Handbook for the Assurance of Radar Cross Section Measurements

L. A. Muth & R. C. Wittmann & B. M. Kent
[Editor's note: This is Part 1 of the "Handbook for the Assurance of Radar Cross Section Measurements." The original document was published by NIST in DRAFT Form using Scientific Word v2.5. This rendition was converted first to Rich Text Format (RTF), then the document was converted to HTML, with formatting clean-up applied manually. Hopefully, the formatting that resulted does not detract from the readability of the document. Comments regarding content should be directed to Dr. Lorant Muth, NIST, (303) 497-3603, -- Roger Davis, 25 Feb 98]
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[Part 2: General Requirements for the Certification of Calibration Laboratories (82k)]
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We recommend technical and documentation standards as minimum criteria for the certification of RCS measurement ranges. Topics include, but are not limited to, uncertainty analysis, calibration and measurement procedures, data processing procedures, process and configuration control and managerial structures. We follow closely the ANSI/NCSL Z-540-1-1994 Standard, augmenting the original document to address areas of concern specific to the RCS community.

This document has been written to define documentation and certification standards to guide certification of radar cross section (RCS) measurement ranges. The basis of certification is a comprehensive measurement assurance program (MAP) for RCS ranges designed to sustain and improve measurement quality and reliability. A measurement assurance program implements and documents standard procedures for uncertainty analysis, intercomparison measurements, check standards and control charts, measurement and calibration, data processing, equipment maintenance, conduct of basic research, personnel training, etc. Additionally, it defines criteria for periodic review and revision to ensure state-of-the-art operation. For the user of RCS data, the major impact of MAP and certification is a better understanding of the limitations of the data and improved confidence in the measurements.

ISO 25 ANSI/NCSL Z-540-1 require that technical operations be clearly defined, documented, and demonstrated in a range Measurement Assurance Program (MAP). In addition, the standard requires that the management structure be clearly delineated, and that the responsibilities for implementing MAP components be clearly assigned. We advocate the use of an independent third party review of this documentation to assure all criteria of a measurement assurance program have been adequately addressed.

Compliance with the guidelines outlined in these pages is the central component of the NIST certification program for RCS measurement ranges. Certification by the RCS Certification Board is a recognition that a range has met standard measurement assurance requirements. It is intended that these standards be implemented uniformly to make range evaluations and comparisons easier and more meaningful.

This document is based on the ISO Guide 25, General requirements for the competence of calibration and testing laboratories , and on the ANSI/NCSL publication American National Standard for Calibration - Calibration Laboratories and Measuring and Test Equipment - General Requirements . These documents have been recognized internationally, and thus are well suited for adaptation to the standardization needs of the RCS community. It is essential to consult the original documents to make full use of this adaptation by the RCS community.

The format of this report follows the structure of the documents ANSI/ NCSL Z540-1-1994and Handbook for the Interpretation and Application of ANSI/NCSL Z540-1-1994. Sections of the ANSI/NCSL Z540-1-1994 directly applicable to the RCS community are quoted in full (in Sans Serif font, labeled [NCSL Z540-1]] ); similarly, text from the interpretive guidance of the ANSI/NCSL Z540-1-1994 Handbook are quoted in full (in small font, indented, labeled [NCSL Z540-1 Handbook]), but other sections are merely referenced by the phrase See Handbook [2] to save space. Thus, we identify sections that are especially important in the context of RCS measurement assurance. When the original text has been replaced to address the needs of the RCS community, the corresponding sections have been labeled with the phrase RCS Standards. Sections not relevant to the RCS community have been preserved in the structure, but the original text has been replaced with the phrase not applicable. The ANSI NOTES and Interpretive Guidance have sometimes been supplemented by RCS NOTES standards. Sometimes the original Interpretive Guidance is abbreviated or omitted, having been judged as not relevant to the RCS community. A sample outline of implementation of these standards by an RCS measurement range is presented in Appendix A, which describes the content and format of an RCS Range Book that documents the overall and daily functioning of a range.

