An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> Abstract A study of volcanic activity and its effects on the atmosphere is one of 28 interdisciplinary investigations, for the Earth Observing System (EOS), due to be launched in 1997 and 1999. The volcanology investigation will include long- and short-term monitoring of selected volcanoes, the detection of precursory activity associated with unanticipated eruptions, and the detailed study of on-going eruptions. The data collected will allow us to address two aspects of volcanism: volcanic padforms and the atmospheric effects of eruptions. A variety of instruments on the two NASA EOS platforms, together with supplemental data from the Japanese and European platforms, will enable the study of local- to regional-scale thermal and deformational features of volcanoes, and the chemical and structural features of volcanic eruption plumes and aerosols. This investigation fits well within the overall goal of the EOS Project, which is to study the regional and global interrelationships between components of the Earth System, because it specifically investigates the links between volcanism, atmospheric chemistry and short-term (1–3 year) climate change. <s> BIB001 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> Lascar Volcano (22°22'S, 67°44'W) is the most active volcano of the central Andes of northern Chile. Activity since 1984 has been characterised by periods of lava dome growth and decay within the active crater, punctuated by explosive eruptions. We present herein a technique for monitoring the high-temperature activity within the active crater using frequent measurements of emitted shortwave infrared (SWIR) radiation made by the spaceborne along-track scanning radiometer (ATSR). The ATSR is an instrument of low spatial resolution (pixels 1 km across) that shares certain characteristics with the MODIS instrument, planned for use as a volcano monitoring tool in the NASA EOS Volcanology Project. We present a comprehensive time series of over 60 cloud- and plume-free nighttime ATSR observations for 1992–1995, a period during which Lascar experienced its largest historical eruption. Variations in short wavelength infrared flux relate directly to changes in high-temperature surfaces within the active crater. From these data, interpretations can be made that supplement published field reports and that can document the presence and status of the lava dome during periods where direct, ground-based, observations are lacking. Our data agree with less frequent information collected from sensors with high spatial resolution, such as the Landsat thematic mapper (Oppenheimer et al. 1993) and are consistent with field observations and models that relate subsidence of the dome to subsequent explosive eruptions (Matthews et al., 1997). Most obviously, Lascar's major April 1993 eruption follows a period in which the magnitude of emitted shortwave infrared radiation fell by 90%. At this time subsidence of the 1991–1992 lava dome was reported by field observers and this subsidence is believed to have impeded the escape of hot volatiles and ultimately triggered the eruption (Smithsonian Institution 1993a). Extrapolating beyond the period for which field observations of the summit are available, our data show that the vulcanian eruption of 20 July 1995 occurred after a period of gradual increase in short wavelength infrared flux throughout 1994 and a more rapid flux decline during 1995. We attribute this additional, otherwise undocumented, cycle of increasing and decreasing SWIR radiance as most likely representing variations in degassing through fumaroles contained within the summit crater. Alternatively, it may reflect a cycle of dome growth and decay. The explosive eruption of 17 December 1993 appears to have followed a similar, but shorter, variation in SWIR flux, and we conclude that large explosive eruptions are more likely when the 1.6-μm signal has fallen from a high to a low level. The ATSR instrument offers low-cost data at high temporal resolution. Despite the low spatial detail of the measurements, ATSR-type instruments can provide data that relate directly to the status of Lascar's lava dome and other high-temperature surfaces. We suggest that such data can therefore assist with predictions of eruptive behaviour, deduced from application of physical models of lava dome development at this and similar volcanoes. <s> BIB002 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a research facility instrument provided by the Ministry of International Trade and Industry (MITI), Tokyo, Japan to be launched on NASA's Earth Observing System morning (EOS-AM1) platform in 1998. ASTER has three spectral hands in the visible near-infrared (VNIR), six bands in the shortwave infrared (SWIR), and five bands in the thermal infrared (TIR) regions, with 15-, 30-, and 90-m ground resolution, respectively. The VNIR subsystem has one backward-viewing band for stereoscopic observation in the along-track direction. Because the data will have wide spectral coverage and relatively high spatial resolution, it will be possible to discriminate a variety of surface materials and reduce problems in some lower resolution data resulting from mixed pixels. ASTER will, for the first time, provide high-spatial resolution multispectral thermal infrared data from orbit and the highest spatial resolution surface spectral reflectance temperature and emissivity data of all of the EOS-AM1 instruments. The primary science objective of the ASTER mission is to improve understanding of the local- and