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Classical SAR (Synthetic Aperture Radar) imaging techniques [1, 2] based on free space propagation may suffer significant distortion when a target of interest is located in a complex environment such as behind a building wall, underground or embedded in foliage. An independently derived analytical solution for electromagnetic wave propagation through a uniform dielectric wall or a uniform dielectric half-space is obtained by the authors. A new and computationally efficient model-based iterative SAR image refocusing algorithm based on the above solution is developed. The algorithm permits non-uniform spatial sampling of imaging data, and cases where a radar unit may be in the radiating near-field of a target. This algorithm is applied to both simulated and measured data. Resulting SAR images are shown to be significant improvement over those generated by the classical free-space back-projection technique.
A three-dimensional (3-D) imaging capability based on a linear FM measurement radar has been developed. This capability provides a means of resolving radar scattering centers in three dimensions, allowing the more accurate feature location and enabling the possibility of separating target returns from undesired environmental clutter. An existing portable radar cross section (RCS) measurement system was modified to incorporate a 3-D imaging capability. This modification allowed the system to remain highly portable and provide quick turnaround time with a typical measurement cycle comprising 20 minutes of data collection, followed by viewable 3D imagery within 5 minutes. The entire measurement system is comprised of a planar scanner and a single equipment rack. A 3-D RCS data set varies by frequency, azimuth, and elevation, and is obtained by scanning the radar antennas in azimuth and elevation. Innovative development of useful data visualization tools was one of the key efforts in this project. Visualization approaches include employing a mesh computer aided design (CAD) model aligned in 3-D space to the image data. The image is mapped to the surface of the model and the user can then move around the model to view it from any aspect in real time.
We introduce a prototype test cell for generating wide band standard fields suitable for antenna and immunity testing. The Co-conical Field Generation System (CFGS) is a conically-expanding coaxial transmission line terminated in a well-matched, high-power, distributed load. By maintaining the symmetrical nature and simple geometry of the cell, a well-defined uniform field can be established with minimal higher-order mode generation. Designed to operate from DC to 40 GHz, the CFGS is ideally suited for the testing of broadband antennas of limited size in a rapid and accurate manner.
The purpose for this advanced development program was to advance the flatness level on an RF/IR Beam combiner. The developed beam combiner minimized transmission losses for RF signals between 1 GHz and 40 GHz and maximized total transmission for RF signals between 8 GHz and 18 GHz. The combiner maximized IR reflectance for IR radiation (2µm to 13 tm). Two 12 inch units were delivered to NAWC-WPNS for evaluation. The combiners produced an average transmission losses in the range of 0.4 dB between 1 and 33 GHz and 0.8 dB between 33 GHz and 40 GHz. Reflectivity in the Infrared was measured at 87% with the use of a 3.39 µm laser source. The combiners were manufactured on PolyOxyMethyle (POM); they are highly crystalline structures, very flat (mold driven), with unique acetal resins structures. POMs are a variant of thermo plastics, are made by free radical polymerization initiated by a peroxide or azo catalyst, or by redox polymerization. Four basic polymerization processes may be used to produce good RF transmission acrylic resins. Using POM as the host material, a Frequency Selective Surface (FSS) using a low pass configuration, was Gold sputtered on the host material surface. The results produced a mirror like surface, highly visible and IR reflective, and very transmissive in the RF domain. These combiners are to be used for the anechoic chamber testing of dual mode missile seeker systems. The missile systems required in an anechoic chamber measurements, far field illumination from both IR signals and RF signals. The dual mode beam combiner allows spatially coherent signals to illuminate the missile seeker under test. Results of these development, seems to indicate that larger combiners can be fabricated on optically flat materials (e.g. fused silica) with flatness of 12 µm. This will allow the next generation seeker heads, operating with large focal plane arrays, to be stimulated in an anechoic chamber environment.
Plasmas inside the TJ-II Stellerator are generated by heating the electron cyclotron resonance waves with a high-power millimeter-wave beam from gyrotron generators and through two transmission lines. Both lines have been tested at nominal power level and they are currently in operation. This paper is devoted to the low-power testing of the transmission lines performed before their operation at high power level. A corrugated horn antenna was designed to generate a pattern that simulates the gyrotron output. In order to evaluate the set up, a twofold approach was taken. On one hand, the antenna pattern was measured and compared with the predicted one. On the other hand, the beam propagation through the mirror line was measured and simulated using Huygens diffraction theory. The comparison of the theoretical and experimental results from both the corrugated antenna and propagation through the transmission line are presented in this paper.
The automobile antenna industry is facing two rapidly growing trends: (1) the incorporation of effective, low cost, AM/FM conformal antenna designs and (2) the antenna capability to handle diversity FM radio receivers. The development of techniques for testing automotive conformal diversity antennas' performance becomes necessary to evaluate and compare them. Testing techniques to obtain the antenna Input Impedance (Zin), Standing Wave Ratio (SWR) and Mismatch Loss (MML) as well as the azimuth gain patterns and the combined diversity signal (maximum of the diversity signals) are described. Experimental results for the Annular Slot Windshield Diversity Antenna using polarization diversity are shown. It is demonstrated that the Annular Slot Windshield Diversity Antenna can be used effectively to reduce multipath fading.
