AMTA Paper Archive

In the past, most radar-cross-section imaging has been done after data has been taken. At best, this off-line processing generates images that are returned to a customer the next day. Many projects can benefit significantly by having concurrent imaging and data acquisition. This allows for real-time cause and effect type diagnostics without rescheduling range time. As RCS range time becomes increasingly more expensive and difficult to schedule, real-time imaging provides the project engineer with a valuable tool to optimally use his range time. A technique has been developed to render real-time radar cross section images while acquiring data. All image processing is performed to achieve a fully enhanced image. Focussing, interpolation, and windowing are all used to give a detailed image. The system uses a Hewlett Packard 8510B for data taking and Hewlett Packard computers for data acquisition and image processing.

RCS instrumentation systems capable of combining wide-band and ISAR techniques to obtain two-dimensional images are widely used to perform RCS diagnostic and measurement functions. Objects involving rotating structures, such as blades of propulsion systems complicate the diagnostic task. The paper address the utilization of diagnostic RCS systems to meaningfully determine the radar signatures of objects with rotating components and presents results obtained from a generic data set, typically available from wide-band RCS instrumentation systems. The results provide valuable insight to the signature of objects with rotating components.

As in any real world measurement application, there are performance trade offs to be made between hardware and software gating in RCS measurements. Hardware gating adds significant complexity and cost to the measurement system and, incorrectly applied, can result in degraded measurement performance. A measurement system based on software gating is the simplest and most cost effective to implement but may have performance limitations on some types of ranges. The objective of this papers to provide useful guidelines to use in choosing between an HP 8510B RCS measurement system based on software gating, hardware gating, or a combination of the two approaches.

This paper considers hardware and software issues associated with accurate RCS, antenna, and near field antenna measurements. In particular we examine methods for making accurate measurements at high speed using existing network analysis equipment, such as the HP8510B. Techniques which allow for fundamental mixing are examined from the viewpoint of enhanced dynamic range and speed. Harmonic mixing techniques are also discussed and limitations related to IF bandwidth and harmonic locking are presented. The realtime requirements of software systems for these applications are presented and operating system considerations are analyzed. Interface attributes are examined with a view toward use with multi-tasking operating systems and the real-time requirements of high speed measurement systems.

Precise and complete measurements of advanced electromagnetic systems demand dramatically higher data acquisition speeds than those commonly attainable. Specific challenges include requirements for wideband measurements with arbitrarily spaced frequency steps. These types of measurements are often encountered in characterizing EW/ECM systems, radars, communications systems, and in performing antenna and RCS measurements. The Scientific-Atlanta Model 1795 Microwave Receiver offers capabilities directly applicable to solving measurement problems posed by highly frequency agile systems. These problems include: 1) timing constraints 2) data throughput 3) RF interfacing 4) maintaining high accuracy A technique is discussed which shows the application of the Model 1795 Microwave Receiver in its high frequency agility mode of operation. Measurement examples are presented showing the advantages gained compared to previous methods and instrumentation configurations.

A major concern for any user of a compact range for RCS or antenna measurements is the quality of the wavefront over the quiet zone and background chamber levels at the desired frequency band. Amplitude and phase ripple in the quiet zone is an indicator of how well the electromagnetic energy is collimated coherently by the reflector system. The amount of ripple depends on the reflector system, reflector edge treatment to reduce diffraction, frequency band and chamber interactions. Edge treatment techniques such as serrations on the reflector edge helps to reduce diffraction of unwanted energy into the quiet zone. Constructive and destructive interference of diffracted of energy in the quiet zone causes the amplitude and phase ripple. The goal is to reduce the ripple to a minimal amount. Previous studies by the author have compared two-way and angle transform field scanning techniques. The results strongly indicate that both techniques provide good agreement. The two-way method has the disadvantage of strong dependence on the scanning target directivity. A directive target will tend to disregard diffraction from the reflector edges because of its low sidelobes. Its advantage is that there is no need for external mixing equipment for the microwave receiver. The angle transform is simple in configuration consisting of a narrow flat plane or bar mounted on an azimuth positioner and rotated. The disadvantage is a summing of energy in the zero-doppler cell yielding an artifact ripple. Both of these methods also depend upon software gating algorithms including gate shape and width which directly influence the amplitude and phase ripple. The aim of this study is to compare the two-way, one-way field scanning techniques and the angle transform method. Can comparisons be made between the methods? Can a fairly good agreement be made? Are multi-path considerations addressed in one-way scan techniques? Hughes Aircraft will use one of the compact ranges at the Antenna Test Facility of Motorola GEG at Scottsdale, Arizona with the March Microwave (Vokurka) dual reflector system. Field scans will be measured using both the two-way and one-way techniques. The two-way method will use the 8 cm diameter disk as the scanning target, mounted on a horizontally traversed scanner. The one-way method will use a standard gain horn mounted on the same scanner. The angle transform method will use an 8 ft narrow flat plate rotated in the quiet zone. The field scans will be measured and studied at 10 GHz.

