MIRAS

MIRAS (Microwave Imaging Radiometer using Aperture Synthesis) is the sole payload on ESA's Soil Moisture and Ocean Salinity (SMOS) spacecraft. It is a 2-D interferometric passive microwave radiometer operating at L-band (1.413 GHz). ESA describes MIRAS as the first satellite instrument to apply aperture-synthesis imaging at L-band from orbit ^[1].

The instrument uses a Y-shaped array of 69 LICEF (Lightweight Cost-Effective Front-end) receivers arranged on a central hub and three deployable arms, spanning a total diameter of approximately 8 metres ^[2]. Rather than mechanically scanning a real aperture, MIRAS cross-correlates the signals received by all receiver pairs simultaneously to reconstruct a brightness temperature field across a hexagonal snapshot footprint. This aperture-synthesis approach achieves spatial resolution of 35 to 50 km ^[2] across a swath of approximately 1050 km ^[2], enabling 3-day global revisit at the equator ^[2].

The instrument operates in dual-polarisation mode (all-H or all-V) or full-polarisation mode ^[2]. eoPortal reports radiometric sensitivity at boresight of 3.5 K rms (Level-1 brightness temperature), degrading to 5.8 K rms at the swath edge, with absolute accuracy of 2.6 K rms and scene- and position-dependent variation of 1.6 to 4 K ^[3].

MIRAS was built under ESA contract by EADS-CASA Espacio (Spain) as prime, leading a consortium of more than 20 European companies ^[4]. SMOS launched in November 2009 and MIRAS has been in continuous operation since December 2009 ^[2]. Primary geophysical products are global surface soil moisture and sea surface salinity, both retrieved from L-band brightness temperature.