An Extremely Large Ka-Band Reflectarray Antenna for Interferometric SAR

The Surface Water Ocean Topography (SWOT) mission, currently in development at NASA’s Jet Propulsion Laboratory, will employ the Ka-band Radar Interferometer (KaRIn) instrument to characterize ocean height at the unprecedented spatial resolution of 2 km, and is designed to provide a global inventory of significant terrestrial water bodies (area >(250m)2) and rivers (width >50-100m). The key enabling technology for this instrument is a pair of large deployable antennas that form the interferometer. This paper describes the development of the largest reflectarray antenna yet developed for a space flight application, a 5m x 0.26m offset-fed reflectarray antenna with a 4.37m focal length. The paper provides details on critical aspects of the development, including RF design and analysis, fabrication, and measurement. We discuss unique requirements imposed by the interferometer which resulted in significant design and verification challenges. Flight hardware measurements demonstrated a gain of approximately 49.5dB, corresponding to an efficiency of 52%, and an azimuth beamwidth of 0.105 degrees with beam pointing knowledge of 5 millidegrees. Accurate characterization of these antenna performance parameters is critical to the success of the KaRIn instrument.