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The plane wave quality of a compact range (CR) is usually specified in terms of the crosspolar level and the magnitude and phase ripple in the test zone. The way these deviations from the ideal plane wave affect the measurement of different antenna types can be treated by the application of the reciprocity principle between the transmitting and receiving antenna in a measurement set-up. By the application of the sampling theorem, it is found that the measured antenna pattern can be expressed as a summation of the plane wave spectrum components of the field at the test zone weighted by the true radiation pattern of the antenna under test (AUT) evaluated at the CR plane wave directions in the rotated coordinate system of the AUT. The inverse procedure can be used to extract the CR plane wave information (and therefore the CR field at the test zone by means of the Fourier series) from the measurement of a standard antenna with a known radiation pattern.
Time-frequency distributions (TFD) describe a signal in terms of its joint time and frequency content. In this paper, it will be shown that TFDs are particularly useful for the analysis of frequency-dispersive electromagnetic scattering. A TFD based on the wavelet transform (WT) of the scattering data is presented. As an example, measured scattering from a waveguide cavity is considered. It is shown that the wavelet TFD can provide good time resolution for specular/point scattering features, and good frequency resolution for resonant features. Application to the scattering data from the KC-135 aircraft in flight shows that the WT is capable of detecting the resonant modes of the engine outlets of the aircraft.
To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.
There has been a great deal of interest in microstrip antennas and arrays in the past decade or so due to their low cost, light weight, and conformability. Most research to date on microstrip antennas has been focused on developing techniques for characterizing their radiation properties. However, interest in evaluating the scattering properties of such antennas is increasing. The RCS of three configurations of circular patch antennas have been measured versus frequency and are compared to Moment Method predictions; a single open-circuited element, a single element terminated in a 50 ohm load, and a 3 x 3 array of open-circuited elements. In most cases, the measurements and predictions are in good agreement.
The design of many modern RCS instrumentation systems is driven by the time required to complete a measurement which establishes the throughput rate of the RCS facility and therefore impacts the operating cost and efficiency. Time considerations are of particular importance when wideband systems are used to measure large targets with low RCS because multiple observations are required to span the frequency band or to increase sensitivity by coherent integration. Although significant improvements have been made to minimize inefficiencies in instrumentation systems, the fundamental limit of measurement time is governed by physical considerations of power, energy, noise, target dimension, and RCS. Evaluating the performance of a particular radar design can be facilitated by comparing the predicted measurement time with a theoretical optimum. The purpose of this paper is to develop estimates of the minimum measurement time under optimum conditions. Although likely precluded by practical considerations, the theoretical limits provide estimates of the maximum degree of radar performance and measures of optimality in practical systems.
ORBIT's String Reel Target Manipulation System is used to support and rotate a target during RCS measurements. One of the challenges in this kind of RCS measurement is to accurately determine the position of the target in space, since the weight and moment of inertia of the target and the string flexibility do not allow measuring its position with conventional methods (linear encoder, etc.). In order to overcome this problem, the Non-Contact Optical Measurement System (NCOMS) has been developed and tested at ORBIT. The system provides the capability for precision tracking of the target position (X, Y, Z) and orientation (ROLL, PITCH, YAW). NCOMS is a computer-controlled system and operates by using two standard CCD cameras (stereo technique), as well as by use of a single camera with insignificant accuracy degradation. Another advantage of NCOMS is that the system operation does not require accurate camera positioning. The only requirements for CCD camera installation are target visibility and use convenience.
The presence of the sea surface has a powerful influence on the scattering characteristics of marine targets during radar cross section (RCS) measurements. To obtain accurate RCS measurements of a large, distributed marine target, the radar site must satisfy various requirements. The major requirement is to provide quality RCS data without strong multipath distortion of the target return signal. In this paper multipath effects on a large scatterer measured at both low-and high-elevation radar sites are summarized. It is observed that multipath effects contribute strongly to the RCS of the target measured at a low elevation radar site. The data show large RCS fluctuations of more than 15 dB when a scatterer is measured at difference altitudes or ranges. The quality of the data measured at a low-elevation radar site then becomes questionable, which creates difficulties in assessing the true RCS of the target. For diagnostic purposes, it may be necessary to change the target range or altitude several times to make a credible assessment of RCS. The same target measured at a high-elevation site has less multipath influence on the RCS data, making assessment of the true RCS feasible.
