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This paper presents the results of NCTR research performed at the POSTECH compact range. The radar cross section data of five scaled aircraft models, such as F4, F14, F16, F117 and Mig29, have been measured over a frequency region of X-band and an angular sector of 29.6o. Afterwards, one-dimensional radar signatures at several aspects of each target are obtained by modern spectral estimation techniques, including MUSIC, Fast Root-MUSIC, TLS-Prony, matrix pencil, TLS-ESPRIT. The proposed features are based on the central moments of a given radar signature distribution, and they can provide scale and translation invariance, which are essential for the improvement of NCTR performance. After the appropriate post-processing, the proposed features are classified by the Bayes classifier. Results show that our proposed technique has a significant potential for use in NCTR or ATR areas.
The National Bureau of Standards pioneered in the development of practical planar near-field measurement techniques for antennas. The basic theory was originally developed to determine a diffraction correction for a microwave measurement of the speed of light. Subsequently, this theory was adapted to antenna measurements, and NBS undertook the development of techniques for characterizing antennas from measurements in the near field. Implementation required development of (1) precision near-field scanners for measuring the phase and amplitude of EM fields over a precisely determined measurement plane, (2) efficient computer algorithms capable of processing large quantities of data, and (3) error analyses for reliably estimating the uncertainties in the computed antenna characteristics.
An empirical study on Planar Near-Field Scan Plane Truncation applied to the measurement of a large phased array radar antenna saves test time per antenna. Lockheed Martin has been manufacturing, aligning, and verifying the AEGIS SPY-1B/D phased array radar antenna for the past 17 yrs . A custom built planar nearfield scanner system (ANFAST II) was designed and built specifically for this purpose. Existing raw near-field measured data sets were cropped in both the X and Y scan planes, processed to the far field, and compared with the un-truncated data to determine the error sensitivity vs near-field amplitude level truncated. Near-field measurements were then acquired at the truncated scan plane dimensions and compared. It was demonstrated that 100 hrs of test time could be saved by applying this technique without adversely effecting the antenna measurement uncertainty. This paper discusses the application of the truncation technique, results of the experiments, and practical limitations.
Flexing of cables in planar near-field test systems may introduce significant phase errors to the measured vector values of the field. Submm-wave receivers require several flexible cables to be connected to them. The phase errors originated in the bending cables get multiplied and added to the phase of the final detected submm-wave signal. A complete submm-wave antenna measurement system with on-the-fly measurement of the phase errors in a flexing microwave cable is presented. The phase error measurement is based on the use of a pilot signal. Correction of the detected vector values is done as a postprocessing step. Quiet-zone fields and the corresponding phase error planes have been measured at 310 GHz for two different-sized CATRs based on a hologram. The measured maximum phase errors were 7o and 11o for 30 cm and 60 cm holograms, respectively.
Widespread deployment of cellular phones and use of wireless devices such as personal digital assistants, in-vehicle installs of Global Position-ing System (GPS) receivers, and the upcoming deployment of mobile satellite digital audio has sprung a revitalized interest in faster, more af-fordable measurement techniques for antennas. This paper presents information on several new Spherical Near-field antenna measurement ranges developed by ATDS-Howland.
Although the theory is straightforward, practical implementation of spherical near-field scanning for evaluating test chambers presents some significant challenges. Among these are the requirement for accurate probe positioning and the difficulty in minimizing support-structure blockage. We report on recent NIST efforts to mitigate these difficulties and present our most recent results.
Holographic back-projections of planar near-field measurements to a plane have been available for some time. It is also straightforward to produce a hologram from cylindrical measurements to another cylindrical surface and from spherical measurements to another spherical surface1-7. In many cases the AUT is approximately a planar structure and it is desirable to calculate the hologram on a planar surface from cylindrical or spherical near-field or far-field measurements. This paper will describe a recently developed spherical hologram calculation where the farfield pattern can be projected on any plane by specifying the normal to the plane. The resulting hologram shows details of the radiating antenna as well as the energy scattered from the supporting structure. Since the hologram is derived from pattern data over a complete hemisphere, it generally shows more detail than holograms from planar measurements made at the same separation distance.
In previous AMTA Symposia, the Air Force Research Laboratory reported on a successful effort to fabricate, measure, and predict the precise radar cross section (RCS) for various cylindrical calibration targets [1]. In this paper, we apply what we have learned about calibration cylinders to the study of a 3.048 meter ogive body of revolution. Recall that an ogive is simply the arc of a circle spun on its axis. The radar signature of this shape is extremely small in the direction of the "point", even at low frequencies. A few years ago, AFRL had the subject ogive built for an RCS inter-range comparison between AFRL and the NRTF bistatic RCS measurement system [2]. In this paper, we utilize this ogive body to assess both the quality and accuracy of VHF RCS measurements and predictions performed using multiple calculation schemes. In the end, reconciling the ogive measurements and predictions led us to reassess how composite objects are "conductively coated" to simulate a perfect electric conductor. This insight resulted in refinements in the process for measuring and predicting the ogive at low frequencies where electrical size and electromagnetic skin depth considerations are important.
