## 1 Introduction  
Optical imaging spectroscopy or hyperspectral remote sensing for Earth observation relies on the use of spectrometers for the measurement of the solar radiation reflected by Earth system components in contiguous spectral channels [1, 2]. The reconstruction of atmospheric and surface absorption features from spectroscopic measurements enables the identification and quantification of land, water and atmosphere constituents with characteristic spectral signatures, which makes imaging spectrometers suited for a wide range of Earth observation applications. The potential of airborne and spaceborne imaging spectrometers has long been exploited to monitor the Earth's surface and atmosphere and to provide valuable information for the better understanding of a large number of environmental processes (e.g., [3, 4]). Those applications include, for example, vegetation monitoring and ecology (e.g., [5, 6, 7, 8, 9, 10, 11]), geology and soils (e.g., [12, 13, 14, 15, 16, 17]), coastal and inland waters (e.g., [18, 19, 20, 21]), mapping of snow properties [22, 23] and archaeological prospection [24, 25]. Most of the developments in imaging spectroscopy in the last few decades have been based on airborne spectrometers covering the visible to near-infrared (VNIR) and, often, shortwave-infrared (SWIR) spectral ranges (roughly, 400-1000 nm and 1000-2500 nm, respectively). In particular, the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) [26, 27], designed and operated by the NASA Jet Propulsion Laboratory in California, has been used since the late 1980s in a large number of imaging spectroscopy experiments and field campaigns. Other airborne imaging spectrometers widely used in the last few years are HyMAP [28], the Compact Airborne Spectrographic Imager (CASI) [29] and the Airborne Prism Experiment (APEX) [30, 31]. Unfortunately, the recognized potential of imaging spectroscopy is currently not counterbalanced by an equivalent availability of spaceborne imaging spectroscopy data. Two technology demonstration missions, Hyperion onboard NASA's Earth Observing-1 (EO-1) spacecraft [32] and the Compact High Resolution Imaging Spectrometer (CHRIS) on ESA's Proba-1 microsatellite [33], have been the main providers of space-based hyperspectral data over the last few decades. Even though both missions have largely exceeded their planned one-year lifetime, they are still very valuable sources of publicly-available spaceborne hyperspectral data. Hyperion, launched in November 2000, is a grating spectrometer measuring in the 400-2500 nm spectral range with a 10-nm spectral sampling distance (SSD), a 30-m ground sampling distance (GSD) and a 7.7-km swath width. CHRIS, in turn, is a VNIR prism-based spectrometer launched onboard Proba-1 in October 2001. In its hyperspectral mode, the CHRIS/Proba system acquires 14 \(\times\) 14 km\({}^{2}\) scenes in the 400-1000-nm spectral range with an SSD of about 10 nm and a GSD of 34 m. Coexisting with Hyperion and CHRIS/Proba, the Medium Resolution Imaging Spectrometer (MERIS) [34] onboard ENVISAT operated during 2003-2012. Although only 15 (binned) spectral channels are made available to standard users, MERIS was conceived of as an imaging spectrometer measuring in the 400-900-nm spectral range with 1.25-nm spectral sampling. Lately, the Hyperspectral Imager for the Coastal Ocean (HICO) [35], developed by NASA and the U.S. Office of Naval Research, has provided imaging spectroscopy data in the VNIR over coastal regions since its deployment on the International Space Station in September 2009 until the end of the mission in March 2015. Furthermore, the Indian Hyperspectral Imager (HySI) onboard the Indian Micro Satellite-1(IMS-1) [36] and the Chinese HJ-1A [37] provide VNIR hyperspectral data, although with only limited access for international scientific users.