Wednesday, 17 January 2007: 4:30 PM
Polarization Selection For Weather Phased Array Radar
217A (Henry B. Gonzalez Convention Center)
In weather radars, polarization is an extremely important parameter for assessing the composition of the atmosphere. The relationship between simultaneous horizontally and vertically polarized returns has shown to provide quantifiable estimates of rainfall. This capability is now being integrated into the WSR- 88D parabolic antenna radar system. Studies provide some evidence that understanding of atmospheric content of moisture in many states (other than rain) can be determined by full characterization of the radar return (H-H, H-V, V-H and V-V) in a simultaneous or sequential methodology. Multiply polarized, wide angle scanned solid state phased array antennas is a technology new to the weather radar application. These new agile beam systems can play a key role in improving warning time for hazardous weather by providing quantifiable information to the forecasting models. However, the selection for the polarization approach (dual circular vs. dual linear) is different for the parabolic dish and phased array antenna (PAR). Whereas a parabolic antenna polarization configuration is fixed for all scan angles, the polarization of the phased array is determined by the behavior of its vertically and horizontally polarized radiating elements over the extent of the scan region. Both the direction (perpendicularity) and the rate of change of the component polarizations from the PAR elements change significantly over the scan volume. This makes it exceedingly difficult to maintain circular polarization over a wide angle scan regions. The inherent filtering/rejection of opposing circular polarizations are thus jeopardized for PAR systems at scan angles far from broadside. PAR linearly polarized components are well behaved over the scan volume, and their respective sensitivities can be easily calibrated. Optimum dual linear polarization can be achieved through calibration of the relative amplitude and angle of the orthogonal polarizations comprising the radiated field. Since calibration is executed at the center of the antenna beam, there will be a residual polarization error that grows from a minimum at beam center to a maximum at angles off beam-center. An analysis has been made of the magnitude of the residual polarization error of the antenna as a function of the antenna beamwidth over the scan coverage area for a PAR with a cross-polarized radiating element.