Radar return data from various types of aircraft were collected and analyzed during varying flight profiles to determine the presence of consistent, dominant radar returns of point scatterers on the aircraft. These measurements were performed by integrating two separate X-band radars into one system with the ability to simultaneously track and image aircraft. Selected processed data from both radar systems were analyzed and are presented as a function of time, azimuth and elevation angle, and range. I/Q data, high-range resolution (HRR) profile data and inverse synthetic aperture range (ISAR) data are presented for selected flight profiles of helicopters, propeller aircraft, and jet aircraft.
The implementation of low observable (LO) materials and the fielding of aircraft with controlled signatures creates a new degree of difficulty for maintaining, executing prompt accurate inspections and achieving meaningful evaluations. To address this problem, Sensor Concepts, Inc (SCI) has prototyped a new radar system, (the SCI-Xe) to provide a test bed for a lighter, smaller RCS measurement and imaging system. The hardware consists of a suitcase containing RF hardware, computer and display and a hand-held or rail-mounted unit containing two X/Ku band antennas. In the rail-mounted application, imaging is followed by registration and image differencing, which allows an operator reproduce a baseline measurement geometry and evaluate RCS changes. The hand-held application forms a synthetic aperture by moving the antennas by hand. This can be used to quickly investigate an object under test.
This paper presents a first-principles algorithm for estimating a target’s far-field radar cross section (RCS) and/or far-field image from extreme near-field linear (1- D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. Wavenumber migration (WM) is an approach that was first developed for the problem of geophysical imaging and was later applied to airborne SAR imagery [1], where it is often referred to as the “Range Migration Algorithm (RMA)”[2]. It is based on rigorous inversion of the integral equation used to model SAR/ISAR imagery, and is closely related to processing techniques for near-field antenna measurements. A derivation of WM and examples of approximate farfield RCS and image reconstructions are presented for the one-dimensional (1D) case, along with a discussion of the angular extent over which the far-field estimates are valid as a function of target size, measurement standoff distance, and near-field aperture dimensions.
In order to better estimate the uncertainties in measured RCS for the Boeing 9-77 Compact Range, we study the responses from three high-quality objects, i.e., two ultraspheres of 14” and 8” in dia., plus the 4.5" squat-cylinder, each supported by strings. When calibrated against each other in pairs, the differences between measured RCS and predicted values are taken as the uncertainties for either object. Two standard-deviations from the target, reference, and background, as computed from repetitive sweeps, are taken as the respective uncertainties for the signals. Using the root-sum-squares (RSS) method, the error bars are found to be between + 0.1 to 0.2 dB for most of the frequency F, from 2 to 17.5 GHz. We also analyze the responses from a thin steel wire (dia. 0.020"), supported by fine fishing strings (dia. 0.012"), at broadside to the radar. When the ‘wire and string’ assembly is oriented vertically, the HH echo from the 3-ft metal wire alone happens to be comparable to the HH from the 30-ft dielectric strings. Varying with F4, the combined RCS in HH for the assembly spans a wide range of 38 dB from 2 to 18 GHz. The error bounds are found to bracket the measured traces even when the signals are barely above the noise floor.
We review a century of radio metrology research and development in the U.S. that paralleled the birth and evolution of radio/wireless and other electromagnetic technologies. The interplay between the scientific and technological advances and the research, measurement and standards development programs at the National Institute of Standards and Technology (formerly the National Bureau of Standards (NBS)) was a factor that facilitated both commercialization of products and implementation of systems for the public benefit.
Antenna tracking systems are an important part of practical system designs. The goal of antenna tracking for communication applications is to provide sufficient accuracy to limit pointing loss, while for radar applications, to determine the target’s position as accurately as possible. Antenna tracking systems are reviewed describing both open and closed loop designs. Corresponding measurement techniques to quantify system performance are described.
