AMTA Paper Archive
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Characterization of Space Shuttle Ascent Debris Based on Radar Scattering and Ballistic Properties, Part I Evolution of the NASA Debris Radar System
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.
NONLINEAR CALIBRATION OF POLARIMETRIC RADAR CROSS SECTION SYSTEMS
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.
Accurate radar distance measurements in dispersive circular waveguides considering multimode propagation effects
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.
Technique to Remove Cable Ringing From Short Range RCS Measurements.
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.
Noise Radar Correlation Patterns of Human and Non-Human Objects at Various Look Angles
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.
Practical Illumination Uncertainty Assessment in Ground-based Radar Measurements
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.
Two Dimensional Scattering Analysis of Data-Linked Support Strings for Bistatic Measurement Systems
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.
RCS Measurement Facilities Certification Program Update
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.
DIGITAL NOISE RADAR PROTOTYPE DEVELOPMENT
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.
Polarimetric calibration of indoor and outdoor polarimetric radar cross section systems
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.
Sharp extraction of energy of bright points of a target
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.
Quality Analysis and Comparisons of Radar Reflectivity InformationTypes
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.
NEAR-FIELD TO QUASI-FAR-FIELD TRANSFORM THROUGH PARALLAX
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.
Recent Developments in Miniaturized Planar Harmonic Radar Antennas
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.
Approaches for Signature Measurement Uncertainty Analysis
Over the last few years, we have implemented several methodologies pertaining to uncertainty analysis of RF and Optical measurements. These methodologies are currently in use within the radar cross-section, electro-optic/infrared, and material measurement laboratories at the Air Force Research Laboratory. In this paper we discuss from a top level some of the approaches we have implemented, and identify some important issues one needs to address before beginning an uncertainty analysis. We illustrate one such approach as it applies to the estimation of radar cross-section uncertainty.
Equivalences between MIMO and physical/synthetic radar arrays and its implications in the selection of imaging algorithms
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 LOW-POWER, REAL-TIME, S-BAND RADAR IMAGING SYSTEM
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.
Rapid RADAR Test Range Development Using Lean Engineering Techniques Case Study for the Dynamic Advanced Radar Test (DART) Facility
This paper describes how the Dynamic Advanced Radar Test (DART) facility was designed, constructed, integrated and validated within budget in a 12-month time frame using lean engineering techniques. The facility is a world-class Radar seeker test facility. These techniques allowed the DART team to enhance capabilities without adding cost or complexity. The purpose of this paper is to identify a new paradigm in Radio Frequency (RF) range development, whereby all variables are accounted for early in the process, thus preventing and avoiding time consuming and costly mistakes. This process relies on lean engineering Accelerated Improvement Workshops (AIW) and Production, Preparation, Process (3P) workshops to guide the design process. Allowing all stakeholders to be owners through these intense workshops is vital. Additionally since formal evaluation tools and methodologies guide the workshops, improvement opportunities are maximized while minimizing risk.
Active Array Antenna Noise Temperature Measurement
Active phased array antennas are often considered for many applications in radar and communications, particularly in millimeter wavelengths. The ability of active phased array antennas to be reconfigured with different beam shapes and pointing directions makes them attractive to increase the flexibility of the next generation of communications satellites so that they can adapt to the needs of a fast changing communications market. The antenna noise temperature of an active array is an important performance parameter, which is difficult to measure compared to a classical passive antenna. Moreover, for a satellite antenna, which has to be evaluated in an anechoic chamber before integration on the spacecraft, the ability to characterise the noise contribution of the antenna itself independent of the environment noise would be very interesting as it would allow better prediction of the antenna performance when it is deployed in orbit on the spacecraft. The paper describes the results of a study undertaken for the French Space Agency (CNES) to devise a new method for the measurement of the noise temperature of a Ka band active phased array antenna when mounted in a Compact Antenna Test Chamber (CATR). An important objective of the study was to find a method which did not rely on the substitution of the antenna under test with a reference antenna, which is the method often used in practice. The method of measurement of noise was based on digital processing of signal to noise ratio rather than analogue detection of noise level, which improves the measurement precision.
In-situ Measurement of the Antenna Pattern for the Haystack Auxiliary Radar utilizing a Ground Based Recording System
Measurement of the antenna pattern of the Haystack Auxiliary Radar (HAX), an experimental Ku band radar system developed by the Massachusetts Institute of Technology Lincoln Laboratory for deep space experimentation, was recently carried out utilizing a ground based, mobile recording system. The HAX radar system uses a 12.19 m parabolic antenna placed inside of a radome which is located on Millstone Hill in Westford, Massachusetts. The recording system, which includes a Ku-band analog front end and a high-speed digitizer with 500 MHz instantaneous bandwidth and long duration recording capability, was located at the summit of Mt. Wachusett, 36.1 km southwest of HAX. Several azimuth and elevation antenna pattern cuts were acquired by transmitting towards a wide-band ground based recording system placed down range while rotating the HAX antenna. Throughout these pattern measurements the radar was operated in a reduced power pulsed CW mode. Continuous wide-band recordings from the slowly scanned pattern measurements were taken and the data was processed to detect individual pulses, retaining only the portions of the recordings containing detected pulses. Post-processing of the pulsed CW data allowed for measurement of the antenna pattern with a significant dynamic range, characterizing both the mainbeam of this antenna and the far-out sidelobes.
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