Impact of transient atmospheric phenomena on radar interferometric processing of Sentinel-1 SAR satellite images

In the last decades, the development of space geodesy methods has allowed much more accurate observations of planetary surface dynamics than before. The various SAR satellites, like global navigation systems, make their observations in different microwave frequency ranges (1-10 GHz). The Earth's atmosphere is transparent to the microwave signal, but the factors affecting wave propagation (propagation direction and velocity) in the medium are time-dependent, the medium is anisotropic, inhomogeneous and, in the case of the ionosphere, dispersive. Without the correction of such atmospheric artifacts the resulting signal delay is evaluated as a displacement during processing, which can be in the order of tens of meters.

In order to get information about the actual geophysical processes from the displacement values derived from satellite data, the effects on wave propagation must also be taken into account. Radar interferometric methods are particularly suitable for detecting processes with velocities in the order of a few mm/year, but are limited by the lack of quantitative knowledge of the signal delay in the wave propagation, which is of particular importance for the study of processes on a regional scale.

Wave propagation in the neutral atmosphere is mostly distorted by refraction due to water vapour, and the correction is complicated by the dynamic variation of the water vapour content and the inaccurate knowledge of the atmospheric water vapour. In the ionosphere, in addition to Faraday-rotation and electron density dependent refraction, the dispersive nature of the medium is another source of error.

Transient atmospheric phenomena (frontal and thunderstorm systems, ionospheric disturbances, sporadic E layers, etc.), which are predominantly inhomogeneous in nature, further complicate the correction of their effects, but also provide an excellent opportunity to study them. The Sentinel-1 satellite images cover an area  of 250 km x 250 km with  a resolution of 5 m x 20 m. This resolution may prove useful for studying atmospheric inhomogeneities.

In radar interferometric processing, virtual displacements generated by atmospheric phenomena can be investigated in areas that are assumed to be geologically stable and contain well-identified objects that provide strong signal reflection. For the latter, corner reflectors  points specifically designed for this purpose have already been developed.

In the area of Sopron (Hungary), there are 4 such installed permanent artificial reflectors. By including these points and by comparing measurements from the local ionosonde and meteorological station, we have studied the influence of atmospheric phenomena on radar interferometric processing and the applicability of radar interferometry for the study of atmospheric phenomena.