Circular polarization for L-band radiometric soil moisture retrieval

For microwave radiometric retrievals of surface soil moisture at L-band, the preferred polarization is horizontal polarization, because it has higher sensitivity to the soil moisture than does vertical polarization. However, L-band observations of the Earth's surface from space may experience significant Faraday rotation. We describe the consequences of using circular polarization to L-band Synthetic Thinned Array Radiometer (STAR) observations of soil moisture. While sensitivity to soil moisture is somewhat reduced, the sensitivity to Faraday rotation is eliminated. Circular polarization also presents the engineering advantage of requiring only one type of receiver/antenna pair in a 2-D STAR, instead of three for any linear polarization. We present a sensitivity analysis and a derivation of the design requirements for a circularly polarized antenna for use in a STAR. An implementation of the antenna for an airborne system, STAR-Light, is also presented.