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A fast algorithm is presented for the generation of 3D current density images of antennas utilizing arbitrary antenna measurement data. The images represent a broadband equivalent current distribution which radiates the same fields as the true current distribution on the antenna structure in a broad frequency band. These equivalent currents convey important information about the antenna. The imaging algorithm can efficiently handle arbitrary measurement geometries and probe characteristics. It is inspired by the Multi-Level Fast Multipole Method (MLFMM). The near-field compensation and probe correction is realized using a modified adjoint operator based on adaptive fast multipole translations. Due to the modifications, a priori knowledge about the field observation density can be exploited and the generated image becomes more accurate. The complexity of the proposed approach is identical to a fast Fourier transform (FFT) based imaging algorithm. Numerical examples are given to prove
Two of the three algorithms require an estimate of the element pattern, which they assume to be common to all the elements. We describe our measurement of our array’s element pattern, as well as the use of the IsoFilter™ to center the element pattern and limit the edge effects.
In this paper we present an optical imaging tool, the Overlay Imaging Aligner (OIA), developed to aid in the mechanical alignment of antenna components in the mm-wave and low-THz frequency regimes (50-500 GHz) where the millimeter and sub-millimeter wavelengths pose significant challenges for alignment. The OIA uses a polarization-selective, machine-vision approach to generate two simultaneous and overlaid real-time digital images along a common axis. This allows for aligning two antenna components to within fractions of a wavelength in the mm-wave and THz frequency regimes. The overall concept, optical design, function, performance characteristics and application examples are presented. Preliminary data at specific frequencies in the WR-2.2 band are presented that compare the alignment achieved with the OIA to an electrical alignment.
Anechoic chamber characterization typically requires measuring the level of extraneous signals within an arbitrarily defined quiet zone volume. From this data, measurement uncertainty due to the presence of extraneous signals can be quantified for various test scenarios. For the anechoic chamber designer, however, it is equally important to determine the magnitude and source of the extraneous signals so that they can be minimized or controlled. This paper discusses improvements in Spherical Near-Field Chamber Imaging as applied to anechoic chamber design and characterization. Measurement system improvements to improve image resolution are described. Data sampling requirements to eliminate processing artifacts is discussed. Critical sampling probe characteristics limiting UHF measurement capabilities are outlined. Test data on an outdoor range and on a large anechoic chamber are presented and discussed.
This communication addresses the design and implementation of a low-perturbation and high dynamic range near-field (NF) imager with increased measurement speed. The imager is equipped with an array of optically modulated scatterer (OMS) probes, each incorporating a commercial-off-the-shelf photodiode chip and a minimum scattering antenna, i.e. short dipole. In the OMS probes, transmission of modulating optical signals is performed using an optical fiber coupled to the photodiode, which is invisible to microwave signals. The imager measurement speed is also improved as the OMS array eliminates the delays associated with probes translations, in addition to fast switching of modulating light between the probes. Fast switching is accomplished by an array of fiber-pigtailed laser diodes. Improved dynamic range and linearity in the NF imager are achieved by adding a carrier canceller within the imager receiver front-end, eliminating the carrier signal and leaving the sidebands intact. This canceller also improves the isolation between input/output ports of the imager providing a potential for higher signal amplification. The performance assessment of the NF imager, including its linearity and result accuracy is made by comparisons with a known field distribution.
In the last decades radar imaging techniques have been widely studied. Electromagnetic imaging is a very promising technique for many practical application domains (medical, surveillance, localization …). As an example, many RCS imaging systems have been developed for compact range indoor RCS measurement layouts. In this paper, a preliminary comparison of near field RCS images from Multiple Input Multiple Output (MIMO) arrays and monostatic radar is presented. The main objective of this study is to make use of efficient radar imaging algorithms, which were originally conceived for SAR systems, with MIMO arrays (ex. back projection) in order to develop real-time imaging applications based on MIMO array systems. The study was conducted with a one-dimensional MIMO array composed of 14 transmitting and receiving antennas. The goal of the optimization is to obtain radar images as similar as possible to those from monostatic radar. This paper presents the experimental layout, the imaging algorithms and the experimental results. As a conclusion, the imaging capabilities of MIMO arrays are discussed.
General Purpose Graphical Processor Units (GPGPUs) provide increased processing capability for applications with a high degree of data parallelism. In the past the few years, GPGPUs have become readily available in the commercial market, and off-the-shelf programming tools (e.g. CUDA from the NVIDIA Corporation and Jacket from Accelereyes, LLC) have made them more accessible to the technical community. SAR and ISAR imaging algorithms are inherently computationally intensive. In order to overcome performance limitations of CPUs and traditional DSPs, simplified, computationally-efficient algorithms are often used, but at the expense of the phase information available within the raw data. We have demonstrated that GPGPU acceleration of SAR/ISAR processing has greatly improved processing times of a less-efficient (but more flexible) algorithm, making its use more practical. We have shown that GPGPUs can provide performance improvement in excess of 30X for a backprojection-based SAR/ISAR imaging technique.
