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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.
The design of a specialized reflector antenna set that supports dual polarization, dual beam widths, and an integrated wideband monopulse tracking capability in the X-band range is described in this paper. The reflector antenna code available at The Ohio State University has been used as the design tool. The design of such an antenna has posed several challenges in the feed and reflector assemblies. The requirement for an integrated wideband monopulse has resulted in a feed array that contains 5 rectangular feed elements with a center-to-center spacing of 1" and a diamond configuration. The 5 feed design has been selected to enable a shared feed array and reflector surface for both transmit and receive beams that eliminates the need for a high-power wideband receiver protector in the radar system. The center feed element is used for transmit waveform and the 4 outer elements are used as receive elements only. Each feed element operates with horizontally and vertically polarized waveforms, requiring a total of 8 waveguide input ports. In this paper, the challenges of the dual beam widths, dual polarized, integrated RCS and tracking antenna are delineated and the tradeoffs among several design configurations are shown. The final design is selected based on the performance predictions using The Ohio State University Reflector Antenna Code. The performance of the antenna has been validated at the OSU compact range for pattern and gain. Both the design and measurement data are presented in this paper.
Instruments for Earth observation working from W-Band up to mm-wave frequencies mainly use quasi-optical feed feeds (QOF) to illuminate the corresponding reflector antennas. The final design of the QOF for the Cloud Profiling Radar System (CPR) for the EarthCARE satellite has been finalized. The QOF is designed to perform polarization and frequency tuning, as well as the separation of transmit and receive channels. The final design verification was performed theoretically by Astrium with QUAST, a new add-on to the GRASP software, especially developed by TICRA for a fast and accurate set-up and analysis of quasi-optical networks. Within the paper, at first a short description of the QOF working at 94.05 GHz will be given. Secondly, the modeling of the QOF will be explained. At last the RF test setup will be described and comparisons between resulting calculated and measured antenna pattern will be shown.
Anechoic chambers simulate open air test conditions and are advantageous for testing avionics systems in a secure, quiet electromagnetic environment. The 412th Electronic Warfare Group’s Benefield Anechoic Facility (BAF), located at Edwards AFB, California was designed for testing systems in the radio frequency (RF) range from 500 MHz to 18 GHz. For frequencies below 500 MHz, the installed radar absorbent material (RAM) does not effectively absorb incident RF energy, thereby allowing undesired RF scattering off the chamber’s floor, ceiling, and walls. This leads directly to measurement inaccuracies and uncertainty in test data, which must be quantified for error analysis purposes. In order to meet the desired measurement accuracy goals of antenna pattern and isolation measurement tests below 500 MHz, RF scattering must be mitigated. BAF personnel developed a test methodology based on hardware gating, range tuning and improved RAM setup, to improve chamber measurement performance down to 100 MHz. Characterizing the chamber using this methodology is essential to understanding test zone performance and thus increases confidence in the data. This paper describes the test methodology used and how the test zone was characterized with resulting data.
The rise in the number of active antennas used in radar applications calls for changes to the common absorber treatment used in chambers. The electronic circuits that are imbedded into these scanning arrays are non-linear in nature so they must be tested at the correct power outputs to get the correct pattern behavior. The combination of higher power and narrow beams means that areas of the anechoic treatment in a chamber can be subjected to high power densities. High power absorber has been used in the industry for many years. The substrate used in these absorbers makes the material very expensive. While in the past it was common to use this material only in regions where the main beam was going to be illuminating the absorber treatment the new electronically scanned arrays will have main beams that can illuminate several areas of the chamber. In cases where Near Field systems are used the absorber material will be in a radiation region where the main beam has not been formed, but the Absorber is closer to an array that is radiating high power so a large area of higher power absorber is needed to treat the chamber. In the present paper the authors present a medium power absorber 3kW per square meter (versus 775w per square meter) using the same material used in common RF absorber. Mechanical changes to the absorber are performed to increase the thermal dissipation of the EM energy absorbed. A series of measurement of the absorber is performed with a without additional air flow for cooling. The result is an absorber that can handle higher power densities without the need for exotic substrates.
This is the second of a two-part paper discussing the NASA Debris Radar (NDR) system developed to characterize debris liberated by the space shuttle during its ascent into space. While initial NDR missions proved the extent of the debris detection and tracking challenge, improvements in NDR hardware, software, and mission operations resulted in very successful debris detection and tracking. These successes lead to a new challenge of processing and analyzing the large amount of radar data collected by the NDR systems and extracting information useful to the NASA debris community. Analysis tools and software codes were developed to visualize the shuttle metric data in real-time, visualize metric and signature data during post-mission analysis, automatically detect and characterize debris tracks in signature data, determine ballistic numbers for detected debris objects, and assess material type, size, release location and threat to the orbiter based on radar scattering and ballistic properties of the debris.
