Antenna data compression using multi-dipole antenna

Communications: directive radio wave systems and devices (e.g. – Directive – Including antenna pattern plotting

Reexamination Certificate

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Reexamination Certificate

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06437737

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to the compression of antenna voltage data acquired with a receiving element of an antenna system. More particularly, the present invention relates to a data compression method for determining a set of complex weights which, in cooperation with the position and orientation of one or more idealized linear dipole antennas, permit voltage data measured with a receiving antenna to be represented with minimal loss of accuracy using minimal computer storage memory.
In general, data compression refers to the process of reducing the amount of data required to represent a given quantity of information with sufficient practical accuracy. A practical problem solved by data compression is efficient image transmission where the underlying objective is to remove redundant data. The reduced data may thereby permit more efficient manipulation, storage, and transmission of information. At a later stage in the handling of this information, the compressed image may be decompressed to reconstruct the original image or an approximation of it. Image compression techniques play a crucial role in numerous applications including televideo conferencing, remote sensing, document and medical imaging, facsimile transmission, among others.
There exist a number of error-free compression techniques with which one may exactly reconstruct the compressed image. These techniques typically provide compression ratios on the order of from 2:1 to 10:1. Lossy encoding approaches on the other hand achieve increased compression ratios but result, by definition, in some degradation of the reconstructed image. Depending on the data, a minimal tradeoff in accuracy may yield significant increases in compression ratios.
Antenna data compression refers to the compression of complex voltage data empirically derived using an antenna and source of RF radiation. The voltage data are collected in a controlled environment and are representative of the voltages one would expect to measure in a field application using the same or a substantially similar antenna. The data are a record of the phase and amplitude of the complex voltages generated at an antenna array when irradiated with an emitter of known power, frequency and polarization. The relative orientation of the array and emitter are varied to acquire a sampling of data representative of the field of view of the antenna. Preferably, the data are characterized by a high signal-to-noise ratio at each of one or more distinct frequencies falling within a predetermined bandwidth. The voltage data are essential in passive, direction-finding systems that are used by ground-based radar and airborne vehicles. Angles-of-arrival are two body-relative polar angles representing the direction of the incoming wave. In the absence of a physical model, the voltage data are required for angle-of-arrival determinations of incoming electromagnetic signals when using search techniques relying on comparisons between measured and reference voltages.
In the present application specifically, the compression of the voltage data refers to the process by which the voltage data are mathematically modeled for later reconstruction in a fast and efficient manner using minimal computer storage memory. With no compression, the amount of memory required to adequately represent the reference voltage data with sufficient accuracy and resolution exceeds the practical memory and data transfer capacity of many volumetrically constrained applications such as with modern missiles and highly portable systems.
In missile and sensor pod applications, the problem of compression is exasperated by the complexity of the voltage data acquired by means of conformal antenna elements. Individual antenna elements are commonly mounted to, or in proximity of, the metallic or dielectric body of a missile, sensor pod or sensor suite housing. The presence of the body may partially shadow the antenna elements in addition to perturbing the field pattern in ways that are difficult to represent and predict with practical accuracy. As a result, both the phase and amplitude of antenna data may deviate substantially from those of simple models of sub-element antennas, such as the infinitesimal dipole model.
A second problem associated with voltage data acquired from antennas in missile applications arises from the fact that it is often necessary to place antenna elements within the constraints of special orientations as required for practical configurations with non-planar surfaces. The antenna elements are typically made to conform to the cylindrical or conical shape of the missile or sensor housing, giving rise to an array of elements having a diversity of polarization orientations. In the previous generation of missiles, arrays were comprised of receiving elements with substantially identical polarization angles resulting in an almost complete absence of polarization diversity. Each of the antenna elements possessed an equivalent polarization mismatch with the incoming wave giving rise to interferometric measurements that are a function of the difference in path length traveled by the incoming wave-front as it impinges upon the particular antenna element pair alone. The phase difference between pairs of antennas in a polarization-diverse array includes additional contributions because of the polarization mismatch between receiving antennas produced by the mounting orientations. The voltage received at each antenna is therefore a relatively complicated function of the angles of arrival and of the polarization of the incoming wave.
There are currently numerous transforms with which one may represent signal data. Among the most notable and commonly applied are the Fourier transform or Discrete Fast Fourier Transform (DFFT), as well as the Cosine transform. These transforms permit one to efficiently reconstruct a given signal exactly where: (1) the original signal is band-limited and (2) the data are acquired at a rate sufficiently high to satisfy the Nyquist sampling criterion. Where the signal data possess high frequency content, a large number of DFFT expansion terms must be retained in order to maintain the integrity of the original data. For purposes of antenna data compression, the Fourier transform is unsuitable for the reason that the voltage data are high frequency data. In the present application, for a typical quality, the Fourier and Cosine transforms provide unacceptably low compression ratios of four to one over the original data.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide a method for accurately representing the voltage data signals of an antenna system comprising a plurality of RF sensing elements with minimal data and minimal loss of accuracy. The complex voltage data signals are represented by the amplitude and phase of the received RF signal as measured by the plurality of sensing elements having differing and substantially known orientations of polarization. The voltage is primarily dependent on the two angles-of-arrival, two polarization parameters specifying the polarization of the incoming electromagnetic wave, and the frequency of the received wave.
This and other objects are achieved by representing each RF sensing element of an array of elements as the superposition of a plurality of “sub-element antennas” with variable positions and orientations. The sub-element antennas are not physical antennas, but are mathematical representations of infinitesimal antennas distributed in free space. The sub-element antennas are preferably infinitesimal linear dipoles, but may be generalized to represent elements of elliptical polarization.
As physically realizable elements, the RF sensing elements are used in airborne vehicles to receive RF radiation propagating from one or more emitters. These receiving elements, when working in cooperation, comprise an array for performing direction-finding. Each receiving element has associated with it a distribution of sub-element antennas for modeling the voltage measured by the rece

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