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This paper reports on a study undertaken to assess the effects of range amplitude tapers on the measurement of low and ultra-low sidelobe levels and gain. It has been shown that low test zone phase tapers are required for the measurement of low and ultra-low sidelobe levels. A few papers have addressed the effect of amplitude errors but not for the measurement of low sidelobe levels. These papers have concluded that amplitude errors have much less effect than phase errors. This paper addresses antenna measurement ranges such as compact ranges where phase taper has been significantly reduced, but amplitude errors remain. The amplitude taper on some modern compact range configurations has not only, not significantly improved, it has often taken on a more complicated “double hump” shape. The effects of these modern amplitude tapers are demonstrated.
Compact range systems have been widely used for high quality RCS measurements. However the taper and cross-polarization effects can lead to significant measurement errors especially as the target approaches the border of the target zone. The taper error is mainly caused by the feed’s finite beamwidth, and the cross-polarization error by the feed’s cross-polarized radiation and the offset configuration of the reflector. A method to correct these errors is presented. In order to perform taper and cross-polarization error corrections, one has to be able to predict the target zone fields and determine the locations and complex strengths of the various scattering centers associated with the target. The correction can then be done by compensating for the taper and cross-polarization effects for each localized scattering center. Several measurements have been taken, corrected and then compared with the theoretically expected results to validate this technique.
The scanning-probe spherical near-field system at GE Aerospace uses a roll over azimuth positioner with probe horn on a cantilevered arm to scan spherical sector centered over a stationary antenna. The main sources of measurement errors in this system are: 1. Signal drift, 2. Deviations of recorded angles from commanded values, 3. Differences of actual sample positions from ideal ones. Unless corrected, these arrors may alter the transformed field.
In coherent measurements, one measures the interference of signals from test and reference paths. These techniques are widely used in RF image measurements of antennas and radar cross sections. The success of a coherent measurement depends highly upon the stability of the path length difference between test and reference signals as well as the quality of the reference signal. The stability and quality are hampered when the experiment has to be conducted with a long reference path length, particularly at outdoor ranges. A new measurement scheme, based on the scattering process initiated by two coherent beams, will be presented here are has the advantage over others in reducing the problems associated with the path length difference instability.
Amplitude taper removing by software implementation has been made beyond the quiet zone region of a compact range reflector where the phase variation is still small. To remove amplitude taper effect in RCS measurement, actual amplitude taper of the range s first obtained by theoretically calculating the field distribution from the given range geometry and confirming with field measurement result. The processed target RCS contour is later implemented with the actual amplitude distribution around the region where the target is located. It is found that with the software implementation of amplitude taper removing the effective quiet zone of the compact range has been able to extend up to the size of the reflector diameter.
The task of making accurate antenna measurements is complicated by the numerous sources of measurement error in the antenna test range. In addition to the test system performance, the overall measurement uncertainty depends strongly upon the range configuration and user-selected operating conditions. A correct understanding of these systematic and random error sources can help optimize the test range, instrument configuration, and measurement technique to achieve the highest levels of measurement accuracy. This paper describes dominant error sources present on an antenna test range and gives methods for quantifying their effects on measurement accuracy.
The utility of wideband RCS data for characterizing scattering mechanisms of complex objects has been established by wide-spread applications. The fundamental data from which the final products are derived consist of calibrated scattered fields measured coherently as a function of frequency and aspect angle. By processing these data, one-dimensional range or cross-range reflectivity profiles can be derived; by further processing, two-dimensional images can be derived. Modern RCS instrumentation systems capable of rapidly measuring and processing wideband data provide more object information than is conveyed by the RCS pattern, which has been the traditional descriptor of scattering behavior. The procedures of one- or two-dimensional imaging inherently involve integration processes, constituting many-to-one mappings in which data from a large set are collapsed to produce an individual pixel of the image. For example, a particular pixel of a range response is derived from the total object response “integrated” over a band of frequencies; similarly, a pixel of a two-dimensional image is derived from the object response “integrated” over frequency and angle. The exposure of a local feature of the object signature, obtained by collapsing the fundamental data, comes at the cost of obscuring the global descriptor. This paper explores techniques for presenting large amounts of information on single displays which retain both global and local features of the scattering process. These tools provide to the RCS analyst options for extracting and interpreting significant information from the measured data without arbitrary degrees of integration which can mask essential details represented in the data. The display methods utilize color coding to increase the amount of information conveyed by a single plot. Because color reproduction is not available for the proceedings, the paper is to be distributed at the conference.
This paper analyzes the data rate requirements for RCS imaging systems as a function of measurement parameters and identifies the measurement conditions most likely to tax a system’s capability. Data rate estimates can assist in determining hardware and software design requirements and guide the selection of data storage devices to maintain high throughput rates.
Transient signature representation of scattered fields and their interpretation have become common in downrange and crossrange scattering center identification. A review of the basic concepts for one dimensional transient analysis is presented. The topics included are the frequency-time domain dual representation, general characteristics of transient signatures and temporal mechanism extraction.
We discuss recent advances in signal-to-noise and signal-to-clutter enhancement technology applied to RCS measurements, with an emphasis on post-processing techniques. Then, we outline a technique we refer to as Interactive Coherent CLEAN Editing (ICCE). ICCE permits the analyst to segregate scattering features of the model under test into various groups. Clutter sources, such as the target support pylon, can be subtracted with potentially less error and more flexibility than other techniques. Limitations and the current status of ICCE are discussed.
