Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
The Microwaves and Radar Institute regularly performs calibration campaigns for spaceborne synthetic aperture radar (SAR) systems, among which have been X-SAR, SRTM, and ASAR. Tight performance specifications for future spaceborne SAR systems like TerraSAR-X and TanDEM-X demand an absolute radiometric accuracy of better than 1 dB. The relative and absolute radiometric calibration of SAR systems depends on reference point targets (i. e. passive corner reflectors and active transponders), which are deployed on ground, with precisely known radar cross section (RCS). An outdoor far-field RCS measurement facility has been designed and an experimental test range has been implemented in Oberpfaffenhofen to precisely measure the RCS of reference targets used in future X-band SAR calibration campaigns. Special attention has been given to the fact that the active calibration targets should be measured under the most realistic conditions, i. e. utilizing chirp impulses (bandwidth up to 500 MHz, pulse duration of 2 µs for a 300 m test range). Tests have been performed to characterize the test range parameters. They include transmit/receive decoupling, background estimation, and two different amplitude calibrations: both direct (calibration with accurately known reference target) and indirect (based on the radar range equation and individual characteristics). Based on an uncertainty analysis, a good agreement between both methods could be found. In this paper, the design details of the RCS measurement facility and the characterizing tests including amplitude calibration will be presented.
Comparisons of the far-field results from two different ranges are a useful complement to the detailed 18 term uncertainty analysis procedure. Such comparisons can verify that the individual estimates of uncertainty for each range are reliable or indicate whether they are either too conservative or too optimistic. Such a comparison has recently been completed using planar and spherical near-field ranges at Nearfield Systems Inc. The test antenna was a mechanically and electrically stable slotted waveguide array with relatively low side lobes and cross polarization and a gain of approximately 35 dBi. The accuracies of both ranges were improved by testing for, and where appropriate, applying small corrections to the measured data for some of the individual 18 terms. The corrections reduce, but do not eliminate the errors for the selected terms and do not change the basic near-to-far field transformations or probe correction processes. The corrections considered were for bias error leakage, multiple reflections, rotary joint variations and spherical range alignment. Room scattering for the spherical measurements was evaluated using the MARS processing developed by NSI. The final results showed a peak equivalent error signal level in the side lobe region of approximately -60 dB for both main and cross component patterns for angles of up to 80 degrees off-axis.
Now-a-days, far-field ranges are being used to measure antenna radiation patterns. Two main types of ranges used are used for these measurements: direct and indirect illumination. In either case, the accuracy of the measurement is dependent upon the quality of the range quiet-zone fields. In direct illumination, phase and amplitude taper cause discrepancies in the fields. For indirect illumination, only amplitude taper must be accounted for. Additionally, stray signals and cross-polarization will further distort the quiet-zone fields and lead to measurement errors. This new methodology starts with the measured antenna data and a priori knowledge of the incident fields and estimates an Effective Aperture Distribution (EAD). The EAD compensates for these sources of error and can be used to predict the far-field radiation pattern of the antenna under test. Analytical results are presented for taper and stray signal analysis.
The next generation of antennas will benefit from advanced instrumentation receivers capable of providing simultaneous analog and digital IF inputs, better TR pulse synchronization and high resolution pulse profiling. One such receiver uses a synergistic combination of a tightly coupled FPGA based beam controller, high performance analog digitizers, multiple FPGA based digital signal processors and a new mathematical programming environment. The FPGA signal processor provides direct digital downconversion, high resolution pulse processing and dynamically reconfigurable time and frequency gated matched filter signal integration. The signal processing functions are fully scriptable, providing spectral analysis, various other types of transform analysis, instantaneous demodulation, pulse characterization, noise estimation and more. Advanced mathematical tools combined with novel user interface technologies provide multiple intuitive views into the test setup, error analysis and measurement environment.
Reflections in anechoic chambers can limit the performance and can often dominate all other error sources. NSI’s MARS technique (Mathematical Absorber Reflection Suppression) has been demonstrated to be a useful tool in the fight against unwanted reflections. MARS is a post-processing technique that involves analysis of the measured data and a special mode filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical near field or far-field range. It has also been applied to extend the useful frequency range of microwave absorber down to lower frequencies. This paper will show typical improvements in pattern performance, and will show results of the MARS technique using data measured on numerous antennas.
