The primary data source of our InSAR investigations is the Sentinel-1 mission, which is part of the Sentinel Earth observation satellite series prepared and operated in the frame of the Copernicus program of the European Commission and the European Space Agency (ESA). The constellation with two satellites (Sentinel-1A and Sentinel 1-B) sharing the same orbital plane and a 180° orbital phasing difference, continues the C-band SAR Earth Observation heritage of ERS-1/2 and RADARSAT-1/2. At the same time it offers more favorable conditions in terms of practical applications for spatial coverage, return time, fast data transfer and reliability (Sentinel-1 User Handbook, 2013).

The Hungarian introduction of the InSAR technology and the preparations of its applications were initiated by the Satellite Geodetic Observatory in 2000. We built up the knowledge base, human resources were allocated and trained, and the IT infrastructure had been procured and installed. We use the professional GAMMA software, which provides all kinds of InSAR solutions and data products in an integrated way. We established, developing and maintaining an integrated reference marker infrastructure, the Sentinel Geodetic Base Network (SENGA), which collocates various geodetic technologies and reference networks.

Interferometric processing of the radar images can be grouped into Differential SAR Interferometry (DinSAR) which uses only two SAR images at different time epoch over the same area, and PS-InSAR, which stacks several differential interferograms for the long time-span or continuous phase analysis.

Persistent Scatter Interferometry (PS-InSAR)

Persistent Scatter Interferometry analyses the phase change of the signal back-scattered from stable reflecting points, which are present on all studied images over long time (usually using at least 25 images). Persistent scatterer (PS) InSAR overcomes the decorrelation problem by identifying resolution elements whose echo is dominated by a single scatterer in a series of interferograms. Thus the coherence loss due to time difference can be reduced and the atmospheric effects can be removed by iteration. In this point of view PS-InSAR plays an important role in the extending the scope of the available data. On the one hand, there is a possibility for a larger period of time (years or decades) for conducting comprehensive phase histories. On the other hand interferometric pairs with longer baseline can also be involved into the processing. Persistent scatterers with continuous and stable reflection are usually man-made structures (buildings, metallic objects) that can be used to determine the displacement (in the satellite line-of sight direction) with an unprecedented resolution (up to 500-1000 points/km2) and at high precision (0.1-0.2 mm/year). There is no need for in-situ measurements and geodetic marker installation, so the technology can be effectively applied in areas (cities, built-in areas) where the capabilities of other methods are limited. Unfortunately rural areas (forest or agricultural cultivation) are less useful because of the limited availabilty of permanent scatteres, although some artificial objects or special scattering objects designed and implemented for this purpose allows the investigation of such territories.

Physical structure of the Sentinel Geodetic Reference Network (SENGA)

The corner reflectors are passive markers, which efficiently reflect the satellite radar signal and they can be easily identified and monitored on the radar images. The SENGA network is composed of trihedral rectangular reflectors whose technical details (coordinates, inner leg length, azimuth and height) are available in the point descriptions.