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In order to assess the suitability of long thin metal rods as calibration devices for both co-polarized and cross-polarized (abbreviated as co-pol and x-pol) RCS measurements, we study RCS data from rods at broadside and compare them with 2D theoretical predictions. We find that the 45° tilt angle is optimum for calibration purposes. Near grazing incidence to a horizontal rod, the first traveling wave lobe in the HH pattern is a very prominent feature. Its angular location and amplitude have been measured as a function of frequency and compared with theory. A formerly unexplained error due to a contaminated calibration is identified.
The accurate measurement of Radar Cross Section (RCS) requires precise calibration "artifacts" as well as carefully executed measurement procedures. The Air Force Research Laboratory (AFRL) reviewed several existing common RCS calibration artifact standards and practices, and identified a number of improvements. Employing a modified "dual calibration" check procedure pioneered by AFRL, this paper demonstrates improved RCS calibration fidelity for a wide variety of static RCS calibration measurement applications. Our calibration results are verified through an industrial inter-laboratory (range) measurement program employing selected calibration artifact standards.
The Georgia Tech Research Institute (GTRI) under contract to the U.S. Air Force 46 Test Group, National Radar Cross Section Test Facility (NRTF) at Holloman AFB, NM, has designed and developed an Automated Field Probe System (AFPS). The AFPS operates as a one-way probe for evaluation of the electromagnetic field at the test zone and provides a mobile capability to rapidly, smoothly, and accurately probe the field at the various test-areas. The AFPS provides the ability to probe over an area as large as 40-ft x 40-ft all under computer control from the radar(s) while sweeping over 1-18 GHz and 34-36 GHz for both H and V polarization. The RF, phase reference, and control signals from the radar are transmitted to the AFPS over a microwave fiber optic link. This paper will describe the design and performance of the AFPS. Quick-look data products will be included in the presentation.
The purpose of this project was to determine the effects of fast neutron bombardment on the radar cross section of metal and dielectric spheres. The energetic neutrons interact with lattice atoms and, in the energy transfer that results, initiate a displacement cascade that effectiveiy damages the crystalline structure of the target material. The induced damage may change the RCS of the target via changes in the conductivity or relative permittivity. Theoretical lattice damage estimates are provided for fast neutron fluences of 1015 n/cm2 and 1016n/cm2. Limitations and potential improvement of damage estimates and measurements are also discussed.
To develop standards for radar cross section measurements a complete uncertainty analysis is needed. We derive the radar cross section error equation and examine sources of measurement errors that contribute to the overall uncertainty in calibrations and measurements. We obtain expressions for upper- and lower-bound errors and uncertainties that are generally valid for monostatic measurements on any unknown target using any standard calibration artifact. The general procedure can be extended to bistatic measurements. Some experimental procedures to determine the uncertainty due to background subtraction are presented and discussed.
There is a growing interest in generating radar images as data collection is in progress. Such a tool is particularly useful for radar cross section verification purposes where the turnaround time is very important. With the availability of faster processing hardware, real-time radar image formation is now feasible. This paper describes the architecture, operation, and performance of a real time imaging (RTI) system that generates SAR or ISAR images while the data collection is in progress. Real-time performance of the system is benchmarked in terms of image-size and quality (imaging technique), image update rate, and image latency. Several examples of RTI are provided using a Lintek elan radar system.
The interferometric measurements for the structure change detection of a dam due to water level change and to seasonal temperature variation is presented. The instrument used is the Linear SAR (LISA) of the European Microwave Signature Laboratory, which allows two synthetic apertures, one linear of 5 meters length and another circular of about 2 meters. The microwave instrumentation, based on a vector network analyzer and on a pair of wide-band antenna, allows a dual polarized measurement in a frequency band, ranging from 500 MHz to 6 GHz. In this particular context, fully polarimetric measurements have been performed in the frequency band from 5.2 to 6 GHz. From the selected measurements parameters a spatial resolution on the structure of about 30 by 30-cm is achieved. Measurements have been repeated at 7 different dates in the period from June to September. From the set of obtained images a large number of differential interferograms was been formed corresponding to different deformation conditions of the barrage. Results showing the deformation pattern, clearly visible on the whole imaged portion of the structure, are presented. The comparison between measured displacements by D-InSAR and those from the barrage monitoring system in the selected points where traditional tools are installed are in good agreement.
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.
This paper presents the theoretical derivation of the G/F parameter, a simple method for measuring this parameter in a Compact Anechoic Antenna Range, a study case, and the error associated with the G/F approximation as a function of antenna temperature and system noise figure.
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.