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The U.S. Navy has considerable experience in the radar cross section (RCS) measurement of dynamic targets. An understanding of the possible error sources and their relative magnitudes is critical to obtaining accurate and repeatable results. In addition to the usual potential sources of error in RCS measurements of stationary items, considerations with dynamic targets include target range and angle tracking, calibration, and various environmental effects. The primary considerations are identified and discussed, and an error budget is developed for a particular test scenario.
In order to assess the suitability of long thin metal rods as calibration devices for both co-polarized and cross-polarized (abbreviated as co-pol and x-pol) RCS measurements, we study RCS data from rods at broadside and compare them with 2D theoretical predictions. We find that the 45° tilt angle is optimum for calibration purposes. Near grazing incidence to a horizontal rod, the first traveling wave lobe in the HH pattern is a very prominent feature. Its angular location and amplitude have been measured as a function of frequency and compared with theory. A formerly unexplained error due to a contaminated calibration is identified.
The accurate measurement of Radar Cross Section (RCS) requires precise calibration "artifacts" as well as carefully executed measurement procedures. The Air Force Research Laboratory (AFRL) reviewed several existing common RCS calibration artifact standards and practices, and identified a number of improvements. Employing a modified "dual calibration" check procedure pioneered by AFRL, this paper demonstrates improved RCS calibration fidelity for a wide variety of static RCS calibration measurement applications. Our calibration results are verified through an industrial inter-laboratory (range) measurement program employing selected calibration artifact standards.
To develop standards for radar cross section measurements a complete uncertainty analysis is needed. We derive the radar cross section error equation and examine sources of measurement errors that contribute to the overall uncertainty in calibrations and measurements. We obtain expressions for upper- and lower-bound errors and uncertainties that are generally valid for monostatic measurements on any unknown target using any standard calibration artifact. The general procedure can be extended to bistatic measurements. Some experimental procedures to determine the uncertainty due to background subtraction are presented and discussed.
The control circuit encoding (CCE) technique [1,2] has been proposed as a method of remotely calibrating a phased array antenna. This patented technique uses an orthogonal coding scheme to measure the amplitude and phase of all array elements simultaneously. The capacity to measure all elements simultaneously is more efficient than single element measurements since measurement time is minimized. This paper describes an experimental verification of the CCE technique. Accurate control of amplitude and phase distribution in an array is important because it allows for low sidelobe array designs that can be maintained over the life cycle of the system. Also discussed is our method for estimating statistics of calibration performance using a stepped null approach. The results demonstrate that the CCE method is a viable approach for calibrating a phased array.
Proper operation of a planar NFR (near field range) includes probe correction as part of the processing of the measured data to result in accurate far field angle patterns, particularly for low cross polarized patterns. The far field transform of the near field data produces the angular spectrum which is the product of the plane wave transmission coefficient pattern of the AUT (antenna under test) with the plane wave receiving coefficient pattern of the probe. Probe correction consists of dividing the angular spectrum by the complex probe angle pattern resulting in the pure far field pattern of the AUT [1]. For best accuracy of co and cross polarized AUT patterns one needs to use accurately measured probe complex co and cross polarized patterns in probe correction for each NFR test frequency. The most accurate probe measurements are usually obtained from specialized test laboratories. However, if the number of frequencies is large, this may create problems due to cost or schedule. Because of this it is typical to procure probe calibration at only a few frequencies spanning the test band for each AUT even though pattern measurements are needed at several additional frequencies falling between the calibration frequencies. A typical strategy at any given test frequency is to perform probe correction using the nearest-neighbor-frequency probe calibration data. This strategy produces some unknown error in the processed probe corrected far field patterns of the AUT at each non-calibrated frequency. Inthis paper we will show a method for estimating the non-calibrated frequency probe correction error for co and cross polarized patterns with examples.
The NIST 18 term error budget is used to estimate the magnitude of each individual source of error and then combine them to the total uncertainty for the planar nearlield range designed for antenna characterization of the ERIEYE Airborne Early Warning System. The ERIEYE AEW System consists of two large phased array antennas, one at each side of the Dorsal Unit which is located on the top of the airplane fuselage. T/R-modules are connected to the antenna waveguides to control the beamsteering and the very low sidelobe level. The sidelobe level is supervised by a calibration during operation, using a table of calibration data. The table of calibration data is produced by iterative computer runs of programs performing the two transformations Near-field-to-Far-field and Far-field-to-Waveguide Excitation - the characterization. Characterization to very low sidelobe level in the calculated farfield is possible when using for instance planar nearfield technique to measure an active antenna. The errors at the planar nearfield range are misleadingly compensated for by the characterization. Therefore a minimization together with a continuous control of the noise level is necessary.
A helicopter-based radar cross section (RCS) measurement system was designed and demonstrated during the past year. The system was a novel combination of modified and un-modified commercial off the shelf (COTS) equipment and software, a minor amount of new hardware, and extensive prior experience. Validation was accomplished using known calibration standards and existing test practices relevant to this type of system, and data were collected and processed for a number of targets of opportunity. The primary subsystems include the measurement radar, the helicopter, antennas and associated mount, boresighted video and recorder, and the calibration tools. The SCI1000 radar was employed because of the combination of its excellent performance at the desired test target range and its minimal physical and power demands. The Bell 500 helicopter was chosen for its size and its wide availability on the world market. Data products were RCS vs. aspect, downrange profile history, and two-dimensional imaging following pre-processing by a robust motion compensation algorithm.