A unique wideband, dual-beamwidth, X-Band antenna has been developed by STAR Dynamics Corporation in support of a Ground-to-Air Radar Signature (GTARS) measurement system. The GTARS radar system provides precision dynamic RCS measurement and radar imaging capabilities for maneuvering in-flight aircraft. This specialized antenna and radar system provides the capability to track and measure dynamic radar target signatures and parameters that are not practical to measure on a static ground-based RCS measurement facility. The GTARS radar requirements posed significant challenges for the antenna design, among which are the capabilities to measure and track targets in wide and narrow fields of view (FOV) with simultaneous linear co- and cross- polarizations. Precision target tracking is required during dynamic measurements to maintain an accurate beam centered on the target during its flight. Consequently, STAR Dynamics has developed an offset reflector antenna with dual polarized five-aperture eight-port feed network that maintains the antenna beam precisely centered on the maneuvering target. The dual beamwidth functionality is achieved by two separate reflectors while each reflector provides multiple channels for simultaneous radar signature measurement and monopulse tracking using the eight-port feed array.
Allwave Corporation, 3860 Del Amo Blvd., #404, Torrance, CA 90503, USA
During the STS-107 accident investigation, radar data collected during ascent indicated a debris event that was initially theorized to be the root cause of the accident. This theory was investigated and subsequently disproved by the Columbia Accident Investigation Board (CAIB). However, the data itself and the lack of understanding of what debris data in radar meant to the shuttle program, required further analysis and understanding. The Space Shuttle Program Systems Engineering and Integration (SE&I) Office commissioned the Ascent Debris Radar Working Group (ADRWG) to characterize the debris environment during a Space Shuttle launch and to identify/define the return signals as seen by radar. Once the capabilities and limitations of the existing radars for debris tracking were understood, the team researched proposed upgrades to the location, characteristics, and post-processing techniques needed to provide improved radar imaging of Shuttle debris. The research phase involved in assessing the threat ultimately evolved into an inter-agency cooperation between NASA and the Navy for shared use of radar assets to the benefit of both agencies. Additional cooperative agreements were made with the Air Force and Army regarding various support aspects to the debris radar efforts. An aggressive schedule of field testing preceded the initial operations of the system during the STS-114 Return to Flight (RTF) mission in July of 2005.
This paper presents a new approach to the inversion of boundary value (BV) problems in an infinite conductive, homogeneous media. Our interest is to investigate the possibility of imaging underwater electromagnetic sources from remote electromagnetic sensor data. Specifically, given two polarizations of the electric/magnetic fields on a cylindrical surface exterior to the electric and magnetic sources, we develop a frequency domain, back-projection technique that allows for the complete determination of the electric and magnetic fields in the region between the BV surface and the sources. This is an inverse problem and Tikhonov regularization is used to obtain an accurate filtered, back-propagated solution. In this approach two components of the measured field yield the six components of the field closer to the source. Of particular interest is the active part of the Poynting vector that is constructed from the back-projected fields, providing the power per unit area radiated from the sources. We believe it may be of immense practical use in diagnosis of electromagnetic sources underwater. We test the theory with a numerical experiment using a linear array of either magnetic or electric dipole sources excited in a frequency range of 1 to 1000 Hz in seawater that generate two cylindrical holograms (30m radius) of the axial polarization of the magnetic and electric fields, respectively. The complete (all polarizations) electric and magnetic fields are predicted along with the real and imaginary parts of the Poynting vector on a cylindrical back-propagation surface (20m radius). These simulations show that very accurate results are obtained even with low signal-to-noise levels. Work supported by the Office of Naval Research.
A first analysis of the equivalences between Multiple Input Multiple Output (MIMO) and physical/synthetic radar arrays is presented. The establishment of these equivalences is addressed to make use of efficient radar imaging algorithms, which were originally conceived for SAR systems, with MIMO arrays. The main advantage of MIMO arrays is that, with a reduced cost and complexity of the antenna feeding network, they offer imaging capabilities very close to those of SAR and physical radar arrays. This makes MIMO radar a very interesting option in real-time imaging applications (e.g., surveillance of small areas). The paper will present some numerical simulations using some reference scenarios where the imaging capabilities of MIMO arrays will be assessed. A comparative analysis with the well-known SAR and uniformly spaced radar arrays will be presented. Here the study is made with one-dimensional radar apertures, and subsequently will be extended to two-dimensional radar apertures. The analysis of the performance of the MIMO arrays is based on a Matlab simulation tool that is used to optimize the array topology and also to form the radar images of a synthetic scenario. The optimization technique is based on a genetic algorithm, using a fitness function measuring the degree of uniformness and uniqueness of the loci of the phase centers of the tx/rx pairs of the MIMO array. Results show that the found topologies show a performance close to uniformly spaced physical radar arrays.
