A novel technique for Earth-observation microwave interferometric radiometry using satellite formation flight is presented. The concept allows large apertures of unprecedented sizes to be synthesised by means of antennas mounted on several free-flying platforms performing interferometry. The size of the synthesised aperture is determined by the furthest distance between the free-flying satellites. The concept is applicable to wide range of microwave frequencies, meaning that the large aperture may be applied to achieve unprecedented spatial resolution, to extend Earth observation radiometry to unprecedented long wavelengths, or to enable radiometry in the geostationary orbit. Two such concepts are presented, and as an example they are applied to geostationary radiometry at 10 GHz. The first concept employs a rotating Y-shaped interferometric radiometer in formation with a constellation of nine free-flying microsatellites. The effective diameter of the synthesised aperture is 14.4m, which produces a spatial resolution of 79.5 km at 10 GHz from the geostationary orbit. The total mass of the constellation can be within 2 tonnes, which may be deployed on a single launch vehicle. The second concept is a constellation of six formation-flying radiometers, which produce apertures of 28.8m, and produces a spatial resolution of 39.8 km at 10 GHz. While this configuration is capable of producing larger effective apertures, the total mass can exceed 5 tonnes, and may require multiple launches and rendezvous at the operational orbit. Both of these concepts can be scaled up for larger apertures, and are bound primarily by the number of satellites deployable in the constellation. The free-flying nature of the concept means they are susceptible to interferometric performance degradation by array deformation. The effect of deviation on the radiometric resolution is explored and found, concluding that deviation as small as 0.1A can lead to performance degradation of up to 20 K. Annual Al' required to maintain such constellations are also found at up to 2.23 m/s annually.
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