A compact, dual-frequency, dual-polarization, wide-angle-scanning antenna system has been developed as part of an airborne instrument for measuring rainfall. This system is an upgraded version of a prior single-frequency airborne rain radar antenna system and was designed to satisfy stringent requirements. One particularly stringent combination of requirements is to generate two dual-polarization (horizontal and vertical polarizations) beams at both frequencies (13.405 and 35.605 GHz) in such a way that the beams radiated from the antenna point in the same direction, have 3-dB angular widths that match within 25 percent, and have low side-lobe levels over a wide scan angle at each polarization- and-frequency combination. In addition, the system is required to exhibit low voltage standing-wave ratios at both frequencies.
The system (see figure) includes a flat elliptical scanning reflector and a stationary offset paraboloidal reflector illuminated by a common-aperture feed system that comprises a corrugated horn with four input ports - one port for each of the four frequency-and-polarization combinations. The feed horn is designed to simultaneously (1) under-illuminate the reflectors 35.605 GHz and (2) illuminate the reflectors with a 15-dB edge taper at 13.405 GHz. The scanning mirror is rotated in azimuth to scan the antenna beam over an angular range of ±20° in the cross-track direction for wide swath coverage, and in elevation to compensate for the motion of the aircraft.
The design of common-aperture feed horn makes it possible to obtain the required absolute gain and low side-lobe levels in wide-angle beam scanning. The combination of the common-aperture feed horn with the small (0.3) focal-length-to-diameter ratio of the paraboloidal reflector makes it possible for the overall system to be compact enough that it can be mounted on a DC-8 airplane. The input ports are oriented orthogonally and carefully positioned and the depths of the corrugations in the feed horn were chosen carefully, all in an effort to minimize the overall level of cross-polarization and side-lobe level in the system.
For optimum performance, the feed phase center would ordinarily be kept at the focal point of the offset paraboloidal reflector. It would be possible to do so in single-frequency operation, but it is not possible to have a single feed phase center for both of the widely separated frequencies of this system. Instead, the feed horn is designed so that the combination of locations is optimal in the sense that it yields an optimal combination of gains, matched 3-dB widths, low cross-polarization, and low side-lobe levels. The elliptical shape of the scanning mirror was chosen, from among a number of superquadric shapes, as the one that results in the lowest overall side-lobe levels.
This work was done by Ziad A. Hussein of Caltech and Ken Green of Microwave Engineering Corp. for NASA's Jet Propulsion Laboratory. NPO-30506
This Brief includes a Technical Support Package (TSP).
Dual-Frequency Airborne Scanning Rain Radar Antenna System
(reference NPO30506) is currently available for download from the TSP library.
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Overview
The document titled "Technical Support Package for Dual-Frequency Airborne Scanning Rain Radar Antenna System" (NPO-30506) is a comprehensive report prepared under the auspices of NASA's Commercial Technology Program. It aims to disseminate the results of aerospace-related developments that have potential applications beyond their original context, particularly in scientific, technological, and commercial fields.
The primary focus of the document is on a dual-frequency airborne scanning rain radar antenna system, which operates at both Ku and Ka frequency bands. This innovative radar system is designed to enhance the measurement and analysis of precipitation, providing critical data for meteorological research and weather forecasting. The dual-frequency capability allows for improved accuracy in distinguishing between different types of precipitation, such as rain, snow, and ice, which is essential for understanding weather patterns and their impacts.
The report includes detailed technical specifications, operational principles, and potential applications of the radar system. It emphasizes the importance of advanced radar technology in addressing challenges related to climate monitoring, hydrology, and atmospheric research. By utilizing dual-frequency measurements, researchers can gain deeper insights into precipitation processes, leading to better predictive models and improved weather-related decision-making.
Additionally, the document outlines the collaborative efforts involved in the development of the radar system, highlighting contributions from various NASA centers and partnerships with other organizations. It serves as a resource for researchers, engineers, and industry professionals interested in the latest advancements in radar technology and its applications in environmental monitoring.
The Technical Support Package also provides information on accessing further resources through the NASA Scientific and Technical Information (STI) Program Office, encouraging ongoing exploration and utilization of NASA's research outputs. It includes contact details for the STI Help Desk, ensuring that users can seek additional assistance or information as needed.
In summary, this document encapsulates the innovative strides made in airborne radar technology, showcasing the dual-frequency scanning rain radar system as a pivotal tool for enhancing our understanding of precipitation and its implications for weather forecasting and climate science. It reflects NASA's commitment to sharing knowledge and fostering advancements that benefit a wide range of scientific and commercial endeavors.