The worldwide system now used in the aviation field as a landing aid is simply called the ILS, or instrument landing system. This paper is about the "null reference" type of vertical guidance component of the system. It operates in a frequency band near 333 MHz by transmitting signals from two antennas on a tower near the aircraft runway. The lower antenna, (and its image) produces a broad beam (the reference) along the approach to the runway. The upper antenna (also with its ground image) produces a vertical guidance signal with a null along the desired approach angle (or glide slope, typically 3 degrees). The reflection zone for these antennas is a critical component of the system. A problem has been discovered for the case of a layer of wet snow on the reflection zone. As the layer of snow warms up and changes from the frozen state to a water-snow mixture, the dielectric constant of the layer of snow changes over a very wide range. At some point in this process, the reflection coefficient of the layer of snow over the wet ground passes through zero at the design approach angle (3 degrees). At this time, the vertical width of the guidance null becomes much larger than normal. An aircraft will lose its normal tight control over the vertical approach angle, and may experience significant errors in the approach angle without any indication of the problem. The time for the phenomena to occur is so short that as of this date, no experimental proof of the phenomena has been obtained. The theory for these phenomena will be shown, and examples where aircraft crashes may have occurred in such conditions win be discussed. Some experimental evidence will be presented.
The control circuit encoding (CCE) technique [1,2] has been proposed as a method of remotely calibrating a phased array antenna. This patented technique uses an orthogonal coding scheme to measure the amplitude and phase of all array elements simultaneously. The capacity to measure all elements simultaneously is more efficient than single element measurements since measurement time is minimized. This paper describes an experimental verification of the CCE technique. Accurate control of amplitude and phase distribution in an array is important because it allows for low sidelobe array designs that can be maintained over the life cycle of the system. Also discussed is our method for estimating statistics of calibration performance using a stepped null approach. The results demonstrate that the CCE method is a viable approach for calibrating a phased array.
Omnidirectional antennas are typically used as Tracking, Telemetry and Command (TT&C) antennas for satellites. However, the omnidirectional patterns of TT&C antennas located on satellite structures are susceptible to substantial scattering and polarization mismatch loss, especially at the initial mission stage. Consequently, it is very important to properly evaluate the extent of these effects for each of the initial mission configurations. In this paper, measurement techniques to achieve proper evaluation of scattering level and polarization mismatch loss for TT&C antennas of NASA's Tracking and Data Relay Satellite (TDRS) are presented. The paper encompasses a test approach, a test procedure and test results. Application of these test techniques is essential to the TDRS TT&C antenna qualification program.
Phase retrieval is an important issue related to the reconstruction of SAR/ISAR images, when phase information is lost or unavailable. In this paper, an iterative algorithm is formulated which demonstrates the ability to perform phase retrieval with minimal set of constrains on the imaged object. This iterative algorithm requires only rough knowledge of the size of the imaged body and the amplitude of the received, far-field, radiation in the various frequencies and/or aspect angels (for I D or 2D image). By applying iterations between the two planes of the imaged body and the plane of the RADAR reflections (as a function of aspect angles and frequencies), a good reconstruction of the phase and the amplitude of the imaged body as well as the phase of the received radiation, are obtained. The algorithm can be used in the problem of imaging body in motion where motion compensation is difficult or in applications involving mm wave images, where phase information is lost in the turbulent atmosphere.
This paper presents an approximate, practical technique for the compensation of antenna pattern amplitude taper effects that occur in near field RCS data. The technique uses inverse synthetic aperture radar (ISAR) data sets. Complete pattern determination uses an iterative approach over target rotation angle and frequency bandwidth, with a series of near field ISAR images as input to obtain the corresponding corrected, near field, frequency/azimuth pattern data. Assumed is direct target illumination using a source with a known angular illumination pattern. The technique and its application environment in the Boeing Near Field Test Facility is described. It is then demonstrated using a near field data collection range of 100 feet from the target center of rotation. The approach is shown to be effective for target sizes with cross range extents extending to the one-way 3 dB points of the illumination taper (two-way 6 dB points). Demonstration of compensation performance and a study of accuracy achievable versus the near field image parameters used is presented.
We present innovations based on pattern recognition technology that significantly reduce the level of human intervention and increase data throughput when processing radar images of airborne targets. Time consuming operator intervention is normally required to insure that images are centered and non-aliased and wireframe overlay drawings are properly registered with the target image. We have developed techniques that produce high-quality images without operator intervention. These include a template registration algorithm that can reliably orient the outline drawing with a radar image even in the presence of image artifacts such as jet engine modulation (JEM). In addition, we have developed methods that remove the average Doppler responsible for crossrange image displacement or aliasing and methods that resolve downrange ambiguities. Examples are shown which illustrate these processes applied to images of a jet aircraft in flight.