regional-scale processes occurring on or near the Earth's surface and lower atmosphere, including surface-atmosphere interactions. Specific areas of the science investigation include the following: (1) land surface climatology; (2) vegetation and ecosystem dynamics; (3) volcano monitoring; (4) hazard monitoring; (5) aerosols and clouds; (6) carbon cycling in the marine ecosystem; (7) hydrology; (8) geology and soil; and (9) land surface and land cover change. There are three categories of ASTER data: a global map, regional monitoring data sets, and local data sets to be obtained for requests from individual investigators. <s> BIB003 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> Infrared satellite data provide information regarding the thermal state and nature of activity at active volcanoes. Such radiometry has been hampered, however, by the tendency of many currently operational sensors to saturate over high-radiance targets. Here the simulated response of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) to high-temperature volcanic features is presented and compared to that of the Landsat Thematic Mapper (TM), which has provided the bulk of useful data since the mid 1980's. We find that ASTER's improved spectral resolution and increased dynamic range will markedly reduce the problem of sensor saturation common in TM-based studies, thus providing valuable data previously unavailable. Saturation will still occur however, when the fractional area of high-temperature (∼1000°C) material approaches ∼1% (∼9 m²) of ASTER's 30-m short-wave infrared pixels. <s> BIB004 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> The synoptic coverage offered by satellites provides unparalleled opportunities for monitoring active volcanoes, and opens new avenues of scientific inquiry. Thermal infrared radiation can be used to monitor levels of activity, which is useful for automated eruption detection and for studying the emplacement of lava flows. Satellite radars can observe volcanoes through clouds or at night, and provide high-resolution topographic data. In favorable conditions, radar inteferometery can be used to measure ground deformation associated with eruptive activity on a centimetric scale. Clouds from explosive eruptions present a pressing hazard to aviation; therefore, techniques are being developed to assess eruption cloud height and to discriminate between ash and meterological clouds. The multitude of sensors to be launched on future generations of space platforms promises to greatly enhance volcanological studies, but a satellite dedicated to volcanology is needed to meet requirements of aviation safety and volca... <s> BIB005 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> Abstract At present efforts in infrared detector research are directed towards improving the performance of single element devices, large electronically scanned arrays and higher operating temperature. Another important aim is to make IR detectors cheaper and more convenient to use. All these aspects are discussed in this paper. Investigations of the performance of infrared thermal detectors as compared to photon detectors are presented. Due to fundamental different types of noise, these two classes of detectors have different dependencies of detectivities on wavelength and temperature. Next, an overview of focal plane array architecture is given with emphasise on monolithic and hybrid structures. The objective of the next sections is to present the status of different types of detectors: HgCdTe photodiodes, Schottky-barrier photoemissive devices, silicon and germanium detectors, InSb photodiodes, alternative to HgCdTe III–V and II–VI ternary alloy detectors, monolithic lead chalcogenide photodiodes, quantum well and quantum dot infrared photodetectors. Final part of the paper is devoted to uncooled two-dimensional arrays of thermal detectors. Three most important detection mechanisms, namely, resistive bolometer, pyroelectric detectors and termopile are considered. The development of outstanding technical achievements in uncooled thermal imaging is also presented. <s> BIB006 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> This paper presents the operational concept for onboard processing for the HysIRI mission which is an Earth observing mission that includes both thermal infrared instrument and a hyperspectral visible/shortwave infrared instrument, and that is being considered for launch in the next decade. This article describes the potential application of AI techniques for the HyspIRI mission-for both onboard processing and ground-based, automated mission planning. <s> BIB007 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> Analysis of thermally emissive volcanic features using satellite infrared remote sensing has been conducted over recent decades, primarily using shortwave and thermal infrared (SWIR; TIR) radiance data. