the subject AUT, a range of 6 miles with a spurious signal level of -58 dB was necessary to obtain the required accuracy. Measurements have been performed on a significantly shorter range without serious degradation. The antenna was focused for the angle of electronic scan and the resulting pattern measured. The theoretical far field patterns were compared with the calculated focused patterns for the short range. The maximum sidelobe error of 1/2 dB occurred at 60 degrees scan. There was no noticeable degradation in beamwidth, gain, or foresight at any scan angle. A 6-mile range would have produced a 2-dB sidelobe error. The measured range reflection level was -50 dB. The transmit dish with sidelobes of 22 dB was replaced with an array that had 40 dB sidelobes. This change reduced the reflections to below the required -58 dB. The antenna was focused using a range calibration technique and the measurements substantiated the theory.
An RCS measurement error model, calibration procedure and correction algorithm are discussed. A distinction between frequency response reflections and range-target reflections is made. Special emphasis is placed on the selection of the gate span with time gating used with the calibration and test target measurements. Mathematical simulations and actual measurements illustrate the discussion. It is concluded that frequency response related reflections must and range-target reflections must not be included in the gate for the frequency response calibration measurement.
Actually there are several projects that involve Active Array Antenna Concept for Satellite Earth study. A very large active array for SAR proposes is being studied by ESA that includes an amount of T/R or R modules of 1960. The studies of risks of failure or variations of LNAs and HPAs gain carried out by the designer gives as a result the need of implement some type of control of these parameters, so it is necessary to study and select an Internal Calibration concept for this antenna. This subsystem allows to know and correct any variation of gain in amplitude and phase of everyone of the transmitter/ receiver (T/R) and receiver ( R) modules
In recent years there has been an increasing demand for antenna calibrations at millimeter wave frequencies. Because of this the National Institute of Standards and Technology (NIST) has been developing measurement capabilities at millimeter wave frequencies. The development of gain and polarization measurement capabilities have been previously reported. This paper reports on the development of the capability to measure an antenna pattern which has been achieved during the last year. Measurement accuracies of better than 4 dB have been achieved for sidelobes which are 40 dB below the mainbeam peak. NIST is now providing a new measurement service for antenna patterns in the 30-50 GHz frequency range.
Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.
An automated instrumentation system has been configured for the purpose of evaluating advanced composites, radar absorbing materials, and frequency selective surfaces (FSS) in free space. Electrical test frequencies are divided into three bands that range from 18 to 60 GHz for any linear polarization. Software has been incorporated to calculate dielectric properties from the measured transmission and reflection characteristics. Using the HP9836 computer, software was written to automate and integrate the Anorad 3253 positioner with the HP8510 network analyzer. This system allows for the input of up to five incident angles at vertical, horizontal, and cross polarization. The measured transmission loss (amplitude and phase) at multiple incident angles is then plotted for comparison. This paper gives a complete description of the system configuration, calibration techniques, and samples of output data. Material properties are computed and compared to specified and theoretical values. Measured results of an FSS structure are compared to its predicted response.
The National Bureau of Standards (NBS) has been measuring antennas and dealing with the problems of leakage for the past twenty years. This paper will discuss the various methods of detecting leakage, typical sources of leakage, how to correct leakage problems, and the effects that leakage can have on calibration.
It is common practice in antenna measurements to use a moderately directive source (or 'probe') antenna, to minimise the effect of reflections. The illumination of the test antenna then exhibits a degree of non-uniformity. A correction scheme has previously been proposed for spherical near-field measurements. This paper describes a new probe-correction algorithm that can be used in conjunction with spherical near-field or 'conventional' measurements. It is operable with a minimum amount of measured data (for either the test antenna or the probe). It may also be used for probe-correcting calibration measurements using a gain horn.
Results of an experimental study of the interactions between a scattering target and the absorber-coated walls and ceiling of the OSU Compact Range Anechoic Room are reported. A 6 ft. square flat metal reflector was mounted in the quiet zone and oriented at selected angles non-orthogonal to the range symmetry axis. In theory, this target (when non-orthogonal) has a relatively low backscattering signature, and a strong planar bistatic scattering beam which can be pointed at several regions and absorber types in the room. By processing, the bistatic iteration terms can be separated form the plate backscatter, and frequency domain spectra and/or transient response signatures of the different mechanisms produced. Th paper will present calibration information on the actual performance of the bistatic scattering beam of the plate, and measurements of both backscattering and bistatic scattering of the absorber-coated walls in the ESL chamber. Suggested guidelines for use of this as a standard anechoic room diagnostic test will be discussed.
Three test methods have been developed and validated for characterizing materials at VHF and UHF in an indoor environment. The first method employs a resonant strip-line cavity for the independent determination of permittivity and permeability from .15-2 GHz. The planar field geometry and sample configuration permit evaluation of material anistropy. Measurements are taken on an Automatic Network Analyzer (HP 8510 ANA). The second method measures the reflection/transmission (R/T) of planar material samples at UHF. This is a free space measurement performed in an anechoic chamber. Data is taken from .2-2 GHz using two dual ridged horn antennas and the ANA. A calibration method has been developed for the ANA to correct for measurement errors. Off-set shorts and thru delays are used in this technique. The third technique evaluates reflection performance of materials from 150-250 MHz. This technique employs a custom designed corner reflector antenna. Only one such antenna is needed due to the calibration technique. These methods allow a synergistic approach to material development. Candidate material can be evaluated using the cavity or R/T systems. Material designs can then be tested on either the UHF and/or VHF systems.