remote sensing Review The Spatial and Spectral Resolution of ASTER Infrared Image Data: A Paradigm Shift in Volcanological Remote Sensing Michael S. Ramsey * and Ian T.W. Flynn Department of Geology and Environmental Science, University of Pittsburgh, 4107 O’Hara Street, Pittsburgh, PA 15260-3332, USA; [email protected] * Correspondence: [email protected] Received: 26 December 2019; Accepted: 12 February 2020; Published: 23 February 2020 Abstract: During the past two decades, the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument on the Terra satellite has acquired nearly 320,000 scenes of the world’s volcanoes. This is ~10% of the data in the global ASTER archive. Many of these scenes captured volcanic activity at never before seen spatial and spectral scales, particularly in the thermal infrared (TIR) region. Despite this large archive of data, the temporal resolution of ASTER is simply not adequate to understand ongoing eruptions and assess the hazards to local populations in near real time. However, programs designed to integrate ASTER into a volcanic data sensor web have greatly improved the cadence of the data (in some cases, to as many as 3 scenes in 48 h). This frequency can inform our understanding of what is possible with future systems collecting similar data on the daily or hourly time scales. Here, we present the history of ASTER’s contributions to volcanology, highlighting unique aspects of the instrument and its data. The ASTER archive was mined to provide statistics including the number of observations with volcanic activity, its type, and the average cloud cover. These were noted for more than 2000 scenes over periods of 1, 5 and 20 years. Keywords: ASTER; thermal infrared data; volcanic processes; image archive; future concepts 1. Introduction 1.1. ASTER Instrument and History The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument was launched on the NASA Terra satellite on 18 December 1999. Prior to the instrument beginning its operational phase on 4 March 2000, ASTER acquired several images including the first thermal infrared (TIR) image on 6 February (Figure1). That image was of Erta Ale volcano (Ethiopia), and initiated a long association of ASTER data with volcanological observations. ASTER was developed and built in Japan under the Japanese Ministry of Economy, Trade and Industry (METI), and is one of five Earth observing instruments on Terra. The combined science team of Japanese and United States investigators has changed over the years, however always maintaining a strong volcanological component [1,2]. During the past two decades, the data from ASTER have been applied to numerous questions and scales of surface processes, most notably for volcanic activity, e.g., [3–6]. ASTER was designed to observe the surface at multiple spatial and spectral resolutions as well as from different viewing geometries. It is actually a suite of three instruments with independent bore-sighted telescopes, originally having 14 spectral channels in the visible/near-infrared (VNIR), the shortwave infrared (SWIR), and the thermal infrared (TIR) regions [7]. The VNIR instrument (0.52–0.86 µm) has three spectral channels at a spatial resolution of 15 m/pixel paired with one channel Remote Sens. 2020, 12, 738; doi:10.3390/rs12040738 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 738 2 of 40 Remote Sens. 2020, 12, x FOR PEER REVIEW 2 of 39 oriented in a backward look direction for the creation of digital elevation models (DEMs). The SWIR channel oriented in a backward look direction for the creation of digital elevation models (DEMs). µ instrumentThe SWIR (1.6–2.43instrumentm) (1.6–2.43 unfortunately μm) unfortunately failed in 2008,failed butin 2008, originally but originally had six had channels six channels at a spatialat a resolutionspatial resolution of 30 m/pixel. of 30 Finally, m/pixel. and Finally, perhaps and most perhaps important most forimportant many aspects for many of volcanological aspects of remotevolcanological sensing, remote the TIR sensing, instrument the TIR (8.13–11.65 instrumentµm) (8.13–11.65 has five μ channelsm) has five at achannels spatial at resolution a spatial of 90resolution m/pixel. of During 90 m/pixel. the lifetime During ofthe the lifetime mission, of the ASTER mission, has ASTER acquired has overacquired 3.5 millionover 3.5 individual million scenes—approximatelyindividual scenes—approximately 22% of which 22% were of which collected were collected at night. at Here, night. weHere, describe we describe the two-decade the two- historydecade of history ASTER, of specific ASTER, programs specific programs to improve to the improve observational the observational frequency frequency of volcanoes, of volcanoes, and present examplesand present of those examples data. of those data. FigureFigure 1. 