High ground resolution in passive microwave earth observations from space by multiple-wavelength aperture synthesis

A method for spaceborne passive microwave earth observation is proposed, which permits to achieve an improved ground resolution along one direction e.g., along the flight trace. Two widely spaced antennas constitute a two-element interferometer which produces a series of narrow beams with sufficiently separated frequencies yielding an equal number of spatial Fourier components of the observed ground landscape. The technique of multiple-wavelength aperture synthesis, equivalent to the conventional variable-spacing synthesis techniques, is then used to synthesize a single narrow (down to 0.001 radian) beam. The width of this beam is determined by the ratio of the shortest wavelength used to the distance between the two antennas. An arrangement of two single antennas e.g., at a 10-meter distance and of an aperture size of 4 m times 1 m each used at wavelength of 1 cm to 6 cm can yield a ground resolution element of about 0.25 times 3.0 sq km by nadir viewing from an altitude of 250 km. Limitations of the method due to the frequency dependence of the beamwidth and of the brightness temperature of the ground are analyzed, and ways of future improvement of the method are traced. The interferometer is recommended for use in the Space Shuttle.