AMTA Paper Archive
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Full Scattering Matrix Calibration with Error Analysis
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.
Study of Wires and Strings of Finite Sizes
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.
Analytical Error Model for Propagating RCS Measurement Uncertainties, An
In the last few years, a change has occurred in the RCS metrologist concerns for error analysis and the quantification of measurement uncertainty. The specific methods for range characterization and uncertainty estimation are the topics of many passionate technical discussions. While no single treatment can please everyone, most agree a measurement uncertainty program is critical to the understanding of measurement quality, the development of error reduction strategies, and to the planning of range improvement paths. We present the statistical case for the natural grouping of errors into multiplicative and additive classes. We will derive the two cases where one class dominates as presented by LaHaie , and then expand the analysis to include the general case of competing classes. We summarize the role and applicability of this method in estimating measurement quality and discuss how this procedure offers a logical and comprehensive error propagation solution to both top-down and bottom-up range characterization approaches.
Full Test-Zone Field Evaluation Using Large RCS Targets
Large Compact Ranges for test zone sizes of 6 meters or can be used for both payload or advanced antenna and RCS testing. In order to determine the range accuracy, test zone field evaluation is required. For physically large test zone dimensions, scanning of the test-zone fields is difficult and impractical in most situations. Furthermore, the accuracy of planar or plane-polar scanners is usually not sufficient for applications above 10 GHz. An alternative approach is the RCS reference target method where the test zone field is derived from the RCS measurement of a flat plate. Such a target can be manufactured as a single sheet aluminium honeycomb structure with rectangular or circular cross section. Reference targets with large dimensions and high surface accuracy are available. Consequently, test-zone fields can be accurately determined for test zone diameters up to about 10 meters and frequencies up to 100 GHz. In this paper the application of this method will be demonstrated at the Compact Payload Test Range (CPTR) at ESA/ESTEC. Large rectangular plate has been used for field determination within a test-zone of 5.5 meters. A 2 meter diameter circular flat plate has been used to map the residual cross-polarization level within the test zone. It will be shown that valuable information about range performance (amplitude, phase and cross-polarization) can be accurately retrieved from the RCS measurements
Verification of Antenna Radiation Patterns and Scattering Returns (RCS) of Full Size Targets Using Missile Engagement Simulation Arena (MESA) Facility (NAWCWPNS, China Lake CA) Radar System, and a Hardware in the Loop Radar System
Most often when performing antenna and RCS measurements, integrating the results is performed with some type of computer generated simulation or model of the application scenario. In the case of Missile Engagements for Fuze Radars, there is an opportunity to engage full size targets in a near real engagement. The missile fuze antenna can be mounted on the test cart which is able to position the fuze antenna in azimuth, pitch and roll. For instrumentation the MESA Facility has available a PN coded BiPhase multi-range gate radar system. Various Full size targets are available for use in the arena. The target are positioned for a multitude of trajectories utilizing an overhead target positioning system. The Overhead Target Positioning System suspends and moves the targets using a multipoint string system that controls, Pitch, Roll, height, and azimuth positioning. The Overhead Target Positioning System (OTS) is also controlled in lateral movement. (across the range) This paper will show the verification of antenna patterns and RCS returns of full size targets using the MESA Radar system, and verification of these measurements using a hardware in loop fuze radar system simultaneously.
Overview of the Bistatic Coherent Measurement System (BICOMS)
The Georgia Tech Research Institute (GTRI), under contract to the U.S. Air Force 46 Test Group, Radar Target Scattering Division (RATSCAT), at Holloman AFB, NM, has designed and developed a fully polarimetric, bistatic coherent radar measurement system (BICOMS). It will be used to measure both the monostatic and bistatic radar cross section (RCS) of targets, as well as create two-dimensional, extremely high-resolution images of monostatic and bistatic signature data. BICOMS consists of a fixed radar unit (FRU) and a mobile radar unit (MRU), each of which is capable of independent monostatic operation as well as simultaneous coherent monostatic and bistatic operation. The two radar systems are coherently locked via a microwave fiber optic link (FOL). This paper discusses the key system features of the BICOMS.
