| Wide-swath interferometric mode has become standard acquisition mode for the two main Earth-observing radar missions that will be active in the coming decade, namely Sentinel-1 and ALOS-2. Increased swath width, compared to classical Stripmap imaging mode, is achieved at the expense of azimuthal resolution. This makes along-track displacements, and subsequently north-south displacements, difficult to measure using standard split-beam (multiple-aperture) InSAR or image correlation techniques. Alternatively, we show here that the along-track component of ground motion can be deduced from computing a double-difference between backward- and forward-looking interferograms within regions of burst overlap. In addition to the cancellation of tropospheric disturbance by the double-difference operation, angular diversity of Sentinel-1 line-of-sight measurements ($\sim1^\circ$ compared to $\sim0.25^\circ$ with ENVISAT) allows for measurements of the along-track component of ground motion with sub-decimetric accuracy. We demonstrate the efficiency of this method using Sentinel-1 data covering the 2015 Illapel \mbox{M$_w$8.3} earthquake (Chile) from which the full 3D displacement field has been retrieved and validated against observations from} a dense network of GPS sensors. |