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Historically, radar imaging sensors have been divided into two categories, SAR and ISAR systems. Even though they are solving the same imaging prob lems the data collection environment is dramatically dif ferent between the two. Consequently, the particular waveforms selected for the two have been different. The primary waveform for ISAR RCS measurement systems is stepped frequency, while the FM-chirp (linear-FM) waveform has been used much more often in SAR applications. However, recently this boundary has been blurred, in that stepped frequency radars are being applied to long range dynamic measurements, long the domain of chirped waveforms, and conversely the chirped waveform has been applied to target RCS mea surements of both static and dynamic targets. This paper will address the system parameter tradeoffs involved in selecting between the two waveforms for two different applications; (i) near range static target imaging, and (ii) far range dynamic target imaging. The system parameter tradeoffs involve RF bandwidth, PRF, scene size, trans mitter power, doppler frequency spread of target, etc. The advantages, disadvantages, and inherent limitations of each waveform will be analyzed to yield a better understanding of the tradeoffs involved, and the data collection examples will further illustrate these tradeoffs for the two specific applications.
With modern range hardware, it is possible to per form ultra wide band frequency measurements with out changing the range configuration. This has not been possible with existing chamber antennas be cause they have been limited in bandwidth in or der to provide the desired illumination. In addition, these antennas have not considered scattering issues, even though one goes to great lengths to minimize reflections within a chamber. The rolled edge Slot line Bowtie Hybrid (SBH) antenna has been used for ultra wide band applications for many years. How ever, it can not meet the range scattering require ments due to its structure (large rolled edges). In this paper, a new R-Card version of the SBH an tenna is presented. It is fabricated by integrating resistive sheets (R-Cards) into the blended rolled edge concept so that both the ultra wide band and low RCS antenna features can be obtained simul taneously. Further, by employing resistive sheets, the R-Card SBH antenna can provide the desired constant beamwidth to fully illuminate the target zone. Measured and calculated results are presented to demonstrate the performance of this new antenna.
The RCS of extended objects measured in the near field is subject to errors induced by the spherical nature of the incident and scattered wavefields. A number of techniques have been applied to estimate far-field responses from results of monostatic near-field measurements. While the results indicate successful transformations for linear scatterers, the lack of a sound theoretical basis brings into question the appli cability to general objects. The paper explores the theoretical basis of the far-field transformation of RCS data and the consequence of the limited data obtained from monostatic measurements. The limitations of approaches reported to date [1-4] are explored from conceptual and physical con siderations with the goal of establishing reasonable expectations for practical methods. Examples using simulated and measured near-field data are presented to illustrate successes and failures of the algorithms in transforming results to far-field RCS.
Coherent background subtraction is an established method of reducing additive range clutter in radar cross section measurements. In some measurement situations, it is neither practical nor convenient to directly make a coherent measurement of the range background. The Environmental Research Institute of Michigan has devel oped two methods of synthesizing background measure ments for the coherent subtraction of additive clutter in these cases. The first method synthesizes a background for measurements of pylon-supported targets by remov ing unterminated pylon returns using software gating. The second method improves background subtraction by compensating for phase drift between target and back ground measurements. In this paper, these methods of improving the performance and utility of background subtraction will be described and demonstrated on mea sured data.
This paper reports on the results of computer simulations of planar near-field scanning and its ability to achieve an high accuracy test-zone field over a wide range of pattern angles. An quality test-zone field was defined for this study to have less than 0.2 dB peak-to-peak amplitude variation and less than 1.5 peak-topeak phase variation. This investigation sought the minimum scan length, for a given critical angle, ec and separation, S. The minimum scan length determined from this investigation is given by: L = D + 2S(tan(0c)) + 20/cos(0c). This scan length is approximately 60),, larger, for a critical angle of 70 degrees, than previously accepted. It is suggested that the maximum practical value of Sc is between 60 and 70 degrees. The use of raised cosine amplitude and/or quadratic phase windows to the edges of the measurement plane is shown to provide test-zone field quality improvement and/or allow scan lengths approximately 10),, smaller.
Several approaches are known for the identification of noncooperative air-borne targets with radar. Assuming that the tar get can be tracked during a certain flight path, observations from different aspect angles will be obtained. High-resolution radar (HRR) systems use these observations to create one-dimensional range profiles. With Inverse Synthetic Aperture Radar (ISAR) the data from all observed aspect angles are combined to obtain two-dimensional images. In recent years, techniques for resolution enhancement have been developed for both techniques. The choice for one of the two approaches should depend on the applicability of the target representation for identification. ISAR is the most suitable for reproduction on a display and identification by human observers. In case of identification by a machine, for example an algorithm on a computer, the choice is not straight forward. In this paper an overview of the influence of several errors on the performance of HRR and ISAR will be given. The error sources that will be evaluated are: • uncertainty of the absolute distance of the target; • errors in the mutual alignment of observations; • additive noise. The errors are generated numerically and applied to data from simulations and low-noise measurements. The influence of the bandwidth and angular span on the quality of the target reconstruction will be regarded as well as the performance of some high-resolution techniques. Finally, conclusions are drawn concerning the applicability of ISAR and HRR.
