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Compact range systems have been widely used for antenna measurements. However, the amplitude taper can lead to significant measurement errors especially as the dimension of antenna is larger than quiet zone area. An amplitude taper removing technique by software implement is presented for compact range system. A 12 feet by 1.0 feet S-band rectangular slot array antenna is measured in SA5751 compact range system, which provides a quiet zone area with a 4 feet diameter. Results of corrected far-field patterns from compact range are compared with that taken by planar near-field range.
Phase relationships between the three dominant modes on a four armed spiral can be used to perform broad band, direction of arrival estimates, but this requires accurate estimates of the phase behavior of the antenna both in the design stage and for calibration purposes. Unfortunately, imperfections in range design make the measurement and interpretation of phase information extremely difficult. This paper describes an approach where the imperfections of the range and the behavior of the antenna are modelled, and range effects removed from antenna data through antenna motion, and frequency change. This technique obtained tremendous accuracy at the cost of large amounts of data processing.
A technique has been developed for calibrating a monostatic antenna, used for reflection measurements of a dielectric half space. The model is based on a one dimensional, spherical wave, scattering matrix theory. The scattering matrix coefficients are found by spatial integration of the eigenvectors. The system is deemed calibrated when the eigenvectors are linear. The spatial integration process works well enough that when a calibrated antenna is used as a reflection measurement tool, the surface of the half space can be as rough as the surface of coal and the correct dielectric constant and depth of the coal can be found.
In near field antenna measurements, knowledge of the the [sic] probe antenna’s pattern, polarization and gain are of vital interest. To calibrate a probe for near field measurements is a delicate task, especially if the probe is small, i.e. low gain. The near field probe and the parameters general to a probe calibration are presented. The delicate task of obtaining an accurate gain for small aperture antennas as well as the problem of transfering [sic] the calibration from the facility where the probe is calibrated to the facility where it is to be used are focussed [sic] upon For a small aperture, the pattern is that of the radiating aperture. The unwanted scattering may be removed by filtering in the spherical mode domain thus obtaining the true aperture radiation. The gain derived from this may however be of little use in reality since the aperture always needs some form of mounting. Such a mounting may be covered with absorber which may reflect and diffract and thus affect the gain value.
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.
The need to calibrate large antennas and radio stars is driven by needs in satellite communications systems, deep space communications systems and navigation systems. NIST presently is able to calibrate standard gain antennas up to 10 feet in diameter using their planar near-field facility and has sought means to extend their calibration services to larger antennas. During the last ten years, NIST developed an ETMS (Earth Terminal Measurement System) to measure the gain of large antennas using both radio sources and noise sources calibrated by NIST. This ETMS, however, requires that the flux density of the radio sources be accurately known. This often is not the case. NIST is currently involved in two measurement efforts using calibrated standard gain antennas, calibrated noise sources and the gain comparison method to accurately determine the absolute gains of large antennas and accurately determine flux densities of radio stars and planets. Recent progress on these efforts will be discussed.
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