Performance and applications of interferometric aperture synthesis radiometers in remote sensing

By measuring the emission of the Earth at L-band microwave frequencies one can retrieve, in principle, moisture content of the upper soil layers and salinity level of sea water. Until recently such measurements from space have been impossible to conduct basically due to two fundamental limitations: First, in order to sustain a reasonable ground resolution from space the antenna of the receiving system should be on the order of several meters. Second, the antenna should be scanning in order to establish the wide swath desired for remote sensing instruments. To overcome these two fundamental limitations a new concept of aperture synthesis interferometry has been proposed. The concept is familiar from astronomy, but its adoption for remote sensing has turned out not to be a straightforward task. Also, the new type of imagery provided by an interferometer needs digesting in order to be fully utilized with retrieval algorithms for scientific purposes. Characteristic of interferometric data are the errors caused by the unique imaging process. This dissertation tackles some of the fundamental problems related to the adoption of the interferometric concept for remote sensing including the calibration of such instruments, imaging algorithms, and radiometric performance and capabilities of the technology. These issues are addressed by developing an airborne interferometer radiometer, by analyzing its measurements, and by using the data in geophysical parameter retrieval. In addition to this, space borne applications are considered by studying the characteristics of the only operational space borne interferometric radiometer. That instrument is onboard the European Space Agency’s SMOS (Soil Moisture and Ocean Salinity) mission. This dissertation is composed on the basis of seven publications. The first two publications present studies which develop and validate the imaging theory of interferometric radiometers. Publications three and four present functional properties of Aalto University’s airborne interferometer. Propagation of errors in interferometric imaging with a space borne instrument is studied in detail in publication five. The sixth publication studies the end-to-end performance of our airborne interferometer in detail, by analyzing its measurements of various targets. The final publication utilizes measurements from the airborne interferometer for detection of a changing salinity level at the Gulf of Finland.