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The accurate measurement of static Radar Cross Section (RCS) requires precise calibration. Conventional RCS calibration objects like plates and cylinders are subject to errors associated with their angular alignment. Although cylinders work well under controlled alignment conditions, and have very low targetsupport interaction, these devices may not always suitable for routine outdoor ground-plane RCS measurements. We seek a design which captures the low interaction mechanisms of a cylinder, yet can be easily aligned in the field due to its excellent angular insensitivity. In a sense, this target has the best characteristics of both the cylinder and the sphere. This paper will describe the design of a "hypergeoid", a new calibration device based on a unique body of revolution. Calculations and measurements of some elementary hypergeoids are presented.
The National RCS Test Facility (NRTF) has a variety of unique test capabilities. Looking to further expand our testing options at the Mainsite test facility, the NRTF began developing a pit/pylon and rotator shroud test bed capability that would allow for radar cross section (RCS) measurement of test articles that are physically too small to accept a rotator. To reach the desired background RCS levels, the use of an expanded polystyrene foam column was not a viable option. In order to maintain the integrity of the calibrated system and enable the measurements of test articles with and without rotator bays on the same pit/pylon, a pit/pylon and shroud combination was required. Other important considerations that influenced the viability of a pylon system include cost effective mounting/dismounting of test articles, safety of the test articles and personnel, and the effective determination of backgrounds due to a stable and low observable pylon system. Our primary goal was to design and fabricate an inhouse system that met the needs of potential customers while satisfying our own clutter and background criteria. This paper documents the fabrication of the pylon and rotator shroud test bed. The results of an RCS characterization are also presented demonstrating the system’s ability to meet the desired RCS background goals.
Free space, coherent radar cross section measurements on a moving target trace a circle centered on the origin of the complex (I,Q) plane. Noise introduces only small random variations in the radius of the circle. In real measurement configurations, additional signals are present due to background, clutter, targetmount interaction, instrumentation and the average of the time-dependent system drift. Such signals are important contributors to the uncertainty in radar cross section measurements. These time-independent complex signals will translate the origin of the circle to a complex point (I0,Q0). Such data are then defined by the three parameters (I0,Q0), the center of the circle, and st, the radar cross section of the target. Data obtained when a target is moved relative to its support pylon can be separated into phasedependent and phase-independent components using the techniques of (1) three-parameter numerical optimization, (2) least-median-squares fit, (3) adaptive forward-backward finite-impulse response procedure, and (4) orthogonal distance regression applied to a circle fit. We determine three parameters with known and acceptable uncertainties. However, the contribution of systematic errors due to unwanted in-phase electric signals must still be carefully evaluated.
We present a methodology for the design of foam columns useful for the support of targets during static outdoor radar cross section (RCS) measurements. The methodology uses modal solutions along with genetic algorithms to optimize the design of a homogeneous column with resistive layers that provides minimal scattering over the design bandwidth. The methodology widens the design space, allowing for better design trades between electromagnetic and structural column performance. Results are presented for two representative design cases (broadband and spot-frequency narrowband), and the performance of the optimized column design is shown to be significantly better than that of the baseline foam column. Further design improvements are also suggested, including the use of the Born approximation for non-axisymmetric columns.
The bistatic radar signature of military systems is of interest for various applications including performance evaluation of semi-active missile systems, surveillance systems, and survivability assessment. While bistatic radar cross section (RCS) measurements have been made for high frequencies at several U.S facilities, there has been little reported work in low frequency bistatic RCS measurements. This paper presents the results of recent low frequency coherent bistatic RCS measurements from 210 MHz to 1.99 GHz at bistatic receiver angles of 0°, 35°, 70°, 120° and 145°. These measurements were successfully completed at the Naval Air Systems Command Weapons Division Etcheron Valley Range (EVR), formerly known as Junction Ranch (JR), China Lake, California This paper describes the process and provides results of low frequency bistatic RCS measurements on a hemisphere-capped cylinder target. Comparisons are presented of measured data to predicted results from moment method models of the calibration object and the cylinder target. Methodologies used in optimizing RCS data quality are also provided.