The growing need for a mobile radar system able to conduct measurements away from fixed radar ranges has prompted System Planning Corporation (SPC) to develop a mobile MkV radar system. Planned helicopter-based SAR measurements generated a requirement for a ground-based platform to verify functionality of X-band and VHF/UHF data collection and processing systems. Accordingly, SPC developed TruckSAR, a DGPS-equipped mobile testbed to collect side-looking and normal-incidence SAR data. Interleaved step chirp data were collected at 9.0-9.3 GHz (HH polarization) and 150-450 MHz (HH, VV, HV, and VH polarization). The system is self-contained and is proving useful for applications beyond ground and foliage penetration SAR investigations. This paper describes the TruckSAR hardware and data analysis systems. Results of measurements are presented, along with observations of challenges in data interpretation. Promising extensions of this mobile ground-based radar are also discussed.
A versatile instrumentation system for automatically measuring both antennas and performing the Air sensitivity & Channel Power test. The system is capable of being easily reconfigured to perform standard FF antenna measurements using a model tower configuration which includes a dielectric mast with a rotary “head” mounted on an azimuth turntable or automated air sensitivity and channel power measurements for both GSM and CDMA mobile cellular devices. The air sensitivity test module iterates until the desired user defined frame error rate is reached at the preset scan positions and than records the data. The system also contains analysis capabilities for all modes of measurement. The paper will summarize the system configuration and the features of this integrated test system.
Waveguide simulators are widely used for low cost validation of periodic microwave designs and to perform antenna measurements. We have used measurement results of a waveguide simulator to predict both Frequency Selective Surface (FSS) and reflectarray responses. For scan angles close to normal, a suitable waveguide simulator is relatively wide and measurement results are often corrupted. This is often caused by uncontrolled multi-mode operation. The work presented here describes a waveguide simulator, which solves this problem for triple-mode operation. The triple-mode waveguide simulator has three standard waveguide ports and one triple-mode port. This device can be excited on the three standard ports. It produces each of the three propagating modes at the triple-mode port separately. Simulations and measurements on a prototype show good agreement. With our current set-up, three scan angles can be predicted instantaneously and grating lobes can be studied as well.
The system noise temperature is a fundamental parameter of performance of a satellite communications reception antenna. Traditional methods of measuring noise temperature involved the use of thermal noise standards connected to the antenna input. Indirect methods can also be used to derive noise temperature from a G/T measurement. However these traditional methods require special setups and cannot be used continuously. A noise temperature measurement system, which can be used continuously, even when tracking and receiving telemetry signals, is a valuable tool for performance monitoring. A noise adding radiometer method, which was originally design for radio astronomy applications, has been adapted for communications antenna measurement [5] and this can be used continuously. However, this method has some limitations in the degree to which telemetry signals interfere with the measurement system. A study has been undertaken under European Space Agency (ESA) contract, into the design of a noise temperature measurement system, which involves the evaluation of the bit error rate of a test signal. Changes in noise temperature result in change of bit error rate of the test signal. The test signal is spread spectrum modulated so interference between the test signal and operational signals are minimised. The study was executed in two phases. In the first phase a theoretical analysis of the test method was performed. In the second phase a prototype measurement system was developed and evaluated. The paper describes the main results of the theoretical analysis, description of the prototype system and analysis of the test results. The prototype was designed to measure the noise temperature of a standard ESTRACK S/X band ground station antenna. As a follow-on to the original study work is now in progress to produce a fully operational unit which will be installed in the ESA Perth S/X band ground station. The results of evaluation of the prototype have been used to introduce design improvements for the operational unit, which are described and discussed.
Multiple-band tests are important to demonstrate the performance of a complete system, including the ability for distinct sub-systems to simultaneously measure the same scene. This paper outlines an experiment performed under the direction of Eglin AFB. The experiment demonstrates the capability of a test range to perform a combined radio frequency and infrared (RF/IR) direction finding experiment in which the RF system cues the IR system as an object is tracked across the field of view. Such a test requires the combined performance of the test range, an RF system and an IR system, along with a procedure to integrate the three systems into a cohesive experiment. The successful demonstration of the complete system shows the strength of the individual systems, and the process of development identified some areas for advancement in future multi-spectral and multi-system tests. This report focuses on the development and performance of the RF direction finding portion of the test, with minor discussion of the other systems.
Future scientific and earth observation instruments as MASTER, PLANCK and HERSCHEL of ESA/ESTEC are working in the sub-millimeter wave range. For measurement of the instruments, a study named ADMIRALS was performed, mainly to identify the most suitable test facility, procure transmit and receive modules and perform measurements up to 500 GHz. The CCR 75/60 of Astrium GmbH, Ottobrunn, was selected for the facility calibration and the pattern verification with an Representative Test Object (RTO). The measurements were performed in three different frequency bands between 200 and 500 GHz. The mmwave transmit and receive modules were designed, manufactured and tested by Radiometer Physics GmbH (RPG). A cost efficient design was achieved by a modular concept. Within this paper, the design and realization of the modules as well as most characteristic performance parameter will be presented.