1.The The first first Advanced Advanced SpaceborneSpaceborne Thermal Emis Emissionsion and and Reflection Reflection Radiometer Radiometer (ASTER) (ASTER) multispectralmultispectral thermal thermal infraredinfrared (TIR)(TIR) imageimage acquired on on 6 6 February February 2000, 2000, before before the the start start of ofthe the instrument’sinstrument’s science science operational operational phase. phase. Erta Erta Ale volcano,Ale volcano, Ethiopia Ethiopia (summit (summit location: location: 13.60◦ N,13.60°N, 40.67◦ E), is40.67°E), shown using is shown a decorrelation using a decorrelation stretch (DCS) stretch of (DCS the) TIRof the bands TIR bands 14, 12, 14, 10 12, in 10 red, in red, green, green, and and blue, respectively.blue, respectively. Color variationsColor variations are mainly are mainly caused caused by rock by rock and and soil compositionalsoil compositional diff differences,erences, and and only possibleonly possible with the with ASTER the AS multispectralTER multispectral TIR TIR data. data. The The blues blues andand purple colors colors are are indicative indicative of of dominantlydominantly basaltic basaltic lava lava flows. flows. The The smallsmall cluster of white white pixels pixels in in the the lower lower central central part part of ofthe the image image is theis the summit summit lava lava lake lake thermal thermal anomaly. anomaly. TheThe ASTER DCS DCS data data are are overlain overlain on on a visible a visible Google Google Earth Earth imageimage for for context. context. Image Image credit:credit: NASANASA/METI/AIST/Japan/METI/AIST/Japan Space Space Sy Systems,stems, and and U.S./Japan U.S./Japan ASTER ASTER ScienceScience Team. Team. Remote Sens. 2020, 12, 738 3 of 40 Remote Sens. 2020, 12, x FOR PEER REVIEW 3 of 39 1.2. Twenty Years ofof VolcanicVolcanic StudiesStudies UsingUsing ASTERASTER DataData The ASTER Science Team created useful derived sciencescience products that have benefitedbenefited numerous scientificscientific fieldsfields such such as volcanology.as volcanology. These These include include a robust a temperaturerobust temperature emissivity emissivity separation separation algorithm foralgorithm the first for orbital the first high orbital spatial high resolution spatial resolution multispectral multispectral TIR data TIR [8], programsdata [8], programs such as thesuch Science as the TeamScience Acquisition Team Acquisition Request (STAR)Request to (STAR) acquire to data acquir focusede data on focused important on important science questions science ofquestions individual of usersindividual/teams, users/teams, as well as the as abilitywell as to the generate ability DEMsto generate [9]. These DEMs individual-scene [9]. These individual-scene DEMs of 60 byDEMs 60 km of were60 by later60 km composited were later globallycomposited into globally the ASTER into Globalthe ASTER DEM Global or GDEM—version DEM or GDEM—version 3 of which was3 of releasedwhich was in Augustreleased 2019 in August [10,11]. 2019 [10,11]. Arguably, volcanologyvolcanology is one ofof thethe scientificscientific disciplinesdisciplines on whichwhich ASTER data have had the greatest impact.impact. InIn orderorder to to quantify quantify this this impact impact and and gather gather all volcanologyall volcanology related related studies studies together together into oneinto referenceone reference document, document, we have we performed have performed an extensive an extensive literature literature review (Appendix review (SupplementaryA). This found 271materials). peer-reviewed This found publications 271 peer-reviewed from 1 January publications 1995 to from 1 December 1 January 2019, 1995 anto average1 December of nearly 2019, an 11 peraverage year of (Figure nearly2). 11 Papers per year published (Figure 2). prior Papers to the published Terra launch prior were to the considered Terra launch precursory were considered studies, commonlyprecursory describingstudies, commonly how the future describing ASTER how data th woulde future be usedASTER for certaindata would volcanic be used studies. for Thiscertain list ofvolcanic271 publications studies. Thiswas list subdividedof 271 publications into 12 was categories subdivided based into on 12 the categories volcanic based focus on of the the volcanic papers. Thefocus category of the namespapers. and The number category of papersnames inand those numb categorieser of papers (shown in inthose parentheses) categories are: (shown Analogs in (3),parentheses) Calibration are: (4), Analogs Lava Flows (3), Calibration (4), Gas/Plumes (4), Lava (30), GeothermalFlows (4), Gas/Plumes (9), Mapping (30), (54), Geothermal Modeling (16),(9), MonitoringMapping (54), (96), Modeling