As the advances in silicon technology continue to redefine the realm of “practical” for scientists and engineers, traditional techniques for acquiring measurements and processing the exceedingly large data sets generated must be constantly improved, and often times discarded as new concepts replace them. The new class of SuperWorkstations available today provides a convenient means to not only maximize the performance of the compact range instrumentation, but also suggests entirely new techniques and algorithms in data acquisition, storage, processing and interpretation. In considering these advances available through SuperWorkstations, benefits in the area of measurement data acquisition and local storage are detailed, recent improvements in magnetic and visual storage techniques and their application to data archiving are considered, new and unique techniques for scattering center identification in near real time are presented, and finally a discussion of tomorrow’s computer technology and the further impact on the compact range completes the study. This paper examines the efforts currently underway to exploit one such superworkstation, the Tektronix XD88, in the compact range at the ElectroScience Laboratory. In the effort to effectively utilize the superworkstation, many disciplines are coupled together (hardware, software, graphics, video presentation, among others) to augment each other. It is this multidiscipline coupling that will serve to expand the realm and utility of SuperWorkstations in the compact range, and the goal of this brief introduction is to present some aspects of these varied areas to the reader, hopefully motivating the reader to consider further extensions of SuperWorkstations.
Today’s measurement systems are placing ever increasing demands upon the computer systems which control instrumentation and collect data. This paper investigates high speed control of instrumentation for RCS and antenna measurements. Off-loading of I/O from control and data acquisition computers is examined with a view toward improving measurement throughput and simplifying I/O control tasks. These methods are particularly important for multi-tasking systems and networked resources where high speed real time control is burdensome. Attributes of I/O enhancement architectures are examined and tradeoffs between performance and flexibility are reviewed.
This paper presents an overview of the integrated Radar Measurement System (IRMS) installed at the Air Force Radar Target Measurement Facility (RATSCAT) for AFSC/6585 TG/RX Holloman AFB, New Mexico.
This paper will describe the production version of the Model 200 TRACKSAR radar, which provides high-resolution imaging in downrange and crossrange using wideband waveforms and both synthetic aperture radar (SAR) and inverse synthetic aperture radar (ISAR) processing. Several other novel features of the system and technical aspects of performing such measurements will be addressed, and sample data outputs will be presented.
State-of-the-art microwave antennas often incorporate the ability to operate in many operating states and over wide frequency ranges. Adaptive arrays and electronically scanned antennas may have programmable beam shapes, configurations or scan positions resulting in several thousand operational states. In addition, these antennas may also require testing at many frequencies. To be properly characterized, these antennas may require testing in all these states. The amount of data required to characterize these antennas, coupled with the requirement to maximize antenna range throughput and minimize costs, puts ever increasing demands on the test equipment designed to perform these measurements. This paper describes a new automated microwave measurement system utilizing a high speed measurement receiver and an 80386 PC based computer to rapidly test these new generation antennas. The recently introduced Scientific-Atlanta Model 2095 Microwave Measurement System incorporates a unique data acquisition coprocessor (DAC) for high speed test device control, instrument timing, frequency control, data buffering and transfer to the system controller. Antenna measurements on multi-beam and multi-port antennas can be made in a fraction of the time associated with other types of test systems. The carious timing parameters of the Model 2095 are described with special emphasis on how these independent and variable factors inter-relate to each other. A method is presented to calculate total test time, given the test requirement and timing for state change of the AUT. Examples of typical test scenarios are presented as a further aid in understanding system timing.
This paper describes an automated radome test and evaluation system, which quickly and accurately measures the electrical boresight shift and loss caused by the presence of a radome in front of a monopulse antenna. The system was required to measure the boresight deflection through all 60 spatial relative angles between the antenna and the radome. The conventional methods of radome characterization were useless for this range of relative angles (mechanically impossible). To overcome this problem, a unique dynamic tracking method was developed. In this methos, the antenna is mounted on a dual-axis gimbal attached to the radome. The gimbal by itself is mounted on a second dual-axis positioner. The antenna gimbal scans the radome through all the required relative angles, while the monopulse error is continuously measured and used to control the radome positioner, in order to return the antenna to the boresight position. The readings of the angles and the values of the monopulse error establish the boresight deflection results, which are highly accurate because the apparent (deflected) source is accurately tracked, and the antenna is boresighted to it. The system measures all the 60 angles in 70 minutes time, at an accuracy of 0.3mRAD.
This paper examines antenna pattern measurements of RF frequency antennas (300 kHz-3 GHs) using an integrated source/receiver and measurement control software. Current microwave measurement systems provide sufficient measurement capability but are often too expensive to be used on ranges which require test frequencies of less than 3 GHz such as aircraft communications, cellular radio, GPS, and satellite telemetry antenna. Several system block diagrams based on the HP 8753 network analyzer will be examined with respect to system performance, measurement accuracy, and cost. System considerations for outdoor RF ranges such as RFI susceptibility will also be addressed.
A Hewlett Packard 8410 Network Analyzer was modified to be used as an automated far-field antenna range receiver. By using external mixers, analog to digital signal conversion, and an external computer/controller, the HP8410 is capable of measuring signals as low as -110 dBm. The modified receiver is an intergral part of an automated far-field range which features computer controlled test antenna positioning, system measurement parameters, and data acquisition, as well as customized measurement file management. The system described was assembled and made operational taking advantage of off-the-shelf hardware available at minimal cost.
The properties of a focused scalar horn-lens antenna are presented. The behavior of the field from the lens to the far field is determined from electromagnetic principles and measured antenna patterns at the focal distance are shown.
An s-parameter measurement system and a procedure are described for making fast s-parameter measurements on multi-state devices. A sample test problem is considered and the application of the system and the procedure to this test problem is discussed. The important features of the system are described and timing measurements of system operation are presented.