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RCS Standards - National Measurement Assurance Program

This Standard sets out general requirements that a radar cross section range must meet for certification to ISO 25 and NCSL Z540-1. These requirements are based on a fully developed and documented range measurement assurance program (MAP), which broadly defined, is a program that implements requirements and documents procedures to help maintain and improve data quality at a measurement facility. A MAP includes, but is not limited to,

a) documented calibration and measurement procedures,

b) documented range uncertainty analysis,

c) documented data processing procedures,

d) documented interlaboratory comparison studies,

e) documented range test and evaluation,

f) well developed basic researchplan, and

g) documented management andquality control tracking methods.

It is implicit in any range MAP that it enjoys the full support of management, and that it will be subjected to periodic review and revisions. For more detail on MAP, see and .

Under the Department of Defense demonstration project, the third party accreditation or certification body will be the RCS Certification Board. For the purpose of the remainder of this document, the term certification implies that ranges have met the adopted RCS documentation standards (based on ISO 25 and NCSL Z540-1 guidelines). The certification board will consist of a joint team of National Institute of Standards and Technology (NIST) and industrial technical representatives who will review the an RCS range for compliance with the standards. The NIST representatives will be members of the Antenna and Materials Metrology Group, Electromagnetic Fields Division. The precise certification board membership will vary somewhat, but must minimally consist of 3 NIST and 2 non-NIST technical representatives. This document details some of the technical requirements for certification.

This Standard and any procedure or document executed in implementation thereof shall be in addition to and not weaken or detract from other contract requirements [NCSL Z540-1].

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ANSI/NCSL Z540-1-1994 : Calibration Laboratories and Measuring and Test Equipment - General Requirements.

ISO 8402 : 1986, Quality-Vocabulary

ISO/IEC Guide 2 : 1986, General terms and their definitions concerning standardization and related activities.

International vocabulary of basic and general terms in metrology (VIM) : 1993, issued by BIPM, IEC, IFCC, ISO, IUPAC, IUPAP and OIML. ISBN 92-67-01075-1 1993

ISO 9000 Series : 1987 (ANSI/ASQC Q90 Series), Quality management and quality assurance standards. ISBN 92-67-10172-2 1992 (In compendium form)

ISO/IEC Guide 25 : 1990, General requirements for the competence of calibration and testing laboratories.

ISO Guide 30 : 1981, Terms and definitions used in connection with reference materials.

MIL-STD 45662A : 1988, Calibration Systems Requirement.

Guide to the Expression of Uncertainty in Measurement : 1993, issued by BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, and OIML. ISBN 92-67-10188-9 1993

NCSL Recommended Practice RP-1; Establishment and Adjustment of Calibration Intervals: 15 November 1989.

NCSL Recommended Practice RP-3; Calibration Procedures Content and Format: August 19,1988.

NCSL Recommended Practice RP-7; Laboratory Design: July 25, 1993.

NCSL Recommended Practice RP-9; Calibration Laboratory Capability Documentation Guideline: 19 July 1989.

NCSL Recommended Practice RP-11, Reports and Certificates of Calibration: 23 October 1991

NCSL Recommended Intrinsic/Derived Standards Practice RISP-1, Josephson Voltage Standard: November, 1993.

NCSL Recommended Intrinsic/Derived Standards Practice RISP-2, Triple Point of Water Cell: May, 1995.

ISA Recommended Practice ISA RP52.1; Recommended Environments for Standards Laboratories: 1975

MIL-STD 105E; Sampling Procedures and Tables for Inspection by Attributes: 10 May 1989.

WHAT'S NEW IN TRACEABILITY; Ernest L. Garner and Stanley D. Rasberry; Reprinted from Journal of Testing and Evaluation, November, 1993.

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This section has been supplemented by RCS NOTES to clarify the role of the item being defined in the RCS certification process.

