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This paper compare some of the features and capabilities of gated CW and pulse radars for RCS and imaging measurements. At the conceptual level, these two types of radars are very similar. The primary conceptual difference is that a pulse radar has a relatively high bandwidth receiver while a gated CW system has a relatively narrow bandwidth receiver. The measures of performance of an RCS and imaging system include sensitivity, measurement time, clutter rejection, dynamic range and accuracy. Other considerations such as inter-pulse modulation may be important in some cases. For some applications, typically where long ranges are involved, a pulse system has significant performance advantages. For many applications, the performance advantage of a pulse system is not significant, particularly when viewed in light of the large difference in cost. This is particularly true of Quality Assurance applications which are normally characterized by both short range and lower budgets. Typically, the price of a gated CW system is in the range of ¼ to ½ the price of a comparable pulse system. This paper discusses general similarities and differences in the fundamental operating characteristics of the two systems. Specific performance measures are discussed including system sensitivity, gate performance, clutter rejection, and measurement times. Other considerations such as pulse modulation are discussed. A summary of the various considerations is presented in order to give the reader an understanding of the applications for which a gated CW system is more appropriate.
Portable near-field measurement systems can provide significant flexibility to both large companies seeking to increase their antenna test capabilities, and small companies looking for their first investment in a test range. There are many unique applications for portable near-field antenna measurement systems in addition to their use for standard antenna performance measurements. Some additional applications include flight-line testing, anechoic chamber quiet zone imaging, and EMI testing. Many of NSI’s near-field systems have been portable designs, capable of being set up in a small lab or office and easily relocated. Key features required for use of a portable system are rapid setup, simplicity of use, low cost, and accuracy. This paper will be focused on practical experience with installing, calibrating, and operating portable near-field measurement systems. It will also cover tradeoffs in their design, and usage in a variety of applications.
Advanced Radar Cross Section (RCS) Data Analysis, consisting of comparisons of measured RCS data to predictions, multiple plot overlays, imaging, etc., it is most often performed off-line. This causes a lag in data acquisition time by as much as several days. McDonnell Douglas Technologies Incorporated’s (MDTI) Radar Measurement Center, a large target (40 feet) indoor RCS measurement facility, used an advanced RCS data analysis system, based on a Tektronix XD-88 superworkstation, for on-line data processing. This system connects over a Local Area Network to the data acquisition computer. This allows the workstation access to each data file immediately after each measurement for processing, without affecting the data acquisition capabilities of the radar system. The hardware used for connections, capabilities of the MDTI-written software, and the capability to store plotted data on VHS videotape directly from the workstation, is described herein.
Recently, super resolution techniques have been applied to image spurious signals in compact range measurement systems. These techniques include parametric modeling of the probe data as well as eigen-space based methods. In these techniques, in incident signals on the probed aperture are assumed to be planar, which may or may not be true. In general, if the separation between a signal source and the probed aperture is more than , where D is the size of the probed aperture, one can assume that the signal incident on the probed aperture is nearly planar. It is shown that this is not necessarily true for super resolution techniques. The signal level also affects the minimum distance requirements. The stronger the signal, the farther its source should be from the probed aperture to achieve the optimum performance.
Accurate scattering and antenna measurements require excellent plane wave purity in the target zone; however all measurement systems are contaminated by various stray signals which result in measurement errors. In this paper, a technique of evaluating the stray signal sources in a compact range using a diagonal plat plate as a test target is presented. The scattering cross section of the diagonal flat plate as a function of frequency and angle of rotation is first measured. Then the time domain response for each projection angle is processed to obtain a two dimensional ISAR image of the plate as well as the stray signals. From the stray signal images, the location and relative strength of the stray signals can be determined. Experimental results from the OSU/ESL Compact Range Facility are presented to demonstrate this stray signal imaging technique.
Much research has been done recently on the interpretation of measured field probe data in order to locate and quantify error sources present in the quiet zone of a compact range. This paper examines an alternative method of analyzing those data by applying spherical phase offsets to focus the field probe data to near-field distances. This method is applied to simulated field probe data for a large compact range. The technique yield the correct [x,y,z] coordinates of multiple scattering sources deliberately introduced into the simulated data.
Classical radar target imaging uses an inverse synthetic aperture radar (ISAR) algorithm based on the two dimensional Fourier transform. The technique has resolution limitations in the time-domain (or down-range) dimension somewhat larger that the inverse of the band-width of the interrogating radar system (depending on the frequency domain windowing function utilized). The resolution in the cross-range domain (or doppler-domain) is related to the inverse of the aspect angle sector over which the target is observed. This paper will present radar target imaging techniques based on modern autoregressive (AR) spectral estimation algorithms (superresolution) which overcome these limitations. Techniques are shown for the generation of ISAR images even with severly [sic] limited frequency or angle domain data. Images will be shown where the quality of the image does not degrade even when the bandwidth of the original data is reduced by a factor of 16. Thus clear images are produced using these techniques with data where the classical Fourier-based techniques produce only “fuzzy blobs”
Two-dimensional RCS imaging systems utilize wide-band, ISAR processing to spatially isolate scattering sources on complex objects. Although the measured data consist of the frequency and angle responses of the entire object, the image process allows the possibility of extracting the responses of scattering components which comprise the total signature. These methods of image editing generally involve the application of spatial filters to the image, followed by a reconstruction of the angle and frequencies responses associated with the filtered image. The objective of these procedures is to determine the responses of localized scattering sources or to delete the contributions of scattering sources on the overall signature of a complex object.
