AMTA Paper Archive

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.

The calibration and measurement of bistatic reflectivity at short range (3.3 m) presents challenges that are significantly different from the usual test range measurements (typically monostatic at 100 to 150 m). In order to overcome this an object-free calibration procedure has been applied, eliminating crosstalk, reducing other interferences and removing errors associated with the RCS and alignment of calibration objects. It is based on calibrating the transmitter and receiver antennas as a pair by directing the antennas toward each other. The method thus requires that the antennas can be separated. Furthermore the signal level needs to be handled e.g. by the separation distance or attenuators. The bistatic reflectivity measurements were performed by placing a clutter sample on a turntable which is located at the centre of a bistatic arc. This configuration enables us to do ISAR images. Background contributions were discriminated using a combination of synthetic resolution and zero-doppler filtration. The sensitivity variation across the antenna footprint was handled by calculating an equivalent area from measured off-axis antenna sensitivities. Reflectivities have been measured for a metallic test surface and for grass. The metallic test surface had been manufactured to correspond to typical theoretical bistatic clutter models.

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.

Compact ranges are well suited to perform accurate indoor RCS measurements. These facilities are limited at the lower end of their bandwidth by the size of the parabolic reflector. Therefore, when RCS characterizations are required in the UHF band, RCS measurement facilities usually operate large horns or phased array antennas in a near field measurement layout. However, these calibrated near field measurements cannot directly be compared to the plane wave RCS characteristics of the target. One way to compare simulation and measurement results is to take the near field radiation pattern of the antenna into account. This paper first presents the design of a phased array antenna developed for indoor UHF RCS measurements. Then a model of this antenna is derived and a simulation of the experimental layout is performed. In parallel, near field RCS measurements of a canonical target were performed with this phased array antenna in an anechoic chamber. As a conclusion, a comparison between simulation and experimental results on this particular canonical target is discussed.

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) [1], Method of Moments (MoM) [2], Multilevel Fast Multipole Method (MLFMM) [3], Transmission Line Method (TLM) [4] 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 [5], 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).

Traditional ISAR imagery measures a rotating target as a function of frequency and angular orientation - typically azimuth at a fixed elevation, with attitude accurately instrumented. For dynamic measurements, knowledge of angular motion (or attitude in general) may be lagging, insufficient or absent. K-space itself, combined with an image focal quality indicator, provides a unique geometry for separating the multi-dimensional problem of estimating rotational motion coefficients into individual estimations localized to separate regions of k-space. In this paper, a polynomial representation is reformulated to separate the annular width of the subtended aperture from a series of control points which effectively isolate regions of influence over k-space. Angular motion terms are estimated to tune subapertures of k-space such that overall image focal quality is maximized. These posited polynomials are inverted, and the associated RCS data are linearized via bandlimited resampling. Both blind and semi-blind (insufficiently instrumented angular motion) cases are addressed.

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.

The DGA-MI and the CEA/CESTA collaborated to measure and compute the RCS of a specific target. It is an empty PEC cylinder with a rectangular aperture. Resonant frequencies are expected to appear when the frequency of the illumination signal increases. Th

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.

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.

Large size, light weight, broadband convex RF lens was developed to meet far-field requirements for antenna measurements. The Lens was fabricated from low loss, low density meta-materials and has diameter of D=2 m, focusing distance 2.4m and weight of just 50 kg with operational frequency 0.8 to 6 GHz. The lens is able to produce a plane-wave zone with an approximate size of 0.7D, allowing a 2m diameter lens to test antennas up to 1.4m in relatively small anechoic chamber. Another possible application of large size, lightweight RF lens is RCS measurements that include bi-static measurements. Results of quiet zone measurements for different frequencies are presented.

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.