New results from numerical simulations and preliminary experiments pertaining to the dynamics of conical rotation of a string-suspended target are reported. For the simplest case of a model target suspended by three strings at three points, we find that its equilibrium position and the pitch, roll and yaw are uniquely dependent on its center of gravity (CG). By assuming that the yaw angle is mainly provided by the rotation of the upper turntable (UTT), we study the change in pitch and roll by controlling the strings while using a precision optical measurement system (POMS) to monitor the actual position and angular orientation of the target. In addition, two natural modes of small oscillations for the transverse and the longitudinal motions of the model target are observed to be of periods 1.45 and 2.46 seconds, respectively. The electrical-mechanical response time of the string-reel systems is found to be about 1.8 + 0.2 seconds. The results will be useful for the conics algorithm development.
A recently completed Hughes program successfully demonstrated an airborne multi-spectral (VHF through X-Band) Synthetic Aperture Radar (SAR) measurement of the radar cross section (RCS) of an aircraft in flight, producing two-dimensional (2-D) diagnostic RCS images of the test aircraft. Ground-to-air imaging of full-scale aircraft was demonstrated by Hughes in 1990. In early 1992, a Hughes A-3 aircraft made air-to-air radar images of a test aircraft in flight. To date, Hughes has collected imagery on nine aircraft from VHF through X-Band, including nose, side and tail aspects at several elevation angles. Reference (2) describes the VHF/UHF capability of the imaging system and this paper will describe the image processing steps developed and will display S- and X-Band radar images with resolution as fine as 6 x 4 inches. The images presented in this paper are dominated by a few very large cavity-type scatterers and do not show the ultimate sensitivity and fidelity of the system. The air-to-air images do demonstrate the spectacular diagnostic utility of this technology.
A recently completed Hughes program successfully demonstrated an airborne multi-spectral (VHF through X-Band) Synthetic Aperture Radar (SAR) measurement of the radar cross section (RCS) of an aircraft in flight, producing two-dimensional (2-D) diagnostic RCS images of the test aircraft. The Air-to-Air Radar Imaging Program was a multi-phase program to develop, demonstrate and exploit this new technology for the design and evaluation of advanced technology aircraft. Radar images with resolution as fine as 6 x 4 inches were produced. To date, Hughes has collected imagery on nine aircraft from VHE through X-Band, including nose, side and tail aspects at several elevation angles. The ability to generate a radar image while in flight is a significant technical achievement. The VHF images presented demonstrate the utility of the system but the images do not show the ultimate sensitivity and fidelity of the system because the aircraft presented in this paper are dominated by a few very large cavity-type scatterers. The ability to measure the VHF/UHS RCS of an aircraft in flight and to make high resolution images is one of the major accomplishments of this program. VHF/UHF in-flight images, never achieved before this program, are a powerful diagnostic tool for use in aircraft development.
In ISAR applications data is acquired on a circular grid. In further processing, data on a rectangular grid is obtained by interpolation. This causes the loss of data outside the interpolated area. The latter can be corrected by extrapolation, but this can give incorrect information. A new technique s proposed which uses a larger rectangular area than in the above mentioned case. Some parts of this rectangle are calculated by extrapolation. Because most of the data in the larger rectangular area consists of original data, only minor parts are extrapolated. Consequently, this method is expected to be more reliable than traditional extrapolation techniques. Simulations have shown that the data obtained by the new interpolation - extrapolation scheme provide a considerable improvement to the amplitude - and phase accuracy across the enlarged rectangular grid.
This paper contrasts indoor and outdoor implementation of efforts during upgrades of VHR RCS measurement capabilities. Sites studied are two McDonnell Douglas Technologies Incorporated, Range Measurements Services facilities. Indoor. Radar Measurement Center (San Diego, CA) is a large compact range. Equipment-Harris Corporation Model 1630 Collimator System, Scientific Atlanta Model 2090 radar. Outdoor. Microwave test facility (Victorville, CA), large ground plane facility. Equipment-Steerable dipole feed dish, System Planning Corp, Mark III radar.
