Communications: directive radio wave systems and devices (e.g. – With particular circuit – Spectrum analysis
Reexamination Certificate
2001-11-20
2003-06-17
Lobo, Ian J. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
With particular circuit
Spectrum analysis
C342S090000, C342S195000
Reexamination Certificate
active
06580388
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electromagnetic and acoustic signatures of objects, more particularly to methods and apparatuses for determining such signatures for complex objects.
Signature reduction of large systems and vehicles is critical to achieving the desired effectiveness of future military systems. As the U.S. Navy progresses towards low observable system designs, new and innovative methods and technologies are needed to meet growing signature reduction needs. The radar or acoustic signature of a body such as a three-dimensional (3D) complex structure can be reduced by shape modification and/or by application of radar or acoustic absorbing material.
Existing numerical methods and computer codes are not adequate or sufficiently accurate for such purposes, since the signature levels have reached a point where second order effects become important. Typically, signature prediction techniques like Physical Optics (PO) or Physical Theory of Diffraction (PTD) are high frequency approximations, and exact methods like Method of Moments (MoM) or Finite Difference Time Domain (FDTD) are computationally intensive and impractical for large objects. Moreover, in many cases, absorbing materials or systems designed to reduce signature are difficult (or impossible) to model accurately using available prediction models, and the only recourse is to use costly full-scale measurements.
In response to U.S. Navy needs to reduce stack and antenna signatures of U.S. Navy ships, Carderock Division of the Naval Surface Warfare Center (NSWCCD) is in the process of developing a low observable (LO) exhaust system with satellite communication (SATCOM) antennae embedded in the associated topside structures. Concept designs for a Low Observable Multi-Function Stack (LMS) are being, developed by the U.S. Navy as part of a FY98 Advanced Technology Demonstration (ATD) program. The present invention is a product or spin-off of the research and development work of the LMS project.
The feasibility of meeting future ship Radar Cross-Section (RCS) signature goals with the LMS was evaluated by the U.S. Navy by performing parametric studies of the LMS shroud shape. The parametric studies showed that the LMS shroud would require radar absorption. A Radar-Absorbing Structural (RAS) material satisfying Radar Cross-Section (RCS) requirements was proposed and developed for the LMS. Bistatic measurements (the accepted method of characterizing the performance of radar absorbing materials) of the proposed LMS material showed that it satisfied the nominal radar attenuation requirements.
A simplified scaled version of the LMS was fabricated using proposed LMS material to evaluate the monostatic radar scattering response. The scaled version of the LMS was a truncated pyramid with approximate dimensions of 6 feet wide by 6 feet long and 3 feet high. The resulting RAS truncated pyramid was measured at the Pt. Mugu radar reflectivity compact range. The RCS measurements of the truncated pyramid showed surprisingly large backscattering from the proposed LMS material.
Attempts to reproduce the RCS measurement results of the truncated pyramid using the measured bistatic absorption of the LMS material as an input to the high frequency Radar Target Signature (RTS) code were not successful. Within the RTS code, the effect of radar absorbing material (RAM) on the radar signature of a scatterer is determined by extracting radar signal attenuation values from a table of measured or calculated bistatic absorption data.
The truncated pyramid or any other target is considered in the RTS code as a collection of basic geometrical shapes, called “primitives” (such as flat plates, elliptic cylinders, truncated cones, etc.), with the total signature of the object being simply the coherent sum of the signature contributions of each of the individual primitives. The assignment of RAM signal attenuation values to any primitive shape on the model geometry is one of the RTS features. For the assigned material, radar signal attenuation is defined as a specular bistatic response for the appropriate radar frequency, incidence angle, and polarization.
However, some materials and structures (such as the proposed LMS material) have a significant unexpected non-specular scattering with undesired monostatic radar returns. The effect of the non-specular scattering is to dominate what would normally have been very low RCS aspects of the truncated pyramid, thus controlling it's median RCS. A problem thus presents itself as to how topredict such monostatic non-specular radar returns, and to identify RCS signatures of complex entities such as ship size systems made of such materials and other non-uniform structures.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide method and apparatus for rendering signature determinations for complex entities which do not admit of conventional techniques (such as involving computer modeling) for accomplishing such purposes.
It is a further object of the present invention to provide such method and apparatus so as to avoid the necessity of effectuating full-scale measurements of such complex entities.
It is another object of this invention to provide such method and apparatus for rendering signature determinations for complex entities which, due to their material and/or structure, have associated therewith radar cross-section signatures characterized by significant monostatic non-specular radar returns.
The present invention provides a methodology for determining a signature of a complex object. An important benefit of the present invention is that it accounts for non-specular scattering and the accompanying monostatic radar returns.
A notable feature of the present invention, unknown in the art, is the extrapolation of signature information from one object to another object. The inventive methodology uniquely includes an extrapolation of the radar cross-section (RCS) signature (or acoustic signature, for acoustic applications) of a “sample” object (such as a scaled-down model of the LMS shroud, a flat RAS panel, or a section of an antenna array) to a full-scale “complex” object (such as a ship size system) which the sample object represents. Typically according to this invention, the sample object is simpler than is the complex object. According to a principle of the present invention, inasmuch as the present invention's “three-dimensional scattering elements” each represent a part of the complex object (e.g., system), the inventive methodology can use either or both of measured sample object signatures and predicted sample object signatures to make extrapolations.
The known methodology for predicting signature data involves (i) taking measured or calculated bistatic signature data from a sample object, and (ii) applying such bistatic signature data to a target object so as to obtain a coherent summation of individual primitives. The present invention provides a new methodology, according to which signature data is extrapolated from a sample object to a complex object (e.g., target). The present invention involves (i) taking measured or calculated monostatic signature data from a sample object, and (ii) extrapolating such monostatic signature data to a complex object so as to obtain an incoherent summation of three-dimensional scattering elements, wherein the three-dimensional scattering elements are reflective of the monostatic signature data. Advantageously, the inventive methodology succeeds in predicting radar-cross section signatures of complex objects which account for monostatic non-specular radar returns from such complex objects; the inventive methodology thus succeeds where the known methodology fails.
According to typical embodiments of this invention, the inventive methodology comprises the actions and rudiments set forth in the following four paragraphs. It is emphasized that the present invention succeeds in estimating either an electromagnetic (e.g., radar) scattering signature or an acoustic scattering signature.
F
Bird William R.
Stoyanov Aleksandr J.
Stoyanov Yuri J.
Tsang Valiant F.
Kaiser Howard
Lobo Ian J.
The United States of America as represented by the Secretary of
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