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.
Polarimetric radar cross section systems are characterized by polarimetric system parameters Eh and Ev. These parameters can be measured with the use of rotating dihedrals. The full polarimetric dataset as a function of the angle of rotation can be analyzed with a nonlinear set of calibration equations to yield the system-parameter complex constants and the four polarimetric calibration amplitudes. These amplitudes appropriately reproduce the system drift and satisfy a drift-free system con.guration criterion very accurately. The results indicate that the nonlinear approach is better than the previously studied linear approach, which yielded system parameters that are seriously distorted by drift.
This contribution deals with guided radar distance measurements in the .eld of industrial tank level control. The aim is to achieve a submillimeter gauging accuracy even when conducting the measurement within thehighlydispersive environment of large and thus overmoded cylindrical waveguides. In this case normally multimode propagation causes a decrease in measurement precision. Therefore, the effects of intermodal dispersion are fundamentally reviewed and based on these results, two different approaches for overcoming the drawbacks of this measurement scenario are derived. On the one hand a prototype of a novel concept for compact mode-preserving waveguide transitions is presented, ef.ciently avoiding the excitation of higher order modes. By applying this concept, free-space optimized signalprocessing algorithms canbe used advantageously. On the other hand, an alternative correlation-based signal processing method is presented. The method is able to exploit the otherwise parasitic dispersion effects to enhance the measurement precision even in constellation with a simple waveguide transition. Finally, the trade-off between the signal processing’s and waveguide transition’s complexity is highlighted and discussed. Measurement results in a frequency range of 8.5 to 10.5 GHz are provided for different kinds of waveguide transitions proving the capability of both approaches.
Cable ringing is a concern in all short range Radar Cross Section (RCS) measurements. The standard method to reduce the RCS return from cable ringing is to minimize the cable length and add additional attenuation at either end of the cable. For VHF to L-band measurements, where the overall longer system ringdown times from both the antenna and cable can dominant the measurement background, this paper demonstrates another solution to eliminate the cable ringdown from the target measurement area for short range RCS measurements. This paper describes how using a cable length that is at least the same physical length of the range you want to measure can eliminate the cable ring down from the target measurement area. The cable length, which has the same physical length as the measurement range, provides a clean measurement target area with an additional margin depending on the group velocity of the cable used. Since the cable loss can be minimized with lower loss coaxial cables from VHF and L-band frequencies, using a longer single cable is a very viable solution to eliminate cable ring down from the target measurement area.
In a search and rescue effort following a natural disaster, rubble and debris within the search environment can obscure human victims. A digital noise radar operating at UHF can penetrate the types of materials typically found in these situations with relatively little loss. This paper compares and contrasts measured correlation values of human and non-human objects taken from a digital noise radar. A noise radar works by cross correlating the received signal with a replica of the transmit signal. A high correlation indicates range to the target. Measured results with the noise radar show that patterns of peaks and valleys exist near the range of the targeted object. This paper looks at the correlation patterns generated by two different sized hollow metal tubes and a human at various look angles. The results show that the correlation patterns of the tubes are similar, but the correlation patterns of the human differ. Full characterization of human and non-human noise radar correlation patterns could confirm the presence of a human victim and information about his location within the rubble.
The models governing ground-bounce illumination have been well-understood for decades. Previous ref-erenced work examined the application of these mod-els to three-dimensional geometries and showed that illumination variation is a spatially-dependent func-tion of frequency which requires knowledge of the target geometry and scattering characteristics in order to evaluate uncertainty. This paper leverages that work and further develops a Short-Time Fourier Transform (STFT) method to extract frequency do-main scattering characteristics from target data di-rectly. By utilizing at least two antenna heights in two separate measurements, ground-bounce geometries allow for useful target feature characterization. This enables an assessment of spatial target uncertainty in the image domain as well as for RCS; it further shows promise for illumination error mitigation. This paper demonstrates this potential in simulation to support measurements collected at the National RCS Test Fa-cility (NRTF), Holloman AFB NM.
This paper reports the recent investigation of data-link strings as supporting structures for antennas used in a bistatic radar cross section (RCS) measurement system. Although several candidate strings exist, analysis of the strings’ scattering contribution needs a generic string model to make comprehensive comparisons. A simple theoretical two-dimensional (2D) dielectric coated cylindrical wire model is initially utilized to predict and compare scattering characteristics of various data-link string structures. In addition to the simple model, a non-destructive measurement method is proposed for extracting the material properties of the string material. Using the analytic 2D model of a dielectric clad wire as the generic string model, the unknown permittivity is computed from reflectivity measurements taken with a focused beam system. Extracted permittivity values are then used in a full-wave electromagnetic solver to validate the model. Measured and simulated results are shown to have excellent agreement for the 2D RCS, radar echo width, of different strings and polarization configurations.