Accurate calibration methods are of essential importance in RCS measurements. First, absolute RCS determination (in dBsm) can be carried out accurately provided a correct algorithm is used describing the RCS dependence of some reference target at all frequencies. Unfortunately, this technique gives error-free calibrated data at one position only. In this paper a new technique for qualifying of RCS ranges will be described. A reference target with well-known RCS response is used during the calibration measurement. The amplitude and phase distributions are then computed for all required positions within the test zone. Finally, an error estimate in measured RCS responses can be made by using two other application programs.

The Vulnerability Assessment Laboratory (VAL) anechoic chamber at White Sands Missile Range, New Mexico was reconfigured and refurbished during the last part of 1988. This paper will be a facility description of the state-of-the-art Special Electromagnetic Interference (SEMI) investigation facility. Electromagnetic susceptibility and vulnerability investigations of US and, in some cases, foreign weapon systems are conducted by the EW experts in the Technology and Advanced Concepts (TAC) Division of VAL. EMI investigations have recently been completed on both the UH-50A BLACKHAWK and AH-64A Apache helicopters in the chamber. The paper will cover the facility's three anechoic chambers, shielded RF instrumentation bay, computer facilities for EM coupling analyses, and the myriad of antenna, antenna pattern measurement, amplifier, electronic, and support instrumentation equipment for the chambers. A radar cross section measurement and an off-line RCS data processing station are also included in the facility.

This paper evaluates the RCS errors associated with measuring a large flat plate which is illuminated by a compact range reflector with significant edge diffraction stray signals. This is done by evaluating the true fields incident on the plate and then using a physical optics technique to predict the backscattered fields. Results are compared with and without the edge diffracted fields present. A simple analytic expression is developed which can approximate the size of this potential error.

Image gating or editing is often used to determine the effect of an isolated scatterer on the RCS in the frequency and aspect-angle domain. In this paper, theoretical computations indicating limits in the image-gating procedure will be presented. The process provides the image-gating capability in combination with phase-corrected (focused) imaging. Targets consisting of two-point scatterers with well-known RCS response have been used. One of the scatterers is gated out and the resulting RCS versus frequency or aspect angle is determined and compared with its theoretical value. Limits in terms of minimum bandwidth or minimum distance in resolution-cell sizes are defined. The influence of several gate shapes and windows have also been examined. Experimental investigation has been carried out in order to verify the theory.

Under many circumstances it is necessary to experimentally estimate the radar cross section of targets in a cluttered environment. A significant reduction in the clutter can be obtained when cross range filtering can be done. In this experimental RC measurement concept, scattering measurements are performed using a moving radar antenna. Thus scattering as a function of target plus clutter versus aspect angle in the near field can be measured. Next, a back projection algorithm can be used to estimate the scattering as a function of position in the neighborhood of the target. The known range to which the signal is to be focussed is used to project back to the target area. An estimate of the RCS at points along a line in the plane of the target is computed. The clutter responses can then be removed from the data, and the remaining target-only values projected forward again (possibly to the far field) to estimate the RCS of the target alone.

An RCS measurement error model, calibration procedure and correction algorithm are discussed. A distinction between frequency response reflections and range-target reflections is made. Special emphasis is placed on the selection of the gate span with time gating used with the calibration and test target measurements. Mathematical simulations and actual measurements illustrate the discussion. It is concluded that frequency response related reflections must and range-target reflections must not be included in the gate for the frequency response calibration measurement.

Serrations are used on Compact Antenna Test Range reflectors to reduce the effects of edge diffraction. It has been found that the traditional triangular shape for these serrations is not optimal and that more continuous shapes should perform better. To verify this, RCS measurements were performed on test targets consisting of strip reflectors terminated by end sections of various shapes. The RCS vs. angle data were corrected for the field irregularities caused by the measurement range and then converted to the induced current distributions on the targets, from which the fields in front of the targets were calculated using Physical Optics. These fields are equivalent to the test-zone fields of an actual Compact Range. The results are compared with theoretical data. The agreement is good.