This paper introduces a broadband free space material measurement system in Temasek Laboratories at National University of Singapore (TL@NUS). The system is designed by TL@NUS and ST Aerospace for measuring permittivity, permeability, reflection and transmission properties of electromagnetic materials and structures from 1 to 40 GHz. The measurement system includes a pair of double convex spot-focusing lenses, horn antennas, a network analyzer and two arms that can be moved along a circular arc. The two arcs of the arms allow measurement to be done with different incident angles. Each of the double convex lenses is made from two plano-convex dielectric lenses of 77 cm in diameter. The plano-convex lenses can collimate the field from the source horn into uniform plane wave thus also allowing both mono-static and bi-static electromagnetic scattering measurement to be done in very limited space. The system is housed in an anechoic chamber of dimension 6.7 m (D) × 6.6 m (W) × 3.8 m (H) to reduce unwanted reflections and interference signals from the surroundings. Typical measurement results are presented in this paper for dielectric materials, magnetic materials, frequency selective surfaces, and metamaterials.

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.

We present a method for measuring ultra-high frequency radio-frequency identification (UHF RFID) tag differential RCS that has the potential for being easier and more accurate than current and proposed methods [1-2]. Our method is based on accurately characterizing the reflection states of a modulated load, accounting for transmission losses between the load and an antenna, and using a well-known, low gain antenna. This has the benefit of using a well characterized “golden tag” reference (i.e., repeatability), while being more linear in power response, independent of reader signal, and independent of manufacturer or process changes. Characterizations of the losses in the reference scatterer allow for direct comparisons between tags on different test beds.

Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.

Numerous applications for antenna, radome and RCS measurements require a very accurate positioning capability to properly characterize the product being tested. Testing of weapons (missiles), guidance systems, and satellites, among other applications, require multi-axis position accuracies of a few thousandths of an inch or degree. For global positioning, spherical error volumes can be extremely small having diameters of .002 inches to .005 inches. This paper addresses the issues that must be resolved when highly accurate mechanical positioning is required. Many factors such as thermal stability, axis configuration, bearing runout and mechanical alignment can adversely affect the overall system accuracy. Additionally, when examined from a global positioning system perspective, the accuracy of the entire system is further degraded as the number of axes increases. Successful system implementation requires carefully examining and addressing the most dominant error factors. The paper will cover current tools and techniques available to characterize and correct the contributing errors in order to achieve the highest possible system level accuracy. A recently delivered 4 ft radius SNF arch scanner, which achieved ± .0043° global positioning accuracy, will provide insight into these methods and show how the dominant factors were addressed.

Indoor RCS measurement facilities are usually dedicated to the characterization of only one azimuth cut and one elevation cut of the full spherical RCS target pattern. In order to perform more complete characterizations, a spherical experimental layout has been developed in 2007 at CEA for indoor near field monostatic RCS assessment. This experimental layout was composed of a 4 meters radius motorized rotating arch (horizontal axis) holding the measurement antennas while the target was located on a polystyrene mast mounted on a rotating positioning system (vertical axis). The combination of the two rotation capabilities allowed full 3D near field monostatic RCS characterization. A new study was conducted in 2011 in order to achieve a more accurate positioning of the measurement antenna. The main objective is to enhance the RCS measurement performances, especially the environment subtraction directly related to the positioning repeatability of the measurement antenna. This new mechanical design has therefore been optimized to allow a +/-100° azimuth range with an angular positioning repeatability of less than 1/1000°. To achieve this level of accuracy, several keys design elements were considered: robust mechanical design, position control system… This paper describes the new experimental layout and the results of a positioning accuracy assessment campaign conducted using a laser tracker.

Radar Cross Section (RCS) and Antenna measurement ranges share many common features and are often used for both purposes. Calibration of these dual-purpose ranges is typically done using the substitution method for both RCS and antenna testing, but with separate RCS and antenna standards. RCS standards are typically based on a geometric shape having a well known theoretical value – and corresponding small uncertainty. By contrast, antenna standards typically must be “calibrated” in a separate antenna calibration system to be used as a gain standard, often yielding higher uncertainties. This paper presents an efficient method for transferring an RCS measurement calibration to an antenna measurement range configuration, allowing a range to be used for both purposes with a single calibration. Insight into the best ways to re-configure the instrumentation between RCS and antenna testing is included. Validation measurements from a compact range are included along with an uncertainty analysis of the method.