# An Overview of Infrared Remote Sensing of Volcanic Activity
## 4 Historical Perspective on Sensors Used for Volcanic Observation
Several geostationary satellites were launched in the 1970s with infrared observing capabilities for meteorological purposes. Of these, the NASA GOES series has been most prolifically used for volcanic observations. The chief advantage of sensors on such systems is that they have an extremely regular image acquisition (four times per hour for GOES and Meteosat), but being geostationary, they are at higher altitude meaning, they have a poorer spatial resolution (4 km for the GOES Imager and 5 km for Meteosat's Visible and Infra-Red Imager (MVIRI)). Since 1998, an automated system at the Hawai'i Institute of Geophysics and Planetology has been operational, with the task of processing GOES data for monitoring global volcanic activity, and since this time, various studies have confirmed its utility for extracting volcanic activity chronologies (e.g., [51; 52]). SEVERI, the infrared-capable instrument on-board the most recent Meteosat satellites (the Meteosat Second Generation), has similarly been used for such purposes and, indeed, has formed part of an automated volcano monitoring system called HOTSAT, which is implemented, along with MODIS data, when rapid activity detection and monitoring is required [53; 54]. In 1986, NASA announced plans for an Earth Observation System (EOS) that would acquire low-Earth orbit observations [55]. Infrared observations fell within the plans, and even prior to its commencement, scientists were optimistic about the role this system could have in the observation of volcanic activity [32]. The EOS satellites were launched from 1999 onwards, and as the data came online, scientists rapidly harnessed them for volcanological applications. The four EOS sensors with infrared observing capabilities were ASTER (on-board the Terra satellite) and MODIS (on-board the Terra and Aqua satellites), as well as the ALI and Hyperion sensors (on-board the EO-1 satellite). Each sensor displayed different characteristics that made them suitable for volcanic observational purposes. ASTER was met with particular optimism within the volcanological community, given that it provided observations in the SWIR (six bands of 30-m resolution) and the TIR (five bands of 90 m), and with a swath width of 60 km and revisit frequency of 16 days, it constituted a follow on to Landsat TM [56; 57; 58]. As of 2008, imagery acquired from ASTER's SWIR bands became unusable; the TIR bands remain functioning at the time of writing. Of the other EOS sensors, ALI possessed ten bands from the VNIR to the SWIR, with a spatial resolution of 30 m, while Hyperion shared the 30-m resolution, but possessed a ground-breaking 172 SWIR bands. Early on, the relatively fine spatial resolution of ASTER's infrared bands was shown to be useful in detecting eruption-precursory thermal anomalies at Chikurachki Volcano, Russia [58], while [19; 59] demonstrated that despite its sub-optimal revisit frequency of 16 days, long-term analysis of its data could be used to characterise the behaviour of Shiveluch and Lascar volcanoes, respectively,with its SWIR and TIR bands showing utility for detecting different thermal components at each volcanic surface. [PERSON] and [PERSON] [60] also demonstrated that within several hours of acquisition, ASTER imagery could be used to estimate the volcanic explosivity index (a scale used to quantify eruption explosiveness) of an eruption, although this relied on ASTER viewing the surface at an appropriate time. The two identical NASA MODIS instruments were launched three years apart on-board the Terra (1999) and Aqua (2002) platforms, each possessing 16 infrared bands with a spatial resolution of 1000 m and, given a larger pixel size, a larger swath and shorter revisit period (12 h); they had an envisaged design-life of five years [61]. The MODIS sensors have become the most commonly-used sensors for regular and reliable observations of thermally-anomalous volcanic activity.