AMTA Paper Archive

This paper presents some current measurement results obtained as part of a research program to investigate the theory, technique, apparatus and practicality of monostatic near-field radar cross-section measurement (MNFRCSM).

The two measurements PCAL / PMRC and PTARG / PMRT are ratioed and the PMRC / PMRT term accounts for changes in both power or phase since calibration, because the mid-range is of fixed RCS size and phase. Using this technique, Scientific-Atlanta has been able to hold calibrations to within 0.5 dB amplitude and 8 degrees phase for as long as 12 hours. This includes outdoor range effects.

The Compact Range RCS Measurement System is comprised of the Harris Shaped Compact Range and the Hughes Short Pulse Coherent RCS Measurement System. The range offers a 10 foot spherical quiet zone with less than ±0.25 dB amplitude ripple, 0.2 dB amplitude taper, and ±2 degrees phase ripple. The short pulse system offers a pulse width as small as 5 nsec with range gate increments of 100 psec minimum. The system has a sensitivity of –70 dBsm without integration and –120 dBsm with 50 dB of coherent integration. System linearity is better that ±0.5 dB over the 70 dB instantaneous dynamic range. The Shaped Compact Range offers nearly 98 percent illumination efficiency with negligible spillover which minimizes the required anechoic chamber size and the amount of absorbing material necessary. The block diagram of the system is shown in Figure 1.

The accurate measurement of radar target scattering properties is becoming increasingly important in the development of stealth technology. This paper describes a low cost imaging Radar Cross Section (RCS) instrumentation radar capable of measuring both the amplitude and phase response of low RCS targets. The RCS instrumentation radar uses wideband FM wave-forms to achieve fine range resolution providing RCS data as a function of range, frequency and aspect. With additional data processing the radar can produce fully focused Inverse Synthetic Aperture Radar (ISAR) images and perform near field transformations of the data to correct the phase curvature across the target region. The radar achieves a range resolution of 4 inches at S-band and a sensitivity of –70 dBsm at a 30 ft range.

There has been renewed interest in RCS measurements recently especially for the evaluation of low backscatter targets. In order to accurately measure such targets, one needs to evaluate the system performance for such applications. One such performance check is the field quality measured within the target test volume. The question is then asked, “What is a satisfactory amplitude and phase requirement?” The normal 1 dB amplitude specification is not satisfactory because it doesn’t indicate whether the error is due to a field taper or ripple. Taper indicates a uniform phase but variable amplitude; while, ripple indicates the presence of a stray signal. This paper indicates why one should require less than a 1/10th dD ripple error for low RCS target measurements; whereas, a dB taper is satisfactory.

Inverse synthetic aperture radar (ISAR) imaging is used to produce high cross-range and down-range resolution on objects undergoing a change of aspect angle relative to the radar. In this application, the ISAR technique was used on an outdoor ground-bounce radar cross-section (RCS) measurement range. The objective is to locate, identify, and quantify the scattering properties of the target-under-test (TUT). The TUT is mounted well above ground on a target pole and can rotate in azimuth and elevation. The TUT’s rotational motion about an axis perpendicular to the radar line of sight is used to produce the cross-range resolution. For range resolution, a high-bandwidth frequency stepped waveform is used. The data are processed entirely in the digital domain with an algorithm that consists of a procedure to remove the dispersive properties and amplitude variations of the complete end-to-end range response, followed by a two-dimensional, polar-to-rectangular resampling filter and a two-dimensional fast Fourier transform (FFT). The processor has achieved images with amplitude and distortion products that are below the system’s noise floor with up to 48 dB of processing gain. The radar imagery is presented to the RCS engineer on a high-resolution color graphics terminal with true-perspective color-coded RCS displays in logarithmic amplitude or linear phase scales. The design of the ISAR processing algorithm is described in this paper as are the results for both simulated and actual radar data.

In any antenna or RCS measurement range, the walls, floor, and ceiling are covered with radar absorbing material (RAM) so that spurious scattering will be reduced. The bistatic scattering characteristics of these walls etc. are often not accurately known, however. This situation is exacerbated by the techniques often used to measure the scattering characteristics of the RAM used on the walls etc. The measurement techniques are typically “arch type” measurements, where the scattering from a section of absorber (often 3x3 feet) is compared to that scattered by a conducting plate of the same size. These type measurements are often corrupted by edge and corner diffraction terms and the results are often not very accurate.

This paper briefly discusses the calibration techniques used in the Sandia National Laboratories Radar Cross-Section Test Range (SCATTER). We begin with a discussion of RCS calibration in general and progress to a description of how the range, electronics, and design requirements impacted and were impacted by system calibration. Discussions of calibration of the electronic signal path, the range reference used in the system, and target calibration in parallel and cross-polarization modes follow. We conclude with a discussion of ongoing efforts to improve calibration quality and operational efficiency. For an overview description of the SCATTER facility, the reader is referred to the article Sandia SCATTER Facility, also in this publication.

Versatile test bodies are extremely useful for RCS measurement facilities for many reasons, some of which are listed below: 1) evaluate the performance achievable for a given measurement facility 2) measure the RCS of components normally mounted on a ground plane, and 3) terminate a target pedestal in order to measure its cross-section since most pedestals are designed to attach directly to a target. In order to perform all of these functions a versatile test body should have flat sections to mount components efficiently, it should have a known smooth cross-section with angle of incidence from very low values to large ones, it should not use absorber that could attenuate the signal meant to illuminate the component pieces being tested, etc. Several such test bodies have been studied, some of which will be described.