A Wideband Optically Multiplexed Beamformer Architecture (WOMBAt) was developed and characterized at the Crane Naval Surface Warfare Center Active Array Measurement Test Bed (AAMTB) facility. The project included development and integration of the WOMBAt photonic beamformer with the Active Array Measurement Test Vehicle (AAMTV). The AAMTV is a 64-channel transmit-receive (TR) module based phased array beamformer that is integrated with the AAMTB facility 12’x9’ planar near-field scanner. The AAMTV provided phase trimming and a small amount of electrical delay while the WOMBAt provided longer optical delays using commercial-off-the-shelf (COTS) components typically manufactured for the telecommunication industry. By integrating the WOMBAt with the AAMTV, a highly flexible test environment was achieved that included system calibration, multi-frequency scanning, and antenna pattern analysis. This paper presents antenna pattern results showing less than 0.7 dB of amplitude variation over the frequency range from 9 to 10 GHz at each of the measured nominal steering angles. The beamformer was steered to greater than ±69 degrees with an observed beam squint from 9 to 10 GHz of less than 1 degree.
Determination of Scan Element Pattern (SEP) and of Scan Impedance (SI) of wideband arrays is desirable, in addition to patterns and gain. Scan Element Pattern gives array gain versus scan angles and frequency, while Scan Impedance is the impedance versus scan angle and frequency that must be matched. Some organizations have been measuring SEP in transmit mode, with all elements terminated and the center element driven. This procedure gives erroneous results, as the mutual couplings are all passive. The way of properly measuring SEP is to place the array in a gain measurement setup as a receive antenna, so that all elements are terminated and properly excited. The nominal center element is connected to the receiver; the Scan Impedance mismatch is included in SEP. Knowledge of Scan Impedance is important, as it controls the impedance matching possibilities. It is however difficult to measure. Network analyzers (HP8510) measure impedance both (S11 and S22) by transmitting a signal and measuring the reflected signal, thus do not allow operation in a mode with all elements excited. A full feed network can be employed, with the network modified to allow measurement of the current and voltage at the center element. This method is seldom used. Because of the importance of SI, use is often made of waveguide simulators, and simulation codes. The infinite array Floquet unit cell codes must be used with caution as these codes omit edge effects; these may be very important in some types of coupled arrays. A planar array code is used to simulate both transmit (single element excited) SEP, and receive SEP. Data on SEP and SI are presented.
Software GPS receiver development has been undertaken. We are particularly interested in improving the GPS signal-to-noise/interference ratio using a beam forming techniques. The phase relationship among the antenna array elements requires careful calibration. In this study, we will report a phase calibration technique for a 2 by 2 GPS antenna array using both simulated and real GPS signals. This technique is based on the GPS signalprocessing algorithm developed for the software GPS receiver. A four-channel digital data collecting system was used in the experiment. For a simulated GPS signal, the experiment was conducted in an anechoic chamber in which a GPS simulation system was facilitated. For real GPS signals, we conducted the experiment on a rooftop to receive the signal from GPS satellites. The calibration verified the coherent nature of the signals among the elements. The results also allowed the source's direction to be determined.
Many system applications require very high gain antennas with extremely narrow beams that must scan electronically over a limited angular range, which in terms of beamwidths is very wide (several hundreds of beamwidths). Large, array-fed reflectors can provide high gain, large aperture, but their scan capability is limited to a few beamwidths. A conventional array, however, is capable of obtaining wide scan performance but required a very large number of elements making them prohibitively expensive. Here a novel hybrid, reflector-array antenna approach is presented that can yield the performance of the full array yet with much fewer elements and controls. To prove the concept and to verify theoretical findings, a two-dimensional hybrid reflector antenna was designed, fabricated and tested. This paper describes the results of this testing.
Accurate UHF phased array antenna patterns are difficult to achieve due to high level multipath present in the far field measurement test range. Special range geometry’s and source arrangements have been devised over the years to mitigate the measurement errors produced by test range multipath. In this paper we will describe new measurement results achieved using Aperture Synthesis illumination method designed to optimize and control the influence of ground reflections and in turn reduce quietzone amplitude ripple. Measured phased array patterns at 418, 434, 449, and 464 MHz will be shown for a 64- element array.
