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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.
This paper presents the feasability (sic) to explore the Focal Plane (FP) of a Compact Antenna Test Range (CATR). We first introduce the interest of getting very fast the Far Field Pattern of an antenna with a 2D modulated scattering array located at the focus of a CATR. Then, we discuss the geometric, electrical and optical constraints involved when using this technique. A comparison with a classical measurement performed at ESA-ESTEC is shown and we conclude by emphasizing the potentialities of this technique.
The idea of developing a large compact range with scanned quiet zone has been adopted by several international organizations (ESTEC, MBB, Ford Aerospace, etc.) and is expected to be an useful (sic) measurement tool. The Front-Fed Cassegrain configuration is likely to present good scanning capabilities due to its long equivalent focal length and the small curvature of both reflectors. However, some kind of degradation in the test zone is expected in the form of phase aberrations as a function of the lateral feed displacement and frequency, as well as an increase of the Xpolar level. In this paper, the phase aberration and the Xpolar component introduced by a non-centered Front-Fed Cassegrain configuration is analyzed in a GO-GO basis. It is found that the scanning concept can be applied up to a certain frequency limit in which a gradual reduction of the quiet zone dimensions is observed as the feed displacement (plane wave scanning angle) is increased.
As many institutions and companies have constructed anechoic chambers in the past few years, there has been little work done to codify the specification requirements. Often chambers have been constructed from woefully inadequate specifications resulting in chambers that may be too costly, unable to meet the performance criteria, and in some cases, be unsafe. This paper shall present various model specifications and guidelines to properly specify a chamber complex. Compact ranges, tapered chambers, as well as traditional rectangular chambers will all be examined. How to specify absorbing materials and quiet zone sizes, as well as tradeoffs associated with them, will be discussed. Finally, a guide for coping with facility concerns such as civil, structural, RF shielding, HVAC, electrical, and fire protection will be presented. Examples of good specifications and inadequate specifications will be demonstrated and reviewed.
The target support system at Boeing Defense & Space Group's 9-77 Compact Range includes a new string support system. The string support system consists of twelve string reels, six each of the High Capacity String Reels (HCSR). The string reel system is used to suspend and manipulate a target for radar cross section (RCS) measurements, primarily at frequencies below 1.5 GHz. The string reel system is capable of supporting targets up to 10,000 pounds and 40' in length and width. The manipulation and handling of targets, is a major consideration in a RCS measurement test plan. The following paper discusses the newly installed string reel system, enhancements to the 9-77 Range equipment which directly affect the use of the string support system, and future developments planned for the system.
Performance trade-offs are investigated between the use of clustered waveguide bandwidth feeds and the use of one multi-octave bandwidth single aperture feed in a prime focus compact range for dual linear polarization. The results show that feed structure may be used for advantage for the particular test requirements of compact range systems for Radar Cross Section Measurement.
Knowledge of the field character in a range is essential to the understanding of measurements performed. Field probe systems are commonplace for small compact ranges. Outdoor ranges have their systems and methods. A large compact range has unique needs. Available systems are not only fairly expensive, but normally time consuming to install. The McDonnell Douglas Technologies facility implemented a probe system designed to meet the particular needs of the facility.
This paper describes the target handling system that was developed for use in the compact range facility at the University of Pretoria. The system was locally designed and manufactured and comprises of a lift platform, RCS pylon and utility trolley. The pylon utilises a unique design approach resulting in a structure with very high stiffness and surface finish.
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.
The radar cross section (RCS) of a target depends on nature environment as well as many physical variables. The objective of a compact range is to exclude environmental effects on RCS measurements of a target. It is also true for time gated RCS measurements as well. RCS obtained in above manners is more suitable for a space borne than for a ground based target. The contribution from surrounding environment is an inseparable part of RCS for a ship, truck, bridge, and building. We need a suitable method to characterize RCS of a ground based target and its dependence on the environment. The uncontrollable natural change makes environmentally dependent RCS results difficult to compare for a ground based target measured at different time instants. A way to reduce the uncertainties induced from changes is to exhaust all possible RCS measurements before the change. A measurement of this kind is referred to as a concurrent RCS measurement, which in a sense is equivalent to take an optical picture of a rapidly changing object with a strobe light. The step frequency radar located at Chesapeake Bay Detachment of Naval Research Laboratory is such a radar, which is equivalent to at least 45 single frequency radars operating simultaneously from 2.0-18.0 Ghz. Last year, we briefly mentioned this radar in our presentation. We will make a detail discussion of this radar and its capability on concurrent RCS measurements.
Edge and corner diffraction and non-planewave illumination both cause measured free space relativity data to deviate from the infinite sample/planewave result which is predicted when using the Transmission Line Methos (TLM) for planar surfaces. The amount by which each of the two factors perturbs the measured data depends on the measurement system used; compact ranges, near field focused antennas and far field antennas on an NRL arch are all susceptible to the effects of non-planewave illumination and perimeter diffraction. Perimeter diffraction is virtually eliminated in the case of a near field focused system or where the sample is semi-infinite; however, the truncated illumination inevitable yields additional angular planewave components. In a far field system, the quadratic phase variation at the sample surface is shown to cause significant errors in the depth of resonant nulls. A uniform illumination is required to accurately map the depth of resonant nulls, but the consequent perimeter diffraction causes errors in null position. Perimeter diffraction does not cause errors in the null depth providing the illumination in uniform.
