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RCS

Combined microwave/millimeter wave RCS compact range based measurement facility, A
J.F. Aubin,C.J. Arnold, November 1997

ORBIT/FR has recently installed and qualified a combined microwave (2-18 GHz) and millimeter wave (92.5-95.5 GHz) RCS system in an existing compact range based chamber. The facility is used for scale model reflectivity measurements on a wide variety of targets. The system features a unique, high power hardware gating system at the millimeter wave band that contains an integrated compact range feed assembly specifically designed to optimize RCS performance. Changeover between the microwave and millimeter wave bands is possible by placement of the appropriate compact range feed assembly on the feed stand, with locating pins being utilized to assure repeatable performance of the feeds in the compact range system. The system utilizes the FR959 RCS Measurement Workstation and HP 8530/85330 "turbo" based receiver system. Appropriate upconversion and downconversion hardware is integrated into the millimeter wave gating system to allow a common set of HP 8360 series sources and the HP 8530 IF receiver to be utilized for operation in both bands. The system is capable of producing high quality ISAR images at the millimeter wave frequencies, as well as in the microwave band.

Integrated antenna/RCS/EMI compact range based measurement facility, An
D.R. Frey,A. Charland, J.R. Aubin, R. Flam, November 1997

ORBIT/FR has recently delivered an integrated facility capable of being used for Antenna, Radar Cross Section (RCS), and EMI measurements to the Naval Underwater Warfare Center in Newport, RI. The facility includes a shielded anechoic chamber, a compact range system capable of producing a 6 foot diameter quiet zone, multi-axis positioning equipment, and a complete complement of Antenna, RCS, and EMI measurement instrumentation and data collection hardware/software. The facility is capable of operation over a frequency range of 100 MHz to 50 GHz, with compact range operation feasible above 2 GHz. The facility can be reconfigured to go between antenna and RCS measurements in any band using both frequency band and antenna/RCS mode switching. In addition, automatic positioning of the appropriate compact range feed to the reflector focal point is available. EMI measurements require minimal relocation of absorber in an isolated area of the chamber floor. Performance of the system is optimized by location of critical RF equipment on the compact range feed carousel or on the positioning system rail carriage. This system offers a unique combination of performance and convenience for making all three types of measurements.

Indoor RCS measurement capability at VHF in the Boeing 9-77 range
M.D. Bushbeck,A.W. Reed, D.E. Young, K.J. Painter, November 1997

This paper discusses Radar Cross Section (RCS) measurement capability at Very High Frequencies (VHF) in the Boeing 9-77 Range in Seattle, Washington. This indoor facility provides a unique asset to the RCS measurement community. Initially operational in 1989, the 9-77 Range was upgraded in 1995 to include a VHF measurement capability. This was achieved using a 56 foot square array of 256 elements, for RCS measurements at frequencies from approximately 140 to 220 MHz, with a 40 foot quiet zone. In this paper, we discuss results from the characterization process used to verify the initial capability and ongoing operation of the RCS measurement system at VHF. We include data demonstrating the sensitivity, stability and dynamic range of the system. We also present samples of recent field probes, and background subtraction and stability measurements. A comparison is made between calculated and measured canonical target signatures.

Some top-down experiments for range characterization
W.D. Burnside,E. Walton, I.J. Gupta, J.D. Young, November 1997

Range characterization is becoming a very important topic for the operators of RCS measurement ranges. Techniques for characterization can be expensive and time consuming. We present a top down approach that recognizes that the range construction and optimization is the responsibility of the range operators. Once the range is operating satisfactorily from the point of view of the range operator, then characterization of t he range performance as achieved can be done. Measurements are proposed that perform this characterization rapidly and inexpensively.

Technique for collecting and procesing flight-line RCS data, A
G. Fliss,J. Burns, November 1997

Recently, several deployable, ground-to-ground col­ lection systems have been developed for the assessment of aircraft RCS on the flight-line. The majority of these systems require bulky rail or scanning hardware in order to collect diagnostic imaging data. The measurement technique described in this paper, while not a "cure-all", does eliminate the need for bulky hardware by allowing the collection system to move freely around the target while collecting radar backscattering data. In addition, a nearfield-to-farfield transformation (NFFFT) algorithm is incorporated in the process to allow the collection of scattering data collected in the near field to be processed and evaluated in the far field. The techniques described in this paper are a part of a data conditioning process which improves the data quality and utility for subsequent analysis by an automated diagnostic system described elsewhere in this proceedings [1]. The techniques are described and demonstrated on numerically simulated and experimentally measured data.

