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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.
This paper discusses the instrumentation techniques that can be used for the measurement and characterization of antennas that are to be tested in a pulsed-RF mode of operation. A pulse-parameter chart is presented that illustrates all possible ranges of pulse width and pulse repetition frequencies for antennas operating in a pulsed mode. An antenna operating in a pulse mode will have pulse parameters that lie somewhere on the pulse parameter chart. This paper defines five different measurement regions of the pulse-parameter chart, and presents the measurement techniques for measuring pulsed antennas that operate in each of these regions.
Typical high-resolution dynamic target imaging radars have frequency scan rates that do not properly sample the modulation from rotating structures such as aircraft propellers, engine turbines and helicopter blades. This results in the scatterer modulation energy being aliased. Moreover, if the chirp rate is too slow blurring and of the scatterer can occur in the image. Often the utility of this data for RCS signature analysis is questioned. This paper addresses the utility of images generated from undersampled data of modu lating scatterers. Experimental results using various combinations of chirp scan, modulation, and target-body rotation rates are presented. Fast scan rates, typical of the Linear-FM waveform, are compared to the slower scan rates commensurate with step frequency wave forms. Images are shown illustrating how the different chirp speeds alter the two-dimensional image of a mod ulating target.
Inverse synthetic aperture radar (ISAR) images of dynamic targets can be generated using stepped- frequency radars [1,2]. However, a stepped-frequency waveform requires many pulses transmitted over tens of milliseconds to achieve range resolution. This has the undesirable property that a target's rotating parts (such as propeller blades and jet engine compressor blades) can move significantly during this time. This results in of the Doppler sampling, and shifting and blurring in range, (range-Doppler coupling), which degrade the image quality. The Naval Command, Control and Ocean Surveillance Center, RDT&E Division (NRaD) has added a linear frequency modulated (LFM) waveform to its X-band imaging radar. This radar measures a 500 MHz bandwidth in 600 ns. The received signal is baseband converted, digitized, and stored. Data from this radar have been successfully processed into ISAR images that do not exhibit many of the undesirable properties of stepped frequency measurements
ISAR imaging has proved to be a _ significant diagnostic tool for the evaluation of RCS signatures because of its ability to resolve scatterers in both the cross range and down range dimensions. There is a growing desire to extend the imaging capability to include the vertical dimension of a target, or three dimensional (3D) imaging. Several techniques have been suggested with varying degrees of success and complexity. These techniques include triangulation from two or more ISAR images of the same target taken at different elevation angles, tomographic algebraic reconstruction, and true 3D ISAR imaging using the FFT. Each technique requires progressively more data and more complex algorithms, but results in more resolution. This paper examines these various techniques, and evaluates their advantages and disadvantages based on actual implementations using simulated data.
Three-dimensional imaging capability has recently been added to METRATEK's Model 200 RCS Diagnostic Radar. This paper describes the rationale and methodology for producing three dimensional images and gives sample images taken with the system.
Radar with the 2-D Fourier trans- form of the scattered field data in frequency and/or have poor resolution. A modified brid method and a modified 2-D AR technique are proposed to high radar images us- limited backscattered field data. The final image presents the scattering properties of the target in a quantitative way. The peaks in the image represents the positions of centers contributing to the backscattered field. Furthermore, the amplitudes of the peaks correspond to the intensities of the scattering centers.
A proper knowledge of clutter characteristics is critical to the design, development, and test of military seeker and radar hardware. The Clutter Mapping System under construction at Flam & Russell, Inc. is simple yet powerful tool for the evaluation of potential radar sites or the analysis of current sites. It provides a maximum 40 foot synthetic aperture that can image a 60 degree sector of terrain out to a 20 mile range and beyond. Aside from this primary mission, it has the capability to perform RCS measurement of non-cooperative ground targets or to serve as a tactical, quickly deployed imaging system. Totally self contained, and transportable, this system can fulfill a wide variety of RCS measurement needs.
