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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.
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.
This paper presents a conceptual overview of the instrumentation system and signal processing involved in dynamic RCS and Imaging measurement systems.
This paper will address low RCS target mounting systems. Structural and electromagnetic aspects will be considered. The 4:1 vs the 7:1 ratio ogival shell pylons will be evaluated with consideration given to structural integrity, electromagnetic scattering, and positioner size. Measured and analytic data will be used in these evaluations.
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.
A unique RCS field probe system is described which determines: 1) the two way phase and amplitude field taper, and 2) the RCS measurement error within the quiet zone. The RCS of a suspended target is measured by the radar at selected locations or while moving in the quiet zone. The field taper is obtained from a time gated target return. The quiet zone RCS error for a target is obtained by comparing RCS measurements from anywhere in the quiet zone with the target RCS measured at the center of the quiet zone. A quiet zone containing a high quality illumination field was measured and found to have more than a 5 dB quiet zone RCS error. The RCS error magnitude is dependent upon the radar variables which are determined by the target size. There is a significant difference between the implied RCS error based on the illumination field quality and RCS measurement error caused by the additional contributions of multipath and target dependent clutter that are peculiar to each facility. Accurate RCS measurements require detailed knowledge of the test facility's multipath, target dependent clutter characteristics, and the target's bistatic signature.
To construct an image of a complex target, increasingly smaller range cells are desired. To decrease range cell size and improve resolution the bandwidth must be increased. The bandwidth of RCS measurements utilizing an HP8510 based collection system is limited to a maximum of 801 frequency points. This paper will present a technique to extend the bandwidth by using off-line processing to overcome this hardware limitation. Fully focused ISAR images formed at millimeter wave frequencies, with the addition of eternal mixers, will be demonstrated. Bandwidths of 5, 10, and 14 GHz, measured from 7-17 GHz and 26-40 GHz will be shown. The comparison of these focused images, with 3.2 cm, 1.6 cm, and 1.1 cm resolution will illustrate a powerful engineering tool to analyze closely spaced scatterers.
In the last few years, the interest in Radar Cross Section (RCS) measurements has increased rapidly. The development of high-performance Compact Ranges (CR) has made possible measurements on large targets down to very low RCS levels (below -70 dBsm). RCS imaging is a powerful tool to determine the location of scattering sources on a target. The response of the target is measured as a function of the frequency and aspect angle. A two-dimensional Fourier transform then gives the reflection density as a function of down-range and cross-range. If the response is measured vs. azimuth and elevation, even a complete 3-D image is possible. For high-resolution imaging (large bandwidth, wide aspect-angle span) a direct 2-dimensional Fourier transform gives rise to errors caused by the movement of the scatterers during the measurement. These errors can be corrected by applying a coordinate transformation to the measured data, prior to the Fourier transforms. This so called focused imaging allows further manipulation of measured data. However, the measurement accuracy can be a limiting factor in application of these techniques. It will be shown that the Compact Range performance as well as positioning accuracy can cause serious errors in high-resolution imaging and thus in interpretation of processed data.
We have reviewed the sampling-interval requirement associated with the algorithmic problem of extrapolating near-field radiation measurements to the far zone and concluded that the far-zone sampling rule (d?=?/D) works as well in the Fresnel portion of the near zone. In addition, we find that the angular window, W, over which the Fresnel-zone field must be measured is approximately W - 2D/R radians in width, where D is the nominal diameter of the antenna and R the range at which the near-field data are taken. This guideline is valid when one uses an integral extrapolation scheme, as opposed to a modal one, since the paraxial approximation gives some assurance that field contributions from points outside the sampling window will contribute negligibly to the far-zone amplitude. We have also looked at the sampling requirements associated with extrapolating near-field RCS measurements to the far zone and concluded that windowing techniques can reduce the magnitude of the bistatic scanning task dramatically.
This paper presents a simple and straightforward technique which significantly improves the performance of some anechoic absorbing materials. The method is easily applied to existing absorbers and chambers and does not change the basic design of the material. The technique involves the proper placement of additional absorbing materials between the shaped structures of the absorber to reduce major scattering contributions. These scattering mechanisms are demonstrated in the paper with measured evaluation data for various absorber types and sizes. The effectiveness of the technique has been best realized for pyramidal shaped absorbers 24 inches and longer and for normal plane-wave incidence. Improvements in the absorber's reflectivity of up to 30 dB have been demonstrated. An example illustrating the method for the reduction of the backwall RCS level of a compact-range chamber is presented.
A free-space RF absorber material (RAM) has been developed and optimized for frequencies above 30 GHz. It is particularly suited for use on equipment and fixtures for RCS, antenna, radiometric, and quasi-optical testing. The material has unique geometry which yields enhanced RF performance when compared with conventional wedge or pyramidal absorbers. Mechanically, the material is elastic, resists damage from flexing or repeated contact and is non-flammable and non-toxic. It offers advantages in size, durability, and mechanical uniformity over previously available products. Data describing RF and mechanical performance are presented.
