Calibration is the process of determining the response of your measuring device to known, controlled signals so that the variables you want to measure can be better determined. From this, it follows that the precision and accuracy of your measuring device is determined by the precision and accuracy of its calibration. This calibration data is then used to ensure the most accurate measurements are made of unknown sources. As very high accuracies are required from radiometers, radiometry can be thought of as being primarily about calibrating the instrument well. The accuracy and stability required from the SMOS mission for reliable salinity and soil moisture measurements is achieved by the careful selection of calibration procedures.

MIRAS has two types of in-orbit calibration: external, in which the satellite makes a manoeuvre to point the instrument to the cold sky, and internal, in which noise is injected to the receivers.

Learn more about MIRAS calibration in the multimedia book:

See the full history of the SMOS routine calibration events which indicate the quality of SMOS data acquired since 2010.

Download documents detailing the MIRAS configuration and in-flight calibration activities:

• Summary of the calibration activities for the routine phase
• MIRAS Instrument Calibration Campaigns

The signal observed by SMOS isn't only a function of soil moisture and ocean salinity; other factors influence the radiation emitted by the Earth. To ensure that SMOS observations are correctly converted into valid geophysical values, these other effects need to be carefully accounted for. Therefore, various activities and dedicated field campaigns are carried out across the world so that in situ ground truth data are available for SMOS data validation purpose.

Learn more about SMOS validation campaigns in the multimedia book: