According to Stephen Grebby, they applied InSAR to satellite data acquired by the Sentinel-1 mission, in order to map how the entire dam and tailings structure was moving during the 17 months preceding the collapse. By studying the observed movement, they found that different parts of the Brumadinho dam were moving at different rates, some of which accelerated during the two months prior to collapse. Analysing the velocities at which these areas were moving and how they changed over time then enabled them to predict the possible time of failure. In this case, they found that if the dam was being systematically monitored using this technique, the failure date could have been predicted to within a week of it happening. Crucially, this prediction would have been possible around 40 days prior to the collapse, allowing time for a warning to be raised that the dam was becoming unstable.