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A Ground to Air Imaging Radar system (GAIR) used to perform diagnostic imaging and total RCS measurements on low observable airborne targets has been developed by the Environmental Research Institute of Michigan (ERIM). In order to ensure accurate measurement of the scatterers contributing to a target's radar signature, proper calibration in imperative. The use of external calibrators to measure the end-to-end system transfer function is the ideal way to perform a system calibration. However, this is a more difficult and challenging task with a ground based radar viewing an airborne target, as opposed to a traditional airborne SAR which views an array of ground based trihedral corner reflectors. This paper will discuss the internal and external calibration methods used in performing an end-to-end system calibration of the GAIR. Primary emphasis is placed upon the external calibration of the GAIR and the three independent measurements utilized: a ground based corner reflector, a sphere drop, and an in-scene calibrator. The system calibration results demonstrate that the GAIR is an accurately calibrated radar system capable of providing calibrated images and total RCS data. Moreover, only the ground and internal measurements are required on a daily basis in order to maintain system calibration
A small compact range measurement facility has been installed at the Environmental Research Institute of Michigan (ERIM) for research aimed at improving RCS measurement and radar imaging techniques. This paper describes the facility, which is referred to as the Experimental Range Facility (ERF). The ERF has two instrumentation radars; a Flam & Russell FR959 gated CW radar and a Hughes MMS-300 pulsed radar. The radars are connected to a suite of workstations, which support a variety of internally and externally developed radar imaging and data exploitation software. The ERF is also equipped with sophisticated target positioning control and sensing equipment.
The U.S.Army Redstone Technical Test Center (RTTC), Test and Evaluation Command, has developed a comprehensive antenna metrology and Radar Cross Section (RCS) evaluation facility. This facility features the compact antenna test range technique for millimeter wave measurements and the near-field scanning technique for microwave measurements. This paper described RTTC's use of these measurement techniques, instrumentation with PC Windows based automation software, anechoic chambers, and types of tests performed. Planned future thrust areas are also discussed.
A complete description is given of the unique radar cross-section (RCS) measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. The uniqueness of this chamber comes from its ability to independently move the transmit and receive antennas, which can each be moved to any position within their respective ranges of motion to a resolution of about 0.05 degrees. The transmit antenna is fixed in azimuth, but can be moved in elevation: the receive antenna is free to move in both azimuth and elevation. Additionally, the target can be rotated in azimuth by means of an azimuth positioner. Analysis has been performed to determine the impact of chamber effects on measurement accuracy. The most notable chamber effect comes from the two large aluminum truss structures, which are the mounting supports for the transmit and receive antennas. Fortunately, the scattering from these structures can be readily separated from the desired target return through the use of range (time) gating. Time domain results are presented showing the effects of these structures.
Numerous monostatic radar cross-section (RCS) calibration routines exist in the literature. Many of these routines have been implemented at the RCS measurement facility built at the Houston Advanced Research Center in The Woodlands, TX. Key monostatic results are presented to give an indication of the measurement accuracy achievable with this chamber. Unfortunately, bistatic calibration routines are not nearly as common in the literature. As with the monostatic routines, a number of bistatic routines have been implemented and typical results are presented. Additionally, descriptions are given for some of the reference targets along with their support structures that are used during calibration.
Composites of the electrically conducting polymer polypyrrole with paper, cotton cloth and polyester fabrics have been evaluated for use in radar absorbing structures. Reflectively measurements on the composites in the range 8-18 GHz and transmission line modelling have revealed impedance characteristics with a common transition region. Relationships between substrate material, polymer loading and electrical performance have been explored. Polarization characteristics have also been measured. The electrical model has been successful in predicting the performance of Salisbury screen and Jaumann multi-layer designs of RAM.
To gain greater insight into the design of surface ships with reduced radar cross-section characteristics, a structure resembling a ship deckhouse was physically modeled and measured. The structure was represented as a truncated pyramid. Four scaled pyramids were fabricated, all identical except for the radii of the four vertical (slanted) edges. The pyramids were measured at the University of Massachusetts, Lowell Research Foundation, submillimeter laser compact range. Measurements were made a scaled X-band using a laser-based system that operates at 585 GHz with the pyramids scaled at a ratio of 1:58.5. These shaper were measured at 0.75 degrees depression angles on a smooth metal ground plane at both HH and VV polarizations. The goal of this study was to determine if small changes in the radius of the curvature of the slanted edges could significantly affect the radar cross-section of the pyramid. In this paper the results of measurements of the pyramids will be presented. The data are compared with computer code predictions and the differences are discussed.
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.