Position correction on large near-field scanners using an optical tracking system
Large scanners used for near-field antenna measurements require careful attention to the design and fabrication process to maintain probe position accuracy. This paper discusses the design, implementation, and results of a novel optical probe position tracking system used by NSI on a number of large near-field scanners. This system provides measurement of the probe X, Y and Z position errors, and real-time on-the-fly position correction. The use of this correction can significantly enhance measurement accuracy, and can reduce the cost of building large near-field scanners.
High performance medium gain antenna applications in the compensated compact range
The Compensated Compact Range (CCR) has been proven to be a high performance test facility for payload and large satellite antenna measurements. To efficiently measure complete antenna farms with various types of antennas on the spacecraft in the same test campaign led to the growing demand for testing e.g. Global Horn antennas on the spacecraft in the CCR.
As a matter of fact, medium gain antennas feature a small aperture simultaneously requiring larger test angles. Therefor, main interferer like "direct leakage" between the CCR feed and the antenna under test have to be quantified and their impact on the measurement accuracy have to be reconsidered.
The presented paper will investigate theoretically the feasibility of testing high performance widebeam antennas in the Top-Fed-Cassegrain double reflector system. Qualified measurement results of INTELSAT Global Horn Antennas featuring medium gain and extreme crosspolarization purity will be presented.
Demonstration of test zone field compensation in an anechoic chamber far-field range
Test zone field (TZF) compensation increases antenna pattern measurement accuracy by compensating for non-plane wave TZFs. The TZF is measured over a spherical surface encompassing the test zone using a low gain probe. The measured TZF is used in the compensation of subsequent pattern measurements. TZF compensation is demonstrated using measurements taken in an anechoic chamber, far-field range. Extraneous fields produced by reflection and scattering of the range antenna field in the chamber causes the TZF to be non-planar. The effect of these extraneous fields on pattern measurements is shown. Measured TZFs are also shown. TZF compensation results for pattern measurements using a high-gain, X-band slotted waveguide array are presented.
Three antenna gain methods on a near field range
The Three-Antenna gain method is commonly used on far-field ranges to determine an antenna's absolute gain. This is especially true when no other calibrated antenna is available. This method has been used for years by calibration laboratories such as NIST to calibrate probes and gain standards for far and near-field ranges. In some cases, the calibration is too costly or does not meet the schedule requirements of the near-field test range. An alternative is to calibrate the probe or gain standards directly on the near-field range. In this paper we present the results of a study done to show the accuracy of the Three-Antenna gain method when used on a near-field range. An extensive error analysis is presented validating the utility of this method.
Surface adjustment of modular mesh antenna using near field measurements
The advantages of mesh antennas include good storability and low mass for large on-board antennas over 10M in diameter. Their weak point is that surface adjustment is necessary to attain high accurate surface. Surface adjustment traditionally involves the repeated measurement of surface node position with a theodolite system and subsequent cable adjustment. These steps take much time. This paper describes a surface adjustment scheme that uses near field measurement for a modular mesh antenna composed of mesh, cable network and supporting structure. The node positions of the antenna are obtained by back projection of the far field pattern generated from the near field pattern. The cable network has low sensitivity to changes in local node position. The results of tests show that the surface accuracy needed to achieve the required RF performance can be obtained quickly without theodolite systems.
High-speed measurement of T/R modules used in phased array antennas
As mobile and satellite phased array antennas move from to concept production the demands on test station throughput increases dramatically. Completely characterizing a Transmit/Receive (TIR) module may require thousands of S-parameter measurements under CW and high-power pulsed conditions, as well as, harmonics, spurious, and noise figure measurements. The measurement throughput of instrumentation used in characterizing the prototype TIR modules simply may not be capable of handling the added volume of a production environment.
The volume of measurements, the multiport nature of the device, and the integrated TIR module control make it necessary to reexamine the traditional approaches of separate network analyzers, noise figure meters, and spectrum analyzers. The result is a high-speed modular test ystem that completely characterizes the device in a single connection. The system contains a single receiver and a dedicated controller that utilizes the instrumentation in the most efficient method while maintaining or increasing the accuracy of traditional approaches.
This paper describes the high-speed test stations that have been designed and built and are currently in use in several production facilities. Test system architecture is discussed and measurement throughput numbers are given and compared to conventional approaches.
Application of flexible scanning in advanced APC techniques
Present day accuracy requirements on high-performance antenna measurements are difficult to meet on any type of compact range. Numerical correction techniques can offer a good solution. An easy and effective method is the Advanced APC-technique. This method requires patterns to be measured on different locations in the test zone so that disturbances of the plane wave can be distinguished. In case of suitable distances, the "true" pattern can be derived from measured amplitude and phase data. Usually, scanning is performed in longitudinal direction. The advantage is that mutual coupling can be distinguished well, but the field ripple in this direction due to extraneous fields varies much slower than in transversal direction. Consequently, first sidelobes can be corrected more efficiently when transversal scanning is performed. Therefore, in this paper a new and flexible way of positioning is proposed depending on the location of extraneous field sources.
