AMTA Paper Archive
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Analysis of Multiple-Pulse-in-the-Air Strategies to Mitigate Windmill Clutter on an RCS Measurement Radar Range
The installation of power-generating wind turbines near outdoor radar cross-section measurement ranges for low-observable targets presents a number of problems for accurate measurements on those ranges. The turbines, with towers up to 100 meters tall and blades 45 meters long, are enormous scatterers that raise clutter levels well above returns from LO targets. The movement of the rotors, rotating about both a horizontal and vertical axis, generates a dynamic and unpredictable Doppler smear that cannot be mitigated with phase coding techniques or Doppler filtering. Lastly, the large wind farms over which the turbines are installed lowers the unambiguous-return PRF to extremely low levels, raising test times and dropping rotation speeds below acceptable levels. A method of varying pulse spacing into a burst mode is presented that maintains data throughput in RCS collection while avoiding the clutter returns of downrange wind turbines. This burst mode has the radar transmit and receive a string of closely-spaced pulses, and then idle while that string of pulses propagates through the wind farm. By careful selection of timing parameters, clean-range clutter levels can be maintained with little to no degradation in test time.
Near-field Antenna Measurement of an Active Phased Array Antenna for a New-Generation Weather Radar
We are developing a new-generation weather radar to observe and predict short-term weather phenomena like severe storms, gust and so on. Therefore, an active phased array antenna (APAA) with digital beam-forming (DBF) receivers could be used for the new-generation weather radar to reduce the observation time. In Toshiba Corporation, 33m x 16m vertical near-field antenna range including the digital instrumentation receivers have been working for multi-beam DBF antenna measurements. This near-field antenna range is used to evaluate the performance of an APAA. In this paper, we describe the characteristics of this new-generation weather radar and the APAA. And we demonstrate the antenna measurement set-up using the near-field antenna range and the measurement results of this antenna.
Efficient Method for Representing Antenna Pattern Illumination in Method of Moments (MoM) Radar Cross-Section (RCS) Predictions
The use of computational electromagnetics (CEM) prediction codes in the analysis and interpretation of RCS measurements has become increasingly prevalent. This is in large part due to rapid advances in computing capability over the last several years, particularly for rigorous techniques such as the method of moments (MoM). In many instances, however, these codes are still limited to plane waves and/or elementary dipoles as the sources of target illumination. Modeling of the illumination from an arbitrary antenna therefore requires meshing and solution of the combined antenna-target geometry for each frequency and aspect angle, with an associated increase in the computational complexity of the problem, even if the interactions between the antenna and the target are negligible. In this paper, we describe a method by which measurements or predictions of the antenna pattern are used to develop an equivalent representation of the antenna in terms of an array of non-interacting elementary dipole current sources in a MoM code that uses RWG basis functions. The representation can then be used to efficiently derive the antenna’s illumination on the target as a function of frequency and aspect angle with only a minor increase in the computational burden relative to plane wave illumination. Results are presented using antenna pattern predictions for an ETS-Lindgren 3164-01quad-ridged VHF antenna which illustrate the accuracy and efficiency of the technique.
Multiplexed Pulsed Transmit and Receive RF Measurement System for Active Phased Array Testing
Radar antennas are typically required to operate in transmit and receive modes, and may or may not support both CW and pulsed signal operation. In active antenna applications, these modes may require different operating parameters, which currently dictate testing the antenna independently in transmit and receive using different test system configurations. In testing highly-integrated active arrays, electrical and thermal considerations make it preferable to test the antenna in its nominal Tx/Rx (Transmit/Receive) operating mode as opposed to transmit-only or receive-only. An extension to the NSI Panther 9100 RF measurement system has been developed to support multiplexed transmit and receive, pulse-mode measurements with different measurement parameters during the course of a single data acquisition. This capability allows pulsed transmit and receive tests to be interleaved using a single measurement setup, reducing overall test time and improving the real-world accuracy of the test results.
Measurement of Vegetation to Characterize its Volume Backscatter and Attenuation
A method is presented to accurately characterize the backscatter and attenuation properties of vegetation using high resolution measurements with the vegetation placed on a turntable. By this method we obtain a controlled scenario of realistic vegetation. To obtain high cross range resolution, 2D-ISAR technique was used. The full obtainable resolution is then defined by the registered bandwidth (2 GHz) and aspect angle width. 2D-ISAR images were produced from which areas of interest were gated out where the vegetation backscatter coefficient was calculated. This, along with antenna tapering compensation and distance compensation allowed us to accurately normalize the vegetation backscatter coefficient. The received signal power was made independent of range and system parameters by calibration. Hence the received power signal can be written to be only dependent on the backscattering radar cross sections. The resulting values of the volume backscattering and extinction parameters are presented for reeds and birch vegetation at HH and VV polarization.
