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The indoor compact range has proven to be quite successful in measuring the radar cross section (RCS) of various targets. As the performance capabilities of the compact range have expanded, the use of larger, heavier, and more sophisticated targets has also expanded. Early target dimensions were limited by the size of the useful test area, as well as the capacities of the low RCS pedestal mount used. Today, our anechoic chamber has a large useful test area, thus the size and weight of targets dictate that a new method be employed in target handling and positioning, as well as target mounting to a low RCS pedestal. Work was recently completed here at the Ohio State University ElectroScience Laboratory to remodel our anechoic chamber to allow for the new generation of targets and the demands that they place on the anechoic chamber. This work included the addition of a one ton motorized underhung bridge crane to our anechoic chamber, the design and construction of an hydraulic assist to smoothly and precisely raise and lower the target for the final linkup of the support column and the receiving hole in the target, the design and installation of a one ton telescopic crane in the chamber annex to link with the main chamber crane, the design and installation of the necessary microwave treatments to minimize the impact of the remodeling on accurate RCS measurements, the development and installation of a sloping raised floor, the design and manufacture of a track guided rolling cart to shuttle operating personnel to and from the target area, the replacement of the existing radar absorbing material, the improvement of the ambient lighting in the chamber to facilitate film and video tape documentation, and the development of new target mounting schemes to ensure ease of handling as well as secure mounting for vector background subtraction.
An RCS measurement system based on the HP 8510 and a Compact Range reflector system has the following limitations: high clutter levels limit the maximum transmit power and therefore the system's sensitivity, the maximum number of frequency points limit the maximum resolution and/or range length, and the proper separation of clutter and test target data requires taking data describing the entire range, even for a desired CW measurement, thus increasing measurement times significantly.
Characteristics of the Harris Model 1606 Compact Range are summarized and considered for applicability to RCS measurements. Measured characteristics of quiet zone performance (amplitude and phase distributions) and standard target RCS data are presented. Of particular interest is a comparison of predicted and measured radar cross section versus aspect angle of some familiar standard targets under various conditions.
This paper presents both radar cross section and polarization scattering matrix measurements on microwave radar navigation targets. The polarization measurements are performed using a unique two-channel facility which allows for measuring the circularly polarized scattering matrix elements at X-band. For the same targets conventional RCS measurements are performed using an automated system comprising a network analyzer (HP-8510) and a desk top computer system (HP-236 or 310). This system allows wide frequency range measurements. Details of these measurement techniques, and results will be presented.
This paper describes an automated, frequency-step, pulsed/CW Radar Cross Section (RCS) measurement system using the HP 8510 network analyzer. The system has been built using the concepts developed at Lincoln Laboratory (1) and is being utilized in an operational capacity. The unique features of this system are the use of (a) a dual-probe antenna for the transmission and reception of RF signals, and (b) a pulse system for separating the target-scattered signals from the incident and background signals. The single antenna configuration provides a true monostatic backscatter measurement. A polarization control circuit makes RCS measurements for all combinations of transmit/receive polarizations possible (linear and/or circular). The pulse system uses pin-diode switches capable of generating a 7-ns pulse width and a repetition rate up to 8 MHz. The pulse system effectively eliminates unwanted signals at ranges other than the target range. Therefore, the full dynamic range of the receiver can be used for the measurement of the target.
Narrow band RCS measurements are usually presented as RCS versus target aspect angle in either a rectangular or polar format. Wide band measurements are not normally analyzed in the frequency domain. The normal procedure is to perform either a one or two-dimensional Fourier transform of wide band data or obtain high resolution information on the location of scattering sources. In this paper, we investigate the possible uses of the wide band data directly. In particular, we show that a natural coordinate system for analysis of these data is a polar format with frequency taking on the polar distance parameter and aspect angle taking on the polar angle parameter. This format is not coincidentally, an intermediate step in the production of fully focused two-dimensional radar images. The polar format frequency domain plots are shown to be effective at categorizing the nature of the physical scattering. This is especially true when combined with image domain filtering to isolate scattering regions of interest. In addition, it can be useful in determining anomalies in the radar measurement system performance, and in assisting the analyst to explain unexpected image domain results.
There is renewed interest in the idea of determining the near-zone and far-zone bistatic RCS of complex targets from near-field data. This paper addresses the issue of efficient acquisition and processing of the requisite scattered near-field electric field data for determining the wide-angle bistatic RCS of electrically-large targets. Toward that end, several potential combinations of target illumination and near-field scanning techniques are considered in this paper. The techniques considered encompass mechanical and electronic scanning methods using single probes, linear probe arrays, and planar probe arrays to accomplish the near-field scanning, combined with either (a) compact range illumination or (b) "synthesized" plane wave illumination employing a single probe, a one-dimensional (1-D) probe array, or a two-dimensional (2-D) probe array. A general Spherical Angular Function (SAF) integral formulation of near-field bistatic coupling/scattering is presented, and an approximate "deconvolution" technique for electrically-large targets is described.
The techniques of near-field antenna pattern measurement can be extended to near-field RCS measurement. The motivation for doing so is precisely the same as that for near-field antenna measurements; i.e., the convenience of an indoor antenna range, and an improvement in accuracy. Although the near-field measurement problem is solvable in principle in a manner analogous to the near-field antenna problem, it requires a significantly larger amount of time to take the necessary data, and to subsequently process the data to obtain useful quantities. BDM is currently involved in an on-going program to evaluate the feasibility of near-field bistatic RCS measurements. At the time of this writing, a complete set of mathematics has been formulated to handle the probe correction and data processing. The hardware has been built, software development is near completion, and the analysis of canonical scattering objects has been completed. Experimental data soon to be taken for these objects will be presented. It is hoped that the technique will prove to be a practical approach to RCS measurements.
