Metrology for Radar Cross Section Systems

Goals

The Metrology for Radar Cross Section Systems Project assists the U.S. Department of Defense (DOD) and industrial radar cross section (RCS) measurement ranges to create and implement a National DOD Quality Assurance Program to ensure high-quality RCS calibrations and measurements with stated uncertainties.

Customer Needs

RCS measurements on complex targets, such as aircraft, ships, and missiles, are made at different types of RCS measurement ranges, including compact ranges (indoor static), and outdoor static or dynamic facilities. Measurements taken at various ranges on the same targets must agree with each other within stated uncertainties to increase con- fi dence in RCS measurements industry-wide. Although the sources of uncertainty are well known, a comprehensive determination of the magnitudes of uncertainties in RCS calibrations require well formulated procedures that measurement ranges can use to determine their uncertainties. Customer needs include:

Calibration Artifacts — RCS users need improved calibration artifacts that are dimensionally traceable and calculable and exhibit wide dynamic range.

Calibration Procedures — Calibration procedures and data analysis techniques are needed to minimize range uncertainties at both government and industrial RCS ranges. The implementation of improved procedures and the determination of range uncertainties at every RCS measurement range are essential if the U.S. RCS industry is to maintain its world leadership.

Technical Strategy

The complex measurement systems and measurement practices at RCS ranges should be documented uniformly throughout the industry so that meaningful comparison of capabilities and important range-to-range differences are recognized. The framework of a RCS Range Book, in the context of a DOD RCS Self-Certification Program, is used to ensure community-wide compliance.

We provide RCS Range Book reviews for the DOD and industrial RCS ranges. These in-depth reviews provide guidance to the RCS community as they pursue their industry-wide certification program. The uncertainty analyses pursued by the U.S. RCS ranges are based on the pioneering NIST work in this area.

We have continued to work closely with selected RCS measurement ranges to develop and standardize procedures to determine RCS calibration and measurement uncertainty for both monostatic and bistatic RCS measurements. Fully polarimetric calibration procedures are also being studied.

To support these research activities we have recommended an expanded set of RCS calibration cylinders to calibrate the system at various signal levels of interest using a single artifact. To support polarimetric calibration research, we recommended a set of calibration dihedrals that can be used to determine system parameters needed to analyze polarimetric calibration data.

We seek to fully assess the technical merit and deficiencies of existing calibration and measurement procedures, data-analysis techniques, and uncertainty analysis. We plan to publish recommendations for improvements in these areas. We plan to further explore known problems in areas such as dynamic sphere calibration, polarimetric calibration, and bistatic RCS calibration.

The annual RCS Certification Meeting held at NIST-Boulder provides a forum for the RCS community to discuss procedural and technical issues on an ongoing basis.

Accomplishments

Sample set of calibration dihedrals.

Cylinder Calibration Set — The RCS community has adopted a basic cylinder calibration set to test the calibration integrity of monostatic RCS systems. Computed radar cross sections for the cylinder set have been obtained. These four cylinders have been measured at a number of government and industrial measurement ranges, with agreement with the theoretical RCS of better than 0.5 decibels. These comparisons demonstrate good repeatability; however, we need more robust independent measurement procedures to determine the measurement uncertainties.
Dihedral Calibration Set — We designed and manufactured a set of calibration cylinders with dihedral cutouts that can be used to calibrate an RCS range within a large dynamic range rather than at a single signal level, thereby improving calibration accuracy within the measurement interval. These artifacts should be useful in a nationwide measurement- comparison program.
RCS Uncertainty Analysis — We have completed a measurement-based RCS calibration uncertainty analysis for the Etcheron Valley Range, NAVAIR, China Lake, CA. This study determined the calibration uncertainty bounds without having to rely on statistical model assumptions that may not be valid for RCS calibrations and measurements.
Polarimetric Calibrations — RCS ranges have reported less-than-satisfactory results with existing polarimetric calibration procedures. We developed a more robust calibration procedure wherein full polarimetric data are obtained using a dihedral rotating around the line-of-sight to the radar. The new procedure allows us to: (1) improve the signal-to-noise ratio and check for alignment problems by exploiting the symmetry properties of the dihedral, (2) correct for the overall angular bias in the rotation angle, and (3) remove the effects of drift to obtain drift-free system parameters.