Achieving the Desired Performance from a Radar Cross Section Pylon Rotator
The desire to acquire Radar Cross Section (RCS) data on full scale models poses a number of challenges to the users of pylon / rotator systems. Typically, these full scale models have significant mass but have a relatively small foot print on which it is acceptable to mount the model to the rotational flange. The challenges to be addressed in this paper include designing a rotator that will have sufficient strength to support the weight of the model and the stress generated by the overturning moment. This rotator must have a sufficiently low profile and small volume so that it will conveniently fit within the model volume but still achieve a sufficient elevation travel to meet test objectives. This rotator must still properly close out the pylon at all elevation angles to prevent unwanted reflections. Additional design considerations include the test conditions and the test environment. A rigorous test requirement can demand special engineering features to mitigate the demands of relatively high scan speeds and extended run times. Environmental concerns including wind loads, temperature, humidity, and contaminants, must be factored into the design of modern RCS rotators. This paper presents the system design approach to address the requirements of a full scale model rotator. The paper examines consequences of selected potential design solutions and demonstrates the importance of performing trade studies.
Reduced Cost, Automated, RTCA-DO-213 Compliant – Commercial Radome Test System
The commercial aviation industry faces several issues in regard to servicing and maintaining the radomes that abound in the aircraft fleet flying today. The first issue is the historically high cost of radome test systems. As a result of this, there are limited numbers of test systems in operation today and some geographic regions have insufficient radome test capacity. Advances in weather radar and increased reliance on them for turbulence avoidance and more efficient route planning around storm systems will increase the importance of ensuring that weather radar systems are performing well and consequently that weather radar radomes are in good condition and have been adequately tested. Because of the potential consequence of flying with a bad radome and the demands of new radar systems, its more important than ever to ensure test systems in use adhere to requirements and to spread awareness of these challenges within the aviation community. Recently, a design effort was conducted specifically geared towards developing a system concept for radome testing that would both provide a robust test capability that fully meets the RTCA-DO-213 after repair test requirements and one that is much lower in cost than traditional systems that are fielded today. This paper describes the issues cited above and provides a description of the low cost -compliant solution
Evaluating the Time Domain Performance of Spiral Antennas Using Near Field Measurements
Ultra wideband (UWB) systems use short pulses in order to achieve high data rate wireless communications and/or radar resolution. Thus, UWB antennas should be designed carefully, both in time and frequency domains, with the system performance in mind. Time domain characterization of an antenna can be performed first by measuring the frequency domain transfer function of a direct link consisting of two identical antennas. Then, the time domain response is obtained by post processing the frequency domain data using the Inverse Fast Fourier Transform (IFFT). This paper discusses frequency and time domain performance of four-arm equiangular and Archimedean spiral antennas operating in mode 2. The frequency domain transfer function is synthesized using complex far field information measured in a spherical near-field chamber from 2GHz to 12GHz. The synthesized approach is validated using simulation and direct link measurements. The quality of radiated pulses is evaluated in terms of fidelity factor over a full field of view, a task not trivial for the direct link measurements.
A 240 GHZ Polarimetric Compact Range for Scale Model RCS Measurements
A fully-polarimetric compact radar range operating at 240 GHz has been developed for obtaining Ku-band RCS measurements on 1:16th scale model targets. The transceiver consists of dual fast-switching, stepped, CW, X-band synthesizers driving dual X24 transmit multiplier chains and dual X24 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 45 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 27” FWHM quiet zone. In this paper a description of this 240 GHz compact range is provided along with an ISAR measurement example.
Multi-Parametric Antenna Test Visualization for Optimization
There are a large number of antennas needed for modern automotive communications systems (AM, FM, FM diversity, TV diversity, remote keyless entry/start, cellular, Bluetooth, automatic toll systems, smart highway information systems, GPS and GPS information systems (traffic information), and radar systems (backup, side impact, lane departure, intelligent cruise control)). Manufacturers are looking for ways to reduce the total number of such antennas by using combinations of a smaller number of antennas. This paper will discuss a software approach that permits the engineer to visualize the antenna performance effects of variations in geometry for a group of antennas based on either test data or simulation. The antennas are typically defined as a set of printed electrically conductive lines on vehicle windows. The number and the location of the lines are parametrically varied and the effect on the RF performance of the total combined received signals for various sets of test data are presented in multi-parametric visualizations. This paper will discuss the development of a tool for multi-antenna testing that can be used by the antenna application engineer to develop an optimized multi-antenna design. Both experimental test data and theoretical simulation data can be used and compared. Examples will be presented.