RCS characterization on a portable pit with a foam column at VHF/UHF
M. Husar,J.H. Eggleston, November 1997

The RATSCAT radar cross section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and customer desires for certified RCS data. This paper discusses the low frequency characterization of the RATSCAT VHF/UHF Measurement System (RVUMS). The characterization was conducted on a portable pit with a 30' foam column at the RAMS site. System noise, clutter, backgrounds and generic target measurements are presented and discussed. Potential error sources are examined. The use of background subtraction and full polarimetric calibration are presented. Potential errors, which can occur from using certain cross-pol calibration techniques, are discussed. The phase relationship between each polarization components of the scattering matrix and cross-pol validation techniques are considered.

Antenna/RCS range evaluation using a spherical synthetic aperture radar
R.C. Wittmann (National Institute of Standards and Technology),D.N. Black (EMS Technologies, Inc.), November 1996

We describe an imaging technique which allows the isolation of sources of unwanted radiation on an antenna/RCS range. The necessary data may be collected by using a roll-over azimuth mount to scan a probe over a spherical measurement surface.

Accurate gain calibration procedure for large antennas
M.A.J. van de Griendt (Eindhoven University of Technology),V.J. Vokurka (Eindhoven University of Technology), November 1996

Gain calibration of circular horns and radiation pattern integration applying patterns in two principle planes only is accurate and does not require large computational or measurement effort. This technique is thus more practical than the integration over the entire angular domain, required in case of rectangular horns. However, for many types of AUT’s, additional errors may occur due to the differences in aperture size of the AUT and standard gain horn. The AUT will in many cases have physically larger aperture dimensions. Consequently, unknown test-zone field variations across this aperture can result in additional errors in gain determination. The new method uses a flat plate as a reference target. An RCS measurement of the flat plate is used to derive test-zone field characteristics over the same physical area as the AUT. Combined with the accurate gain calibration described above, field information is available over the entire area of interest and the accuracy in gain determination is increased. In this paper, experimental results and practical considerations of the method will be presented.

Performance analysis of the image-based near field-to-far field transformation
I. LaHaie (ERIM),E. LeBaron (ERIM), November 1996

At last year’s conference we presented the discrete implementation of an image-based near field to far field transform (IB-NFFFT) for predicting far field radar cross-section (RCS) from spherically-scanned near field measurements, along with some preliminary transform results using numerically-simulated data. This paper quantifies this expected performance in terms of the RCS prediction error (RMS dB difference) using numerically-simulated data for two ten wavelength-long canonical bodies, a thin wire and a conesphere. It will be shown that for the highly-resonant wire target, the NFFFT’s algorithm performance is limited by the multiple interactions resulting from the travelling wave reflections between the end of the wire, except at near broadside aspect angles. Conversely, very good performance is obtained for the conesphere at nearly all aspect angles, except very close to nose and tail-on. We will also shown that the IB-NFFFT algorithm performance is robust with respect to clutter and scan angle coverage.

Performance comparison of the analog and digital ramps in a linear-FM chirp RCS measurement radar
D.S. Purdy (NAWCWPNS),J. Piri (NAWCWPNS), N. Cheadle (NAWCWPNS), November 1996

The designer of a linear-FM homodyne RCS measurement system must consider the nonlinearity present in the chirp waveform. Two basic methods employed in obtaining the chirp waveform are to apply either a digital ramp or an analog ramp to a YIG oscillator source. Nonlinearity can occur as the result of the characteristics of the YIG oscillator and the applied ramp waveform. The point spread functions useful in characterizing the performance of both the digital and analog ramp excited YIG oscillator systems are given. Both range resolution and dynamic range of the measurement system are dependent on the target range and can be adversely effected by the nonlinearity. Theory shows that the point spread function of a digital ramp is suitable for short range RCS measurements. However the analog ramp system has improved performance at extended range. By using the analog ramp, we have been able to improve performance of RCS measurements over the digital ramp. Experimental data from both the digital and analog ramp systems are provided.

