Large-Scale InSAR Monitoring: Status and Challenges

After a slow uptake, spaceborne radar interferometry techniques are becoming a key tool for wide area ground deformation mapping and for frequent, local, monitoring programs. Wide area processing (WAP) allows users to obtain synoptic views of displacement phenomena over thousands - or even millions - of square km, paving the way for new environmental monitoring programs providing invaluable information on a variety of natural and anthropogenic hazards. In this paper, we argue that the growing number of projects at national or even continental scale (such as the European Ground Motion Service - EGMS) is the result of three factors: (1) a proper space segment, allowing systematic SAR acquisitions at global scale suitable for InSAR analyses; (2) increased computational resources, typically via cloud computing, allowing scalable processing chains; (3) new visualization platforms for 4D data, where the temporal dimension can be easily displayed and interrogated. In WAP projects, several aspects - having limited impact at local scale - become extremely important, e.g.: careful estimation and compensation of atmospheric artifacts (exhibiting a variance increasing with the distance from the reference point); proper data calibration via GNSS measurements (important to make the data more easily interpreted and integrated with other information sources and to better estimate vertical and east-west displacement components from ascending and descending satellite passes); data mosaicking (in fact, the final result is usually the merge of the results of different data stacks of SAR images). Although significant advances have been made in recent years in all these processing steps, some challenges remain. Another topic which is becoming increasingly important is the size of InSAR databases. In fact, data screening tools are becoming a must to take advantage of the huge amount of information associated with a typical WAP project. Which points are exhibiting a change in trend over the last few months? What areas are accelerating? Which points suffered an abrupt change in location after the last seismic event? These are typical questions users want to answer in a few seconds and not after hours or even days spent on a GIS platform. After a gallery of examples of different WAP projects at regional, national, and continental scale, the paper reports some suggestions and recommendations to improve the quality, the effectiveness and the usability of future WAP InSAR projects.