# An Overview of Infrared Remote Sensing of Volcanic Activity
## 4 Historical Perspective on Sensors Used for Volcanic Observation
Although their spatial resolution is not ideal, their regular overpasses (up to four times daily when both sensors are considered) have contributed to their widespread use. Various products have been developed that extract and process MODIS thermal data. One such group of products with utility for volcanologists is the MODIS Fire Products, which provide a table of all thermally-anomalous pixels identified within each MODIS scene [62; 63]. The MODIS instruments have now been functioning for over 15 years, and this long time series of regular data has allowed scientists to automatically monitor volcanic activity and to determine baseline activity for every active volcano on Earth (e.g., [64; 65; 66]). The latest NASA satellite to have been launched (in 2011) that possesses infrared imaging capabilities is the S-NPP satellite, which features the VIIRS sensor. The S-NPP was launched as an experimental satellite of the Joint Polar Satellite System (JPSS), coordinated by both NASA and NOAA. This system is envisaged as continuing the satellite record of environmental observations, which commenced with AVHRR in the 1970s and continues with MODIS. The VIIRS possesses two sets of bands with infrared detection capabilities: five high resolution imaging (I) bands (375 m) and sixteen moderate (M) resolution bands (750 m), each covering the MIR and TIR portions of the spectrum. To date, there has only been one attempt at using VIIRS data for monitoring volcanic activity, with [67] showing that its detection capabilities are similar, if not superior, to those of MODIS, while a number of studies have similarly confirmed the utility of its infrared bands for monitoring wildfires (e.g., [68; 69]). One final set of satellites that has been used for volcanic observations is what are termed'small satellites'. Varying definitions exist, but in general, a small satellite is one weighing under 1000 kg, with the small size making them more affordable and potentially commercially mass-producible [70; 71]. The first small satellite mission for hot-spot monitoring was the Bi-spectral Infrared Detection (BIRD) mission of the German Aerospace Center (DLR). The BIRD satellite was launched in 2001 and possessed two infrared bands (one MIR and TIR) for identifying hotspots [72]. The satellite entered a relatively low Earth orbit (570 km), with a swath width of 190 km and a pixel size of 370 m \(\times\) 185 m. Data collected by the sensor (which itself possessed adjustable gain settings) were examined using a customised hotspot detection algorithm. The chief advantages of BIRD were a higher spatial resolution, as compared with sensors, such as MODIS, and the availability of both MIR and TIR detection capabilities (an advantage over ASTER), although these advantages were at the expense of temporal resolution, given its relatively narrow swath [73]. In 2012, a new DLR mission, FireBIRD, commenced, with the micro-Technologieerprobungstrager 1 (TET-1) satellite being launched, possessing two infrared sensors [74]. Although fire observation was the objective of FireBIRD, in late 2014, it imaged the effusion of lava at Stromboli Volcano, Italy, and in this case, its adaptive gain settings meant that while a coincident MODIS image experienced saturation, its data remained unsaturated; and its higher MIR spatial resolution (320 m) provided a relatively more detailed image [75]. Thermal remote sensors utilised for volcanic remote sensing have also been launched by non-U.S. entities. In 1991 and 1995, the ESA launched the European Remote Sensing (ERS) 1 and 2 satellites, each possessing the Along Track Scanning Radiometer (ATSR), which was equipped with MIR and TIR bands and also one SWIR band. In 2002, a third ATSR, the Advanced ATSR (AATSR), was launched following the success of the two earlier versions.