Applications of interferometric radiometry at millimeter wave band

Interferometric imaging is an alternative passive imaging method developed originally from radio astronomy from 1960’s. Comparing with the traditional real aperture system, interferometric synthetic aperture system can realize much better spatial resolution by using thinned array. Since 1980’s, this technique had been introduced in earth observation community, aiming to realize the soil moisture and ocean salinity measurement at L-band from space. The first synthetic aperture radiometer is MIRAS onboard ESA’s SMOS satellite, which is a Y-shape array with 72 antennas, launched in 2009. From 2000’s, several instrument concepts at millimeter wave (53GHz & 183GHz) had been proposed to support the next-generation geostationary orbit(GEO) meteorological satellites, including GeoSTAR by JPL, GAS by ESA and Omnisys and GIMS by NSSC.

Two generation GIMS demonstrators at 53GHz had been developed in NSSC, including 28-element GIMS-I in 2011, and 70-element GIMS-II in 2017. The GIMS-II demonstrator has been implemented based on modular system design, with customized MMIC and digitization ASIC, which largely reduced the mass/power/volume of the whole system and ensure the inter-element consistency and system performance. The antenna/receiver units of GIMS-II are non-uniformly distributed in a circular array with 3.7m diameter. The distribution of the antenna units is optimized to ensure the uniform sampling coverage of the spatial frequency domain. The digital correlator, common LO and other electronic boxes are located in the center part of the array. The overall system keeps uniform-speed rotating during measurement. The specification of GIMS-II demonstrator are as follows: 0.08 degree angular resolution, 1K NeDT@5-minute, 18 degree field of view (FOV). Imaging experiments had been taken on buildings, sky, solar transit, and also on person on the top of building.

Introducing inteferometric technique for the application of security imaging is another interesting topic. Considering the scalable nature of an interferometric system, the demonstration works for GEO atmospheric sounding can be used as a good start point for the designing of a same type instrument for security imaging purpose. Compared with GIMS, several new technical challenges should be considered for the case of security imaging.