The relevant definitions from ISO/IEC Guide 2, ISO 8402, ISO Guide 30, and the International vocabulary of basic and general terms in metrology (VIM) are applicable, the most relevant being quoted below together with further definitions applicable for the purpose of this Standard [NCSL Z540-1].

calibration - RCS Standards
The set of operations which establish, under specified conditions, the relationship between values indicated by RCS measuring systems and corresponding RCS standard artifacts.

1) Calibration procedures must be clearly documented ; deviations from accepted practice must be explained.
2) A calibration result must be accompanied by an uncertainty assessment. Any documented uncertainty analysis procedure that has been reviewed and accepted by the certification board will satisfy this requirement. To aid uniform implementation throughout the RCS community and for ease of assessment, NIST has suggested guidelines to document range uncertainty, see .
3) The result of a calibration is to be recorded in a measurement report.
4) Calibration consistency checks must be implemented. This can be done through the use of
(a) control charts or calibration histories
(b) interlaboratory comparisons
(c) other measurement procedures documented by the range.

calibration certificate or report
Document that presents calibration results and other information relevant to a calibration [NCSL Z540-1].

RCS NOTES- A calibration certificate or report, otherwise known as data report or test report, presents the results of calibration to a customer. The RCS Certification Board will review samples of such reports.

calibration method
Defined technical procedure for performing a calibration or verification [NCSL Z540-1].

1) Technical procedures must be documented in compliance with RCS standards. See RCS Standard Calibration Procedures, Appendix B.
2) Full documentation of the calibration procedure must be available at the range for range personnel, the customer and the RCS Certification Board.
3) The documented calibration procedures will be subject to review by the RCS Certification Board. Deviations from accepted practice must be documented.
4) Calibration data and data analysis techniques will be reviewed by the certification board as part of the certification process.

certified reference material
See Handbook

interlaboratory comparisons
Organization, performance and evaluation of calibrations on the same or similar items by two or more laboratories in accordance with predetermined conditions [NCSL Z540-1].

RCS NOTES-Interlaboratory comparisons of calibrations and measurements on special targets can be used to monitor range performance. This is a valuable tool that can be used to check consistency of uncertainty analyses. Two or more RCS measurement ranges can monitor range measurement performance through such intercomparison studies, which would generally be divided into one of two types:
1) intercomparison studies initiated at the request of the RCS Certification Board to support the certification review process, or otherwise clarify technical performance metrics presented by the range to the certification board.
2) Industry-wide intercomparison studies initiated by mutual consent of two or more ranges to support or assess their state-of-the-art calibration capabilities. If so requested, such studies can also be initiated by or supervised by the certification board.

international (measurement) standard
A standard recognized by an international agreement to serve internationally as the basis for fixing the value of all other standards of the quantity concerned [NCSL Z540-1].

RCS NOTES-Currently there are no nationally or internationally recognized intrinsic RCS calibration standards. See RCS Standard Artifacts and Intercomparisons, Appendix C.

influence quantity
A quantity which is not the subject of the measurement but which influences the value of the measurand or the indication of the measuring instrument. Examples: ambient temperature; frequency of an alternating measured voltage [NCSL Z540-1].

RCS NOTES-Other processes that might influence the results of RCS measurements are: electromagnetic interference, weather conditions, sea states, humidity, system drift and operator error, etc. In many cases, repeatability of an RCS measurement can demonstrate to what extent an RCS measurement system is being affected.

laboratory/calibration laboratory
Body that calibrates or performs calibrations and verifications [NCSL Z540-1].

RCS NOTES-In this document all RCS measurement ranges are considered calibration laboratories.

limits of permissible error (of a measuring instrument)
The extreme values of an error permitted by specifications, regulations, etc. for a given measuring instrument.

NOTE-This term is frequently referred to as "tolerance" in the United States [NCSL Z540-1].

A quantity subjected to measurement [NCSL Z540-1].