Calibration of the Mini-RF Synthetic Aperture Radar System
The Mini-RF instrument on NASA’s Lunar Reconnaissance Orbiter is gathering data toward its science goal of probing the permanently shadowed terrain for the presence of water near the lunar poles. The circular polarization ratio is the central parameter used to characterize the lunar surface using Mini-RF radar returns. Accurate use of this parameter requires on-orbit polarimetric characterization of the instrument. This is done with a combination of measurements. Indirect measurements are performed by pointing the radar at the lunar surface to remove any asymmetry in the surface backscatter. Direct characterization of transmit and receive portions of the Mini-RF radar are performed using Earth-based resources. The Earth-based resources include the: Arecibo Radio Telescope, Green Bank Telescope, and Morehead State University Space Tracking Antenna. This paper describes the measurement approach and a sample of results. The primary measurements of interest include: principal plane antenna patterns, transmit polarization ellipse, receiver amplitude and phase balance, and antenna bore sight direction. These measurement values are incorporated into the Mini-RF SAR data processing to produce quality, calibrated CPR measurements
Analytical Formulation And Problematic Of The Interpolation In The RADAR Imagery
From measurements of RCS of a target as a function of the frequencies and the bearings, it is possible to make RADAR imagery. A common way is to use a bi-dimensional Fast Fourier Transform (FFT2) while this algorithm being very fast. Yet this algorithm demands that the grid on which the RCS is known fulfils some particular conditions. Now such conditions are not respected by the grid of measurement. Consequently an interpolation of this grid is necessary in order to be able to apply the FFT2 algorithm. The choice of the method of interpolation will directly impact the quality of the calculated RADAR image. In this article we propose to study this impact while giving the analytical expression of the interpolation then while giving the analytical expression of the RADAR image calculated from the interpolated RCS and while specifying eventually the method interpolation which limits the degradation of quality of the calculated RADAR image.
Study of a Near Field RCS Imaging System Based on a MIMO Array
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.
Antenna Pattern Measurement of Space-borne W-band Doppler Radar
The cloud profiling radar (CPR) for the Earth, clouds, aerosols and radiation explorer (EarthCARE) mission has been jointly developed by JAXA and NICT in Japan. The development of CPR has required several technical challenges from the aspects of hardware designing, manufacturing and testing, because very large antenna reflector of 2.5m diameter with high surface accuracy, high pointing accuracy and high thermal stability had been required to realize the first space-borne W-band Doppler radar. In order to verify the RF design, we have just begun to perform antenna pattern measurement by using a CPR Engineering Model (EM). For this RF testing, we introduced a Near-Field Measurement (NFM) system with necessary capabilities for high accuracy measurement. This paper will present the summary of preliminary test results of the CPR EM antenna and the other technical efforts being taken for the antenna pattern measurement.
Validation of Measured RCS by Computational Electromagnetics Techniques
Computational Electromagnetics (CEM) Techniques have found wide use in scattering analysis of structures due to the fact that they require less cost and time than doing physical measurements. Numerical methods both in the time and frequency domain such as the Finite Integration Technique (FIT) , Method of Moments (MoM) , Multilevel Fast Multipole Method (MLFMM) , Transmission Line Method (TLM)  and Finite Element Method (FEM), have been known to provide accurate results for Bi-static as well as Mono-static Radar Cross Section (RCS) analysis in general but their practical applicability to specific types of structures is frequently misunderstood thus leading to mistrust in the results obtained. A result comparison between the different techniques is typically the best way of gaining trust in the results obtained, however this involves the general principle of result convergence which must be achieved for each individual solution technique. Using one of the standard benchmark radar targets which is the Cone-sphere , a comprehensive description of how to achieve result convergence for each technique will be presented and the final results will be shown to agree with published measured results [7, 8]. This target will be used in different configurations (with and without a slot) as well as coated with Radar Absorbent Material (RAM).
Wideband Dual Beamwidth, Dual Polarized, Integrated Monopulse and RCS Antenna for Dynamic Radar Signature, Measurements and Tracking
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.
Design of a Wideband Dual Beam Width, Dual Polarized, Integrated RDC Measurement and Monopulse Tracing Antenna
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.
Final RF Testing of the 94 GHz Quasi-Optical Feed for EarthCARE's Cloud Profiling Radar
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.
IMPROVEMENT IN LOW FREQUENCY TEST ZONE PERFORMANCE IN THE BENEFIELD ANECHOIC FACILITY
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.
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