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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.
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.
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.
A new implementation of the planar near-field back projection technique for phased array testing and aperture imaging is described. In the alignment of phased arrays, the aperture field is treated as a continuous distribution rather than using idealized array concepts. The continuous field is then sampled to obtain element excitations. In this way, nonrectangular arrays can easily be accommodated. The method also produces highly interpolated images of apertures that can offer much insight into their nature. Also, any polarization of the aperture field may be obtained if the probe pattern has been characterized. The technique uses large FFTs which are computed very quickly by a workstation located in the facility. Results from an iterative phase alignment of a 12x18 phased array are presented, as well as highly interpolated images of apertures and results which demonstrate the polarization selection.
A technique for aligning a phased array is described. Array element attenuation and phase commands are derived from far-field patterns measured without calibrations. The technique is based on iteratively forming mulls in the antenna pattern in the directions specified by a uniform array illumination. It may be applied in situations where array elements are not individually accessible, or where an array contains no build-in calibration capacity. The alignment technique was evaluated on a far-field range with a linear, 32-element array operating at L-band. The array containing transmit/receive modules with 12-bit amplitude and phase control. Insertion attenuation and phase measurements were comparable to those obtained by conventional techniques. However, the alignment procedure tends to compensate for the effects of nonuniform element patterns and range multipath. Thus, when used to implement other excitation functions, the array sidelobe performance with adaptive calibrations was substantially better.
Calibration is one of the most important activities to be performed during the assembly, integration and testing of a phased-array antenna. Space-based phased-array antennas, conceived for remote sensing applications and for satellite communications, are going to require both on-ground and on-orbit calibration techniques. The paper reviews on-orbit calibration methods being envisaged for an electronically steerable receiving array at S-band, to be embarked onto the forthcoming PSDE/Artemis satellite.
Electronically scanned phased array antennas typically have a large number of beam positions. Accurate on-line monitoring of phased array beam positions can be used to ensure proper antenna and total system performance. Bendix has developed and successfully implemented a beam-position monitoring technique designated the “RF Integral Monitor System”. Use of this on-line technique does not interfere with normal system operation and yields results that are comparable to results obtained on an actual far field antenna range. The RF Integral Monitor technique and specific hardware implementations, for both linear and circular electronically scanned phased arrays, will be described in this paper.
This paper presents an overview of the test and set-up requirements of active Synthetic Aperture Radar (SAR) antennas. The specific antennas under consideration are those that are intended to be used in the next generation of spaceborne SAR C-Band satellites. These antennas are typically 1m to 2m wide and 10m to 20m long, possessing between 3000 to 12000 radiating elements. The paper considers each unit of the active antenna in turn and identifies which tests are to be carried out where. In considering the test of the whole antenna some initial result of focussing techniques, to allow the antenna to be tested in real time at reduced distance, are presented.
To enhance the performance of existing near field techniques the new idea of far field pattern determination from only amplitude distributions of the near field is proposed. In this way the difficulties related to phase measurements are overcome. Some different algorithms are introduced and discussed. In particular, after recalling results for the planar geometry, cylindrical scanning surfaces are considered. The feasibility and the performances of the introduced algorithms are shown through numerical examples.
This paper deals with design and evaluation of Compact Range Antenna and RCS measurement systems. Reflector subsystem and feeders design as well as quiet zone evaluation and system performance qualification are considered. Acquisition, process and presentation software to control the whole system has been developed and successfully implemented. Two systems have been designed and are now at implementation stage. A Gregorian concept Compact Range is now been constructed at RYMSA (Spain). This facility has been fully designed by IRSA and will be operative by the end of 1990. Compact Payload Test Range (CPTR) at ESTEC (ESA) is now been tested. System Instrumentation and PAMAS (Payload and Antenna Measurement and Analysis Software) have been developed.
The reflective properties of a flat circular plate and a long thin wire are discussed in connection with the quality and calibration of the quiet zone (QZ) of a compact antenna test range. (CATR). The flat plate has several applications in the CATR. The first is simple pattern analysis, which indicated errors as function of angle in the QZ, the second uses the plate as a standard gain device. The third application makes use of the narrow reflected beam of the plate to determine the direction of the incident field. The vertical wire has been used to calibrate the direction of the polarization vector. The setup of an optical reference with a theodolite and a porro prism in relation to the propagation direction of the incident field is presented as well.
A method for obtaining the individual element excitations of an array antenna from measured radiation patterns is presented. Applications include element failure diagnosis, phased array antenna calibration, and pattern extrapolation. The measured far-field information is restricted to visible space which does not always contain the entire Fourier domain. A typical example is phased array antennas designed for large scan angles. A similar problem arises during near-field testing of planar antennas in which case the significant far-field domain is restricted by the scanning limitations of the near-field test facility. An iterative procedure is then used which is found to converge to the required solution. The validity of the approach has been checked both using the theoretical radiation patterns and real test cases. Good results have been obtained.
A novel and very powerful measurement technique is presented which allows the determination of antenna radiation and scattering by radar-cross-section (RCS-_ measurements. The antenna under test is treated as a loaded scatterer using a polarization dependent network model that allows a complete antenna description in terms of scattered, radiated and absorbed waves. A load variation principle is used to determine the network model parameters and all commonly used antenna parameters like gain, antenna polarization, axial ratio, polarization decoupling, input impedance and also structural scattering can be derived from the backscatter measurement without using any additional standard antenna. With the antenna network description it is furthermore possible to examine the antenna behavior for arbitrary excitation or loading on their waveguide or radiation port.
The quiet zone performance of the Harris 1606 compact Range Collimator has been reported in the literature for 2 through 35 GHz 1,2. This paper discussed our achievements in the past year with the 1606 at 95 GHz. We will summarize the improvements in our fabrication and alignment methods that have yielded excellent performance at these frequencies using an intermediate size multi-panel main reflector. Quiet zone performance data will be presented from recent measurements on the Millitech Corporation’s Millimeter Wave Antenna Test Range in South Deerfield, MA and from the Harris 1606 Capital test equipment range.
A 75 foot diameter offset paraboloidal outdoor compact range reflector was designed for operation up to 95 GHz and installed at Ft. Huachuca, Arizona. The need for high frequency operation required that a highly accurate reflector surface be maintained in the desert’s harsh thermal and wind environment. The use of thermal modeling to predict the temperature distribution in the structure, along with extensive finite element analysis to determine the structure’s distortions from thermal, wind and gravity loads were integral to the reflector design. Using the above tools, thermal isolation techniques were developed to minimize the harmful effects of the thermal environment on surface accuracy. A surface error budget based upon both calculations and measurements shows an overall rms error of 4.9 mils under optimal environmental conditions, degrading to only 6. Mils under the worst operating conditions.