This paper proposes an approach for the wireless industry to use in assessing its measurement facilities to help ensure that they are providing measurement results that are accurate and repeatable, with a knowable error and uncertainty. This approach is based upon the successful development of a certification program for US RCS facilities based upon an ISO 17025-like standard. Key pieces of this program include a documentation standard for defining the facility's capabilities and operation, and a Report of Measurement and an accompanying Uncertainty Analysis. This paper will discuss the similarities and differences between an existing RCS certification program and the proposed wireless program, to include technical distinctions between the two programs. These distinctions are based upon such factors as a 1-way instead of 2-way propagation paths, the various modulation schemes in use today and the different types of measurements such as Specific Absorption Rate that are not considered in RCS measurements.
This paper describes some improvements in the measurement process of the NIST 18 term error analysis that reduces the required measurement time and also improves the sensitivity of some of the tests to the individual sources of uncertainty. As a result, the measurement time is reduced by about 40 % and some of the estimated uncertainties are also improved without a reduction in the confidence levels. The reduction in measurements is accomplished by using one measurement for two or more error terms or using centerline rather than full 2D data scans for some of the terms.
A new algorithm for the amplitude-only characterization of Compact Antenna Test Ranges (CATRs) is presented. The algorithm applies a successful strategy to retrieve the missing phase of the field in the quiet zone. Particular care is devoted to facing the issue of the typically large electrical dimensions of CATRs and to obtaining the necessary accuracy by the use of an “efficient” representation of the radiated field. This is accomplished through a Jacobi-Bessel expansion of the aperture field which allows to keep low the overall number of unknowns and to improve the accuracy and the reliability of the algorithm. The presented numerical analysis, based on realistic CATR simulations by means of GRASP8-SE, shows the feasibility of the algorithm to estimate amplitude and phase of the quiet zone field within an acceptable accuracy.
Satellite TV reflectors for home use, provided to the public by service companies such as DIRECTV, have many features which must be adequately characterized prior to design release, including: • Multiple Beam Frequency Re-use • FCC Sidelobe Envelope Verification • Circular Polarization Isolation These features must be adequately tested at frequencies up to Ku band and beyond. The use of a far-field range is impractical, as some of the reflectors measure several feet in diameter, and thus requires a range length of several hundred feet at Ku band. Near-field testing requires a full scan to determine a single cut for evaluation of FCC compliant sidelobe performance. Thus, a compact range is a logical alternative for measurement of this class of antennas. The compact range can provide a quick assessment of multiple beam coverage performance and pass/fail analysis against FCC sidelobe curve specifications. In addition, the feeds for these antennas often use Low Noise Block (LNB) Downconverters that are built in as part of the feed assembly. Measuring the output of an LNB does not yield the phase information required to determine all polarization parameters. A spinning linear measurement with some unique processing was implemented on this range to determine the full polarization characterization, using some elementary assumptions about polarization sense. This paper describes the implementation of a compact range based measurement facility for satellite antenna testing, with emphasis on the circular polarization measurement of the LNB assembly, capability for comparison against FCC sidelobe levels, and measurement of offset beams featuring frequency re-use capability.
A common apparatus for microwave characterization of intrinsic material properties is the coaxial airline. Often the largest source of measurement uncertainty in the coaxial airline is from air-gaps between the sample and fixture. Previous analyses of air-gaps in these fixtures have been restricted to analytical quasi-static approximations that assume very small air-gaps. In this work, finite difference time domain (FDTD) simulations were used to study the systematic error caused by air-gaps in coaxial airline measurements of dielectric permittivity. The fundamental mode in a coaxial airline is a circularly symmetric TEM wave. Thus body of revolution (BOR) symmetry was assumed, reducing the required computational effort. Comparison of two-dimensional BOR-FDTD to three-dimensional FDTD showed excellent agreement. BOR simulations were conducted for a variety of gap sizes and sample permittivities to catalogue systematic ‘measurement error’. The quasi-static gap models were evaluated with these simulations, showing that traditional corrections are effective only with small gaps and low to moderate permittivity. The conventional wisdom for non-magnetic samples is that dielectric inversion from the transmission coefficient is the most accurate. The transmission-only inversion was compared to the Nicolson-Ross-Weir algorithm, showing that the opposite may be true – that inversion of dielectric properties from both transmission and reflection can be more accurate when gaps are present.
