## 2 SAR Data and SAR Data Archives
From a scientific point of view, data policies represent strong constraints that explain why data archives, at Piton de la Fournaise and other volcanoes worldwide, may not be complete. The fact that La Reunion is an isolated island located in the Indian Ocean is both an advantage and a drawback: On the one hand, there are generally few acquisition requests before or after the satellite flyover of La Reunion, which makes scheduling conflicts unlikely and rare. On the other hand, the SAR instruments on board the satellites are likely to stay on standby mode during the entire Indian Ocean flyover when no acquisitions are explicitly requested. Therefore, not many background acquisitions are being acquired over La Reunion Island. As for many other isolated volcanic ocean islands, no ERS1 or ERS2 data have been acquired as the satellites were not equipped with large onboard storage capacities and because any real time data downlink station wastoo far from the island at the time. Likewise, no ENVISAT ASAR swath 1 data have ever been acquired at Piton de la Fournaise since those had unfavorable viewing geometries. Especially the new generation SAR satellite systems (Figure 3b) deliver high-resolution and high-quality data with acquisition designs that are optimized for InSAR applications. For instance, the separation of SAR scenes in space (represented by the normal baseline \"Bn\") is usually kept below critical distances that would cause coherence loss (Figure 4). This is oftentimes achieved by flying the satellites within orbital tubes and/or by operating satellite constellations of the same kind of SAR systems. Resolutions, both temporal and spatial, are continuously improving, and dedicated SAR satellite modes for one and/or the other preference are available. Some space agencies allow target-specific tasking of future acquisitions (oftentimes following a successful data proposal), ensuring data takes with the most suitable acquisition modes, geometries, and timings. Since 2007, the most commonly used SAR sensors at Piton de la Fournaise are the ones aboard the TSX and CSK (X-band), Sentinel-1 (S1) (C-Band), and ALOS-2 (ALOS2) (L-Band) satellites (Figure 3b). Combining these data allows taking advantage of the individual strength of the satellite systems. The two S1 (S1A and S1B) satellites, for example, are reliably collecting data every 6 days (in different sensor modes), which are available to registered data users right upon acquisition and distributed free of charge. This makes the system closest to near-real time data availability today. However, in particular S1 TOPS/IW data have comparably coarse pixel spacing (Table 1), which may cause surface displacement patterns to be less clearly defined. TSX and CSK provide better spatial resolutions, but the X-band becomes saturated faster with respect to displacement gradients, and the data underlie more difficult distribution policies and restrictions. ALOS2 SPT data combine both high spatial and potentially high temporal sampling, and they feature a better overall coherence over vegetated areas (L-band) (Figure 4) as well as a lower sensitivity that allows for near-field displacement detection. Those data, though, are often delivered with longer delays. InSAR-derived deformation measurements at Piton de la Fournaise are in overall agreement with GNSS data, as closely investigated and proven by [PERSON] et al. (2019). The average annual coherences in X-band, C-band, and L-band at Piton de la Fournaise volcano are sufficient for InSAR applications (Figure 4). Except for vegetated areas along the volcano's eastern flank, the steep walls of the caldera, the two summit craters, and newly emplaced lava flows, coherence in all three bands remains high within the Enclos Grand Brule collapse structure and throughout the year. Depending on the frequency and timing of eruptions and SAR acquisitions throughout the period of investigation, information on the age of lava flows, i.e., the timing of their emplacement earlier or later during the year, is reflected in mean coherence maps.