Incremental build of a planar near-field range
The Sacramento Air Logistics Center at McClellan AFB has developed near-field (NF) antemia test capability over the past three years. With assistance from the National Institute of Standards and Teclmology (NIST), McClellan has assembled a modem planar near-field antenna range using components from various vendors. Although the LH( division of McClellan AFB) team's current range has been operating for over a year, it is being continuously improved for measurement accuracy, user-friendliness, and safety.
This paper will briefly discuss the evolution of McClellan's near-field program, and then focus on the building of the LH near-field antenna range. Radio-frequency (RF) issues, such as RF design and electromagnetic shielding will be covered. Precision measurement teclmiques such as positioning accuracy and temperature control are discussed. Finally, relevant safety and constrnction issues affecting the McClellan facility will be examined.
Qualified and high performance test results of the cylindrical outdoor near-field test range
One of the world's most sophisticated antenna test ranges is now fully operational. This was designed by the Deutsche Aerospace (DASA) and is operated by Siemens Plessey Systems (SPS).
The presented paper will describe the pioneering design philosophy adopted to ensure the stringent performance features.
Although this facility is located outside, it allows extremely high precision probing of cylindrical near field of large and very complex antenna systems, with turning diameters up to 16 meters and up to 20 GHz.
Besides the RCS optimized 36 m large scanner tower the significant highlights of this facility consist of a comprehensive air-conditioning system for all accuracy dependent components, a permanent autoalignment system, which ensures high precision cylindrical measurements and an interleaved high speed data collection system, which delivers a maximum of data performance within a minimum time frame.
Test results including a pattern comparison of the Ref erence Antenna between measurements in DASA facilities and the SPS Cylindrical Near-Field Test Facility show good range performance. The evaluation of the range performance data demonstrates the measurement integrity of the facility and proves to be qualified to characterize a wide range of antennas.
Precise computer controlled conical rotation of string supported targets
New results on very precise, computer controlled manipulation of string supported targets suspended from an upper turntable (UTT) in the Boeing 9-77 compact radar range are presented. A computer program was developed that uses the precision optical measurement system (POMS) information for feedback to automatically control the conic_pitch and conic_roll of arbitrary radar targets to within ± 0.066° (RMS) of the desired pitch and roll. The system provides quick and accurate maneuvering of targets to any desired static position with accuracy in the static yaw, pitch, and roll of ± 0.01°. Automatic volumetric field probes are also possible using a sphere suspended from computer operated strings. Sphere movement can be continuous or stepped along any desired path and is controlled to within ±0.05 inches anywhere within the quiet zone (± 14 ft high by ± 20 ft wide by ± 25 ft).
Portable 4.5m x 2.0m near-field scanner, A
Portable scanners used for near-field antenna measurements are usually incapable of providing a large scan area with a high degree of probe position accuracy. This paper discusses a 4.5m x 2.0m portable scanner developed by NSI with a probe position accuracy on the order of 2 mils (0.050 mm) rms. An NSI patented optical measurement system measures the X, Y, and Z position, and provides real time position correction capability. This lightweight, portable scanner combined with optical correction provides enhanced accuracy while reducing overall antenna measurement system costs and improving test chamber flexibility.
Pattern measurement of ultralow sidelobe level antennas
The development* of a real time electronic system to accurately measure the pattern of high gain, ultralow sidelobe level antennas in the presence of multipath scatterers is described. Antenna test ranges contain objects that scatter the signal from the transmitting antenna into the main beam of a receiving antenna under test (AUT), thereby creating a multipath channel. Large measurement errors of low sidelobes can result. The design and computer simulation of an Antimultipath System (AMPS) is complete. Fabrication of a feasibility demonstration model AMPS to operate with rotated AUTs to suppress indirect (scattered) components and permit accurate pattern measurements is almost done. Results to date show the likelihood of measuring sidelobe levels 60 dB below the main beam.
* This project is sponsored in part by the Air Force Material Command under Rome Laboratory Contract Nos. F30602-92-C-0009, Fl9628-92-C-0130 and F 19628-93-C-02 l4.
New approach for modeling of radar signatures
The identification of targets with radar is frequently based on a priori knowledge of the RCS characteristics of the target as a function of frequency and viewing angle. Due to the complex ity of most targets, it is difficult to predict their RCS signature accurately. Furthermore, complex and large reference libraries will be required for identification purposes. In most cases, a complete knowledge of the RCS is not required for successful identification. Instead, a target representation composed of the contributions of the main scattering centers of the target can be sufficient. This means that a corresponding target representation based on an estimation with Geometrical Optics (GO) or Physi cal Optics (PO) techniques will contain enough information for target identification purposes.
In this paper, a new technique is described which is based on a reconstruction of the scattering centers. These are found at locations where the normal to the surface points in the direction of the angle of incidence. The RCS at these positions depends mainly on the local radii of curvature of the surface. Further more, PO and GO approximations are known as high-frequency techniques, assuming structures that are large compared to the wavelength. At low frequencies, which may be of interest for certain class of identification procedures, and for small physical radii of curvature, the RCS prediction is often difficult to determine numerically. Results from measurements show that this approach is also valid at lower frequencies for the classes of targets as mentioned, even for structures that are significantly smaller than the wavelength. As a consequence, it is expected that even complex targets can be represented adequately by the simplified model.