Two cylindrical phased array antennas were characterized at the NAVSEA Crane.s Active Array Measurement Test Bed (AAMTB) facility. The antennas include the Full Performance Antenna (FPA) and the Ultra Light Antenna (ULA) that are intended for land mobile test sites for the United States Department of Defense. These air breathing, low-cost antennas are candidates for a new communication system. Crane.s role as the program Technical Advisor (TA) includes integration and performance testing at the component level, antenna level, and system level. This paper discusses issues related to the antenna-testing phase including pattern measurements, G-F, and high power safety concerns. The final goal of the integration and testing phase was to verify that the antenna RF performance specifications were met. To this end, conventional cylindrical near-field pattern testing was adequate for many items such as beam width, pointing angle, and side lobe levels. However, two issues required additional effort: G-F measurement and high-power transmit safety concerns. Since the majority of required measurements could be made using the near-field chamber and the antenna required special controllers and prime power sources, it was desirable to make all measurements in the same location. Hence, a new measurement process was required for G-F using a near-field range and the high-power safety concerns needed to be addressed.
Back projection techniques have been used extensively in planar near-field ranges and to a lesser degree in spherical near-field ranges. Recently a back projection technique allowing back projection from spherical near-field data onto a planar surface has been published and implemented. This paper explores this technique further through the presentation of measured data for a large microstrip array antenna. The results demonstrate how the technique can be used to investigate anomalies in the feed structure of the array.
Z-position errors are generally the largest contributor to the uncertainty in sidelobe levels that are measured on a planar near-field range. The position errors result from imperfections in the mechanical rails that guide the motion of the measurement probe and cause it to deviate from an ideal plane. The deviations ä z (x, y) can be measured with precise optical and/or laser alignment tools and this is generally done during installation and maintenance checks to verify the scanner alignment. If the measurements are made to a very small fraction of a wavelength in Z and at intervals in X and Y approximating one half wavelength, the sidelobe uncertainty can be estimated with high confidence and is usually very small. For Z-error maps with lower resolution the resulting error estimates are generally larger or have lower confidence. This paper describes a method for estimating the Zposition error from a series of planar near-field measurements using the antenna under test. Measurements are made on one or more planes close to the antenna and on other planes a few wavelengths farther away. The Z-distance between the close and far planes should be as large as the probe transport will allow. The difference between the holograms calculated from the close and far measurements gives an estimate of the Z-position errors. This approach has the advantage of using the actual AUT and frequency of interest and does not require specialized measurement equipment.
We have measured the amplitude and the phase of the electric field on a planar area very near (about 0.3 wavelengths) to the aperture of a X-band standard gain horn antenna using a photonic sensor and transformed the aperture field distribution to the far field pattern. The measured aperture field distributions and antenna patterns agreed well with those calculated by the method of moments. Comparing the far field patterns by the photonic sensor and the conventional open-ended rectangular waveguide probe reveals that the antenna measurement using the photonic sensor has advantages over the conventional probe.
High precision near field measurement systems in dual polarization have stringent requirements on the probe performance in terms of radiation pattern shape, on-axis and off-axis polarization purity and port-to-port isolation. In general, these specific requirements can be fulfilled using single polarized probes for narrow frequency bands (about 10% relative bandwidth) and using mechanical rotation for polarization diversity. Consequently, several sealed probes and complicated procedures are necessary to cover the operational frequency band of most common antenna applications leading to inefficient and time-consuming measurement procedures. SATIMO has developed high precision wide-band, dual polarized near field probes covering the frequency range from L to Ka-band to overcome this problem. Two different probe technologies have been applied, each particularly well suited for the appropriate low (L to X-band) and high (X to Ka-band) frequency range. The low frequency probe design is based on a compact corrugated horn with capacitive orthogonal excitations. The high frequency probe design consists of an axially symmetric corrugated horn, a square to circular wave-guide transition, and a wide-band, high isolation ortho-mode junction (OMJ) exciting the orthogonal polarizations. Probes in C-band and Ku-band have been delivered to and tested by ALCATEL SPACE INDUSTRIES in their planar near field antenna test range in Toulouse. The C-band probes have operational bandwidths of 25% covering the entire commonly used transmit and receive frequency bands for C-band communication satellites. The Ku-band probes have operational bandwidths of 40% covering the entire commonly used transmit and receive frequency bands for Ku-band communication satellites.
