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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.
Antenna pattern measurements are dominantly influenced by the presence of extraneous fields in the test zone. A fast and simple way to recognize problems in pattern measurements provides the Antenna Pattern Comparison-technique (APC). This method usually consists of recording azimuthal patterns on different positions across the test zone. Differences in the amplitude data give a rough indication for the magnitude of the interfering signal. The "Novel APC-method" (NAPC) employs both amplitude- and phase-data so that it becomes possible to separate the direct and the extraneous signals from each other. It will be shown that this method is eminently suited to correct radiation patterns of high-gain and low-sidelobe antennas. For verification purposes corrected patterns are compared with time-dated ones and the resemblance is excellent. It is concluded that the NAPC-method is promising and powerful technique for accurate antenna pattern determination, mainly because it can be easily implemented for most applications.
The Hughes Aircraft Company Compact Range facility for antenna and RCS measurements, scheduled for completion in 1993, is described. The facility features two compact ranges. Chamber 1 was designed for a 4 to 6 foot quiet zone, and Chamber 2 was designed for a 10 to 14 foot quiet zone. Each chamber is TEMPEST shielded with 1/4 inch welded steel panels to meet NSA standard 65-6 for RF isolation greater than 100 dB up to 100 GHz, with personnel access through double inter locked Huntley RFI/EMI sliding pneumatic doors certified to maintain 100 dB isolation. While Chamber 1 is designed to operate in the frequency range from 2 to 100 GHz, Chamber 2 is designed for the 1 to 100 GHz region. Both RCS measurements and antenna field patterns/gain measurements can be made in each chamber. The reflectors used are the March Microwave Dual Parabolic Cylindrical Reflector System with the sub-reflector mounted on the ceiling to permit horizontal target cuts to be measured in the symmetrical plane of the reflector system.
RCS measurement accuracy is degraded by reflections occurring between the feed antenna, the range, and the radar subsystem. These reflections produce errors which appear in the image domain (both 1-D and 2-D). The errors are proportional to the RCS magnitude of the target under test and they are present in each of the typical range calibration measurements. Current 2-term error models do not predict or account for the above errors. An improved 8-term error model is developed to do so. The model is based on measurable reflections and losses within the range, the feed antenna, and the radar. By combining the improved error model with the commonly used 2-term RCS range calibration equation, we are able to quantify the residual RCS errors. The improved error model is validated with measured results on a direct illumination range and is used to develop specific techniques which can improve RCS measurement accuracy.
The three cable method for removing the amplitude and phase variations of microwave cables due to temperature change and movement can offer a substantial improvement in antenna measurement accuracy. Implementation details of the method are provided for a planar near-field range. Items specifically addressed are range configuration, hardware requirements, data collection methodology, identification and assessment of error sources, and data reduction requirements.
One antenna characteristic that is difficult to predict accurately is the antenna temperature. There are two basic reasons this is true. First, the effect of the full volumetric radiation pattern of the antenna must be taken into account. Secondly, the antenna temperature calculation requires knowledge of the noise power incident on the antenna, from the environment in which it is operating. This paper describes a measurement program which was undertaken to establish the accuracy of a model which is being used to predict antenna temperature for earth based reflector antennas. The measurements were conducted at 11 GHz, using an 8-foot diameter Cassegrain reflector antenna in an outdoor environment. The measurements are compared to predictions generated by The Ohio State University Reflector Antenna Code. Use of the reflector code allows the full volumetric pattern of the antenna, including all sidelobes, backlobes and cross-polarized response, to be included in the calculation. Additionally, the contribution to the antenna temperature from the various regions of the pattern can be calculated separately and analyzed.
The applications of conventional reflector type compact antenna test ranges (CATR), becomes increasingly difficult above 100 GHz. The main problems are the tight surface accuracy requirements for the reflector, and therefore the high manufacturing costs. These problems can be overcome by the use of a new hologram type of compact range, in which a planar hologram structure is used as a collimating element. This new idea is described, and its performance is studied with theoretical analyses and measurements at 110 GHz.
This paper addresses measurement requirements for space communication antennas and identifies antenna parameters most influenced by indoor compact range quiet zone quality. These parameters include sidelobe level, beam pointing, and gain. The compact range mechanisms limiting measurement accuracy are identified and discussed. Proven methods for characterizing quiet zone performance are described and demonstrated through illustration and example. Analysis is presented which related quiet zone quality characteristics to antenna measurement accuracy. The paper summarizes typical measurement results and error levels achievable for modern compact range systems. Methods for improving compact range performance for satellite antenna testing are also presented.
METRATEK has completed a highly successful program to prove the feasibility of high-resolution, air-to-air diagnostic radar cross section imaging of large aircraft in flight. Experience with the system has proven that large aircraft can indeed be imaged in flight with the same quality and calibration accuracy that can be achieved with indoor and outdoor ranges. This paper addresses the results of those measurements and the Model 100 AIRSAR radar and processing system that were used on this program.
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.
