We describe an L-band, cross-track scanning radar and radiometer that is designed to map soil moisture and ocean salinity, both important components of the water cycle. The instrument will also map sea ice density and thickness, which is an important factor in ocean-atmosphere heat exchange in polar regions. Soil moisture, a direct component of the water cycle, is strongly related to ecosystem health and is a factor in prediction of regional precipitation. Measurement of sea surface salinity, when added to knowledge of oceanic precipitation will provide the potential for estimating oceanic evaporation. Salinity is also a strong forcing factor in the oceanic thermo-haline circulation, and thereby in climate variability.

The RadSTAR instrument we describe combines an L-band, digital beam-forming radar with an L-band synthetic aperture, thinned array (STAR) radiometer. The long wavelength is needed for soil moisture in order to obtain the required penetration into the soil, so as to measure the root zone moisture. In the case of salinity, the salt water emission characteristics require the long wavelength. The RadSTAR design approach makes shared use by both instruments of a single array of patch antenna elements. The result is a very compact instrument with matched beams and cross-track scanning. The cross-track scanning enables mapping of these Earth surface characteristics while over-flying areas of interest in an airplane or spacecraft. Due to the long wavelength, development of a compact instrument is a significant design consideration, either for airborne operation or for eventual use in space. The RadSTAR instrument, now under construction, will be ready for initial use in late 2003.