Minimize MIRAS Payload Calibration

The calibration of the SMOS payload: MIRAS "Microwave Imaging Radiometer using Aperture Synthesis" is achieved throughout a combination of internal calibration modes and external manoeuvres to point the instrument to deep-sky. During the external manoeuvre the Noise Injection Radiometers (NIRs) located in the MIRAS' central hub structure, are accurate calibrated in order to use their measurements in nominal pointing to absolute calibrated MIRAS' measurements: images of Earth surface brightness temperature in Kelvin. The main NIR's calibration parameter is the internal diode noise temperature (Tna) which is estimated during external manoeuvre. Its variation directly affect the final level of brightness temperature image hence the need to monitor the Tna evolution since the beginning of SMOS mission. Further monitoring of MIRAS' absolute calibration is performed with in-situ L-band brightness temperature measurements over Concordia station in Antarctica (Domex experiment) and globally over Oceans by using sea surface emission forward model. Results of MIRAS calibration long-term monitoring are presented in the figures collection available here.

 

Chapter Editor

IDEAS+, SMOS team

 

 

Noise Injection Radiometers (NIRs) internal diode noise temperature (Tna) for vertical polarisation receiver (V) evolution since the beginning of the mission for the three NIR unit: AB (blue), BC (green) and CA (red). At the beginning of the mission Tna has evolved differently among the three NIR units until October 2014 when warm-NIR calibration was introduced. In this configuration the Sun is located slightly above the instrument horizon, improving NIR thermal stability and calibration accuracy noticeably. The unit CA has shown to be more stable, since beginning of the mission, hence is the only one used for brightness temperature absolute calibration.

 

Noise Injection Radiometers (NIRs) internal diode noise temperature (Tna) for horizontal polarisation receiver (H) evolution since the beginning of the mission for the three NIR unit: AB (blue), BC (green) and CA (red). At the beginning of the mission Tna has evolved differently among the three NIR units until October 2014 when warm-NIR calibration was introduced. In this configuration the Sun is located slightly above the instrument horizon, improving NIR thermal stability and calibration accuracy noticeably. The unit CA has shown to be more stable, since beginning of the mission, hence is the only one used for brightness temperature absolute calibration.

 

L-band Brightness Temperature evolution over Concordia Station (Dome-C Antarctica). SMOS measurements at 42 incidence angle averaged every 18 days at surface horizontal polarisation (blue line) and at surface vertical polarisation (green line). In-situ DOMEX measurements at 42 incidence angle averaged over the same period at horizontal polarisation (red line) and at vertical polarisation (purple line). Reference value for July 2010 is subtracted. SMOS brightness temperature evolution in vertical polarisation is very well correlated with in-situ measurements, stability, since the beginning of the mission, is within about 1K. The brightness temperature in horizontal polarisation is less stable and impacted by geophysical condition at surface level as confirmed by DOMEX measurement evolution. In particular, the drift in horizontal polarisation around beginning of 2015 was due to a change on surface geophysical condition due to snow accumulation since November 2014 and rapidly evolution of snow density on 22 March 2015 when a strong wind event has rapidly changed the surface condition and surface emissivity around Dome-C. Slightly different values for DOMEX 2017 dataset are due to a new three loads calibration schema used for data processing. Differences in vertical and horizontal polarisation top of atmosphere absolute value between SMOS and DOMEX measurements are under investigation.

 

Hovmoller plot for L-band brightness temperature differences between SMOS measurements and ocean forward model over Pacific Ocean (between 160 and 220 degree longitude) for ascending orbit direction and horizontal polarisation at antenna frame since June 2010. World Ocean Atlas 2009 climatology was used in the ocean forward model. The straigt blue line below the equator visible till December 2014 was due to Radio Frequency contamination around 180 deg longitude.

 

Hovmoller plot for L-band brightness temperature differences between SMOS measurements and ocean forward model over Pacific Ocean (between 160 and 220 degree longitude) for descending orbit direction and horizontal polarisation at antenna frame since June 2010. World Ocean Atlas 2009 climatology was used in the ocean forward model.

 

Hovmoller plot for L-band brightness temperature differences between SMOS measurements and ocean forward model over Pacific Ocean (between 160 and 220 degree longitude) for ascending orbit direction and vertical polarisation at antenna frame since June 2010. World Ocean Atlas 2009 climatology was used in the ocean forward model.

 

Hovmoller plot for L-band brightness temperature differences between SMOS measurements and ocean forward model over Pacific Ocean (between 160 and 220 degree longitude) for descending orbit direction and vertical polarisation at antenna frame since June 2010. World Ocean Atlas 2009 climatology was used in the ocean forward model.

 

Evolution of the L-band brightness temperature differences between SMOS measurements and ocean forward model over Pacific Ocean (area between 160, 220 degree longitude and -60, +60 degree latitude) for horizontal polarisation at antenna frame since June 2010. Noisy result from May 2017 onwards was due to configuration change in metric processing (up to 40Km from coastline instead of 100Km). World Ocean Atlas 2009 climatology was used in the ocean forward model.

 

Evolution of the L-band brightness temperature differences between SMOS measurements and ocean forward model over Pacific Ocean (area between 160, 220 degree longitude and -60, +60 degree latitude) for vertical polarisation at antenna frame since June 2010. Noisy result from May 2017 onwards was due to configuration change in metric processing (up to 40Km from coastline instead of 100Km).World Ocean Atlas 2009 climatology was used in the ocean forward model.

 

Evolution of the L-band brightness temperature differences between SMOS measurements at descending and ascending orbit direction over Pacific Ocean (area between 160, 220 degree longitude and -60, +60 degree latitude) for horizontal polarisation at antenna frame since June 2010.

 

Evolution of the L-band brightness temperature differences between SMOS measurements at descending and ascending orbit direction over Pacific Ocean (area between 160, 220 degree longitude and -60, +60 degree latitude) for vertical polarisation at antenna frame since June 2010.

 

Evolution of the L-band brightness temperature differences between SMOS measurements at descending and ascending orbit direction over Pacific Ocean (area between 160, 220 degree longitude and -60, +60 degree latitude) for first Stokes since June 2010.

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