Antenna and Radar Cross Section measurements require a large amount of data collection. Network Analyzers are often used to characterize these systems, and although these data ideally are collected automatically by computer it is not unusual for a single characterization to require many hours or even days to perform. We describe a technique for speeding up these measurements by at least an order of magnitude. Clearly making measurements in an hour that formerly took a day or making measurements in a day that formerly took two weeks is extremely appealing. The method we describe may be used for applications which require a large number of automatically performed measurements with sequentially swept frequencies, but which find lack of speed in tuning the network analyzer to be a limiting factor. Antenna, and Radar Cross Section measurements benefit substantially since frequency response measurements must be repeated many times to provide spatial characterization.
Recent advances in RCS measurement techniques, microwave hardware receiver technology and computer capability have drastically altered the price and performance considerations of turn key RCS measurement systems. Access to 'real time' data and processing improvements are a few of the issues addressed in lower cost and compute intensive configurations available in today's marketplace. This paper explores a systems approach to the wide variety of components configurable for 'state-of-the-art' RCS measurements. High performance, flexibility and productivity are emphasized.
The ISAR image is a domain that possesses many of the spatial physical characteristics of the target. Certain procedures can be performed in the image domain that are equivalent to physical operations on the target. These operations include the ability to modify the amplitude of the scatterers that are represented in the image and, after performing these modifications, subjecting the image to an inverse transformation that recovers RCS data of the whole body as a function of frequency and aspect angle. The RCS plots obtained by transforming the edited image are representative of similar modifications made to the physical body and are of value in eliminating the need for many model modifications and retests in low-observable model development. This paper describes, using simulated and actual target data, some of the procedures that can be fruitfully applied in this type of analysis.
In this paper a new system consisting of a single parabolic reflector and a point source will be presented. Such a system is capable of producing a cylindrical wavefront over a wide frequency range. Moreover, physically large text-zone dimensions can be realized. The principle of operation is identical to that of the near-field/far-field cylindrical scanning, however, the far-field antenna pattern or RCS response can be computed more efficiently by performing a simplified transformation procedure in one dimension only. It will be shown that such a system is suitable for both antenna and RCS measurements. Finally, experimental RCS data will be presented.
The performance of an outdoor, ground-plane RCS measurement range can be degraded by fluctuations in the atmospheric reflectivity N. These fluctuations can introduce error into RCS measurements, particularly when they do not manifest in the radar return from the secondary calibration standard. A propagation anomaly study at the RATSCAT RCS range compares the N-fluctuations -- obtained from meteorological instruments and separately from RF receivers -- at several levels above the ground. The fluctuation mechanisms are discussed in terms of temperature lapse rates, "constant-N" cell sizes, wind velocity, and rough ground effects. The optimal RF sensor height for propagation anomaly indications is found to depend on the cell size. This has implications for the positioning of secondary calibration standards.
It is common practice to window RCS data prior to inverse Fourier transformation into an image. Windowing reduces image sidelobes at the expense of some loss of resolution. When the window shape is adjusted to give the best resolution-sidelobe tradeoff for the given application, however, the apparent RCS of features in the image varies unless the correct calibration of normalization is applied. This paper discusses the proper calibration and normalization techniques to use with RCS imaging. These techniques permit efficient generation of images that accurately depict the RCS of significant target features, independent of the data window shape.
An instrumentation radar system suitable for collection of backscatter characteristics of targets in an indoor chamber was built and installed in the Ohio State University ElectroScience Laboratory. The radar is a pulsed system with continuous coverage from 2 to 18 GHz, and spot coverage from 26 to 36 GHz. The system was designed to have maximum flexibility for various test configurations, including complete control of the transmit waveform, H or V transmit polarization, dual receive channels for simultaneous measurement of like and cross polarization, greater than 100 dB dynamic range, and convenient data storage and processing. A personal computer controls the operation of the radar and is capable of limited data reduction and display functions. A mini-computer is used for more widely sophisticated data reduction and display functions along with data storage. This paper will present details of the radar along with measured performance capabilities of the system.
Modern analysis techniques of radar scattering data or radar cross section (RCS) data often include transformation to the time domain for the purpose of understanding the specific scattering mechanisms involved or to isolate or identify specific scattering points. The classic technique is to transform from the frequency domain to the time domain using an inverse (Fast) Fourier Transform (IFFT). Often, however, the scattering centers are too close together to resolve or the requirement for accuracy in the measurement of the differential time delay is too high given the IFFT inverse bandwidth. This paper presents a technique for determining the time domain response of a radar target by processing the data using modern autoregressive (AR) spectral analysis. In this technique, the scattering from a radar target in the high frequency regime is shown to be autoregressive. This paper will show examples using the maximum entropy method (MEM) of Burg.
The monostatic RCS of ogival tilted pylon was calculated in a two stage computational process. First, a two-dimensional model of an infinite cylinder of ogival cross-section was employed. At the second stage, the effects of finite length and inclination were incorporated. The RCS was predicted by two independent methods, namely, the method of equivalent currents and the method of moments. Excellent agreement between the results of the two methods was found in the overlap domain of their respective validity. The results indicate a weak dependence of the RCS on the ogive ratio. Similarly, it was found that in the case of an infinite straight ogival cylinder the effect of frequency variation is negligible. The main contribution to RCS reduction is derived from the finiteness of the pylon and its tilt. It also becomes evident that beyond a certain tilt angle the marginal decrease of RCS does not justify the increasing mechanical complexity.