Range resolution of a radar image can be obtained by use of wide-band signal (linear FM or chirp waveform) and cross-range resolution by object rotation which synthesized a large antenna aperture (the so called ISAR method, refer [1]). Although both cross-range profiles can be resolved by rotation of the abject about two mutually orthogonal axes, however, the data manipulation would be quite cumbersome and the measurement implementation would require a mechanical support system by which the objet [sic] can be independently tilted and rotated relative to the radar axis. In this paper, the algebraic reconstruction technique (ART)[2] for tomography is used to resolve the vertical cross-range profile (along the axis normal to the ground) while the horizontal cross-range profile still resolved by ISAR method. Applications of the ART to a simple circular pattern and a complicated emblem pattern of the CSIST show that ART is a suitable approach and easier than ISAR method to obtain the second cross-range resolution.
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.
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.
Increased productivity and higher resolution imaging capabilities are becoming of greater concern for RCS ranges. The ideal measurement scenario involves taking data on all desired frequencies for a target combination in a single rotation. This could involve one or more frequencies in several bands, imaging data on more than one band or very high resolution imaging data covering several bands. Placing several feeds in a cluster at the focal point of an offset fed com-pact range can provide these capabilities. The effects of feed clustering such as beam tilt are discussed along with cluster sizes that provide little if any degradation in compact range performance. Experimental data is shown that gives an indication of the quality of data that may be obtained. The concepts are also applicable for outdoor ranges that have an array of antennas offset from range boresight.
The Hughes Aircraft Company conducted a study to characterize the backscattering performance of wedge shaped anechoic absorbers for use in treating the sidewall regions of RCS chambers. ISAR imaging techniques were utilized to obtain a diagnostic results at near-grazing incidence angles which were not possible with conventional testing methods. These techniques allowed for separation and identification of individual scattering sources from each of the evaluated samples. As a result, the backscattering from an entire wall of absorber can be simulated by evaluating only a few samples. Absorber performance data was collected over frequencies from 2 to 40 GHz. Results from this study clearly show that differences in absorber fabrication methods have a significant impact on the performance of the materials. Various approaches for impregnating, loading, and cutting the absorber have also been evaluated. Gaps, formed during installation, at the joint between two pieces of material are shown to significantly degrade performance, whereas, offsets and glue lines are shown to have less of an effect, provided the absorbers are uniformly loaded.
Comparative measurements have been made in a compact range to determine the performance improvements that can be achieved when adding a hardware gate to a CW-based measurement system. Starting with conventional stepped frequency CW measurements made in the time domain mode, high resolution downrange data was collected to determine the background levels of the compact range. This was followed by comparative measurements under the same conditions adding a narrow pulsed hardware gate to reject inter-horn coupling and high returns from the compact reflector. A second mode of comparison was examined by collecting aspect data with a specific range gate fixed about the target. Software gated measurements required more points to insure alias free operation, while the hardware gated measurements allowed fewer points which reduced measurement time without sacrificing any accuracy. Finally, imaging measurements were made with both software and hardware gating to compare the measurement time and 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.
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 near field synthetic aperture imaging technique using three main beam suppression methods is used to locate and quantify the sources of stray signals in a compact range. First, main beam cancellation by subtracting the complex average of the measured field for the overall probe aperture is used. Second, a software on-axis null is generated by preprocessing the data. Third, an antenna with a broadside null is used as the prober. It is shown that the software on-axis null enhances the resolution of the spurious scatterer images and is able to detect small spurious scattering centers, such as the surface discontinuity at the top of the reflector, which are otherwise undetectable. Probe data with two metallic tapes placed on the compact range reflector is used as another example to show the performance of the nulling technique.
A mini compact range system has been built for NASA, Langley Research Center. The performance of the system was evaluated at the Ohio State University by probing the fields along a vertical cut and a horizontal cut. The probe data showed that the target zone fields contain stray signals, which do not originate from the reflector surface. The probe data was imaged to locate the sources of the stray signal. Both conventional Fourier techniques as well as the MUSIC algorithm were used to image this data. The results of this study are discussed in this paper. It is shown that at the back end of the chamber, the absorber scattering can be quite significant. The aperture blockage due to the feed structure also contributes to stray signals in the target zone.
A microwave holographic analysis system is shown to have successfully resolved the surface deformations on an 8' symmetric Cassegrain reflector antenna known to have significant surface deformation problems. The technique is based on the Fourier transform relationship between the aperture field of an antenna and its radiated far-zone field. A signal processing technique dubbed "pattern simulation and subtraction" is discussed that increases the resolution in the transformed aperture domain by removing unwanted signals from the aperture distribution. Measurements taken on the Cassegrain reflector at 11 GHz in the OSU-ESL Compact Range provided excellent amplitude and phase stable data to be processed by the holographic analysis system. Surface deformation profiles generated by this system were then compared to an optical measurement of the main reflector surface. Excellent agreement was obtained with a worst case deviation in the adjusted profiles being 0.05 ?.
Measured component RCS results are frequently dominated by the test body and target mounting structures. This paper will present a measurement technique that will improve measurement accuracy using a less complex and expensive test body. The design of the test body and measurement geometry allows isolation in both range and cross range from the static return of the room and mounting structure. This is accomplished by first creating an ISAR image of the target and test body, gating the image in two dimensions, then transforming back into the frequency and spatial angle domains to determine the scattering levels of the target by itself. Details of this technique, covering both its advantages and limitations, will be discussed. Data will be presented to verify the approach and illustrate the level of performance attainable using this technique.