NPL has been providing antenna gain standards since the late 1970's, initially to service internal needs for microwave field strength standards. To meet the increasing industrial demand for the calibration of microwave antennas in areas such as satellite communications and radar, NPL has developed an antenna extrapolation range. The current facility, which is due to be replaced by the end of the year, is used to measure the gain of microwave antennas in the frequency range 1 to 60 GHz, often with a gain uncertainty as low as ± 0.04 dB. Axial ratio, tilt, sense of polarisation and pattern measurements can also be made in the same facility, while for larger antennas a planar near-field scanner is used. Of the many measurement techniques for determining the gain of an antenna, the most accurate is the three antenna extrapolation technique [1,2] which was developed at the National Institute of Standards and Technology (NIST) at Boulder, Colorado, and is the method used at NPL. This is an absolute method as it does not require a prior knowledge of the gain of any of the antennas used. Since calibration data is often required across a wide frequency band, the measurement techniques and software have been developed to allow measurements to be performed at a large number of frequencies simultaneously. This reduces the turn round time, the cost and the need for interpolation between measurement points.
A variety of unique instrumentation radars have been developed by the RF & MMW Systems Division at Eglin Air Force Base in order to support both static and dynam ic Radar Cross Section (RCS) measurements for Smart Weapons Applications. These systems include an airborne multispectral instrumentation suite that was used to collect target signatures in various terrain and environmental conditions (95 GHz Radar Mapping System - 95RMS), a look-down tower based radar designed to perform RCS measurements on ground vehicles (MMW Instrumentation, High Resolution Imaging Radar System MIHRIRS), two high power (35 & 95 GHz) systems capable of mapping/measuring both attenuation and backscatter properties of Obscurants and Chaff (MMW Radar Obscurant Characterization System MROCS: 1&2), and a Materials Measurement System (MMS) which provides complex free space, bistatic attenuation and reflectivity data on Radar Absorbing Materials (RAM), paints, nets and specialized coatings/materials. This paper will describe the instrumentation systems, calibration procedures and measurement techniques used for data collection as well as several applications which support modelina and simulation activities in the Smart Weapon community.
Errors arising in the measurement of reflection coefficient are identified and analyzed. The presence of multiple reflections due to poor connectors, transmission line discontinuities, and terminal loads is described, modeled and applied. Various measurement scenarios are analyzed, and measured results are presented as a guide for laboratory troubleshooting and as a validation of the measurement models. Improvements to Vector Network Analyzer calibration methods are proposed, including computer corrected calibration for one-port radiating elements and elementary improvements to two-port TRL calibration. An extensive error evaluation of the somewhat forgotten slotted line measurement is finally presented as a robust alternative, and computer automation, acquisition, and calibration of this measurement is outlined.
A fully-polarimetric compact range operating at 524 GHz has been developed for obtaining Ka-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, C W , X-band synthesizer driving dual X 4 8 transmitmultiplier chains and dual X 4 8 local oscillator multiplier chains. Software range-gating is used to reject unwanted spurious responses in the compact range. A motorized target positioning system allows for fully automated sequencing of calibration and target measurements over a desired set of target aspect and depression angles. A flat disk and a dihedral at two seam orientations are used for both polarization and R C S calibration. Cross-polarization rejection ratios of better than 45 d B are routinely achieved. The compact range reflector consists of a 1.5m diameter aluminum reflector fed from the side to produce a 0. 5 m diameter quiet zone. Targets are measured in free-space or on a variety of ground planes designed to model most typical grou nd surfaces. A description of this 524 GHz compact range along with 30 ISA R measurement examples are presented in this paper.
ARCS acquisition software for antenna and RCS measurements has been modified such that it is now based on LabWindows/CVI of National Instruments. With open system architecture, industry-standard tools and platform flexibility, new ARCS software delivers all components which are required for an advanced antenna and RCS measurement system. This means tht the portability and modularity of the software is increased considerably. Such a concept has the major advantage of simple adaptation/modification by the user, for instance by adding new menu pages. The virtual instrument concept of CVI guarantees easy adaptation of the newest interface technology, such as USB and firewire. Furthermore, there is a large base of instrument drivers which can be readily used to extend the measurement capabilities of ARCS in a minimum of time Special care is taken in the design of the user interface. This is to avoid complex procedu res for entering measurement parameters. Even less experienced operators must be comfortable with the software and be able to perform complex calibration and data acquisition procedures. Finally, a large number of application programs is written for advanced antenna and RCS calibration, microwave holography, ISAR imaging and frequency extrapolation techniques.
