During the major 30 March to 1 May 2007 distal eruption that excelled common Piton de la Fournaise eruptions by an order of magnitude in terms of lava flow volume, the Dolomieu summit crater collapsed. This event destroyed the upper reservoir and distribution system and changed the eruptive behavior of the volcano. Between 2008 and 2010 only low-volume summit or near-summit eruptions and many intrusions occurred. This was followed by a rest period of 41 months between 2011 and June 2014 before another phase of nearly continuous refilling and high activity began [21], which is ongoing as of 31 December 2019. Deformation monitoring at Piton de la Fournaise started in 1980 with the installation of (initially only a few) permanent tiltmeters and extensometers, as well as a frequently measured levelling network, all of which were employed and are operated by the Institut de Physique du Globe de Paris (IPGP) through the Piton de la Fournaise Volcanological Observatory (OVPF). The monitoring network had been progressively improved in order to sample the volcano's deformation field thoroughly in time and space. In 2004 a first telemetered Global Navigation Satellite System (GNSS) network was employed, and in 2007 the deformation monitoring network at Piton de la Fournaise included 8 continuous tiltmeters, 4 continuous extensometers, and 12 continuous GPS stations [22]. As of the time of writing, ground deformation is measured by 21 permanent GNSS stations (in fact, 24 including three stations that are not shown on the map: \"REUN\", which is located about 15.5 km northwest of the central volcanic cone, \"TRCG\", and \"FEUG\" that are also located outside of the caldera) in addition to 9 tiltmeters (plus one additional tiltmeter \"RER\" that is located about 8 km northeast of the summit cone and is not shown on the map), 3 extensometers, and a network of 75 stainless steel benchmarks that are surveyed by GNSS campaigns immediately after eruptions (Figure 2). While permanently installed ground-based instrumentation delivers point-wise, highly precise, and continuous records of surface displacements in real-time and 3D, Interferometric Synthetic Aperture Radar (InSAR) sheds  
Figure 1: The location and timing of all located seismic events of a minimum magnitude of \(-0.6\) that were recorded on the Piton de la Fournaise Volcanological Observatory (OVPF) network between early March 1996 and early September 2019 (marked by colored dots) plotted onto the shaded relief of Piton de la Fournaise volcano with the approximate location of the three main rift zones (shaded areas). The profiles A-B and A’-B’ illustrate the depths of the seismic events and show clusters of increased seismic activity within the volcano’s interior. Inset shows the location of Piton de la Fournaise (PdF) and Piton des Neiges (PdN) on La Réunion Island (OVPF-IPGP). light on the spatial extent of ground movements over large areas and at a high measurement point density throughout a volcano's eruption cycle. The first SAR image acquired of Piton de la Fournaise volcano dates back to 1996 [23], and since the beginning of systematic ASAR acquisitions in 2003, SAR satellite data are routinely collected in the framework of the Indian Ocean InSAR Observatory (OI2), an Observation Service of the French National Service of Volcanological Observation (INSU/CNRS), and processed at the Observatoire de Physique du Globe at Clermont-Ferrand (OPGC). In addition, since late-2017, OVPF has operational InSAR and multitemporal InSAR (MT-InSAR) processing capacities on site, and processing is done with a focus on the long-term assessment of the volcano's eastern flank stability. All SAR data cover the entire Enclos Grand Brule collapse structure, except for COSMO-SkyMed (CSK) and TerraSAR-X (TSX) SpotLight (SPT) data (the spatial coverages of which are indicated by orange and light green frames in Figure 2, respectively). Details on available SAR data and archives are provided in the following Section 2.