What a difference a day makes: when and where are daily satellite observations of morphology and deformation needed during volcanic eruptions and unrest?

Daily high-spatial-resolution satellite imagery at active volcanoes could be used to anticipate eruptions and save lives, but is only rarely available or used in real time. Specifically, daily repeat coverage of ground deformation, topography, and surface morphology at volcanoes is now possible using high-spatial resolution optical and radar imagery (< 10 m/pixel). However, multiple satellites are needed—either constellations of four or more sibling satellites or a “virtual constellation” using several different types of satellites. As part of the Global Volcano Early Warning and Eruption Response from Space (G-VEWERS) project of the Committee on Earth Observation Satellites (CEOS), we review case studies at both erupting and restless volcanoes to identify scenarios where near-daily radar (both backscatter images and interferometric products) or optical data captured noteworthy changes, including Merapi and Mount Agung, Indonesia; La Soufrière, St. Vincent; Colima, Mexico; K?lauea, Hawai?i; and Fagradalsfjall, Iceland. Finally, we consider the resources needed to scale this globally. There are 40–50 volcanoes erupting at any given time, but currently only about 10% have daily imaging at high-spatial-resolution. The allocations of high-spatial-resolution imagery available through G-VEWERS and other open datasets (~10,000 scenes/year) are < 60% of what is needed (~17,000 scenes/year) for daily imaging of all restless and erupting volcanoes. We therefore propose criteria to prioritize where daily satellite high-spatial-resolution observations may be useful continuously or under emergency conditions based on the threat and the type of volcano/eruption.