A Small-size, heavy-duty RCS AZ/EL rotator pylon tip
M. Pinkasy (Orbit Advanced Technologies),A. Geva (Orbit Advanced Technologies), E. Katz (Orbit Advanced Technologies), J. Torenberg (Orbit Advanced Technologies), M. Mena (Orbit Advanced Technologies), November 1996

So far, Azimuth-over-Elevation rotators on RCS pylon tips were of large size (typically 10” for 500 lb. load, over 2’ for a 6000 lb. load). Therefore, RCS measurements of small but heavy targets were very difficult if not impossible to perform. The new design supports loads of 5,000 lb. with an Azimuth turntable diameter of only 136 mm, close to the pylon’s maximum width. The Azimuth and Elevation axes mechanisms are hidden inside the pylon body. The Azimmuth rotator is mounted on the top surface of the elevation main plate. The Elevation plate is attached to the pylon tip on one side and on the other side to the actuator, which is attached to the base of the tip. The actuator drives the Elevation plate to the required rotation angle. Even with its small size, the new design does not compromise on performance. The Azimuth axis moves a full 360° continuous motion at 22 deg/min with 0.03° accuracy, 0.03° backlash and 0.01° repeatability. The Elevation axis moves in a 0°-40° sector at 1.5 deg/min with 0.05° accuracy, 0.05° backlash and 0.01° repeatability.

ISAR imaging using UWB noise radar
E. Walton (The Ohio State University ElectroScience Laboratory),S. Gunawan (The Ohio State University ElectroScience Laboratory), V. Fillimon (The Ohio State University ElectroScience Laboratory), November 1996

It is possible to build a very inexpensive radar which transmits wide band radio noise. On receive, the signal is cross correlated with a delayed version of the transmitted signal. In this paper we will discuss the design and operation of a UWB noise radar which was installed in the OSU compact RCS measurement range. Scattering measurements were made for a number of targets over 360 degrees of aspect angle. Calibration was performed, and then the data converted to ISAR images. Example ISAR images will be shown.

Diagnostic imaging radar system for the F-117A stealth fighter
T.P. Benson (System Planning Corporation),E.V. Sager (System Planning Corporation), November 1996

The U.S. Air Force is currently building deployable Diagnostic Imaging Radar (DIR) systems to perform quality control (QC) low-observable (LO) measurements of the F-117 fighter. Each system is a stepped-pulse frequency synthetic aperture radar (SAR) built by System Planning Corporation (SPC) combined with analytical software developed by MIT Lincoln Laboratory for generating radar images that will be interpreted to ensure LO integrity. The DIR systems will be used at fixed operating sites such as the F-117A main operating base, the F-117A maintenance depot, and any sites worldwide to which the aircraft may deploy. The F-117A DIR is the first field-level deployable radar cross section (RCS) measurement system for an operational weapon platform that is designed for use by the maintenance squadron. This paper discusses the critical issues of QC measurements for LO systems. It also describes the test requirements that are driving the development of DIR, and highlights the radar and SAR positioner requirements. Also presented is an overview of the diagnostic software and the algorithms used for detecting RCS anomalies and predicting maintenance actions for problem correction by flight-line crews.

3-D imaging of a T-72M at 35 and 95 GHz
W. Parnell (TASC),Darrin Lyon (TASC) John Seybold (TASC) Steven Bishop (Air Force Development Test Center), November 1996

Millimeter Wave (MMW) Radar Cross Section (RCS) measurements of full scale ground vehicles are used to develop and validate scattering models for smart weapons applications (target detection, discrimination and classification algorithms) and Hardware-in-the-Loop (HITL) missile simulations. This paper describes a series of MMW RCS measurements performed at Range C-52, Eglin AFB FL on a T-72M in a field environment using an exiting instrumentation radar (with slight modifications to allow for accurate height adjustment) and in-scene phase reference. The test methodology, instrumentation systems, 3-D Imaging Algorithm and sample data sets at 35 and 95 GHz will be presented as well as a detailed sensitivity analysis and discussion of error effects.

Indoor low frequency radar cross section measurements at VHF/UHF bands
A. Bati (Naval Air Warfare Center),D. Hillard (Naval Air Warfare Center) K. Vaccaro (Naval Air Warfare Center) D. Mensa (Integrated Systems Analysts, Incorporated), November 1996

In recent years there has been much interest in developing low frequency radar cross section (RCS) measurement capability indoors. Some of the principal reasons for an indoor environment are high security, all-weather 24-hour operation, and low cost. This paper describes recent efforts to implement VHF/UHF RCS measurement capability down to 100 MHz using the large compact-range collimator in the Bistatic Anechoic Chamber (BAC) at Point Mugu. The process leading to this capability has given rise to a number of technical insights that govern successful test results. An emphasis is placed on calibration and processing methodology and on measurement validation using long cylindrical targets and comparing the results with method-of-moment computer predictions and with measurements made at other facilities.

