## 2 SAR Data and SAR Data Archives  
The most important SAR data archive for Piton de la Fournaise volcano is the OI2 scientific database that has been developed in order to support both the semi-operational InSAR monitoring of Piton de la Fournaise volcano and the scientific exploitation of long InSAR time series. The OI2 SAR data archive holds 1835 SAR scenes acquired since September 2000 by 8 different SAR satellite systems or constellations (and a total of 13 different satellites) as of 31 December 2019 (Figure 3 and Table 1). In addition, InSAR products such as interferograms and coherence maps dating back to May 2003 are also kept within the OI2 database. All data stored within OI2 are available through a dedicated web interface named CASOAR [24]. The data have been immensely valuable to image and study 52 of the 58 eruptions that occurred since 9 March 1998 (Figure 3). While access to most of the raw data has to be restricted due to the data policies of the responsible space agencies, derived data products are downloadable in geotiff format by registered users. A mirror archive of OI2 has been developed locally at OVPF, ensuring on-site processing and analysis as well as OI2 data and product backup. Even though SAR scenes cannot be distributed through CASOAR, the online platform provides an almost complete listing of all SAR data acquired at Piton de la Fournaise volcano since 2003 (Figure 3 and Table 1). A number of SAR images that were acquired before the development of OI2 are not centrally stored and are, therefore, harder to access for the community. While comprehensiveness is anticipated, not all SAR data acquired since 2003 at Piton de la Fournaise volcano are yet included within the OI2 database. For example, CNES routinely acquired ENVISAT ASAR swath 4 data on both ascending and descending tracks that are not stored within OI2. Also, several Radarsat-1 images may exist that are not maintained within the central archive OI2, and certainly more CSK and ALOS2 images exist than the ones OI2 is holding. Moreover, the German Aerospace Center (DLR) is operating the TanDEM-X satellite (TDX) as a TSX add-on for the collection of digital elevation measurements and the generation of a global digital elevation model (DEM). For this purpose, bistatic SAR data are being acquired in a way that one satellite is transmitting the electromagnetic signal while both TSX and TDX satellites are receiving the echoes simultaneously. Over Piton de la Fournaise volcano, a total of 211 such Co-SSC scenes were acquired before the end of the year 2019, which are listed in Table 1 (but not included in Figure 3). However, not all of them are stored in OI2. Most of the data maintained within the OI2 archive were obtained through data proposals in response to the Space Agencies' announcements of opportunity (AOs). Because data contingents offered through these AOs are usually limited to a certain number of images or to a specific time window, the temporal density of acquisitions often remains below the capacities of the satellite systems. Another source of SAR data are the open archives of the space agencies. For example, archive data acquired by \"old generation\" (i.e., out of order) satellites, such as ESA's ENVISAT, and JAXA's ALOS-1 SAR systems, are now freely available through these space agencies. For the restricted archives, like RADARSAT, CSK, and TSX, data can be purchased, even if no AOs are open for proposal submissions. For example, the COFUR prices for TSX data for scientific use vary between 200 \(\upmu\)c/scene (for 1-30 scenes) and 160 \(\upmu\)c/scene (for \(>\)100 scenes) (according to prices listed under [[https://sss.terrasar-x.dlr.de](https://sss.terrasar-x.dlr.de)]([https://sss.terrasar-x.dlr.de](https://sss.terrasar-x.dlr.de))). Likewise, a CSK StripMap (SM) HIMAGE interferometric datastack of at least 5 scenes over the same area in a twelve-month' time period can be purchased for 1000 \(\upmu\)c/scene from e-GEOS.