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The design of many modern RCS instrumentation systems is driven by the time required to complete a measurement which establishes the throughput rate of the RCS facility and therefore impacts the operating cost and efficiency. Time considerations are of particular importance when wideband systems are used to measure large targets with low RCS because multiple observations are required to span the frequency band or to increase sensitivity by coherent integration. Although significant improvements have been made to minimize inefficiencies in instrumentation systems, the fundamental limit of measurement time is governed by physical considerations of power, energy, noise, target dimension, and RCS. Evaluating the performance of a particular radar design can be facilitated by comparing the predicted measurement time with a theoretical optimum. The purpose of this paper is to develop estimates of the minimum measurement time under optimum conditions. Although likely precluded by practical considerations, the theoretical limits provide estimates of the maximum degree of radar performance and measures of optimality in practical systems.
The presence of the sea surface has a powerful influence on the scattering characteristics of marine targets during radar cross section (RCS) measurements. To obtain accurate RCS measurements of a large, distributed marine target, the radar site must satisfy various requirements. The major requirement is to provide quality RCS data without strong multipath distortion of the target return signal. In this paper multipath effects on a large scatterer measured at both low-and high-elevation radar sites are summarized. It is observed that multipath effects contribute strongly to the RCS of the target measured at a low elevation radar site. The data show large RCS fluctuations of more than 15 dB when a scatterer is measured at difference altitudes or ranges. The quality of the data measured at a low-elevation radar site then becomes questionable, which creates difficulties in assessing the true RCS of the target. For diagnostic purposes, it may be necessary to change the target range or altitude several times to make a credible assessment of RCS. The same target measured at a high-elevation site has less multipath influence on the RCS data, making assessment of the true RCS feasible.
A recently completed Hughes program successfully demonstrated an airborne multi-spectral (VHF through X-Band) Synthetic Aperture Radar (SAR) measurement of the radar cross section (RCS) of an aircraft in flight, producing two-dimensional (2-D) diagnostic RCS images of the test aircraft. Ground-to-air imaging of full-scale aircraft was demonstrated by Hughes in 1990. In early 1992, a Hughes A-3 aircraft made air-to-air radar images of a test aircraft in flight. To date, Hughes has collected imagery on nine aircraft from VHF through X-Band, including nose, side and tail aspects at several elevation angles. Reference (2) describes the VHF/UHF capability of the imaging system and this paper will describe the image processing steps developed and will display S- and X-Band radar images with resolution as fine as 6 x 4 inches. The images presented in this paper are dominated by a few very large cavity-type scatterers and do not show the ultimate sensitivity and fidelity of the system. The air-to-air images do demonstrate the spectacular diagnostic utility of this technology.
A recently completed Hughes program successfully demonstrated an airborne multi-spectral (VHF through X-Band) Synthetic Aperture Radar (SAR) measurement of the radar cross section (RCS) of an aircraft in flight, producing two-dimensional (2-D) diagnostic RCS images of the test aircraft. The Air-to-Air Radar Imaging Program was a multi-phase program to develop, demonstrate and exploit this new technology for the design and evaluation of advanced technology aircraft. Radar images with resolution as fine as 6 x 4 inches were produced. To date, Hughes has collected imagery on nine aircraft from VHE through X-Band, including nose, side and tail aspects at several elevation angles. The ability to generate a radar image while in flight is a significant technical achievement. The VHF images presented demonstrate the utility of the system but the images do not show the ultimate sensitivity and fidelity of the system because the aircraft presented in this paper are dominated by a few very large cavity-type scatterers. The ability to measure the VHF/UHS RCS of an aircraft in flight and to make high resolution images is one of the major accomplishments of this program. VHF/UHF in-flight images, never achieved before this program, are a powerful diagnostic tool for use in aircraft development.
Unique instrumentation is required for dynamic (in-flight) measurements of aircraft radar cross section (RCS), jammer-to-signal (J/S), or chaff signature. The resulting scintillation of the radar echo of a dynamic target requires special data collection and processing techniques to ensure the integrity of RCS measurements. Sufficient data in each resolution aspect cell is required for an accurate representation of the target's signature. Dynamic RCS instrumentation location, flight profiles, data sampling rates, and number of simultaneous measurements at different frequencies are important factors in determining flight time. The Chesapeake Test Range (CTR), NAVAIRWARCENACDIV, Patuxent River, Maryland, is a leader in quality dynamic in-flight RCS, J/S ratio, and chaff measurements of air vehicles. The facility is comprised of several integrated range facilities including range control, radar tracking, telemetry, data acquisition, and real-time data processing and display.