High resolution radar imaging is becoming an increasingly important component of RCS measurement systems. The primary purpose of radar imaging as applied to RCS measurements is to locate and quantify the various scattering components that contribute to the total RCS of a model under test. The technique when properly applied by trained personnel can greatly improve the productivity of measurement programs by reducing the number of measurements needed to find defects in a model, and by rapid improvement in the understanding of the scattering phenomena itself.
This paper explores a design approach to RCS measurements as required for the radar backscatter community. Background will be provided as to the approach and the measurement system experience of the RCS system design team. This will include the approach to computer networking, multiple range configurations and data reduction schemes. The solution under development will detail some of the requirements for the controllers and peripherals needed for the task. System design goals such as CPU independent software design, real time data acquisition and status display, multiple CPU and radar front end networks, system resource control and dynamic graphics design will be explored.
For over a decade the National Bureau of Standards has utilized the Planar Near-field Method to accurately determine antenna gain, polarization and antenna patterns. Measurements of near-field amplitudes and phases over a planar surface are routinely obtained and processed to calculate these parameters. The measurement system includes using a cw source connected to an accessible antenna port and a two channel receiver to obtain both amplitude and phase of the measurement signal with respect to a fixed reference signal. Many radar systems operate in a pulsed-cw mode and it is very difficult if not impossible to inject a cw signal at a desired antenna port in order to calibrate the antenna. As a result it is highly desirable to obtain accurate near-field amplitude and phase data for an antenna in the pulsed-cw mode so that the antenna far-field parameters can be determined. Whether operating in the cw or pulsed-cw modes, one must be concerned with calibrating the measurement system by determining its linearity and phase measurement accuracy over a wide dynamic range. Tests were recently conducted at NBS for these purposes using a precision rotary vane attenuator and calibrated phase shifter. Such tests would apply not only to measurement systems for determining antenna parameters but also to systems for radar cross section (RCS) measurements. The measurement setup will be discussed and results will be presented.
The recent activity and study of the compact range has been increasing the past few years. Both radar cross section (RCS) and antenna measurements have been conducted in the compact range. Important research and analytical investigation has also been done in the design and construction of the reflectors so characteristic of these types of ranges. Edge diffraction from the reflector has been studied and characterized by methods of geometrical optics, geometrical theory of diffraction, physical optics and physical theory of diffraction. Treatment of edge diffraction effects on the reflector have included serrations, rolled edges, and absorbing materials. The primary goal is to obtain as perfect a plane wave as possible in the enclosed chamber with reduction of edge diffraction from the reflector.
The compact range reflector used these days for RCS and antenna measurements have rolled edges [1] to reduce the stray fields diffracted from the rim of the parabolic section. For optimum performance (small edge diffracted fields), blended rolled edges [2] are used. A blended rolled edge ensures that the radius of curvature of the surface is continuous at the junction between the paraboloid and the rolled edge. By selecting an appropriate blending function, one can make the first and higher derivatives of the radius of curvature continuous at the junction [3] which in turn results in a weaker diffracted field. However, the resulting reflector may be too large to be practical. Also, the minimum radius of curvature of the reflector surface in the lit region may become less than one fourth of the wavelength at the lowest operating frequency, which is not desirable. Thus, the choice of blending function and rolled edge parameters is quite important in the design of compact range reflector antennas. In this paper, a procedure to design blended rolled edges for such applications is discussed. The design procedure leads to a rolled edge that minimizes the edge diffracted fields while satisfying certain constraints regarding the reflector size and minimum operating frequency of the system. Some design examples are included.
The offset fed parabola is one type of reflector used in compact radar ranges. Cross-polarization problems have been noted when a parabola is used in near field applications. A good understanding of the near field cross-polarization effects was needed to evaluate this type of reflector for a compact range. We found that the polarization vector was rotated differently at each location in the "quiet zone." The polarization vector rotation is due to the parabolic curvature. In addition, a mathematical model was derived that compares well with the data. A theoretical study of how the RCS measurements of a wing are affected is presented.
Over the last five years a considerable attention has been paid to further developments of Compact Antenna Test Ranges for both antenna and RCS measurements. For many applications, these devices proved to be more attractive than outdoor ranges or near-field/far-field transformation techniques. On the other hand, accurate operation at very low or very high frequencies can cause considerable difficulties. It is the aim of this paper to describe the theoretical limitation of collimating devices, in particular for low frequencies. For this purpose, an idealized collimator will be defined. Using the spectral components analysis a comparison of achievable accuracy will be made between collimators and outdoor ranges. Theoretical limits in the accuracy for RCS measurements will be computed for all applicable frequencies. Finally, a comparison will be made between the experiments on a dual-reflector Compact Antenna Test Range and theoretically achievable limits. Representative targets, like cylinders and rectangular plates have been used for experimental investigation. These data will also be presented.
The development of a high efficiency compact range has made it possible to consider alternative equipment for making radar cross section measurements. Historically, high power radars were required to make measurements on low efficiency, high clutter ranges. Their high power and narrow pulse capability was essential in making precision measurements. Such instrumentation is complex and expensive. There is, however, a relatively inexpensive approach which uses test equipment commonly found in the laboratory. It is centered around an HP8510 network analyzer and an RF switching network.
A novel method for evaluating conductive coatings used for radar cross section (RCS) scale models is presented. The method involves the RCS measurement of a short circuited cavity whose interior is coated with the material under study. The dominant scattering from such a structure occurs from the cavity rim and surface walls internal to the cavity. The method of conductivity testing has excellent sensitivity due to the energy that couples in and out of the cavity. This energy undergoes many reflections with the interior walls and thus very small losses can be detected. Calculations and measurements are shown for several different types of coatings, including coatings of silver, copper, nickel and zinc.
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.