Average Speed Variation Index — ERS ENVISAT Descending Comparison

The derived acceleration maps were drawn up from satellite SAR ERS data (period from 1992 to 2000) and ENVISAT (period from 2003 to 2008). The two sets of data differ for an interval of about four years. In order to better discriminate against areas subject to abnormal movements, it has been decided to develop secondary acceleration measures for the determination of hydrogeological risk. From the average speeds obtained from the ERS and ENVISAT data processing, the areas present in both data sets and with the same geometry have been identified: Descending ERS with ENVISAT descending. Within these areas, comparisons were made between the estimated speeds in the 1992-2000 time interval with ERS data and those estimated in the 2003-2008 time interval with ENVISAT data: the result of these comparisons is an index of the change in the estimated speeds over the two time intervals. The derived acceleration measures provided are two, one related to the ascending observation geometry and the other relating to the descending observation geometry. Each measure contains the velocity variation indices for the points identified and measured by the ERS and ENVISAT calculations within certain areas. The result of the comparison consists of two maps representing the average speed difference measured over the 1992-2000 time interval with ERS data, and 2003-2008 with ENVISAT data. The differences refer to areas of extension of 100 m x 100 m for a total coverage of almost 100,000 sq km in both geometries. The comparison refers to more than 10 million PS in total between ERS and ENVISAT.