The feed support struts often cause noticeable strut lobes in the patterns of reflector antennas. For example, strut lobes are apparent in the measured and calculated patterns presented in Ref. [1] for the 8-foot diameter reflector with a prime focus feed. As pointed out in [1], the calculated strut lobes are higher than the measured ones. The reason for the difference is secondary scattering by the oppositely located strut, which was not modeled in the calculated pattern in [1]. Detailed examination showed a difference of about 2 1/2 dB caused by the secondary scattering for this reflector antenna design. The purpose of this paper is to present measured and calculated patterns which explicitly demonstrate the quantitative effect of the secondary strut scattering. This effort is shown by comparing the measured strut lobe levels with the oppositely located strut removed, i.e., by using 3 struts instead of 4 struts. Calculated patterns are also given in which the secondary scattering is modeled.
Aeronautical SATCOM systems for INMARSAT typically employ circular polarized electronically or mechanically steered multi beam antennas. Characterization of thee antennas requires extensive measurements that differ from conventional antenna pattern measurements. Some of these are: A. Multiple frequently CP gain, axial ratio, and discrimination measurements over a hemisphere for a large number of beams. B. Noise temperature and G/T measurements C. Carrier to multipath rejection D. Intermodulation characteristics E. Receiver and Transmitter system characteristics Details of instrumentation and procedure for these tests are presented with special emphasis on issues such as measurement speed, accuracy and processing of large amounts of data.
This paper discusses a new technology for viewing and measuring the power distribution of a propagating electromagnetic wave. A wave is passed through an absorbing material that absorbs only a small fraction of the energy; the wave is unchanged otherwise. The absorbed energy heats the material so that an engineer can view the power distribution with an infrared camera and get real-time feedback about design changes. Because the engineer is viewing the power distribution of the propagating wave in real-time, normal antenna design schedules are reduce. In agitation, the equipment used in the measurement technique is portable and can be easily calibrated in the field.
A description of antenna measurements performed on an ocean-buoy mounted antenna array is given. The array is designed to measure the E and H fields of a received wavefront at four different heights over the ocean. Four collocated electrically-small loop-dipole antenna pairs at 2 meter height spacings were integrated into a non-conducting buoy support structure. The frequency band was 50-250 MHz. Data was taken with both the Substitution (2-antenna) and 3 Antenna Measurement Methods for comparison purposes. The ground plane range that was used is described as well as the various range setups used to accumulate all of the required data.
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.
The LX/LH organization of McClellan AFB has been using near-field (NF) technology for the past two years to test an Air Force phased array receiving antenna. McClellan uses both a close range surface RF scanner and a larger offset, fain and back-transform near-field scanner. NF testing is done for both trouble-shooting purposes to support repair efforts, and for acceptance-testing to certify the antenna. The purpose of this paper is to show how McClellan interprets its planar near-field data for diagnosing antenna faults. First the various near-field techniques used by the LX/LH organization will be discussed. Following, will be an examination of the antenna defects pointed out by the NF test date. Failures will be traced to the component level where possible. Techniques other than near-field, such as electronic test, will be used to verify these problems. Additionally, the repair methods will be discussed.
Phased array antenna sidelobe levels are evaluated based on the statistics of the differences in element patterns. It is shown that the differences can be treated as random errors. The standard formula for predicting the average sidelobe level of an array due to random errors is valid if the interaction between the element patterns and the excitation function is taken into account. Sidelobes of a linear array with a variety of near-field perturbations are considered. The statistics indicate that for an N-element array, adaptive calibrations may lower the average sidelobe level by a factor of N.
This paper deals with the development of an approach to the design of triad steering antenna arrays which are used in anechoic chambers for hardware-in-the-loop testing of monopulse antenna seeker systems. In the design of a large array, such as those used for hardware-in-the-loop of guided weapons, it is important to optimize the array element spacing. An excessively narrow spacing results in an unreasonable number of required antennas and increased cost, while an excessively wide spacing will induce angle measurement errors in the seeker under test which can be significant. The specific objective of this effort is to quantitatively describe the monopulse discriminant efforts which result when a non-planar field, radiated by an antenna triad, illuminates a monopulse seeker under test. The approach to this problem is to calculate the triad field at the aperture of the monopulse seeker assuming various levels of triad element phase and amplitude error. Using this illumination field and the illumination function of the monopulse antenna, the resulting sum and difference patterns are calculated along with the monopulse discriminant. Software has been developed to perform these calculations. The resulting patterns are compared with the ideal far field pattern and the discriminant bias, or angle measurement error, is quantified.