NOTE-As appropriate, this may be the "measured quantity" or the "quantity to be measured [NCSL Z540-1]."

The set of operations having the object of determining the value of a measurand.

measurement assurance
Measurement assurance is a technique that may include, but is not limited to: 1) use of good experimental design principles so the entire measurement process, its components, and relevant influence factors can be well characterized, monitored and controlled; 2) complete experimental characterization of the measurement process uncertainty including statistical variations, contributions from all known or suspected influence factors, imported uncertainties, and the propagation of uncertainties throughout the measurement process; and 3) continuously monitoring the performance and state of statistical control of the measurement process with proven statistical process control techniques including the measurement of well characterized check standards along with the normal workload and the use of appropriate control charts [NCSL Z540-1].

RCS NOTES-A well-defined measurement assurance program (MAP) is central to RCS range certification. A MAP consists of technical requirements designed to ensure state-of-the-art RCS measurements. It documents measurement procedures, range uncertainty analysis, data analysis procedures, basic research to improve measurement quality, results of laboratory intercomparison studies and managerial structures. A MAP in compliance with accepted standards should be in place before a certification board is convened. For more detail, see Section 1.1, and ….

measurement standard
A material measure, measuring instrument, reference material or system intended to define, realize, conserve or reproduce a unit or one or more known values of a quantity to serve as a reference [NCSL Z540-1].

1) We will use a suite of cylinders, dihedrals or spheres to calibrate RCS measurement systems. Other artifacts can be used for calibration if the computed or measured RCS value of such artifacts are known with acceptably established uncertainties.
2) For certification, a range should calibrate its measurement system using an accepted RCS standard artifact, preferable one that has been used in an interlaboratory comparison study. The use of a well characterized artifact with associated uncertainties will expedite comparison of RCS measurements on different systems.

measuring and test equipment
All of the measuring instruments, measurement standards, reference materials, and auxiliary apparatus that are necessary to perform a measurement. This term includes measuring equipment used in the course of testing and inspection, as well as that used in calibration.

NOTE-In the context of this Standard, the term "measuring and test equipment" is taken to encompass "measuring instruments" and "measurement standards" [NCSL Z540-1].

measuring instrument
A device intended to make a measurement, alone or in conjunction with supplementary equipment [NCSL Z540-1].

mutual consent standard
An artifact or process that is used as a de facto standard by mutual consent of the supplier and customer when no recognized U. S. national standard is available [NCSL Z540-1].

RCS NOTES-We recommend a set of cylinders and dihedrals for calibration of RCS measurement systems. See RCS Standard Artifacts and Intercomparison, Appendix C. However, more complex targets might be used in interlaboratory comparisons.

national (measurement) standard
A standard recognized by an official national decision to serve, in a country, as the basis for fixing the value of all other standards of the quantity concerned [NCSL Z540-1].

RCS NOTES-For the purposes of certification a range will need to measure one or more of the RCS standard artifacts adopted by the RCS industry. Currently these artifacts include cylinders, dihedrals and spheres. NIST will assist in establishing traceability to these artifacts as part of the DoD demonstration project. See RCS Standard Artifacts, Appendix C.

proficiency testing
Determination of the laboratory calibration performance by interlaboratory comparisons or other means [NCSL Z540-1].

RCS NOTES-Interlaboratory comparisons are a part of the certification requirements.

(quality) audit
A systematic and independent examination to determine whether quality activities and related results comply with planned arrangements and whether these arrangements are implemented effectively and are suitable to achieve objectives.

NOTE-The quality audit typically applies to, but is not limited to, a quality system or elements thereof, to processes, to products, or to services. Such audits are often called "quality system audit", "process quality audit", "product quality audit", "service quality audit [NCSL Z540-1]."

RCS NOTES-As part of the certification process the range MAP including documentation, uncertainty analysis, calibrations and data analysis procedures will be reviewed by the RCS Certification Board. As part of the ISO 25 and ANSI/NCSL Z-540-1 compliance process, range customers can request spot checks on range process compliance, or request a review of a specific measurement.

quality manual
A document stating the quality policy, quality system and quality practices of an organization.

