AMTA Paper Archive
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Performance of the model 1606 compact range
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.
Model 1603 compact range: a room sized measurement instrument
Harris Corporation has developed and introduced a miniature version of its shaped compact range called the Model 1603. This model is actually a scaled version of its very large compact ranges. The range features a three foot quiet zone in a very compact configuration, allowing the range to be set up in an anechoic chamber as small as a normal conference room. Performance features are equivalent to those achieved in large compact ranges by Harris, such as the Model 1640 with a forty foot quiet zone. Key features are very low quiet zone ripple, extremely low noise floor, and low cross polarization. This range can be used for the full gamut of precision RCS testing of small models or precision testing of antennas. It should also find wide application in production testing of these items. Harris can also provide turnkey compact range test systems based on the Model 1603 that use available radar instrumentation. Several of these miniature compact ranges have been delivered and are in use.
Remodeling of the ESL-OSU Anechoic Chamber
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.
Evaluation of Anechoic Chambers
This paper describes methods commonly used by anechoic chamber manufacturers to characterize chamber performance. Test procedures depend first on the purpose of the test; second on the purpose of the anechoic chamber and third on the amount of information required. Most anechoic chambers are built for a specific use. In order to prove its design, the test will be done accordingly. In most anechoic chambers one measures the reflectivity level because this is a measure for the accuracy on future measurements when the chamber is in operation. Anechoic chambers can vary from Antenna Pattern Test Chambers to Radar Cross Section Test Chambers, Electronic Warfare Simulation Chambers and Electro Magnetic Compatibility Test Chambers. Each type of chamber will have its specific evaluation technique. Some techniques can be done by the chamber user himself. Other methods need some special equipment that will or can only be used for that particular test method. Some customers want to do their own calibration on a regular basis. They can purchase this special equipment from the chamber manufacturer, if necessary. More complicated methods make use of computer controlled equipment. The data required can be taken in the chamber. This can be done relatively fast. All sorts of information about the chamber characteristics can be obtained in a later stage in a different format by use of the right software. This paper gives possible evaluation methods for different types of anechoic chambers. Detailed information about each method can be obtained from Emerson & Cuming.
Radar cross-section and scattering matrix measurements on microwave radar navigation targets
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.
A Pulsed/CW RCS measurement system using the HP8510 network analyzer
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.
Polar format interpretation of wide band RCS data
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.
ISAR measurement techniques applied to antenna measurements and diagnostics
A pulsed, coherent radar system was used in the inverse synthetic aperture radar mode to obtain 1-way high resolution images of simple antennas. These high resolution images display the amplitude and phase distribution of the received wave. The images were then edited and reconstructed using System Planning Corporation's Image algorithms contained in the SPC RPS software package. The 2-D (range vs. cross range) image data is very useful for detecting defects in antennas and can also 0be applied to modification of illumination conditions such as wavefront sphericity (phase taper) and/or amplitude variabilities (taper, ripple). This technique offers an alternate approach to near field/far field transformation. The technique involves rotation of the antenna under test at a controlled, uniform rate. The antenna port is connected to the radar receiver and the radar transmitter attached to an illuminating antenna. The radar transmits a step chirp wave form. The received signal is recorded to tape and processed off-line on the SPC Image Reduction Facility. A calibration technique was developed using simple wide bandwidth horn antennas. The downrange and cross range resolution of these 1-way ISAR antenna images is half as large as with 2-way radar ISAR for the same bandwidth and angular integration interval. Image data will be shown on reflector-type antennas to illustrate the technique.
Performance specification for diagnostic radar imaging systems
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.
A State-of-the-art radar cross section system controller
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.
Antenna calibrations using pulsed-CW measurements and the planar near-field method
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.
A Comparison of three field probing techniques
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 Effects of an offset fed parabolic reflector on polarization
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.
Formulation of proper standards in radar polarimetry
We have found several crucial inconsistencies in the basic equations of radar polarimetry which are rather common in the current literature on the subject. In particular, the pertinent formulations of the polarization state definitions given in the IEEE/ANSI Standards 149-1979 are in error. These and other inconsistencies and conceptual errors are analyzed very carefully in this presentation. We provide the correct formulae for the proposed revision of the polarimetric standards together with a well-defined and consistent procedure for measuring target scattering matrices in both, mono-static and bi-static arrangements. Further, the proposed procedure can be applied to an arbitrary measurement process in any general elliptical polarization basis.
Coherent signal measurement of time modulated antenna pattern
The Boeing V-22 Osprey tilt rotor aircraft is a candidate platform for use as an airborne surveillance radar system. The impact of radar RF energy scattering from the aircraft's large propellers is a concern due to the potential for interference with an airborne pulse doppler radar where frequency changes are used to discriminate moving targets from ground clutter. In order to ascertain the effects of the scattering, a unique measurement system was devised for recording the time modulated antenna pattern of an array antenna.
Making precision RCS measurements on a compact range using an HP8510 and an RF switching network
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 Method of evaluating conductive coatings for RCS models
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.
Performance of gated CW RCS and antenna measurement
Conventional receivers for pulsed radar systems employ a wideband final filter that is matched to the pulse width and risetime. However for pulsed RCS measurements on small test ranges, instrumentation receivers with narrow IF bandwidth have proven useful. This paper analytically examines the differences between narrowband and matched filter instrumentation receivers and describes typical conditions under which gated CW measurements are made. Useful relationships between PRF and IF bandwidth are derived.
ISAR image quality analysis
In practical ISAR applications the quality of the image obtained depends upon the distortions in the wavefront illuminating the target, effects introduced by the radar-target path, the accuracy of the angle and frequency steps used in obtaining the data, vibration, and multiple reflections from neighboring objects. Results of analysis, simulation and data obtained in an RCS compact range are presented to quantify the relationships of the image degradation introduced by these effects.
Interpretation of two-dimensional RCS images
The objectives of RCS imaging are to spatially isolate and quantitatively measure the strength of scattering mechanisms on complex objects. Although some isolation can be provided directly by using radars with high spatial resolution, most current RCS systems achieve the required resolution by synthesizing the image from measurements of the object response to variations in frequency and rotation angle.
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