A novel algorithm for radar imaging is presented. The method comprises two steps. First, a decomposition of the radar data domain into sub-domains and computation of pertinent low resolution images. Second, interpolation, phase-correction and aggregation of the low-resolution images into the final high resolution one. A multilevel domain decomposition algorithm is formulated. The computational cost of the proposed algorithm is comparable to that of the FFT-based techniques while it appears to be considerably more flexible than the latter.
A nonparametric, two-dimensional spectral estimation algorithm, based on adaptive decomposition using the reweighted minimum norm method, is applied to ISAR imaging. This paper will describe the algorithm and demonstrate its performance using numerical simulations and compact range measurements. It will be shown that the technique can provide robust isolation and extraction of target and/or contamination returns in situations where these returns would not be resolvable using conventional Fourier imaging techniques.
Time domain (TD) antenna measurements have been successfully implemented in far field ranges [1,2]. The short acquisition times and the wide-band nature of the measurements make this regime a potential alternative to classical frequency domain measurements. Due to the measurement versatility offered by compact ranges, the implementation of TD measurements becomes especially attractive. This paper presents the modelling of the compact range performance in TD. In addition, a statistical evaluation criterion to assess the quiet zone quality is formulated. The results obtained show that this type of measurements can be successfully implemented in compact ranges.
The growing need of ultra-wide band measurements has promoted the research on real time domain (TD) antenna measurements. Theory has been already established, but practices still under development until the measurement regime becomes fully operational. In the Delft University Chamber for Antenna Measurements (DUCAT) there have been already provided outstanding results in a TD far-field configuration. A TD far field model of the facility has been developed in order to provide a key to improve the range performance and accuracy. This paper presents the model and considerations for establishing TD error correction techniques.
Compact antenna test ranges intended for low cross polarization antenna measurements require the use of feeds with polarization ratios typically greater than 40 dB across the included angle of the quiet zone as well as across the frequency band of interest. The design for a series of circular corrugated aperture feeds to meet these requirements is presented. The feeds are based on a circular waveguide OMT covering a full waveguide frequency band with interchangeable corrugated apertures to cover three sub-bands. In order to validate the design of this series of scalar feeds, high accuracy cross-polarization data was collected. The primary limiting factor in the measurement of the polarization ratio was the finite polarization ratio of the source antennas. A technique for correcting for the polarization ratio of the source is presented along with measured data on the feeds. The technique begins with the accurate characterization of the linear polarization ratio of the standard gain horns using a three antenna technique, followed by pattern measurements of the feeds, and ends with the removal of the polarization error due to the source antenna from the measured data. Measured data on these feeds is presented before and after data correction along with data predicted using the CHAMP® moment method software.
MI Technologies has developed a technique to achieve very high accuracy cross-polarization measurements using a single reflector compact range. The technique, known as the "Error Correction Code Algorithm" (ECCA) leverages the "ideal" performance of a single parabolic reflector when the feed axis is aligned to the parabola axis. ECCA mathematically corrects for the amplitude taper induced by the feed axis alignment. Historically, 'conventional' compact range polarization purity has been limited to »-30 dBi. The ECCA technique, however, lowers the cross-polarization error to »-48 dBi. This performance has been verified in two separate inter-range measurement comparisons with the National Institute of Standards and Technology. The results of these tests prove ECCA is an extremely accurate technique for low cross polarization measurements and provides a lower cost, superior performance alternative to dual reflector systems when low cross-polarization measurements are required.
The DSS/DASA Compensated Compact Range CCR75/60 is a very accurate measurement facility to measure the RF antenna performance. A special feature of this compact range is the creation of scanned quiet zones with defocused feeds, which allow accurate antenna measurements with tilted plane waves. Even there is a straight forward and accurate method to determine the tilting angle or boresight. The boresight of the facility can be derived from the geometrical configuration using optical references. The agreement between geometrically and RF determined boresight is shown in this paper. The geometrical boresight has been measured optically using only geometrical references. The tilting angle for the scanned quiet zones has been predicted by the antenna design program GRASP. The RF boresight has been determined by plane wave probing.
Modelling of the field in the quiet zone (QZ) of a compact range is a difficult task since the edges of the range reflectors are designed not to radiate into the quiet zone. As a consequence the edges of the range reflectors are complicated to model electromagnetically. In the present paper we compare modelling by physical optics (PO) with modelling by the uniform geometrical theory of diffraction (GTD). The investigation is carried out on a double reflector range with rectangular serrated reflectors. It is found that the range far field determined by PO is best explained by a ray model for reflectors having a double straight edge, which suggests to apply a GTD model on reflectors with straight edges but with attenuated diffraction contribution. Both PO and GTD results are shown and compared to measurements.