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), mounted on the Earth Observation System (EOS) Terra satellite, offers an advance on earlier instruments, having more bands covering the SWIR atmospheric window and offering a wider dynamic range. This paper compares methods used to analyse ASTER SWIR imagery of active volcanoes, using both simulated cases and actual ASTER imagery of Lascar Volcano, and focuses on radiative power estimates. Those based on the Oppenheimer approach are found to be most reliable for simulated surfaces, with the Lombardo and Buongiorno and Dozier retrievals having larger uncertainties in most cases. However, the Dozier Method results in the highest proportion of successful retrievals, the reliability of which is influenced by factors including band comb... <s> BIB008 </s> An Overview of Infrared Remote Sensing of Volcanic Activity <s> Specific Considerations for the Infrared Remote Sensing of Volcanic Activity <s> Abstract In 2007, the NASA Hyperspectral InfraRed Imager (HyspIRI) mission was recommended in Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond (Decadal Survey) to address critical science questions in multiple areas, in particular ecosystems and natural hazards. HyspIRI is comprised of two instruments, a visible to short-wavelength infrared (VSWIR) imaging spectrometer and a thermal infrared (TIR) multispectral imager, together with an Intelligent Payload Module (IPM) for onboard processing and rapid downlink of selected data. The VSWIR instrument will have 10 nm contiguous bands and cover the 380–2500 nm spectral range with 30 m spatial resolution and a revisit of 16 days. The TIR instrument will have 8 discrete bands in the 4–13 μm range with 60 m spatial resolution and a revisit of 5 days. With these two instruments in low Earth orbit, HyspIRI will be able to address key science and applications questions in a wide array of fields, ranging from ecosystem function and diversity to human health and urbanization. <s> BIB009
There are a number of considerations that must be taken into account when acquiring and analysing thermal imagery of volcanic activity, and this section will examine these. Spatial resolution: This term relates to the smallest spatial unit of data that a sensor can collect, and in terms of remote sensing, it relates to the area on the Earth's surface that is imaged by one pixel. It is to some extent a function of the altitude of the sensor, with the area imaged by one pixel being larger the further a sensor is from the surface. Another influence on the pixel size is the type of data being collected, with technological constraints limiting the spatial resolution of longer wavelength sensors. Readers will be familiar with high-resolution Google Earth imagery, which allows small features on Earth's surface to be identified (sometimes features smaller than 1 m in size). Unlike such visual band remote sensing, however, the spatial resolution of infrared imagery is relatively coarse, with the highest resolution that is used for volcanic observations being 30 m (for example, the SWIR bands of NASA's ASTER and Landsat-8 sensors) and with an even coarser resolution for longer wavelength TIR bands (e.g., 1000 m for MODIS). Additionally, even the highest spatial resolution for an infrared sensor currently in orbit is only 5.5 m (the Korea Aerospace Research Institute's Korea Multi-Purpose Satellite, KOMPSAT-3A,  ). The cause of the disparity in spatial resolution between visual and infrared bands is the higher complexity in optical equipment required for the latter, with cooling of the longer wavelength photon detectors, for example, being necessary, which hinders the development of smaller detector elements BIB006  . The significance of the requirement for larger infrared-imaging pixels is that the area imaged by one MODIS TIR pixel, for example, is rather large (1000 m × 1000 m), and when imaging an active lava-flow, such a pixel will detect radiance from various surface components (i.e., hot-incandescent and cooling lava-flows and even non-volcanic surfaces). As the radiance detected within one pixel is an average of the whole area imaged, with signals from sub-pixel components combined into a one 'pixel-integrated' value BIB005 , the higher the spatial resolution of a sensor (i.e., the smaller the pixel size), the greater the level of accuracy and detail that will be discernible (see Figure 2 ). In providing a greater level of detail, smaller pixels image a smaller area of Earth's surface, but this has a downside: that any hot surface imaged will constitute a greater proportion of a small pixel's field of view, therefore increasing the possibility of saturation. As such, despite the added detail provided by higher resolution infrared sensors, those of coarser resolution (e.g., 1000 m) remain useful in the remote monitoring of the most radiant volcanic activity. Temporal resolution: The temporal resolution of remotely-sensed data refers to the regularity with which a point on the Earth's surface can be imaged by a sensor, and this, as well as the spatial and temporal resolution of spaceborne remote sensing devices are, in practice, inversely related  . The reason for this is that the higher the spatial resolution of a sensor, the smaller its pixels and, therefore, the narrower its swath, where the swath is the width of the area on the Earth's surface that a sensor scans on overpass ( Figure 3) . As a narrower swath views less of the Earth's surface at any one time, it will image the entire surface of the Earth with less regularity, hence reducing the temporal resolution. The swath width of NASA's OLI, for example, is 185 km, with a pixel size of 30 m, resulting in a temporal resolution, otherwise termed revisit frequency, of 16 days (i.e., it will view the same point on the Earth's surface, such as a volcano, once every 16 days); NASA's MODIS, in contrast, has a swath width of 2330 km and pixel size of 1 km, resulting in a twice-daily temporal resolution. A reduced temporal resolution is sub-optimal if instantaneous reports of volcanic activity are required (for this, geostationary satellites may be used), but is often adequate for determining trends in volcanic activity over time. a sensor scans on overpass ( Figure 3 ). As a narrower swath views less of the Earth's surface at any one time, it will image the entire surface of the Earth with less regularity, hence reducing the temporal resolution. The swath width of NASA's OLI, for example, is 185 km, with a pixel size of 30 m, resulting in a temporal resolution, otherwise termed revisit frequency, of 16 days (i.e., it will view the same point on the Earth's surface, such as a volcano, once every 16 days); NASA's MODIS, in contrast, has a swath width of 2330 km and pixel size of 1 km, resulting in a twice-daily temporal resolution. A reduced temporal resolution is sub-optimal if instantaneous reports of volcanic activity are required (for this, geostationary satellites may be used), but is often adequate for determining trends in volcanic activity over time. Spectral resolution: This relates to the capacity of a sensor to distinguish different parts of the electromagnetic spectrum and, in many ways, is also inversely related to the spatial and temporal resolution (see Figure 4) , with the dictating factor often being the bandwidth of data transfer. NASA's Hyspiri, for example (a sensor that was due for launch 2013-2016, but is likely delayed until 2022 at the earliest  ), will generate up to 1 GB of data per second of imaging, due to possessing 212 bands in the 380-2500-nm region of the spectrum, and this must all be broadcast to the surface BIB007 BIB009 . Such large data requirements have necessitated its design with a smaller swath width (150 km for the SWIR bands) to reduce the quantity of data it will produce on each overpass, but in turn, this will reduce its temporal resolution. In contrast, NASA's VIIRS acquires a maximum of 10.5 MB of data per second, and given this, and the fact that it has fewer data-collecting bands, this means that it could be designed with a wider swath (3060 km), so that more of the Earth's surface could be imaged on each overpass, thereby enhancing its temporal resolution. An additional consideration is that as the spectral (and indeed spatial) resolution increases, the actual signal detected within a band (or pixel) decreases, thereby reducing the signal to noise ratio. This complication adds to the trade-offs represented in Figure 4 that must be weighed by sensor designers and those interpreting such data. Spectral resolution: This relates to the capacity of a sensor to distinguish different parts of the electromagnetic spectrum and, in many ways, is also inversely related to the spatial and temporal resolution (see Figure 4) , with the dictating factor often being the bandwidth of data transfer. NASA's Hyspiri, for example (a sensor that was due for launch 2013-2016, but is likely delayed until 2022 at the earliest  ), will generate up to 1 GB of data per second of imaging, due to possessing 212 bands in the 380-2500-nm region of the spectrum, and this must all be broadcast to the surface BIB007 BIB009 . Such large data requirements have necessitated its design with a smaller swath width (150 km for the SWIR bands) to reduce the quantity of data it will produce on each overpass, but in turn, this will reduce its temporal resolution. In contrast, NASA's VIIRS acquires a maximum of 10.5 MB of data per second, and given this, and the fact that it has fewer data-collecting bands, this means that it could be designed with a wider swath (3060 km), so that more of the Earth's surface could be imaged on each overpass, thereby enhancing its temporal resolution. An additional consideration is that as the spectral (and indeed spatial) resolution increases, the actual signal detected within a band (or pixel) decreases, thereby reducing the signal to noise ratio. This complication adds to the trade-offs represented in Figure 4 that must be weighed by sensor designers and those interpreting such data. The trade-off between spatial, spectral and temporal resolution for Earth observation imagery. These relationships are a function of data transfer bandwidths; assuming a fixed bandwidth, a sensor possessing a high spectral resolution will need to compromise on either spatial or temporal resolution, and so on. Orbit: There are two types of satellite orbits used for volcanic remote sensing: geostationary and Sun-synchronous. Geostationary satellites, such as NASA's GOES and the ESA Meteosat, orbit the Earth at the Equator at the same speed as Earth's rotation and at an altitude of approximately 35,000 km. By matching Earth's orbital speed, geostationary satellites maintain the same view of the Earth's surface at all times and, being at a high altitude, provide near-hemispheric and therefore regular coverage (every 15-30 min). Sun-synchronous satellites, in contrast, are synchronised with the Sun, orbiting the Earth at a much lower altitude (approximately 705 km) and from pole to pole, in such a way that they overpass the Equator at the same local solar time, but a different longitude, on each orbit. The temporal resolution of such sensors is swath-width dependent, ranging from one day (for MODIS, with a swath width of 2330 km) to sixteen (for Landsat-8, with a swath width of 185 km). An additional orbital consideration is that satellites in orbit move at considerable speed (e.g., 27,500 km/h for low Earth orbiting satellites, such as NASA's Terra  ), thereby rendering image acquisition far from straightforward. To overcome the associated problems, time delay and integration (TDI) technology is used to increase the integration time that a pixel has for viewing the surface, thereby providing a non-blurred image. Atmospheric effects: If accurate radiant