Adaptive antenna has both the amplitude as well as phase (as weights) can be adapted optimally to get required Direction of Arrival (DOA) estimation or directed beam forming. This paper tries to analyze state of the art criteria for Adaptive antenna, suppressing the interference in directions other than desired. We model the Uniform Linear array (ULA) based on simulations of various adaptive and non-adaptive algorithms. We list possible types of errors in brief. Element spacing and mutual coupling influence each other and affect the antenna element pattern. We formulate the array antenna that tries to reduce the error by optimally adjusting the weights. We make an attempt to model mutual coupling. A high precision array antenna can be designed keeping in mind error factors, optimum adjustment of the element interval and mutual coupling. An adaptive antenna optimal weight adjustment is discussed here. Key words: ULA, DOA, DBF.
The Phoenix Lander, a NASA Discovery mission which lands on Mars in the spring of 2008, will rely entirely on UHF relay links between it and Mars orbiting assets, (Odyssey and Mars Reconnaissance Orbiter (MRO)), to communicate with the Earth. As with the Mars Exploration Rover (MER) relay system, non directional antennas will be used to provide roughly hemispherical coverage of the Martian sky. Phoenix lander deck object pattern interference and obscuration are significant, and needed to be quantified to answer system level design and operations questions. This paper describes the measurement campaign carried out at the SPAWAR (Space and Naval Warfare Research) Systems Center San Diego (SSC-SD) hemispherical antenna range, using a Phoenix deck mockup and engineering model antennas. One goal of the measurements was to evaluate two analysis tools, the time domain CST, and the moment method WIPL-D software packages. These would subsequently be used to provide pattern analysis for configurations that would be difficult and expensive to model and test on Earth.
The UWB radar operates simultaneously over large bandwidth and the antenna parameters must refer to simultaneous performance over the whole of the bandwidth. Conventional frequency domain (FD) parameters like pattern, gain, etc. are not adequate for UWB antenna. This paper describes an UWB radar antenna planar near field (PNF) measurement system under construction to get the impulse response or transient characteristic of the UWB antenna. Unlike the conventional antenna or RCS time domain test system, the UWB radar signal instead of the carrier-free short time pulse was used to excite the antenna that can avoid the decrease of the dynamic range and satisfy the needs of SAR and the other UWB radar antennas measurement. In order to demonstrate the data analysis program, FDTD simulation software was used to calculate the E-field of M×N points in a fictitious plane at different times just like the actual oscilloscope’s sampling signals in the time domain planar near field (TDPNF) measurement. The calculated results can be considered the actual oscilloscope’s sampling output signals. Through non-direct frequency domain near field to far field transform and direct time domain near field to far field transform, we get the almost same radiation patterns comparing to the FD measurements and software simulation results. At last, varied time windows were used to remove the influences of the non-ideal measurement environment.
As NASA prepared the Space Shuttle for its first return to flight mission (STS-114) in July of 2005, a number of new visual and radar sensors were used during the critical ascent phase of the flight to assess if unintentional debris was liberated from the Shuttle as it raced into orbit. New high-resolution C-Band and X-Band radars were used to help ascertain the location and speed of released debris. We also used both radars to monitor debris generated by routine flight events such as Solid Rocket Booster (SRB) separation. To assure these new radars did not interfere with flight-critical engine subsystems, an Electromagnetic Interference (EMI) measurement was performed on the Shuttle Orbiter "Discovery" in January 2005, using the Air Force Research Laboratory's Mobile Diagnostic Laboratory (MDL). This portable EM Measurement system performed a large number of attenuation measurements the night of January 17-18, 2005. This paper describes how the attenuation data was acquired, and the methodology used to reduce the data to predict average attenuation of the radar energy from the outside world to the inside of the aft engine bay of the Orbiter. This data was combined with a separate NASA-performed avionics EMI analysis to demonstrate that the new C and X-Band Debris Radars could be operated without adversely interfering with the Orbiter aft bay Avionics systems.
The purpose of this project was to evaluate a section of terrain at our Newport test site to determine multipath areas. This is being done to test the possibility of a new test range on the area evaluated. The goals were to find a low cost process for detecting areas of specular reflection where multipath might occur and testing ways to minimize this multipath. The two main objectives in achieving these goals were to accurately model the terrain and then simulate the transmission of radio frequency (RF) energy at and onto that terrain. POV-RAY, persistence of vision ray tracer, is an open source artist’s program used to create three dimensional (3D) artwork with realistic lighting and shadows. POVRAY was chosen for a variety of reasons. Its 3D environment is easily manipulated in many ways: addition and placement of objects, reflective properties of objects, and animations of the environment. The light waves simulated by POVRAY represented the transmitted RF energy and the 3D environment was manipulated to react to these light waves as if they were RF energy. This paper explains our process and the results thus far.