A passive intermodulation (PIM) near-field XY-scanner for the GSM900 frequency band has been earlier constructed to localize distortion sources in antennas and in other open structures. However, the measured intermodulation level has been relatively high, around 90 dBm. The equipment should be able to measure distortion levels down to 115 dBm with an input power of 2x20W, since the noise floor of a GSM900 base station is typically around 110 dBm. The sensitivity is limited either by thermal noise or by residual intermodulation distortion depending on the sensor coupling. Various causes of residual intermodulation distortion in the PIM near-field measurement are considered and evaluated. Sensitivity measurements of the scanner have been carried out on two test devices. With a sensor coupling of 30 dB, sensitivities of 115 dBm and 105 dBm have been achieved with an electric and a magnetic field sensor, respectively.
A filter-based approach to software range gating is presented. Conventional approaches to range gating are widely used and include hard gates applied in the time domain and running average filters applied in the frequency domain. The potential problems with those methods are well understood and involve (1) sideloberelated distortion of the frequency-domain data caused by hard clipping in time and (2) the dual problems that arise from finite-duration smoothing kernels in the frequency domain. Herein, range gating is formulated as a digital filter design problem. We employ a type-II Chebyshev design, which has a maximally flat pass-band and a specified stop-band attenuation. User parameters include constraints on the smoothness of the passband and the width of the gate transition. Edge effects are minimized by filtering symmetrically extended copies of the measured data. The results are illustrated on data acquired by the JHU-APL compact range.
There is a conflict between the requirement of a very low RCS target support system, and the need for high stability and accurate target setting. To meet the ideal of measuring targets in free space, multiple string suspension systems from overhead gantries have been devised. Despite measures to the contrary, it was found air turbulence and mechanical vibration could produce complex perturbations of the target during ISAR imaging. Over the frequency range of interest (1-100GHz), even sub-millimetre disturbances can produce significant and unwanted image artefacts. Model code was written to provide representative parametric dynamic models for the oscillatory motion of the targets. Modelling results over a wide range of motion patterns, acquisition configurations, and radar parameters allows a quantitative assessment of the limitations and validity of ISAR imagery. Image degradation is affected not only by the amplitude of the target’s motion, but also by its direction, and relationships between the radar frequency sweep rate and characteristic period of oscillation. The benefits to image recovery of data averaging and frequency sweep randomisation are examined. A motion-correction system is discussed, based around a video photogrammetry system that provides a record of a target’s 3-dimensional motion during data acquisition. This work was carried out under the UK Ministry of Defence’s Corporate Research Programme.
Ultra-Wide Band (UWB) antenna descriptors have been used to design an optimal Log Periodic Dipole Array (LPDA) antenna for the Army Research Laboratory’s (ARL) BoomSAR. Although dispersive in nature, the LPDA design offers improvements over existing antennas with broader beamwidth, higher efficiency, and improved off-boresight polarization performance. In the UWB/A SAR application, it is important that the antenna polarimetric spectral response remains coherent and uniform over the aspect angles used in the image formation process. This paper describes the process by which these goals were achieved and presents measured results from the BoomSAR of trihedrals and cylinders that validate the approach.
A Ground-Based Synthetic Aperture Radar (GB-SAR) interferometer system operating at 17 GHz is used to monitor the movement of an active landslide. The selection of the optimal image formation technique for such an imaging system is addressed. The algorithms considered in this study are those previously developed for spaceborne and airborne SAR. A near-field algorithm that forms the image in the time domain is selected as the optimal solution. Furthermore, example results obtained in a measurement campaign in Schawz (Austria) are shown.