The scattered field from an arbitrary target may include a variety of scattering mechanisms such as specular and diffraction terms, creeping waves and resonant phenomena. In addition, buried within such data are target-mount interactions and clutter terms associated with the test environment. This research presents a method for decomposing a broadband complex signal into its constituent mechanisms. The method makes use of basis functions (words) which best describe the physics of the scattered fields. The MUSIC algorithm is used to estimate the time delay of each word. A constrained optimization refines the estimate and determines the energy for each. The method is tested using two far-field radar cross section (RCS) measurements. The first example identifies targetmount interactions for a common calibration sphere. The second example applies the method to a low observable (LO) ogive target.
Full polarimetric scattering measurements are increasingly being required for radar cross-section (RCS) tests. Conventional co-and cross-polarization calibrations fail to take into account the small amount of antenna cross-polarization that will be present for any practical antenna. In contrast, full polarimetric calibrations take into account and compensate for the cross-polarization the calibration process. We present a full polarimetric calibration procedure and a simulation-based performance study quantifying how well the procedure improves measurement accuracy over conventional independent channel calibration.
Probe correction is required to accurately determine the far-field pattern of an antenna from near-field measurements. At Raytheon Primary Standards Laboratory (PSL) in El Segundo, CA, data acquisition hardware, instrument control software, and a mechanical positioning system have been developed and used with an HP Network Analyzer/Receiver system to perform these measurements. Using a three antenna technique, the on-axis and polarization parameters of a linearly (or circularly) polarized probe are calibrated. The relative far-field pattern of the probe is then measured utilizing the two nominal, orthogonal polarizations of the source antenna. All measurements are stepped in frequency and use a time domain gating technique. The probe and the source antenna are optically aligned to the interface and unique, kinematic designed interface flanges allow repeatable mounting of the antennas to the test station.
Recently, RCS measurements were made of several common calibration objects of various sizes in the Boeing 9-77 Range. A study was conducted to examine the accuracy and errors induced by using each as a calibration target with a string support system. This paper presents the results of the study. Two of the objects, i.e., the 14"-ultrasphere and the 4.5"-dia. cylinder, are found to perform the best in that they exhibit the least departures (error) from theory. The measured departures of 0.2 to 0.3 dB are consistent with the temporal drift of the radar in several hours.
Calibration of monostatic radar cross section (RCS) has been studied extensively over many years, leading to many approaches, with varying degrees of success. To this day, there is still significant debate over how it should be done. It is almost a certainty, that if someone proposes a way to calibrate RCS data, someone else will come up with reasons as to why the "new" approach will not yield results that are "good enough." In the case of full scattering matrix RCS measurements, the lack of information concerning calibration techniques is even greater. The Air Force's Radar Target Scattering Facility (RATSCAT) at Holloman AFB, NM,has begun an effort to refine monostatic and bistatic cross polarization measurements at various radar bands. For the purposes of this paper, we have concentrated on our monostatic cross polarization developments. Such issues as calibration targets and techniques, system stability requirements, etc. will be discussed. During several programs we have attempted to collect sufficient data to do full scattering matrix corrections. In a previous paper, "Bistatic Cross-Polarization Calibration," our collected data had a high background which obscured much of the cross polarized return. The data presented here is from a program conducted at RATSCAT recently which utilized the Ka band. Because of the sensitivity of measurements at Ka to many effects, an error estimate was required. This paper presents this error estimation and some results of full scattering matrix correction of RCS data. This analysis is based upon "The Proposed Uncertainty Analysis for RCS Measurements", NISTIR 5019, by R. C. Wittmann, M. H. Francis, L. A. Muth and R. L. Lewis. This paper was aimed at principle pole measurements, e.g. HH and VV. The tabular data presented in the paper are from this paper with additions for errors associated with cross polarization and cross polarization correction.
Recent results from RCS measurements on metal wires, rods and dielectric strings are presented. For a cylinder at broadside to the incident wave, theoretical from 3D formulas converted from 2D exact solutions are used for comparisons with the experiments. The lone-of-sight orientation dependence is described by the polarimetric scattering matrix. Several types of interference effects are analyzed. Of particular interest is finding the suitable objects for the cross-polarized calibrations over a wide frequency range. Details from a 36" wire of radius 0.01" for calibrations in the VHF range are described. While the wire is supported by fine fishing lines, mitigation of the unwanted string echoes is important.
Calibration standards for radar systems are being developed cooperatively by NIST and DoD scientists. Our goals are to develop standard procedures for polarimetric radar calibrations and to improve the uncertainty in the estimation of system parameters. Dihedrals are excellent polarimetric calibration artifacts, because (1) the consistency between dihedral scattering data and the mathematical model of scattering can be easily verified, and (2) symmetry properties of the dihedral data provide powerful diagnostics to reveal system problems. We apply Fourier analysis to polarimetric data from dihedrals over a full rotation about the line of sight to reduce the effects of noise and clutter, misalignment, and other unwanted signals. An extension of the analysis to satisfy nonlinear model constraints allows us to monitor data quality and to further improve the calibration. We obtain the system parameters from the Fourier coefficients of the data in a simple manner. We illustrate these concepts using polarimetric radar cross section calibration data obtained as part of a national interlaboratory comparison program.