Development of a High Performance Offset Gregorian Antenna for the Colorado State University (CSU)-CHILL National Radar Facility

Colorado State University, Fort Collins CO

Investigators

Abstract

Under this MRI, the Principal Investigator will develop, acquire, install and test a new high performance offset Gregorian antenna for the Colorado State University (CSU)-CHILL radar facility. The CSU-CHILL radar facility has been operated for many years by CSU as a multi-user facility available to all qualified researchers. The facility is supported primarily through a cooperative agreement between CSU and NSF. The specifications for the new antenna are stringent and push the antenna design and manufacturing technology. A substantial reduction in sidelobe levels over the current CSU-CHILL antenna is expected. This improvement results from the offset Gregorian design that eliminates blockages by the feedhorn and feed support struts. The cross-polarization levels are expected to be less than -35 dB in any plane. The ratio of two dimensional integrated cross-polarized energy to copolarized energy over all planes is expected to be reduced by around 10 dB compared with the current antenna. These correspond to significant reductions in expected (two-way) sidelobe clutter-to-mainlobe signal power (16-20 dB), and significant improvement in the radiated polarization purity offered by the dual-offset design relative to the prime-focus parabolic reflector antenna. These high antenna performance levels are needed to accurately measure conventional Doppler data and the full polarimetric covariance matrix in the presence of strong spatial gradients of precipitation echo, especially in severe storms. Currently, the relatively poorer sidelobe performance near the planes of the feed support struts tends to cause large errors in many of the key polarimetric measurements even in the presence of modest precipitation echo spatial gradients. The new antenna performance is expected to nearly double the precipitation echo spatial gradient that can be tolerated without significant error in the polarimetric measurements. It will also greatly improve the quality of conventional Doppler data especially in the context of data assimilation by numerical models. The new antenna will permit cutting-edge research using the full covariance matrix to greatly improve the quantitative remote sensing of cloud and storm microphysics, and possibly in the future use of assimilation of polarimetric data by numerical models. It will be the only such research weather radar of its kind in the world at S-band operating frequency. Remote sensing using weather radar has been the impetus for many meteorological advances over the last few decades. Such advances have included gaining fundamental understanding of severe weather phenomena such as thunderstorms, flash floods, tornadoes, hurricanes and microbursts. Much of this knowledge has led to practical societal benefits such as radar warning systems that have been adopted by the operational weather agencies. The CSU-CHILL facility has made substantial contributions to this knowledge base over the years. The new antenna system will help keep what are robust, ongoing research and educational activities at the cutting edge of radar meteorological research.