The use of “Spectral Analysis” algorithm in RADAR imaging is mainly motivated by the fact that RADAR signals are supposed to follow a very convenient model which says that the target echo (its scattering coefficient) can be decomposed into elementary scatterers, and that it can be modelled with a sum of complex exponential functions. This “high frequency” approach leads to the extensive use of Fourier imaging. When resolution becomes poor, due to the fact that the extent of the set of data is too small, one invokes High Resolution Algorithms like MUSIC or ESPRIT. This becomes particularly the case at low frequencies, where such a model is not valid anymore. The aim of the paper is to show that the use of the MUSIC algorithm can be related theoretically to the “characteristic fields” decomposition of the scattered electromagnetic field. The Harrington-Mautz “characteristic currents” theory leads to a decomposition of the bistatic scattering matrix of the target allowing naturally the use of the MUSIC algorithm to reconstruct the target. We show an application of this on a F117 calculated dataset. 3D bistatic images are obtained.
The linear phase interferometer (LPI) has long been a popular means for performing direction finding (DF). Most texts treat the error terms associated with LPI-DF determination with approximation, such that the error formulae given are a generalized bound, useful for system engineering and design. For certain applications, however, it is important to understand the error associated with LPI-DF in more detail. Related work has accomplished this via Monte Carlo simulations for specific comparisons [5, 6]. To provide an improved (and more general) understanding, we have formulated a rigorous receiver noise error distribution that enables direct determination of bias and variance in LPI-DF. The approach can be generalized to an arbitrary number of simultaneous antenna element apertures.
Reflections in antenna test ranges can often be the largest source of measurement errors, dominating all other error sources. This paper will show the results of a new technique developed by NSI to suppress reflections from the radome and gantry of a large hemi-spherical automotive test range developed for Nippon Antenna in Itzehoe, Germany. The technique, named Mathematical Absorber Reflection Suppression (MARS), is a post-processing technique that involves analysis of the measured data and a special filtering process to suppress the undesirable scattered signals. The technique is a general technique that can be applied to any spherical near-field test range. It has also been applied to extend the useful frequency range of microwave absorber in a spherical near-field system in an anechoic chamber. The paper will show typical improvements in pattern performance and directivity measurements, and will show validation of the MARS technique using data measured on antennas in a conventional anechoic chamber.
A low-profile, high efficiency sixteen-element stacked patch microstrip array operating in the L-band frequencies of 1.26GHz and 1.413GHz was designed, fabricated and tested for use in applications to airborne sensors operating on small aircrafts. The array was optimized for element spacing, excitation amplitude taper, low cross-polarization and high beam-efficiency using Particle-Swarm Optimization (PSO) and Finite-Difference Time Domain (FDTD) methods. The design and measurement of sixteen-element array topology, stacked patch elements, and power-divider beam forming network are presented in detail. The study highlights the repeatability measurements and characterization of array with the effect of dielectric radomes in a spherical near-field test facility at UCLA. The results met the requirements of center-frequencies and frequencybands(1.26GHz ± 10MHz, 1.413GHz ± 15MHz), side-lobes, very good beam-efficiency (>90%) and low-cross polarization (<-40dB) in main-beam region of array. The measured results compared well with simulations for the two frequencies. Based on measurement results, the microstrip array design has a potential to be used as a feed for deployable mesh antennas for future spaceborne L-band passive and active sensing systems that can operate at integrated active radar (1.26GHz) and passive radiometer (1.413GHz) frequencies with dual polarization capabilities to study soil-moisture and sea-surface salinity.
Electromagnetic material characterization is the process of determining the complex permittivity and permeability of a material sample. One common method uses measured scattering parameters from a sample mounted in a coaxial transmission line to calculate the material’s permittivity and permeability. If the material uniformly fills the cross-sectional area of the transmission line, then the standard Nicolson-Ross-Weir (NRW) algorithm [1][5] can be used since only a single dominant TEM mode will be supported. However, if gaps exist between the material sample and the coaxial conducting boundaries, higher-order modes are excited which introduce error into material characterization measurements since these modes are not accommodated in the NRW algorithm. This paper proposes two techniques for mitigating the air gap error in coaxial test fixtures. The first method utilizes a quasi-TEM approach whereas the second method invokes a more rigorous mode-matching analysis. In either case, expressions for the theoretical scattering parameters are obtained and are subsequently compared to the scattering parameters obtained via measurement. For both methods, the resulting error between the theoretical and measured scattering parameters is iteratively minimized until the material’s properties are calculated within a specified tolerance.