Planar Near-Field scanning is becoming the method of choice for testing many types of antennas. These antennas include planar phased arrays, space deployable satellite antennas and other antennas either too large to move during the test or otherwise sensitive to the gravity vector. The planar scanner is a major component of the measurement system and must provide an accurate and stable platform for moving the RF probe across the test antenna's aperture. This paper describes basic design requirements for a planar near-field scanner. Based on recent development activity at Scientific-Atlanta several design considerations are presented. Scanner parameters discussed include basic scanner concepts and geometry, scanner accuracy and stability, RF system including cabling and accuracy, load carrying requirements of the RF probe carriage, position and readout systems and drive and control systems. A scanner will be presented which incorporates many of the design features discussed.
Rapid data acquisition is crucial in making comprehensive near-field scanning tests of electronically-steered phased array antennas. Multiplexed data sets can now be acquired very rapidly with high speed automatic data acquisition. To obtain high speed without giving up accuracy in probe position a feature termed subinterval triggering has been devised. To obtain simultaneously reliable thermal drift or tie scan data a feature termed block tie scans has been devised. This paper describes these two features that yield speed and accuracy in planar near-field scanning measurements.
Some of the novel mechanical and electronic subsystems features on a recently installed high specification planar near-field scanner are described together with a discussion of the problems encountered during the commissioning period. The test facility incorporates a number of novel design concepts both in terms of its instrumentation, control and processing subsystems. Features of the facility are the speed of data acquisition and the accuracy of the acquired near-field data. Scan speeds of up to 0.8 m/s and positional accuracies of 30 microns in the Z-axis have been achieved, and the near-field data is acquired, displayed and measured on the fly, hence allowing a typical 3m x 3m scan to be executed and the measured near-field results to be displayed and processed within a period of thirty minutes.
As millimeter wave antenna systems become increasingly popular, engineers are challenged to develop effective methods for testing them. A practical method of designing a millimeter wave antenna measurement instrumentation system is presented in which frequency range, accuracy, dynamic range, and speed are considered.
The frequency accuracy of the HP 8530A receiver and HP 8360 Series synthesizers in ramp sweep is measured using a delay line discriminator. The effect of the frequency error on measurement accuracy is derived for radar cross section (RCS) measurements of one and two point constant-amplitude, scatterers and for background subtraction. The results of swept and synthesized frequency measurements are compared, showing that the errors due to ramp sweep are negligibly small for practical RCS measurements.
As part of an effort to provide improved measurement services at frequencies above 30 GHz, scientists at the National Institute of Standards and Technology (NIST) have completed development of a swept frequency gain measurement service for the 33-50 GHz band. This service gives gain values with an accuracy of ± 0.3 dB. In this paper we discuss an example measurement and the associated errors.
Some proposed attitude control systems will require sub-millimeter position accuracy and GPS signals. Toward this end, two antenna parameters must be determined and optimized. These two parameters are phase center location and phase center stability. The phase center location is defined as the point whose ideal spherical phase front has the minimum RMS difference between itself and the measured phase data. Phase center stability is the effective movement of a GPS antenna’s phase center caused by the deviation of the radiated phase front from an ideal spherical phase front. The RMS difference between the ideal phase and the measured phase is a good measure of phase center stability. This paper describes a method of determining the phase center location of a wide-beam GPS antenna. Once the phase center is determined, the phase center stability throughout the coverage region (usually a hemisphere) is characterized. Finally, some sources of error are identified. Methods of minimizing the effects of these error sources are addressed.
This paper will show that it is possible to make bistatic measurements in a compact range environments using near field scanning. A test scanner is designed and operated. Criteria for the accuracy of positioning and repositioning are presented. Algorithms for the transformation of the raw data into bistatic far field calibrated RCS are presented. Examples will be presented where comparisons with theoretical bistatic sphere data are shown. Bistatc pedestal interaction terms will be demonstrated.
This paper compare some of the features and capabilities of gated CW and pulse radars for RCS and imaging measurements. At the conceptual level, these two types of radars are very similar. The primary conceptual difference is that a pulse radar has a relatively high bandwidth receiver while a gated CW system has a relatively narrow bandwidth receiver. The measures of performance of an RCS and imaging system include sensitivity, measurement time, clutter rejection, dynamic range and accuracy. Other considerations such as inter-pulse modulation may be important in some cases. For some applications, typically where long ranges are involved, a pulse system has significant performance advantages. For many applications, the performance advantage of a pulse system is not significant, particularly when viewed in light of the large difference in cost. This is particularly true of Quality Assurance applications which are normally characterized by both short range and lower budgets. Typically, the price of a gated CW system is in the range of ¼ to ½ the price of a comparable pulse system. This paper discusses general similarities and differences in the fundamental operating characteristics of the two systems. Specific performance measures are discussed including system sensitivity, gate performance, clutter rejection, and measurement times. Other considerations such as pulse modulation are discussed. A summary of the various considerations is presented in order to give the reader an understanding of the applications for which a gated CW system is more appropriate.
Computer codes for the computation of scattering are based on physical, mathematical, and numerical assumptions and approximations that impact the accuracy of the results in ways that are not obvious or quantifiable analytically. This paper stresses the usefulness of a concurrent measurement program to provide reliable RCS data for targets of special interest in establishing the range of validity of the various assumptions upon which a specific computer code is based. This in turn assists in developing “modelling guidelines” restricting the design of computer models for input to the code such that reasonable accurate results are likely to be obtained.