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This presentation will review recent developments in microwave radiometry including the introduction of interferometric radiometry in remote sensing. Microwave radiometers have been used in satellite remote sensing of the Earth since the 1960's. They measure the intensity of thermal radiation emitted by the target, expressed as brightness temperature. Multi-frequency dual-polarized radiometers contain several receivers, each operating over a certain frequency band and receiving thermal emission at either horizontal or vertical polarization. Geophysical characteristics of the target are derived from the observed brightness temperatures by employing theoretical or semi-empirical emission models. Satellite radiometers have been developed by everal countries including USA, Russia, Japan and India. The main advantages of satellite microwave radiometers include their capability to provide data regardless of the lighting conditions and nearly regardless of weather conditions. The spatial resolution of real-aperture radiometers depends on frequency. The most recently launched satellite system, the AMSR-E radiometer, operates from 6.9 to 89 GHz and its antenna diameter is 1.6 m providing corresponding average spatial resolutions from 56 to 5 km. Hence, for a real-aperture radiometer operating at 1.4 GHz a very large parabolic reflector diameter would be needed in order to achieve a resolution of approximately 50 km. The airborne ESTAR one-dimensional interferometric radiometer, developed in USA in the 1990's demonstrated that thinned antennas can be used for achieving high spatial resolution. The European Space Agency's SMOS (Soil Moisture and Ocean Salinity) satellite is scheduled for launch in 2009. The only payload is the MIRAS two-dimensional interferometric 1.4 GHz radiometer, containing tens of antenna/receiver subsystems in Y-shaped configuration. The main goals of the mission are to produce on a regular basis global maps of soil moisture and ocean surface salinity. The operation principle and technical details of the MIRAS radiometer will be discussed in the presentation. Airborne demonstrators with technical characteristics identical with those of the satellite sensor are needed in order to validate the instrument concept and provide experimental data before the launch of the SMOS satellite. This talk will present technical characteristics of the HUT-2D airborne interferometric 1.4 GHz radiometer developed by the Helsinki University of Technology. Results from recent campaigns are presented. Possible future developments in this field are discussed. |