Moment Method Inter-code Comparisons and Angular Sensitivity Studies for NIST Calibration (Squat) Cylinders
Considerable attention has been given recently to the problem of properly calibrating RCS measurements. Traditionally accepted approaches utilize aluminum spheres for ease of placement (insensitivity to orientation) and availability of computationally accurate (Mie series) solutions. In many situations, however, it can be shown that spheres fail as calibration devices. Past AMTA presentations [1, 2, 3] have shown that required mechanical tolerances for spheres are stringent, and can be difficult to achieve. Furthermore, energy can be bistatically reflected from spheres into column or pylon target supports, adding to calibration contamination. One solution may be a more wide-spread introduction of squat cylinders as calibration devices. Outdoor ranges have utilized squat cylinders for years for many of the aforementioned reasons. Advantages and disadvantages exist as always. The reduction of target support interaction and improved mechanical tolerances may be offset by difficulty in providing computationally accurate cylinder predictions and proper cylinder orientation. This work attempts to straightforwardly illustrate how these considerations come into play to assist the range engineer in determining how best to proceed to calibrate his or her data.
Relocation of RCS Measurement Facility Sycamore Canyon Site A Poway, CA to Tucson, AZ and the Techniques used for Measurement Capability Validation
In the process of relocating an RCS range from Sycamore Canyon, Poway, CA to the Raytheon Systems Company plant site in Tucson, AZ, the very important question of measurement validation had to be addressed. This relocation has to be accomplished on a very aggressive schedule in order to keep the impact to measurement schedules at a minimum. A high standard of measurement capability had to be retained. The aggressive relocation schedule poses risks to site selection and subsequent range validation. We will present an outline of our validation plan and our relocation plan from a technical point of view, and discuss our various procedures for measurement and range validation. The philosophy and methodology of the proposed site selection and measurement for the validation of the Tucson test facility will also be presented. This paper will also present the resolution of encountered risks and problems.
BICOMS Antenna Positioner System (APS) and Automated Field Probe (AFP)
BICOMS (Bistatic Coherent Measurement System) is a RATSCAT radar cross section (RCS) range at Holloman AFB, NM. BICOMS includes a Mobile Radar Unit (MRU), Fixed Radar Unit (FRU), and an Automated Field Probe (AFP). The MRU's antenna positioner system moves eight antennas using single pivot elevation/azimuth positioners and screw jack and cable hoist height actuators. The Automated Field Probe (AFP) raster scans a 40 x 40-foot aperture in front of the target under test. A 4- wheel drive scissors lift provides mobility and vertical axis travel. A cable drive moves a carriage horizontally along a 48-foot truss boom, mounted on the lift platform. The system computer controls both axes, as well as microwave data acquisition. All structures and systems feature minimum weight and wind resistance.
Effective Evaluation of Monostatic RCS From Near-Field Data
An efficient algorithm for the RCS evaluation of the Monostatic Radar Cross Section (RCS) from a reduced set of bistatic near-field data is proposed. The algorithm allows to evaluate the monostatic RCS from near field data collected in an angular region centered on the direction of interest, whose amplitude depends on the size of the scatter and the distance of the measurement zone. Numerical examples on two dimensional elliptical cylinders show the effectiveness of the proposed technique.
Uncertainties in Dynamic Radar Cross Section Measurements
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.
Time Domain Processing of Range Probe Data for Stray Signal Analysis
Time domain processing (TDP) is used to analyze the quiet zone fields of antenna/RCS ranges. To carry out the time domain analysis, the quiet zone fields are probed over a band of frequencies. It is shown that TDP is a very effective tool for analyzing probe data. One can not only estimate the time and direction of arrival of various signals present in the quiet zone, but can also estimate their frequency dependence and quiet zone variations.