Traditional range/doppler ISAR techniques have inherent geometric limitations. By using concepts of microwave holography and tomography, a vector-based k space approach allows a more generalized geometry of the sampled Fourier space. By constructing a complete annulus in the polar sampling space, arbitrary apertures up to 360 degrees can be processed for "full body" two dimensional images. This processing also typically exhibits better resolution. The algorithm relies on linear interpolation for potar Cartesian conversion. The general geometric formulation is also readily adaptable to arbitrary antenna configurations.
This paper describes an adaptive array that was designed to improve the carrier-to-interference ratio (C/I) delivered to base station radios by 6 dB in U.S. 800 MHz analog cellular networks. The C/I performance of this kind of system is difficult to verify, because it cannot be characterized in terms of traditional antenna specifications such as beamwidth and directivity. This paper describes a simple C/I measurement strategy in which the antenna under test and a collocated reference antenna are placed into simultaneous operation in an actual cellular network. Relative C/I performance can then be deduced from a statistical analysis of the antenna outputs. This method is particularly well-suited to software radio based systems, because no special test equipment is required to gather the necessary data.
A novel test article, the Transformable Scale Aircraft-Like Model (TSAM), which holds great promise for validating complex computational electromagnetic (CEM) codes more effectively is described. The novelty of TSAM is in the use of removable/replaceable canonical shaped structural components. The complexity in TSAM can be tailored to the modeling capabilities of the CEM code under test allowing discrepancies between measurement and simulation to be more explainable. A set of preliminary measurements on TSAM have been made and the results compared to calculations from the General Electromagnetic Model for the Analysis of Complex Systems (GEMACS) program (1), a standard CEM code.
Near field inverse synthetic aperture radar 3D is performed utilizing data for arbitrary, but known, positioning of the target. The imaging method was implemented and is described. This straightforward approach has many advantages. It geometrically correct in near field. Field corrections can be independently for each frequency, antenna position and point of interest in the target volume. The main disadvantage is that the processing using the algorithm is very time consuming. However, in many cases it is only necessary to perform the analysis on a few cuts through the object volume.
Due to inherent cost, safety and logistical advan tages over dynamic measurements, Ground-to-Ground (G2G, aircraft and radar on tarmac) diagnostic radar measurements may be the preferred method of assessing aircraft RCS for signature maintenance. However, some challenging complications can occur when interpreting SAR imagery from these systems. For example, the effect of ground induced multi-path often results in the measurement of a significantly different image based RCS than would have been obtained by a comparable Ground-to-Air (G2A) or Air-to-Air (A2A) system. Although conventional 2-D SAR images are useful in determining the physical source (down-range/cross range) of scatterers, it is difficult at best to deduce whether an image pixel is a result of direct (desired) or ground induced multi-path (undesired) scattering. ERIM and MRC recently completed an experiment testing the utility of collecting and processing interfero metric (2-antenna) SAR radar data. This effort produced not only high resolution SAR imagery, but also a com panion data set, derived from interferometric phase, which helps to isolate the source (direct or multi-path) of all scattering within the SAR image. Additionally, the data set gives a measure of the physical height of direct scatterers on the target. This paper outlines the experiment performed on a RCS enhanced F-4 aircraft using a van mounted radar. Conventional high resolution imagery (down-range/ cross-range/intensity) will be shown along with down range/height/intensity and cross-range/height/intensity images. The paper will also describe the processing pro cedure and present analysis on the interferometric results. The unique motion compensation processing technique combining prominent point and motion mea surement instrumentation data, eliminates the need for a tightly controlled collection path (e.g. bulky rail sys tems). This allows data to be collected with the van driven somewhat arbitrarily around the target with side mounted antennas taking measurements at desired aspects.
Spatially Variant Apodization (SVA) [l] is nonlinear image domain algorithm which effectively eliminates finite-aperture sidelobes from SAR/ISAR imagery without degrading mainlobe resolution, unlike traditional methods of sidelobe suppression (e.g. Taylor weighting). Dezellum et. al. [2] demonstrated at the 16th AMTA symposium the benefits of SVA for improving RCS analysis of ISAR data. The purpose of this paper is to show that robust super-resolution via bandwidth extrapolation can be obtained in a relatively simple, straightforward manner using SVA, providing further improvement in RCS measurements from SAR/ISAR data. This new super-resolution algorithm (called Super-SVA) can extrapolate the signal bandwidth for an arbitrary set of scatterers by a factor of two or more, with a commensurate improvement in resolution. Super-resolution techniques have been traditionally limited to problems where a-priori knowledge is available and/or the scene content is suitably constrained. Using Super-SVA, no a-priori knowledge of scene content is required. Super-SVA exploits the fact that SVA applied to an image results in finite image-domain support on the scale of the system resolution for an arbitrary set of complex scatterers. Extrapolation of the frequency-domain signal data is then simply a matter of applying frequency-domain inverse amplitude weighting. The fidelity of the deconvolution process can be improved by embedding the original signal data in the extrapolated data and performing further iterations of the process.
