The MIRAS "all-licef" calibration mode

After more than six years of operation, the SMOS (Soil Moisture and Ocean Salinity) European Space Agency (ESA) mission [1] has proven to be highly useful for a variety of scientific applications related to soil moisture over land, ocean salinity and winds over ocean and specific studies over ice covered surfaces. Level 2 data is currently used by many institutions around the world to extract relevant information aimed at improving weather forecast, extreme events monitoring and others. SMOS single payload is the Microwave Imaging Radiometer using Aperture Syntyhesis (MIRAS), an interferometric radiometer consisting of an array of 69 low-gain dual-polarization antennas placed along the three arms of a Y-shaped structure [2]. The fundamental measurement of the instrument is the visibility function [3] in units of Kelvin, obtained after amplitude and phase calibration of the complex cross-correlation between the signals collected by all pairs of antennas. A given sample of this function depends on the relative spatial coordinates of one antenna with respect to the other in a pair. The particular case of both antennas collapsing in a single location is the zero-spacing visibility, which is equal to the antenna noise temperature for this particular element. The MIRAS instrument is designed to use three redundant Noise Injection Radiometers (NIR) specifically to measure this average antenna temperature. The SMOS Level 1 operational processor (L1OP) uses these three measurements in the image reconstruction part. Nevertheless, in its latest version only one of the three NIRs is used, as it shows improved seasonal and long-term stability with respect to using all three. Additionally, the NIRs are used in the calibration of the power measurement system (PMS) installed in all