The Noise Radar (actually an ultra-wide band (UWB) spread spectrum radar) is a radar that generates a very wide band pseudo-random waveform and (optionally) up-converts the waveform to a desired microwave frequency spectrum. The bandwidth may be more than 5 GHz. The digital system generates a pair of GHz bandwidth pseudo-random waveforms. The two waveforms may either be identical or the pair of waveforms can be specially designed to be matched to the radar target and its environment. The first waveform is transmitted without a carrier, or it may be up-converted to a high microwave frequency. On receive; the second waveform is cross correlated with the received signals. Specific design of the two waveforms is possible so that the cross correlation coefficient forms an optimized peak for a particular target or class of targets, or to maximize the difference in the response between clutter and targets of interest. A design where a multi-GHz waveform is generated using a FIFO chip and a serializer chip will be developed. Construction of this radar and sample data will be shown. Detection range versus Doppler images will be presented.
We used a set of dihedrals to perform polarimetric calibrations on an indoor RCS measurement range. We obtain simultaneously hh, hv, vh, and vv polarimetric data as the calibration dihedrals rotate about the line-of-sight to the radar. We applied Fourier analysis to the data to determine the polarimetric system parameters, which are expected to be very small. We also obtained polarimetric measurements on two cylinders to verify the accuracy of the system parameters. We developed simple criteria to assess the data consistency over the very large dynamic range demanded by the dihedrals. We examined data contamination by system drift, dynamic range nonlinearities, and the presence of background and noise. We propose improved measurement procedures to enhance consistency between the dihedral and cylinder measurements and to minimize the uncertainty in the polarimetric system parameters. The final recommened procedure can be used to calibrate polarimetrically both indoor and outdoor ranges.
At the present time at the end of a measurement of RCS of a target, it is possible to obtain either the value of the RCS of the target as a whole for a given frequency, bearing and elevation or a RADAR image of this target. The aim of this RADAR image is generally to locate the bright points that constitute the target and not to estimate the energy of these bright points. That is why the calculation of these energies is not generally the subject of an elaborate rigorous processing. Yet it may be necessary to be able to give the RCS of any part of the target when this target has been measured as a whole. In answer to this need it is necessary to isolate and calculate sharply the energies of the bright points that constitute the target, because the RCS of each part of the target is the sum of the energies of the bright points which constitute it. This article exposes a method of processing which allows this calculation, while resolving the problems linked to the interpolation and to the discrete nature of the measurements and calculations.
This paper describes or deals with a quality analysis and comparison of three radar reflectivity information or data types. The information or data types include radar cross section (RCS) as defined by IEEE Standard 100, the bowtie sector average, and the gross estimate radar return (commonly known as the fuzzball). The paper discusses the uncertainty analysis of measured RCS, and the paper provides analysis on the uncertainty of bowtie sector averages and “fuzzballs” (gross estimate radar returns). The comparison of the information or data types, their quality, uncertainties, and usefulness represents a significant part and focus of the study.
Practical ISAR measurements must often be made in the near-field. Scatterers are illuminated by a spherical wavefront, generating a continuum of incident angles due to parallax. Ignoring this, radar image processing produces geometrically distorted images whose utility diminishes the more deeply into the near-field the measurements are made. The underlying assumption that a target may be accurately modeled as a collection of isotropic point scatterers can enormously widen in angle. Yet, by considering parallax (with attention to phase), near-field measurements can produce quasi-far-field images, whose Fourier transform bears a greater likeness to a far-field RCS signature. A technique is presented and explored whereby each image pixel is focused at angles normal to the incident spherical wavefront by compensating for parallax. The focused coordinates are spatially variant, but for a pixel exactly containing a point scatterer, the resulting focused IQ pairs are identical with those in the far-field.
Harmonic radar has recently been shown useful for remote state sensing in high clutter environments. This new application of harmonic radar with chemically sensitive "tags" allows long-range state sensing of individual low-cost passive (battery-less) sensors, such as corrosion indicators for industrial storage tanks. The "tag" response is sensed at the second harmonic of the radar transmitter, eliminating clutter from undesired objects. A new miniaturized planar harmonic radar tag design has been developed from a low-cost switching diode and low-cost laminate, without the use of shorting vias. An 85% cost reduction over the previous tag design has been achieved while maintaining similar performance. Data are presented from field testing and the laboratory environment comparing the new tag design to the old tag design as well as a basic wire dipole.
The National Radar Cross Section Measurement Facilities Certification Program seeks to raise collectively the quality bar across the community. A program to accomplish this goal was initiated in 1995. It continues with facilities joining the program every year. The program has now entered the recertification phase for facilities that achieved certification five or more years ago. This paper will briefly cover the history of the program, the participants, the certification process and criteria, the recertification process, status, and the way ahead.