A real-time S-band radar imaging system will be shown in this paper that uses a spatially diverse antenna array connected to a highly sensitive linear FM radar system and uses a synthetic aperture radar (SAR) imaging algorithm to produce real-time radar imagery. The core of this radar system is a high-sensitivity, range gated, radar architecture. Previous work has demonstrated the effectiveness of this radar architecture for applications requiring low-power and high sensitivity for imaging through lossy dielectric slabs at S-band and in free space at both S and X bands. From these results it was decided to develop a real-time S-band SAR imaging system. This is achieved by constructing a spatially diverse antenna array that plugs directly into a pair of S-band transmit and receive radar front ends; thereby providing the ability for real-time SAR imaging of objects. The radar system chirps from approximately 2 GHz to 4 GHz at various rates from 700 microseconds to 10 milliseconds. Transmit power is adjustable from approximately 1 milliwatt or less. The image update rate is approximately one image every 1.9 seconds when operating at a chirp rate of 2.5 milliseconds. This system is capable of producing imagery of target scenes made up of objects as small as 1.25 inch tall nails in free space without the use of coherent integration. Previous applications for this radar system include imaging through dielectric slabs. It will be shown in this paper that this radar system could also be useful for real-time radar imaging of low RCS targets at S-band.
Highlights of work at the Naval Research Laboratory in evanescent near-field electromagnetic holography (ENEH) will be presented. This work grew out of extensive experimental work in near-field acoustical holography at our laboratory that has been recognized formally by the Laboratory as one of the 75 most innovative technologies over the past 75 years. This new electromagnetic approach differs from the usual nearfield imaging in that it provides much better than halfwavelength resolution due to the inclusion of evanescent waves. Furthermore ”imaging” to a source surface provides a reconstruction of the surface currents, Poynting vector as well as the E and H field vectors. These quantities are derived from two measured holograms (phase and amplitude) of two polarizations of the electric and/or magnetic fields over a 2-D surface (the hologram). Experimental work in both low (100 Hz) and high frequencies (10GHz) is of interest, although we present here results of the latter along with the theory. Two approaches will be discussed for backtracking the measured fields: one that uses wave function expansions in plane, cylindrical or spherical geometries, highlighting the cylindrical geometry in this paper, and a second more general formulation that uses the field expanded using an array of equivalent dipole sources especially useful in arbitrary geometries. Both approaches represent inverse methods and are ill-posed and require regularization to stabilize the reconstructions. We hope that these methods will provide high resolution new diagnostic tools for antenna analysis, as well as diagnostics for applications in EMC and EMI among others. Currently we are seeking partnership with other laboratories and universities to direct this technology towards problems that could benefit from its unique diagnostic capabilities. Work supported by the Office of Naval Research.
The DGA/CELAR (France) (Centre d'Electronique de l'Armement: French Center for Armament Electronics) is able to measure targets in order to get their RCS (Radar Cross Section). Once this RCS is acquired it may be very interesting to calculate RADAR pictures of these targets because RADAR picture allows emphasizing the bright points. Until now, CELAR produced images in two dimensions, but these pictures have shown their limits in order to locate problems in altitude. This article fills this gap while proposing two methods in order to get an image in three dimensions: a method using a three-dimensional Fourier transform and a method based on interferometry.
Using a discarded garage door opener, an old cordless drill, and a collection of surplus microwave parts, a high resolution X-band linear rail synthetic aperture radar (SAR) imaging system was developed for approximately $240 material cost. Entry into the field of radar cross section measurements or SAR algorithm development is often difficult due to the cost of high-end precision pulsed IF or other precision radar test instruments. The low cost system presented in this paper is a frequency modulated continuous wave radar utilizing a homodyne radar architecture. Transmit chirp covers 8 GHz to 12.4 GHz with 15 dBm of transmit power. Due to the fairly wide transmit bandwidth of 4.4 GHz, this radar is capable of approximately 1.4 inches of range resolution. The dynamic range of this system was measured to be 60 dB thus providing high sensitivity. The radar system traverses a 96 inch automated linear rail, acquiring range profiles at any user defined spacing. SAR imaging results prove that this system could easily image objects as small as pushpins and 4.37 mm diameter steel spheres.
It is widely known to radar engineers that the best radar receiver is a correlation receiver, which correlates the originally transmitted radar pulse with received pulses from the reflection. But the true correlation receiver could not yet be realized in past. The advancement of fiber optical technology changes that. The new and true correlation receiver, which has been referred to as interferoceiver, revolutionizes the technical foundation of radar and electronic warfare. The present talk discusses the use of the interferoceiver in advancing techniques of RCS measurement and RF imaging.
A large compact range facility required a foam column for RCS testing where the center of the quiet zone was six meters above the floor level. The RCS measurement after vector background subtraction, had to be accurate down to a –50 dBsm level from 1.5 GHz to 40 GHz. A foam column was constructed from a single billet of material. The foam column was evaluated as to its RCS level in both whole body and ISAR imaging modes. This paper describes the specification, construction and RCS evaluation of this column in the compact range facility. The column was evaluated at single frequencies and with RCS images from 2 GHz to 36 GHz using a gated CW radar. Data is presented that shows the effects of the column on the response of a calibration sphere and the response of the column itself. A study of the foam column imaging response used as the background for vector background subtraction is also described. Targets in the –60 dBsm range were successfully imaged with vector background subtraction of the foam column.
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.