Rome Laboratory has recently designed and implemented a state of the art automated antenna measurement data acquisition system at the Rome Lab/Newport antenna test facility. A generalized approach to the antenna data acquisition hardware and software was implemented which allows sequencing, control and measurement of test variables in virtually any order without test specific software modifications. The hardware design is based on distributed computers in which real-time data acquisition tasks and near real-time operator control and data analysis tasks are performed independently. The computers operate in a remote client/server configuration in which control information and data are transferred via fiber optic local area network. In this paper, the fundamental approach to the data acquisition system design is discussed and the antenna measurement hardware and software that comprises the final system are described.
The feasibility of realizing a 500 GHz hologram type of compact antenna test range (CATR) for testing the 1.1 m antenna of the Odin satellite is studied. The quiet-zone field is analyzed theoretically by us ing an exact near-field aperture integration method. Due to fabrication errors the slots of the hologram are wider or narrower than in the ideal case. How ever, with reasonable value of fabrication errors the quality of the quiet-zone field is not degraded deci sively. The effect of the displacement of the different parts joined together to form a large hologram is the inclination of the amplitude and phase in the quiet zone. The analysis of the CATR feed scanning in the focal plane to avoid the need to rotate the AUT showed that at least a ±1° change of the direction of the plane wave in the quiet-zone is feasible.
Instrumentation grade, coherent, bistatic, radar cross section (RCS) measurement systems require a reliable low-noise method to link the reference, local oscillator (LO) and intermediate frequency (IF) coherent signals between the transmit and receive subsystems. One approach to this is the use of a fiber optic link (FOL). Phase noise measurements have been performed on a distributed feedback (DFB) type laser transmitter-photodiode receiver link with a delay of up to 2.26 kilometers, operating at 5 GHz, using a standard HP 3048A phase noise test measurement setup. System level tests have been performed, incorporating a FOL into a coherent bistatic instrumentation radar system local oscillator path, and performing image processing on an emulated target A first level analysis was conducted regarding the effects of the thermal noise on the radar perfonnance.
This paper will describe theoretical and experimental techniques for the analysis of the performance of conformal automotive antennas. The theoretical techniques include the application of the method of moments (wire and plate models) and UTD. The experimental techniques include turntable range measurements and mobile on-road measurements. The antennas to be modeled and tested include an AM/FM annular slot windshield film antenna, a generic rear-window heater grid AM/FM antenna, and various configurations of cellular telephone antennas.
Due to the limited size of modern helicopters, airborne antennas must be physically small and lightweight. Slot antennas have been widely used by the aerospace community to meet the size, weight, and aerodynamic requirements when flush-mounted to a platform surface. Having these characteristics, a ferrite-loaded cavity-backed slot (CBS) antenna is an excellent choice for as a tunable low-frequency antenna. Excitation of a magnetostatic mode in the ferrite results in resonances at frequencies below those of the dynamic modes of dielectric-loaded CBS antennas. Frequency agility is achieved by varying the applied DC magnetic bias. Two ferrite-loaded CBS antennas were built and their impedances and radiation patterns were measured. Reasonable (0-6 dBi) with dynamic 3 dB bandwidths in excess of 20% were measured in the UHF band. Air-filled versions of these antennas agree well with Method of Moments (MoM) predictions, but non-uniformity of the magnetic field in the ferrite violates assumptions made in the theoretical model, resulting in discrepancies.
Characterisation of the test zone field in a Compact Antenna Test Range (CATR) is traditionally done by scanning with a probe. The test zone field can thus be measured more or less directly at any position by the probe. This method, however, has some serious disadvantages. In this paper the scanning probe method is compared with a characterisation method using a reference target such as a flat plate, bar or cylinder. It will be shown that from an RCS measurement of the reference target, an accurate test zone field can be determined using Fourier transformation. An analysis of this method together with experimental verifications which validate the approach will be presented. A comparison between the probe and reference target method is also given.
From a study of several radar cross section (RCS) measurement facilities, we identify significant sources of uncertainty and develop methods for estimating their effect. Out goal is to provide a "reasonable" and uniform formalism for evaluating RCS measurements which can be used on a variety of test ranges to produce comparable estimates of uncertainty.
A novel computer simulation methodology to properly characterize the role of probe directivity/pattern compensation in cylindrical near field scanning geometry is presented. The methodology is applied to a linear test array antenna and the JPIJNASA scatterometer (NSCA1) radar antenna. In addition, error analysis techniques of computer simulation and measured have been developed to determine the achievable accuracy in pattern measurements of the NSCAT antenna in cylindrical near field.