Backscatter Analysis of DF Platform Properties, 16-R9780Printer Friendly Version
Inclusive Dates: 01/01/08 07/01/09
Background - SwRI has expertise in implementing direction-finding (DF) systems on complex platforms such as naval ships. By sensing the field at multiple locations on the platforms, these systems determine the direction of arrival of incoming plane wave signals. SwRI has developed a DF algorithm to account for antenna pattern perturbations caused by the complex scattering environment. Unfortunately, these complex platforms have characteristic scattering properties (resonances) that limit the performance of the DF system. With the advent of electromagnetic modeling, different array configurations can be modeled, and the configuration having the least impact from the platform can be chosen, but this practice is difficult because of the complex inter-relationship between the antenna array characteristics and the platform characteristics. The same electromagnetic models developed by SwRI to analyze the performance of DF systems can also be used to determine the radar cross-section (RCS) of the platform. Both the DF and RCS are responses to incident plane waves. The concept being proposed here for investigation is to use the numerical model to determine the RCS for all angles of arrival, elevations, polarizations, and frequencies and use the RCS as a function of these signal parameters to characterize the platform for DF performance.
Approach - Several very good numerical models of naval platforms exist for which DF antennas arrays have been installed and calibrated. The numerical data previously generated for these ships indicates frequencies and azimuths that have consistently limited performance across all sources of data indicating the potential of platform characteristics influencing performance. There are also azimuths and frequencies at which the response data consistently disagrees between the data sources. This indicates platform characteristics that may make it difficult to model using either scale or numerical models. Numerical models of these platforms will be used to generate backscatter responses and surface currents over a set of frequencies, azimuths, elevations and polarizations. Characteristics of these responses and currents will be analyzed to determine their relationship to areas of DF performance problems. Once these relationships are determined, a set of MATLAB tools will be developed to automate the process of searching for the parameters that can cause potential DF problems.
Accomplishments - Models of a simple DF array in the presence of numerous simple scattering structures have been built. These models displayed a few significant results. These scattering objects do significantly affect the DF performance of the array, but do so over a relatively large frequency range making it extremely difficult to discern between scattering mechanisms in complex scenarios. Some scattering structures such as corner reflectors produce very strong backscatter results while not affecting the DF performance. Other resonate structures such as monopole objects affect the DF performance significantly while not producing a strong backscatter response. Based on these backscatter findings, the emphasis of this project was changed to the evaluation of the surface currents on the model. A direct correlation was found between strong surface currents and DF anomalies. Unfortunately, the generation and storage of the surface currents as a function of frequency, azimuth, elevation, and polarization requires an extensive amount of computational run time and disc storage, making this type of evaluation impractical for the large scale models we typically construct.