On the Use of Bistatic TanDEM-X Images to Quantify Volumetric Changes of Active Lava Domes

TanDEM-X is a recent SAR mission, consisting of two almost identical spacecraft flying in close formation. The small distance between the two radar satellites allows two images to be acquired at the same time (bistatic images), strongly reducing the influence of temporal decorrelation, which is one of the major sources of error in repeat-pass interferometric analyses. For the first time, we successfully apply TanDEM-X data to observe topographic changes at active volcanoes by using the image pairs to generate high-resolution digital surface models (DSMs) for each transit of the satellites. Taking the difference between two bistatic DSMs allows us to assess substantial topographic changes and/or sudden ground displacements above the 1m level. As the first test case, we used bistatic TanDEM-X data to assess topographic change due to the major Merapi 2010 eruption. The preliminary estimated volumetric loss of 19 x 10(6) m(3) is reasonable; however, strong phase noise due to geometrical decorrelation and resulting unwrapping errors affect the result. To demonstrate that much smaller topographic changes are observable with TanDEM-X, we further analyzed data acquired before and after a small explosion at Volcan de Colima in June 2011. The estimated volume loss of 2 x 10(5) m(3) fits well to ground truth data.