NATO and former Warsaw Pact nations have agreed to allow overflights of their countries in the interest of easing world tension. The United States has decided to implement two C-135 aircraft with a Synthetic Aperture Radar (SAR) that has a 3-meter resolution. This work is being sponsored by the Defense Nuclear Agency ( DNA) and will be operational in Fall 1995. Since the SAR equipment must be exportable to foreign nations, a 20-year-old UPD-8 analog SAR system was selected as the front-end and refurbished for this application by Loral Defense Systems. Data processing is being upgraded to a currently exportable digital design by Sandia National Laboratories. Amplitude and phase histories will be collected during these over flights and digitized on VHS cassettes. Ground stations will use reduction algorithms to process the data and convert it to magnitude-detected images for member nations. System Planning Corporation is presently developing a portable ground station for use on the demonstration flights. Aircraft integration into the C-135 aircraft is being done by the Air Force at Wright-Pafterson AFB, Ohio
Lincoln Laboratory has developed a high resolution imaging radar in conjunction with Flam & Russell, Inc., of Horsham, PA. This highly mobile, ground-based system is capable of 2-D and 3-D imaging of targets at very close ranges to a synthetic aperture. The radar is fully-polarimetric, and operates over two frequency bands (0.05-2 GHz and 2-18 GHz). The radar is currently being used for target imaging and for foliage and ground penetration experiments. In this paper, the radar system is described. In addition, data calibration and image formation are explained. Sample imagery, both 2-D and 3-D, are shown.
Deutsche Aerospace is developing and testing high performance communications antennas for the INTELSAT program. A large number of antenna measurements must be performed, for two polarizations, multiple frequencies and multiple beams. To measure all parameters in a single rotation of the antenna, a highspeed instrumentation system is required. The instrumentation was upgraded using the latest technology in receivers, sources and control systems. Commercially available components were used for all components. The resulting system can perform a complex antenna measurement consisting of over four million data points within only two hours.
The characteristics of dual circularly polarized, dual plane monopulse, millimeter wave antennas are being measured at the U.S. Army Redstone Technical Test Center. This paper will describe the instrumentation suite which allows for simultaneous collection of sum and differenee data in both planes. Also discussed is special antenna under test interfacing requirements, and compact antenna test range facilities
INMARSAT (International Maritime Satellite Organization) provides satellite communication services to civil aviation operations. Aircraft Earth Station (AES) installations aboard all types of commercial transport and business aircraft must meet strict requirements as specified by INMARSAT. The purpose of these requirements is to ensure that all AES shall be able to perform correctly with standard INMARSAT Ground Earth Station (GES) throughout the INMARSAT space segment, and shall not endanger the integrity of the satellite network. To have INMARSAT approval for multi-carrier mode, the AES must meet specifications for levels of transmitted intermodulation products before satellite access is provided.
Details of a public domain Microsoft Windows-based database program to organize and distribute measured antenna pattern and RCS data are described. Tools for editing, viewing and plotting data are also included.
The Spherical Near-Field measurement technique has been in existence for a number of years. The cost associated with this type of measurement system has often been assumed to be substantial. Herein is presented the system configuration for a low cost Spherical Near-field System whose design goals include the capability for production line testing while retaining simplicity in approach. NSI has been contracted to provide a Spherical Near-field antenna measurement system. This paper focuses upon the design considerations undertaken during the prototype development of that system.
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.
Martin Marietta has developed a new, automated facility for high-volume production testing of the Longbow millimeter wave missile. Two dedicated far field anechoic chambers were designed, both automated to support component test and analysis in the production environment. One standard far field chamber is used to perform the complete characterization of the antenna and rac1orne; it allows very accurate measurements of power sidelobes, monopulse errors, and cross polarization isolation. The completed radar missile sensor group is evaluated in the second far field chamber, which can reach higher-level parameters of the antenna, transceiver, and gimbal. This paper describes chamber and test station capabilities; time reduction benefits; and the novel, new assembly technique which allows for future portability of these chambers with limited downtime.
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.
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).
This paper presents the various aspects involved in the design, development and establishment of Cylindrical Near-Field Measurement(cnfm) facility. A brief description of the hardware and the method of data acquisition are outlined. The capabilities of the CNFM system are brought into focus. The effects of alignment errors are presented. The patterns of various test antennas are presented over different frequency bands.