The design of a multipurpose Antenna/RCS range at GTRI is described. A novel approach to design of the far-field antenna range utilizes the bottom 40-foot section of a 130-foot windmill tower. The top 90-foot section is used as the main support for a slant RCS measurement range offering a maximum depression angle of 32º. A 100-tom capacity turntable, capable of rotating an M1 Tank, is located 150 feet from the 90-foot tower. The rigidity and stability of the tower should allow accurate phase measurement at 95 GHz for wind speeds up to 10 mph. In addition, a 500-foot scale-model range uses the ground plane effect to enhance target signal-to-noise and is designed to be useful at frequencies up to 18 GHz. Initially, the radar instrumentation to be utilized with the ranges includes several modular instrumentation systems and associated digital data acquisition equipment at frequency bands including C, X, Ku, Ka, and 95 GHz. The properties of these systems, which include coherence, frequency agility, and dual polarization, are discussed.

Pulsed systems have been used for many years to eliminate unwanted clutter in RCS measurements, but have not been used much for antenna measurements, even though similar clutter problems are common to both. There are many reasons for this, such as cost, increased bandwidth requirements, lack of necessary hardware, etc. However, with the development of modern pin diode switches, one can construct a low cost pulsed measurement system that simply adds to existing CW equipment. Using the system design presented in this paper, one can eliminate unwanted clutter from antenna measurements simply by adjusting the transmit and receive pulse widths and the delay between them. For example, it can be used to range gate out the ground bounce for outdoor measurements or the backwall for an indoor facility so that one can accurately measure the backlobe of a high gain antenna. The pulsed system is presented along with several measured examples of its use.

Flam & Russell, Inc. has developed a short pulse radar cross section measurement system (Model 8101) which operates from VHF up to L band. This paper describes operation of the system, with emphasis given to the design considerations necessary to minimize susceptibility to a number of problems that have imposed serious limitations on achievable sensitivity at lower frequencies in pulsed RCS outdoor measurement systems. These problems have been, to a great extent, solved in the current system design. The system has been designed for use in outdoor range facilities with a variety of target sizes. A w ideband, high power transmitter is capable of producing pulses 50-350 nanoseconds wide at peak levels of up to several kilowatts. A phase coherent wide bandwidth receiver provides amplitude and phase information at video for sampling. A maximum of four independently located range gates may be selected and set with a resolution of one nanosecond. The data collection system features a three-tier processor structure for dedicated position data processing, target data processing, and system I/O and control, respectively. A real time display of RCS versus position coordinate is available to the operator, as well as a real time indication of the presence of radio frequency interference (RFI). A 60 foot reflector antenna equipped with a duo polarized feed provides full scattering matrix capability with 30 dB of polarization isolation and better than 50 dB of "ghost" suppression. Careful antenna structure and transmission line design has eliminated reverberation or "pulse ringing" problems. A radar "figure of merit" (ratio of peak transmitted power to receiver noise floor for the required pulse bandwidth) of better than 150 dB has been achieved.

In this paper we consider those factors having primary impact on submicrowave RCS measurements in outdoor (ground-bounce range) environments, including: 1. The target illumination problem, reflecting fundamental limits on antenna size and height 2. Measurement sensitivity as limited by thermal noise and radar frequency interference (RFI) 3. Antenna selection at VHF frequencies 4. Ground-bounce effects near Brewster's angle. 5. Clutter (due to either terrain or target support) and clutter suppression techniques. Some improvements to basic RCS measurement range design are analyzed in detail, with emphasis on mobile (variable range) antenna/radar systems.

Prediction methods currently being developed for estimating the Radar Cross Section (RCS) of a complex target are based on the concurrent use of different numerical techniques each being employed in the region where it performs best. Since the high frequency techniques and the numerical methods used in the computation must deal with important rapid phase and amplitude fluctuations of the resultant scattered field, it is sometimes very difficult or impossible to know to what extent the computed solution is valid, unless measurements are available for comparison purposes.

Gating is a widely used technique of improving RCS measurements. However, the exact type of gating used has a dramatic effect on such parameters as dynamic range and clutter rejection. Time Domain Gating offers significant advantages over software gating as used in some network and spectrum analyzers. This paper explores a technique used by Scientific-Atlanta in CW and FMCW RCS measurements. With the adaptation of an external computer controlled hardware gating unit, existing RCS and antenna systems can be retrofitted for significant performance improvements.

The ogival target-support pedestal as shown in Figure 1 is claimed to have a low radar cross section (RCS); yet, it can handle very large and heavy structures. This paper attempts to find out whether this claim is true through analysis as well as measurements. The pedestal backscatter is just one aspect of this study. Another more serious issue is associated with the bistatic scattering by the pedestal which influences the target illumination. * This work was supported in part by the National Aeronautic and Space Administration, Langley Research Center, Hampton, Virginia, under Grant NSG-1613 with The Ohio State University Research Foundation.

This paper discusses some of the applications of high-resolution coherent radar image processing techniques in unimproved indoor facilities. The techniques are particularly useful in situations where traditional indoor range chambers are unavailable or impractical. Experiments in an 18-foot-high warehouse building have shown that useful measurements can be made at close quarters, in a high-clutter environment.

In recent years many of the problems making RCS measurements on a compact range have been addressed [1,2,3]. Factors such as ripple and taper in the target zone have been analyzed and existance of lower level effects such as stray radiation in the chamber. This paper discusses this problem and the way it was addressed in the design of the Harris Model 1606 Compact Range shown in Figure 1, 2 and 3. This range was designed to operate from 2 to 18 GHz with a six foot quiet zone with extension of the frequency range to 95 GHz possible.

This paper reviews procedures and techniques employed for calibrating antennas used in electromagnetic compatibility (EMC) measurements. Details of our measurement procedure and results using the TEM cell and the three antenna methods are described.

This paper presents data from facility evaluation tasks on current projects. The data were obtained on outdoor free-space pattern test facilities, and in anechoic chamber RCS test facilities.