Spurious Fourth Stokes Parameter Signal in the SMAP Radiometer

The radiometer on the NASA SMAP (Soil Moisture Active/Passive) mission is fully polarimetric and operates at L-band in the spectrum window at 1413 MHz protected for passive use only. A unique feature of the radiometer is the fully digital back-end which permits direct computation of the third and fourth Stokes parameters (i.e., real and imaginary part of the correlation of signal at horizontal and vertical polarizations). The SMAP conical scanning geometry has provided the opportunity to look at the global distribution of the fourth Stokes parameter at constant incidence angle (40 degrees at the surface). A striking feature of these maps is the existence of a strong (± 10 K peak-to-peak) spurious signal at coastlines. There is also a weak coupling to Faraday rotation. This paper reports research to explain the existence of these spurious signals.