A 160 GHz polarimetric compact range for scale model RCS measurements
M.J. Coulombe (University of Massachusetts Lowell),J. Neilson (U.S. Army National Ground Intelligence Center), J. Waldman (University of Massachusetts Lowell), S. Carter (U.S. Army National Ground Intelligence Center), T. Horgan (University of Massachusetts Lowell), W. Nixon (U.S. Army National Ground Intelligence Center), November 1996

A fully-polarimetric compact range operating at 160 GHz has been developed for obtaining X-band RCS measurements on 1:16th scale model targets. The transceiver consists of a fast switching, stepped, CW, X-band synthesizer driving dual X16 transmit multiplier chains and dual X16 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and a rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 40 dB are routinely achieved. The compact range reflector consists of a 60” diameter, CNC machined aluminum mirror fed from the side to produce a clean 20” quiet zone. A description of this 160 GHz compact range along with measurement examples are presented in this paper.

Radar cross section range characterization
L.A. Muth (National Institute of Standards and Technology),B. Kent (Wright-Patterson Air Force Base), J. Tuttle (Naval Air Warfare Center) R.C. Wittmann (National Institute of Standards and Technology), November 1996

Radar cross section (RCS) range characterization and certification are essential to improve the quality and accuracy of RCS measurements by establishing consistent standards and practices throughout the RCS industry. Comprehensive characterization and certification programs (to be recommended as standards) are being developed at the National Institute of Standards and Technology (NIST) together with the Government Radar Cross Section Measurement Working Group (RCSMWG). We discuss in detail the long term technical program and the well-defined technical criteria intended to ensure RCS measurement integrity. The determination of significant sources of errors, and a quantitative assessment of their impact on measurement uncertainty is emphasized. We briefly describe ongoing technical work and present some results in the areas of system integrity checks, dynamic and static sphere calibrations, noise and clutter reduction in polarimetric calibrations, quiet-zone evaluation and overall uncertainty analysis of RCS measurement systems.

Radar target scatter (RATSCAT) division low frequency range characterization
M. Husar (Air Force Development Test Center),F. Sokolowski (Johnson Controls World Services, Inc.), November 1996

The RATSCAT Radar Cross Section (RCS) measurement facility at Holloman AFB, NM is working to satisfy DoD and program office desires for certifies RCS data. The first step is to characterize the Low Frequency portion of the RATSCAT Mainsite Integrated Radar Measurement System (IRMS). This step is critical to identifying error budgets, background levels, and calibration procedures to support various test programs with certified data. This paper addresses characterization results in the 150 – 250 MHz frequency range. System noise, clutter, background and generic target measurements are presented and discussed. The use of background subtraction on an outdoor range is reviewed and results are presented. Computer predictions of generic targets are used to help determine measurement accuracy.

Accuracy of RCS measurements
S. Mishra (Canadian Space Agency),C.W. Trueman (Concordia University), November 1996

Some precautions necessary for accurate RCS measurements using a short model range are discussed. Sources of error in these measurements such as non ideal range geometry, misalignment of the target and inappropriate time domain gating are discussed. A simple technique to estimate possible errors in RCS measurements due to factors such as bistatic angle due to finite separation of source and receive horns and finite length of the measurement range, is presented. The range of RCS values that can be measured within defined error bonds is identified.

A Top-down versus bottom-up RCS range certification approach
W.D. Burnside (The Ohio State University ElectroScience Laboratory),E. Walton (The Ohio State University ElectroScience Laboratory), I.J. Gupta (The Ohio State University ElectroScience Laboratory), J.D. Young (The Ohio State University ElectroScience Laboratory), November 1996

A new approach for certification of RCS ranges is discussed. This new approach is based on evaluating the major expected sources of errors in a RCS range rather than evaluating each and every error source and then defining the error bar for a given RCS measurement. The new approach is, therefore, called a top-down approach. Based on our experience with many indoor RCS ranges, we can say that the main sources of errors in RCS measurements are range related. (stray signals, chamber drift, target/mount interactions etc.) One should, therefore, critically evaluate these errors such that the performance level of the range can be verified. A test approach is defined to characterize the range related errors. Various tests are based on the RCS measurement of specific targets, and thus, can be easily performed using standard RCS measurement procedure. This approach will provide range operators with the needed information to justify the use of their range to measure RCS of a given target. Also, one can spend more effort fixing the error sources which lead to large RCS measurement errors.







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