To fulfill the future demand of highly accurate antenna-, RCS- and payload testing, MBB built a new antenna test centre at Ottobrunn (Ref. 1). This paper describes the development and qualification of the large, dual reflector Compact Range (CR) which has a plane wave zone of 5.5 x 5.0 x 6.0 m (w x h x d). It starts with the results of a detailed electrical trade-off study between different CR-concepts, followed by some mechanical/thermal construction aspects of the large, highly accurate reflectors. Finally, some qualification results are shown, covering the frequency range from 3.5 GHz up to 200 GHz (lowest frequency of operation approx. 2 GHz). The achieved plane wave performance (amplitude ripple ±5o, phase ripple ±5o, cross-polarization isolation > 40 dB) verifies the high quality overall system design.

Dielectric straps can support very heavy targets and have a low radar cross section (RCS), especially at low frequencies (below 8 GHz). In this paper, the scattered fields of dielectric straps surrounding a perfectly conducting structure are presented, and the computed results are compared with experimental data. Empirical formulas for the strap scattered fields are also given. These formulas are good for general convex structures and are expected to give a reasonable estimate of the true RCS of the dielectric straps when used as a target support structure.

A stressed-skin inflatable target support provides an improvement over a foam column for radar cross section (RCS) measurements in an anechoic chamber. Theoretical analysis indicates that backscatter from the support is minimized because its mass is reduced below that of a foam column and is distributed to favor incoherent scattering. Compared with a foam column, a pressurized thin shell has superior mechanical stability under both axial and transverse loads. Experimental observations using Mylar -- a low dielectric constant, high tensile strength film -- confirm these results. Spurious reflections from rotational machinery located below an inflatable column are reduced by a layer of absorber within the base of the inflatable support.

The requirement to measure lower radar cross-section (RCS) levels within anechoic chambers has demonstrated the need to further analyze the performance of microwave absorbers. The interactions of the feed system, compact range reflector, target mount, and target/test body with the microwave absorber greatly effect both the measurement accuracy and ambient noise level within the anechoic chamber. Better absorber characterization and understanding leads to improved chamber performance analysis and chamber design modeling. Past absorber studies have evaluated the backscatter performance of most absorber types, however, bistatic performance characterizations have been limited. This paper will discuss a method of obtaining bistatic absorber data which offers the advantages of time gating and synthetic aperture imaging to improve measurement isolation and accuracy. The approach involves illuminating a large absorber test wall about several incidence angles with the plane wave generated by a compact range. A receive antenna is then moved about the test wall and bistatic scattering is observed. The technique provides improved measurement results over methods utilizing NRL arch type systems. Bistatic absorber data has been collected and analyzed over angles from normal to near grazing incidence. Test results will be demonstrated with different absorber shapes, sizes, orientations, and material transitions from wedge to pyramidal. Various bistatic conditions will be analyzed for both polarizations over a number of frequencies.

Evaluation methods for analyzing the performance of anechoic chambers have typically been limited to field probing, free space VSWR and pattern comparison techniques. These methods usually allow the users of such chambers to qualify or determine the amount of measurement accuracy achievable for a given test configuration. However, these methods in general do not allow the user to easily identify the reasons for limited or degraded performance. This paper presents a method based on synthetic aperture imagery which has been found usable for finding and identifying anechoic chamber performance problems. Photographs and illustrations of a working SAR imaging/mapping system are shown. Discussions are also given regarding the method's advantages and disadvantages, system requirements and limitations, focusing processing requirements, calibration techniques, and hardware setups. Both monostatic and bistatic configurations are considered and both RCS and antenna applications are discussed. The SAR system constructed to date makes use of a portable HP-8510 based radar placed on a hydraulic manlift for easy system maneuverability and flexibility. The radar antenna is mounted on an 8 foot mechanical scanner directed toward the area to be mapped. An image is processed after each scan of the receive antenna. Measured data and example results obtained using the mapping system are presented which demonstrate the system's capabilities.

Circularly polarized radar cross-section (RCS) measurements place stringent requirements on an RCS range. Indoor compact ranges without the problems of ground reflections have the potential of making accurate circular polarization (CP) measurements. A simple method for CP RCS measurements is described using broadband meander-line polarizers over the compact range feed horns. Axial ratio and differential phase measurements were performed to evaluate the polarizer fabrication accuracy. Basic scattering shapes were measured to test the performance of the CP measurement system. Comparison of CP measurements with analytical predictions demonstrated the success and limitations of the technique.

Compact range facilities designed for RCS measurements have exhibited a performance-limiting effect commonly referred to as "feed ringing". "Feed ringing" is a phenomenon in which energy is stored in or about the RF feed structure and is sustained for a sufficient period off time after the source is turned off such that its presence contaminates the true target return. This effect has placed severe constraints on the design of the RF feed for the compact range, particularly in regard to its operating bandwidth. This paper presents the design of a lossless, waveguide type RF feed suitable for compact range application with a demonstrated useful bandwidth approaching a full octave.