RCS measurement accuracy is degraded by reflections occurring between the feed antenna, the range, and the radar subsystem. These reflections produce errors which appear in the image domain (both 1-D and 2-D). The errors are proportional to the RCS magnitude of the target under test and they are present in each of the typical range calibration measurements. Current 2-term error models do not predict or account for the above errors. An improved 8-term error model is developed to do so. The model is based on measurable reflections and losses within the range, the feed antenna, and the radar. By combining the improved error model with the commonly used 2-term RCS range calibration equation, we are able to quantify the residual RCS errors. The improved error model is validated with measured results on a direct illumination range and is used to develop specific techniques which can improve RCS measurement accuracy.
The paper discusses the results from a series of experiments to measure the dynamic radar cross section (RCS) for high-velocity targets at millimeter wave (60GHz). The low observable nature and detectability of the threats at millimeter wave are addressed. Date processing will provide calibrated dynamic RCS time series, from which RCS scintillation analysis and detection modeling can proceed. The data collection, reduction, analysis and target Doppler signatures are addressed.
Many current, and near future, antenna and Radar cross section measurement requirements dictate improvement in microwave power amplifier performance and capabilities. Maturing of Traveling Wave Tube (TWT) technology and breakthroughs in modulator and power supply design now enable exploitation of the maximum possible RF performance from TWTs.
Calibrated measured results are presented that characterize the performance of a rhombic shaped TEM parallel plate horn antenna to transmit and receive ultra-wide bandwidth (UWB) waveforms, the standard narrow band antenna parameters, such as gain, are inadequate in characterizing the antenna. In this paper, the antenna is viewed as a transducer in which the transmitting and receiving antenna can be fully described by complex transfer functions. These functions provide a more natural means of characterizing an antenna for UWB applications. The time domain transmit and receive transfer functions of our test antenna are presented in a contour map as a function of angle for the two principal planes, and the responses are correlated to physical attributes. In addition, the waveform dispersion and the total received energy for a bandwidth limited impulse excitation are used to characterize its use for UWB synthetic aperture radar (SAR) applications.
In an airborne radar, the aircraft motion causes the returns from stationary ground clutter to be spread over a significant part of the Doppler space. Without flight testing, it is difficult to develop or verify the clutter suppression techniques required by future airborne radars. In this paper a technique is presented for emulating the angle and Doppler characteristics of airborne radar clutter from a fixed site, for the purpose of ground-based testing. An inverse displaced phase center antenna (IDPCA) is used to simulate the aircraft motion. The inverse displaced phase center antenna described is an 18-element linear UHF array whose phase center can be electronically shifted by means of a switching matrix. The motion emulation capability is demonstrated through the use of this antenna as an auxiliary array in conjunction with a stationary UHF surveillance radar. Examples of the clutter returns received by this system are given.
Mathematical techniques (calibration, background subtraction, software range gating, imaging, etc.) have become integral to the process of generating precision radar cross section measurements. The "reference target method" is a powerful RCS correction algorithm which yields plane wave illumination results from data acquired under an arbitrary but known illumination. This method is analogous to a two dimensional RCS calibration. Measurements of long bars (at X- and Ku-bands) and of a scale model aircraft (at C-band) were performed under the cylindrical wave illumination produced by March Microwave's Single-Plane Collimating Range (SPCR) at Arizona State University. The targets were also measured under the quasi-plane wave illumination produced by a March Microwave dual parabolic-cylinder CATR. The SPCR measurements were corrected using the reference target method. The corrected SPCR measurements are in good agreement with the CATR measurements.
Lincoln Laboratory has developed a high resolution imaging radar in conjunction with Flam and Russell, Inc. or Horsham, PA. The radar is a highly mobile, ground based system that is capable of two and three-dimensional imaging at very close ranges to a synthetic aperture. The radar is fully coherent from 0.1 to 18 GHz and transmits CW pulses that are stepped in frequency across a preselected bandwidth. High range resolution is achieved by coherently processing the returned signals. The radar is being used for target imaging and for foliage penetration measurements.