NOTE - The quality manual may call up other documentation relating to the laboratory's quality arrangements [NCSL Z540-1].

RCS NOTES-An RCS Range Book, containing or referencing the complete documentation of range operations, will be the quality manual at RCS measurement ranges. The structure of a Range Book should be uniformly implemented, but the specific contents of the Range Book may vary from range to range. An electronic version implemented as an interactive Web page with hypertext links could also be created. For the format of the standard Range Book, see Appendix A.

quality system
The organizational structure, responsibilities, procedures, processes and resources for implementing quality management [NCSL Z540-1].

(quality system) review
A formal evaluation by management of the status and adequacy of the quality system in relation to quality policy and new objectives resulting from changing circumstances [NCSL Z540-1].

reference material
See Handbook

reference standard
A standard, generally of the highest metrological quality available at a given location, from which measurements made at that location are derived [NCSL Z540-1].

RCS NOTES- RCS standard artifacts include a set of cylinders, dihedrals or spheres, and other artifacts that have been accepted by the industry. These artifacts are traceable to NIST in some cases. For details, see RCS Standard Artifacts, Appendix C.

A translation of the needs into a set of individual quantified or descriptive specifications for the characteristics of an entity in order to enable its realization and examination [NCSL Z540-1].

The property of a result of a measurement whereby it can be related to appropriate standards, generally national or international standards, through an unbroken chain of comparisons [NCSL Z540-1].

RCS NOTES-The results of calibrations using standard RCS artifacts may be traceable to NIST. For details, see RCS Standard Artifacts, Appendix C.

uncertainty of measurement
Parameter, associated with the result of a measurement, that characterizes the dispersion of the values that could reasonably be attributed to the measurand [NCSL Z540-1].

NOTES [NCSL Z540-1 Handbook]-
1) The parameter may be, for example, a standard deviation (or a given multiple of it), or the half-width of an interval having a stated level of confidence.

2) Uncertainty of measurement comprises, in general, many components. Some of these components may be evaluated from the statistical distribution of the results of series of measurements and can be characterized by experimental standard deviations. The other components, which can also be characterized by standard deviations, are evaluated from assumed probability distributions based on experience or other information.

3) It is understood that the result of the measurement is the best estimate of the value of the measurand, and that all components of uncertainty, including those arising from systematic effects, such as components associated with corrections and reference standards, contribute to the dispersion. This definition is that of the "Guide to the Expression of Uncertainty in Measurement" in which its rationale is detailed (see, in particular, 2.2.4 and annex D).

RCS NOTES -ISO 25 and ANSI/NCSL Z-540-1 dictate the need or requirement for an uncertainty analysis, but do not advocate a particular approach. The RCS certification board will review any documented method for estimating uncertainty. For guidelines on uncertainty analysis procedures, see ....

approved signatory
A person who is recognized by an accredited body as competent to sign accredited laboratory calibration reports [NCSL Z540-1].

Evidence by calibration that specified requirements have been met [NCSL Z540-1].

NOTES [NCSL Z540-1]-
1) In connection with the management of measuring equipment, verification provides a means for checking that the deviations between values indicated by a measuring instrument and corresponding known values are consistently smaller than the limits of permissible error defined in a standard, regulation or specification peculiar to the management of the measuring equipment.

2) The result of verification leads to a decision either to restore to service, or to perform adjustments, or to repair, or to downgrade, or to declare obsolete. In all cases documentation of the verification performed is kept on the measuring instrument's individual record.

3) The term "verification," as defined in this Standard is frequently referred to as "calibration" in the United States.

Go to: GENERAL REQUIREMENTS FOR THE COMPETENCE OF CALIBRATION LABORATORIES--This Standard applies to calibration laboratories in the development and implementation of their quality system. [NCSL Z540-1]

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