Prelaunch antenna calibration of CubeSat MMW/SMMW radiometers with application to the PolarCube 3U temperature sounding radiometer mission

Abstract

Radiometric sounding measurements used to provide data for weather prediction are strongly affected by the main-beam, ohmic, and spillover efficiencies of the optics, as well as inhomogeneities in the scene and background radiation fields. The interpretation of radiometric data is also affected by the accuracy with which these efficiencies can be determined. This necessitates design of highly precise antenna systems whose patterns are exactly known. Subtle changes in feedhorn design parameters, including changes that induce feedhorn mode phasing such as corrugation depth or step diameters, produce pattern changes at the reflector that are not captured by the widely used Gaussian approximation. To provide a more precise determination of the requisite efficiencies for radiometry, this study focuses on the rigorous and precise numerical analysis of the complex diffracted field produced by a feed at the focal point of a lens/reflector focussing element. The analysis leads to the determination of an optimal feed horn and lens/reflector geometry such that the main beam and spillover efficiencies of the system are maximized, and these and the ohmic efficiency are precisely known. The above analyses is applied to the antenna subsystem of the PolarCube 3U CubeSat payload which comprises a spinning offset paraboloidal main reflector and a stationary corrugated feed.