3-D processing and imaging of near field ISAR data in an arbitrary measurement geometry
Near field inverse synthetic aperture radar 3D is performed utilizing data for arbitrary, but known, positioning of the target. The imaging method was implemented and is described. This straightforward approach has many advantages. It geometrically correct in near field. Field corrections can be independently for each frequency, antenna position and point of interest in the target volume. The main disadvantage is that the processing using the algorithm is very time consuming. However, in many cases it is only necessary to perform the analysis on a few cuts through the object volume.
Calibration of bistatic RCS measurements
Calibration of monostatic radar cross section (RCS) measurements is a well-defined process that has been optimized through many years of theoretical investigation and experimental trial and error. On the other hand, calibration of bistatic RCS measurements is potentially a very difficult problem; the range of bistatic angles over which calibration must be achieved is essentially unlimited and devising a calibration target that will provide a calculable scattering solution over the required range of bistatic angles is difficult, particularly for cross-polarized measurements. GTRI has developed a solution for amplitude calibration of both co-polarized and cross-polarized bistatic RCS, as well as a bistatic phase-calibration procedure for coherent systems.
ISAR RCS editing via modern spectral estimation methods
ERIM is investigating the use of modem spectral esti mation techniques for extracting (editing) desired or undesired contributions to RCS and ISAR measurements in two ways. The first approach involves using parametric spectral estimators to perform frequency sweep range compression and signal history editing, while the second involves using the associated stabilized linear prediction filters to extrapolate sweep data and perform "enhanced resolution" Fourier image editing. This paper summarizes our editing algorithms and illustrates RCS editing results using measurements of a conesphere target contaminated by a metal rod and foam support. The reconstructed "clean" conesphere measurements are compared quantitatively against numerically simulated ground truth. Editing was performed using three bandwidths at two center fre quencies to provide insight into the impacts of nominal resolution and scatterer amplitude variation with fre quency on editing efficacy, and to assess the degree to which superresolution algorithms can offset reduced nominal resolution.
Integrating diagnostic imaging radar into development and production programs
Radar cross section measurements must be performed in a wide variety of situations throughout development of a new vehicle. In these days of smaller budgets, it is vitally important that the right things get measured, at the right time in the program, with the right accuracy, and that these measurements be integrated into the development process in the right way. After delivery, the measurement system must be confidently usable by the user organization, with a minimum of outside to ensure that the vehicle is maintained. Many of the key programs in this area were begun before modern measurement technology was known to be capable of providing detailed diagnostic measurements. Consequently, specifications did not consider what can be easily measured with today's modern diagnostic radars. This paper addresses how mcxlern diagnostic radar cross section measurements can be exploite4:l to make the specification, development, pnxluction, and testing phases much more efficient than they have been in the past.
Video photogrammetry in antenna manufacturing
Photogrammetry, as its name implies, is the science of obtaining precise coordinate measurements from photographs. Until recently, photo-grammetry used film photographs taken with specially designed, high-accuracy film cameras. With the development of h igh resolution solid-state imaging sensors, a new era in photogrammetry has arrived. Video grammetry, as it is often called, provides far faster results and greater capability than film based photogrammetry, and therefore eliminates the major impediments to more widespread use of photogrammetry in the antenna manufacturing industry.
Video-grammetry is a powerful enabling technology that not only performs many current measurement tasks faster and more efficiently th an existing technologies, but also, now makes feasible many types of measurements, that pre viously were not practical or possible. The capability for quick, accurate, reliable, in place measurements of static or moving objects in vibrating or unstable environments is a powerful combination of features all in one package.
There are many applications for this emerging new technology in the antenna manufacturing industry. This paper will describe some of the successfu l implementation of video-grammetry into the MSA T program at Hughes Space and Communications Company located in Los Angeles, California.
Accurate boresighting and gain determination techniques
Boresight and gain determination play an important role in antenna measurements. Traditionally, on outdoor ranges, optical methods are used to determine the boresight. Accuracy requirements better than 0.001 degrees are difficult if not impossible to obtain on outdoor ranges using these method since the effect of incident electromagnetic fields are not taken into account. On indoor ranges no technique is available at present that achieves the desired accuracy demands. In this paper, an improved method for boresighting will be presented. It will be shown that using this technique, desired accuracy demands on both outdoor and indoor can be obtained. Furthermore, the method can also be combined with accurate gain calibration. Advantages and disadvantages of this technique will be discussed.
State-of-the-art near-field measurement system
Planar near-field measurements are the usual choice when testing phased array antennas. NSI recently delivered a large state-of-the-art near field measurement system for testing a multi beam, solid state phased-array antenna. The critical sidelobe and beam pointing accuracy specifications for the antenna required that special attention be paid to near-field system design. The RF path to the moving probe was implemented using a multiple rotary joint system to minimize phase errors. Additional techniques used to minimize system errors were an optical probe position correction system and a Motion Tracking Interferometer (MTI) for thermal drift correction.