The dynamic, polarization/frequency diverse, Instrumentation Radar System (IRS) described herein combines the features of an X-band radar tracker with a wideband, fully polarimetric coherent data collection system. Mounted in a transportable trailer, the system can be towed to virtually any site to acquire radar signature measurements on moving aircraft. Specifically, this system can collect the complete, polarimetric target scattering matrix as a function of frequency in real time from all three traditional monopulse channels, as well as from the usually terminated diagonal difference channel. The acquired data can be used for multidimensional images, or for studying the characteristics and performance of monopulse trackers following real targets.
METRATEK has completed a highly successful program to prove the feasibility of high-resolution, air-to-air diagnostic radar cross section imaging of large aircraft in flight. Experience with the system has proven that large aircraft can indeed be imaged in flight with the same quality and calibration accuracy that can be achieved with indoor and outdoor ranges. This paper addresses the results of those measurements and the Model 100 AIRSAR radar and processing system that were used on this program.
A portable measurement system has been designed and implemented to produce focused three dimensional RCS images. The Synthetic Aperture Radar (SAR) system was especially designed to operate in harsh physical and cluttered electromagnetic environments. The acquisition system, signal processing and 3D visualization capabilities are discussed and representative data ranging from simple canonical objects to production hardware are presented. The technique meets its design goal in effectively discriminating undesired clutter.
ISAR images and RCS signatures of aircract-in-flight using a ground based and an airborne radar system are presented. The ground-based measurements were at X-band and were of a Mooney 231 aircraft, which flew in a controlled path in both clockwise and counterclockwise orbits, and successiely with gear down, flaps in the take-off position and with the speed brakes up. The air-to-air measurements were made by a radar installed in the nose of the TA-3B aircraft which followed a KC 135 airplane at a range of approximately 450 ft. and traversed a cross-range angle component of (plus or minus) 30(degrees). The data indicates that these systems are useful tools for RCS signature diagnostics of aircraft in flight.
Recently, super resolution algorithm have been used in radar target imaging to increase the down range and/or the cross range resolution. In the open literature, however, the super resolution algorithms have been applied to simulated targets or very simple targets measured in a test range. In this paper, the super resolution algorithms, namely the hybrid algorithm and the 2-D linear prediction, are applied to more realistic targets. One of the targets is a flat plate model of the F-117 aircraft. The back-scattered fields of the flat plate model were measured in a compact range. The other target is a Mooney 231 aircraft. The aircraft was flown in a circular pattern approximately 10 miles from the radar. It is shown that the super resolution algorithm can be successfully applied to these targets.
A microwave diversity imagery based on parametric modeling of back scattered signal versus frequency and aspect is presented. Forward-backward linear prediction is used to compute the model parameters. After stabilizing the corresponding transfer function, data are extrapolated to adjacent frequencies or aspects. Superior range- and/or cross-range resolution can be obtained by using frequency- and/or aspect-extrapolated data. Cross-range resolution can also be enhanced by extrapolating the frequency data and using data at a higher center frequency. For severly (sic) restricted viewing angles, or for small radar bandwidth, the new imagery can significantly improve the image resolution.
This paper describes an effort to evaluate the effect on RCS of base closures on a metallic frustrum at various depths with conducting and electrically isolated plugs. The tests were conducted at Sandia National Laboratories using System Planning Corporation's (SPC) Mark IV radar from 8 to 18 GHz, in the step chirped Inverse Synthetic Aperture Radar (ISAR) mode. Data reduction was performed on Information Systems and Research's workstation using the KNOWBELL software package. The workstation allowed the study of the imagery data in many different modes, which assisted in determining ways to evaluate RCS matching.
Coherent subtraction algorithms, such as specular subtraction, require precision target alignment with the imaging radar. A few degrees of phase change could significantly degrade the performance of coherent subtraction algorithms. This paper provides an analysis of target position measurement errors have on ISAR data. The paper addresses how traditional position errors impact phase and image focusing. Target rotational positioning errors are also evaluated for their impact on magnitude errors from specular misalignment and polarization sensitive scattering and image phase errors from height-of-focus limitations. Several tables of data provide a useful reference to ISAR data experimenters and users.