An alpha-beta-gamma (a-ß-?) tracking algorithm has been devised to improve the performance of a laser-references planarity control servo. GTRI is developing a field probe for the USAEPG Compact Range at Ft. Huachuca, Arizona. The probe scans a surface whose planarity is controlled by a servo. A reference plane is generated by sweeping a laser beam with a pentaprism. The beam is detected by a photodiode mounted with the probe. The servo nulls any error detected. The servo must correct dynamic errors in the presence of high frequency electronic noise and low frequency atmospheric scintillation. A control algorithm based on the alpha-beta-gamma tracker has been developed and tested by simulation. The algorithm and simulation results are presented.
The target designer using a compact range to verify the predicted RCS of his target needs to know what measurement errors are introduced by the range. The underlying definition of RCS assumes that the target is in the far-field, in free-space, and illuminated by a plane wave. This condition is approximated in a compact range. However, to the extent that these conditions are not met, the RCS measurement is in error. This paper, using the results of the preceding companion paper1, formulates an error budget which shows the typical sources that contribute to the RCS measurement error in a compact range. The error sources are separated into two categories, according to whether they depend on the target or not. Receiver noise is an example of a target independent error source, as are calibration errors, feed reverberation (“ringdown”), target support scattering and chamber clutter which arrives within the target range gate. The target dependent error sources include quiet zone ripple, cross polarization components, and multipath which correspond to reflections of stray non-collimated energy from the target which arrives at the receiver at the same time as the desired target return. These error contributors depend on the manner in which the target interacts with the total quiet zone-field, and the bistatic RCS which the target may present to any off-axis illumination. Results presented in this paper are based on the design of a small compact range which is under construction at RRI. The results include a comprehensive error budget and an assessment of the range performance.
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.
It has been recently shown that an optimized blended rolled-edge compact range reflector can be successfully used to measure two or three foot targets at microwave and millimeter frequencies. In addition to the reflector design, one is faced with many other practical range design issues, such as absorber treatment, target mount and access, feed mount and access, etc. Each of these design aspects has been evaluated and an actual range has been constructed to illustrate the capability of such a system. The feed is mounted on a rotating side door for easy access. The target zone is approached from the rear of the chamber by rotating the backwall. These design concepts allow the range operator to quickly modify the measurement setup, yet still maintain extremely stable results. The simplicity of this design as well as its excellent measurement capability are presented.
Since the first commercial compact range was introduced by Scientific-Atlanta in 1973, the compact range has become a very popular alternative to far-field ranges. In recent years larger and larger compact ranges have been built, increasing the size of antennas that may be tested and lowering the operating frequency. However little has been done in the other direction, to increase the operational frequency and to decrease the size of the compact range. This paper reports on the design and fabrication of a small compact range having a 1 foot test zone and operating at 95 GHz.
A performance simulation for analyzing the measurements of target RCS in a compact radar range has been applied to a small indoor range which will be installed at RRI. A dual reflector collimator has been examined with respect to both quiet-zone quality and the amount of stray energy in the chamber which eventually end up as clutter or multipath interference. The complicated ray geometries, beyond the reach of hand calculation, are discovered by complete tracing of all the rays from the feed source. The ray pats which interfere with target measurements are shown convincingly by graphical display. Vector clutter subtraction is widely used in compact ranges in order to reduce the background clutter to an acceptable level. Some of the effects which limit the effectiveness of clutter subtraction are also addressed in the paper. The sources of measurement errors which are obtained by this simulation are used in the measurement-error budget analysis, which is the subject of the follow-on paper.
Shaped beam illumination of a parabolic, circular aperture compact range reflector using a multi-ring planar array feed is investigated. Since no reflector edge treatment is employed, the entire parabolic surface is available for ray collimation. A technique is presented for the design of array feeds which result in a reflector illumination which is uniform with a specified ripple in the central portion, but zero at the rim in order to eliminate the first order edge diffracted field. Since the excitation coefficient of each element on a ring is constant and real-valued, no complex phase shifts are required, so that a simplified and cost-effective feed network implementation is possible. The quiet zone field is evaluated for arrays comprised of two, three, and four rings of elements. It is demonstrated that the quiet zone field amplitude taper and ripple can be optimized for a specific measurement application by adjusting the amplitude distribution among the rings. Performance is compared with that of a serrated reflector configuration. The array sidelobe level and physical size are examined with regard to overall system integration and implementation.
Multiple mechanisms for the generation of extraneous signals exist in a compact range. These include edge diffraction, scattering from surface imperfections, direct feed radiation, and scattering from absorber or other objects in the range. The field quality in the quiet zone is the resultant of the direct signal and these multiple scattering mechanisms. Since the scattering mechanisms are independent, their effects are often modeled independently and statistically combined to yield an estimate of quiet zone field quality. This paper examines the statistics of multiple independent extraneous signals in a compact range. It is shown that the amplitude ripple produced by an extraneous signal computed as the root sum of the squares (RSS) of the individual extraneous signals does not correctly predict the final quiet zone amplitude ripple. Theoretical results for scattering from multiple thin gaps in the surface of a compact range are presented and statistical computer models are used to demonstrate the computation of the resultant compact range quiet zone.
Large Compact Antenna Test Range (CATR) reflectors are often made up of accurate individually fabricated panels which results in interpanel gaps. The electromagnetic scattering produced by these gaps may cause a degradation of the quiet zone performance. An analytical approach coupled with experimental determination of a key new parameter, the magnetic induced field ratio (MIFR), has been developed to evaluate the effect of the scattering from the interpanel gaps. Depending upon the gap scattering assessment, a decision has to be made whether or not to have the gaps filled. Furthermore, these panels are often painted for aesthetics and to protest against corrosion. Therefore, the effects of paint on the panel has to also be addressed. A novel technique of gap treatment and its evaluation is described.