# An Overview of Infrared Remote Sensing of Volcanic Activity
## 4 Historical Perspective on Sensors Used for Volcanic Observation  
Infrared remote sensing imagers have been relatively common on orbiting satellites since the early 1960s, with the NOAA TIROS constituting the first operational infrared radiometers in orbit [36]. As technology progressed, so too did the number of platforms launched with infrared imaging capabilities, but it was not until 1968 that many American infrared remote sensing systems were decllassified, with data from the TIROS range being the first [37]. Since this period, there has been continuous infrared monitoring of Earth from space [36], with sensors gaining in both spectral and spatial resolution and with data becoming more widely available. The next section will provide a chronology of the discipline, while here, the main satellites and sensors will be presented. The AVHRR instruments on-board the satellites of the National Oceanic and Atmospheric Administration (NOAA) series were key to the infrared remote sensing of volcanism becoming mainstream. The first AVHRR instrument was launched in 1978, and soon after, their use in volcanic studies began (e.g., [38; 39]). The AVHRR instruments have various characteristics useful for the monitoring of volcanic activity, including of most relevance, two infrared bands with a spatial resolution of 1000 m \(\times\) 1000 m and, hence, the potential to detect a wide range of thermally-anomalous volcanic phenomena [1]. Each AVHRR orbits the Earth 14 times a day and, with an altitude of 833 km and a swath width of 2399 km, and provides for repeat imagery of the same surface roughly every 12 h (of clear utility in monitoring rapidly changing volcanic events (e.g., [40; 41; 42])). Although used less widely today given the availability of newer sensors, AVHRR monitoring of volcanic activity continues (for example, [43]) because of the availability of a long time series of data [44] and NOAA's  
Figure 6: (**a**) Band 9 ASTER (SWIR) nighttime image of Lascar Volcano, Chile, with a transect through the radiant signal; (**b**) quantification of the signal along the transect in (a). Note that between Pixels 12 and 16, the signal plateaus, indicating saturation. At Pixel 18, the signal is 0 W m\({}^{2}\) sr\({}^{-1}\)\(\upmu\)m\({}^{-1}\), suggesting it has been temporarily disabled because of saturation in adjacent pixels. policy of retaining two AVHRR satellites in orbit at the same time, effectively enhancing the temporal resolution [36]). In the 1970s, the NASA Landsat programme commenced, although it was not until the 1984 launch of Landsat-5 that functioning SWIR and TIR bands of the Thematic Mapper sensor (TM) were available for thermal studies, possessing spatial resolutions of 30 and 120 m, respectively. The superior spatial resolution of this sensor was achieved with a smaller swath width as compared with AVHRR (185 km versus 2399 km), although this came at the expense of its temporal resolution, which was 16 days. The next successful sensor of the series, on-board Landsat-7, was the Enhanced Thematic Mapper (ETM+), enhanced in terms of its TIR observation capability and superior TIR spatial resolution (60 m). With their higher infrared spatial resolution, the Landsat sensors rapidly became utilized for making more detailed volcanic observations than was possible with AVHRR [45; 46; 47; 48], although the reduced temporal resolution meant that the Landsat satellites were less suitable for observing short-lived events. In 2013, the latest in the Landsat series was launched, Landsat-8, as part of the Landsat Data Continuity Mission (LDCM) [49; 50]. This satellite was equipped with the Thermal Infrared Sensor (TIRS), possessing two bands with a spatial resolution of 100 m. This reduced spatial resolution, as compared with Landsat-7, was a result of a limited development time [49], but despite this, the TIRS has been shown to acquire accurate volcanic observations, which will, for the foreseeable future, ensure that infrared observations of volcanoes will remain available when MODIS expires [20].