## 3 Materials and Methods
### Workflow of the MOUNTS System  
MOUNTS is based on freely available remote sensing data, open source toolboxes, and a modular architecture. The workflow is outlined in Figure 2, and the basic processing information is described hereafter. We refer the reader to the Supplementary Material S1 for more in-depth technical details on various aspects of the processing chain. The entire framework is managed with Python, and is composed of the following elements:  
(1) Data query: the ESA Sentinel Data Hub is queried for new products at regular time intervals (typically every hour) using the Open Search API. Query options include the product type (i.e., platform name, file type), the product sensing time, and the footprint of the area of interest (AOI). Products types used by MOUNTS are the following: Sentinel-1 Level 1 (L1) Single Look Complex (SLC) Interferometric Wide (IW) products, Sentinel-2 Level 1C (L1C) products, and Sentinel-5P Level 2 (L2) Near Real Time (NRTI) SO\({}_{2}\) products. The standard AOI for a specific volcano is defined as a 10 \(\times\) 10 km mask centered around the volcano summit, but can be adapted based on specific requirements (e.g., Kilauea Rift Zone 40 \(\times\) 60 km to monitor flank eruptions). Volcano name, coordinates, and identification number are taken from the Global Volcanism Program (GVP) database [1], maintained by the Smithsonian Institution. The volcanoes monitored by MOUNTS and the associated relevant information for data query and processing are stored in a local database.  
(2) Data download: when a new product is found on the Sentinel data hub, the data is automatically downloaded, and its metadata is stored in a local database.  
(3) Data processing: as soon as a product is downloaded it goes through the dedicated processing chain, which intends to generate both image files (geocoded) and extract key parameters which help visualize volcanic activity. Products are opened and processed using ESA's open-source SNAP software (Sentinel Application Platform) through its Python API. The processing techniques applied to Sentinel-1, Sentinel-2, and Sentinel-5P products are described in Section 3.3.  
(4) Data dissemination: the outputs of the processing chain are displayed on an open-access website (www.mounts-project.com). The web-design is based upon that of MIROVA, whereby all monitored volcanoes have a dedicated webpage displaying the same set of information. A range of  
Figure 1: Utility of different sensor types, both spaceborne and ground-based, used by MOUNTS to monitor various volcanic processes. The occurrence of these may vary strongly from one volcano to another [11; 26], and therefore the timings reported are given in an indicative manner. Processes flagged with an asterisk are those illustrated in this paper. The approximate revisit frequency of each satellite product used is indicated with a colored-marker (see Table 1 for details). Apart from Sentinel-3, all data are available on MOUNTS website. USGS: U.S. Geological Survey; SAR: Synthetic Aperture Radar; SWIR: short-wave infrared (SWIR).  
tools have been incorporated to conveniently visualize the multi-parametric data, and thereby evaluate the volcanic activity (Figure 3).  
Figure 3: Snapshot of the standard webpage identical for each monitored volcano, and description of the menu content.  
Figure 2: Workflow of MOUNTS monitoring system, managing automated data query, download, processing, and dissemination of satellite-based (Sentinel-1 (S1), -2 (S2) and -5P (SP5)) and ground-based (global earthquake catalogues USGS and GEOFON) data at a number of volcanoes worldwide.