SMOS multi-angular brightness temperature measurements at 1.4 GHz are strongly affected by radio frequency interferences (RFI) from radar networks, TV and radio links in what shall be a protected electromagnetic band. These intereferences are numerous over land in Europe and Asia, but can be also encountered in some other areas of Africa, America and Greenland and in some numerous islands over the world. Over the oceans, the signals emanating from land sources can extend very far away from the coasts and have dramatic consequences on the accuracy of sea surface salinity remote sensing in some key oceanic areas like the north atlantic, north pacific and north indian oceans. The signature of RFI in SMOS data is highly variable in time and space and strongly depends on the instrument probing polarization and observation angles. Because of the interferometric principle, local strong RFI signals in the physical space pollute a very extended area in the Fourier domain of the synthetic antenna and contaminate large portion of the SMOS reconstructed brightness temperature images. In particular, RFI sources located in the aliased regions of the image can impact the data in the (extended) alias-free field of view.
Figure 1: these maps represent monthly averaged of the SMOS brightness temperature in terms of first stokes parameter (Th+Th)/2 at an incidence angle of 47.5° and in ascending passes. Over Sea ice the Tb is saturated because it is much higher than over the ocean. On the left: map for May 2011; right: map for May 2012
One of the largest area of contamination is the Northern Hemisphere, in particular over the North Pacific and Atlantic oceans. RFI are mostly induced here by the signals from the military radars of the Distant Early Warning (DEW) line sites. As illustrated in Figure 1, until summer 2011, the later RFI strongly contaminated SMOS data over all Canadian, Alaska waters and in the northern Atlantic (>45deg N) on ascending passes. Descending passes were less affected because of the look-angle of SMOS.Over the years, investigations of exactly where the interferences come from have been made by ESA. National authorities have collaborated with ESA to find out about the origin and how to switch these unlawful emissions off, and so RFIs have waned. Over recent years, authorities from Canada and Greenland were informed, and requested to take actions. Canada started to refurbish their equipment in autumn 2011, while Greenland switched off their transmitters in March 2011. At least 13 RFIs have now been switched off in the northern latitudes.
As illustrated in Figure 2, the switch-offs have led to a significant improvement in SMOS observations at these high latitudes, which were previously so contaminated that accurate salinity measurements were not possible above 45 degrees latitude.
Figure 2: (Top) Objectively analyzed in situ observations, (bottom) SSS from SMOS in ascending passes in May 2011 (left) and 2012 (right).