RCS Measurements of LO Features on a Test Body
The paper presents an example of the design process undertaken to determine the RCS response of LO features mounted on a test body. Although not unique, the example considers the various aspects which determine the accuracy of the final data in the design of the experiment and signal processing. The high quality of experimental results illustrate the potential of using an integrated approach in which the designs of the test body, the measurement process, the signal processing techniques, and validation of results are optimally applied to meet the objective not achievable by conventional means.
Techniques for Improving Background Subtraction at the RATSCAT RAMS Facility
A method for implementing and/or improving background subtraction performance in wideband outdoor RCS range measurements is described. The method estimates and corrects for systematic changes that have occurred between a test and back ground measurement. Results from the application of a phase-only version of the techniques to background measurements from the RATSCAT RAMS facility are presented. Background subtraction performance improvements of as much as 20 dB are demonstrated.
Contributions of Wind Effects and Target Rotation Rates to Range Uncertainty, The
RATSCAT has been heavily involved, as part of the DoD Range Certification Demonstration Program, in examining and documenting the underlying principles of all aspects of the outdoor measurement process. Our goal is to replace historical or "anecdotal" measurement approaches with processes founded on validated and documented procedures. This paper reports on the results of two areas of study. These are the effects on measurements caused by wind and calculation of target rotation rates. When RCS targets are measured outdoors on pylons or columns, some uncertainty will be introduced due to the effect of wind on the target and target support structure. This paper will present the results of an investigation into the errors introduced by wind motion on targets mounted on pylons or columns. When rotation rates are determined for target collection, the usual procedure is to employ a rule of thumb like "collecting three points per lobe" or "meeting the Nyquist criterion." This paper examines these common methods to determining rotation rates, and their impact on the measurement of the peak values of RCS magnitude and phase. Finally, the significance of these two measurement errors will be examined in light of their impact on outdoor range operations as well as on decisions based upon the collected RCS data.
Calibration and Error Budget in RCS Measurements
Uncertainty analysis for fundamental standards is mature, but the cost overhead has, until recently, prevented much of this work being taken up by the UK RCS measurement community. The requirement to verify the radar signature of new equipment has made it necessary to examine in detail the RCS measurement process and to create a methodology for error budgeting. The paper reviews some basic concepts in estimating uncertainties, and describes work on 'squat' cylinder calibration standards that have been manufactured following designs proposed at previous AMTA conferences. The moment method code CLASP has provided the basic theoretical solutions which have been verified on a compact range through reference to a precise 100mm spherical standard. The concept of multiple standard calibrations is discussed, and recommendations are made for overall error budgeting and the intercomparison of range types.
Interlaboratory Comparison Between the RCS Ranges at FOA Defence Research Establishment and Saab Dynamics, An
An interlaboratory comparison is made between radar cross section (RCS) measurements at the test ranges at FOA Defence Research Establishment and SAAB Dynamics, Sweden. The comparison is made in order to increase the measurement and calibration quality at the ranges. An analysis of the deviations in the measured RCS data from the ranges provides a better understanding of the sources of errors. The RCS of two generic targets are measured at the X-band. The targets are simple airplane models, length and width are approximately 1.0 m, with no cavities. A brief comparison between some theoretical results and experimental RCS data are also presented.
Wideband Radar Echoes From Cylindrical Rods
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.
Improvements in Static Radar Cross Section Calibration Processes and Artifacts -- Initial Measurement Results and Validation Through Inter-range Comparisons
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.
Impact of Radiation on Radar Cross Section
The purpose of this project was to determine the effects of fast neutron bombardment on the radar cross section of metal and dielectric spheres. The energetic neutrons interact with lattice atoms and, in the energy transfer that results, initiate a displacement cascade that effectiveiy damages the crystalline structure of the target material. The induced damage may change the RCS of the target via changes in the conductivity or relative permittivity. Theoretical lattice damage estimates are provided for fast neutron fluences of 1015 n/cm2 and 1016n/cm2. Limitations and potential improvement of damage estimates and measurements are also discussed.
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