emissions data from a volcanic surface are required, then the effect of the atmosphere, and of volcanic gases, that the signal passes through between the surface and the sensor must be accounted for. This is because transmitting gasses may both attenuate and augment radiant signals that pass through. Most infrared bands on sensors used to monitor volcanic activity have been strategically placed in 'atmospheric windows', or portions of the electromagnetic spectrum with high transmissivity and, hence, low atmospheric absorption (see Figure 5 ). Even in these windows, however, a small proportion of the signal may be absorbed because volcanic gasses, These relationships are a function of data transfer bandwidths; assuming a fixed bandwidth, a sensor possessing a high spectral resolution will need to compromise on either spatial or temporal resolution, and so on. Orbit: There are two types of satellite orbits used for volcanic remote sensing: geostationary and Sun-synchronous. Geostationary satellites, such as NASA's GOES and the ESA Meteosat, orbit the Earth at the Equator at the same speed as Earth's rotation and at an altitude of approximately 35,000 km. By matching Earth's orbital speed, geostationary satellites maintain the same view of the Earth's surface at all times and, being at a high altitude, provide near-hemispheric and therefore regular coverage (every 15-30 min). Sun-synchronous satellites, in contrast, are synchronised with the Sun, orbiting the Earth at a much lower altitude (approximately 705 km) and from pole to pole, in such a way that they overpass the Equator at the same local solar time, but a different longitude, on each orbit. The temporal resolution of such sensors is swath-width dependent, ranging from one day (for MODIS, with a swath width of 2330 km) to sixteen (for Landsat-8, with a swath width of 185 km). An additional orbital consideration is that satellites in orbit move at considerable speed (e.g., 27,500 km/h for low Earth orbiting satellites, such as NASA's Terra  ), thereby rendering image acquisition far from straightforward. To overcome the associated problems, time delay and integration (TDI) technology is used to increase the integration time that a pixel has for viewing the surface, thereby providing a non-blurred image. Atmospheric effects: If accurate radiant emissions data from a volcanic surface are required, then the effect of the atmosphere, and of volcanic gases, that the signal passes through between the surface and the sensor must be accounted for. This is because transmitting gasses may both attenuate and augment radiant signals that pass through. Most infrared bands on sensors used to monitor volcanic activity have been strategically placed in 'atmospheric windows', or portions of the electromagnetic spectrum with high transmissivity and, hence, low atmospheric absorption (see Figure 5 ). Even in these windows, however, a small proportion of the signal may be absorbed because volcanic gasses, including water vapour, strongly absorb certain wavelengths. As the atmosphere and volcanic gases also themselves have a temperature, they too emit electromagnetic radiation that contributes to that detected by an observing sensor. A further complication is that a proportion of the atmospheric radiance, and indeed daytime solar radiance, will also be reflected sensor-ward, additionally contributing to the signal detected by satellite sensors. Radiative transfer models are available to simulate such atmospheric effects, and these must be applied to the infrared detections made at a sensor if accurate ground-level emissions data are to be retrieved. including water vapour, strongly absorb certain wavelengths. As the atmosphere and volcanic gases also themselves have a temperature, they too emit electromagnetic radiation that contributes to that detected by an observing sensor. A further complication is that a proportion of the atmospheric radiance, and indeed daytime solar radiance, will also be reflected sensor-ward, additionally contributing to the signal detected by satellite sensors. Radiative transfer models are available to simulate such atmospheric effects, and these must be applied to the infrared detections made at a sensor if accurate ground-level emissions data are to be retrieved. Figure 5 . Atmospheric transmissivity in the infrared region of the spectrum. Note the limited transmissivity around 6.0 µm, attributable to absorption by water vapour. These data are based on a 'standard atmosphere'. Source:  . Emissivity: This is the ratio of energy emitted from a particular body, to that radiated from a perfect emitter (a blackbody) at the same temperature and wavelength; in effect, the term quantifies the radiating efficiency of a surface. An object with an emissivity of 1.0 is a perfect emitter, emitting radiation with 100% efficiency; all other objects emit with less efficiency with the effect that the apparent (or radiance) temperature of such objects, if measured remotely and derived using the Planck equation (Equation (1)), is less than the true kinetic temperature of the object. This means that to derive the true temperature of a volcanic, or indeed any surface, remotely, then its emissivity must be known. The emissivity of volcanic surfaces has been shown to vary widely depending on mineralogy, temperature and the waveband of observation (e.g., it has been shown to be 0.92 in the SWIR bands for cooled andesitic lava at Lascar in Chile BIB002 and close to 1.0 for the active lava lake at Erta Ale in Ethiopia  ). Accurate emissivity values are essential, as errors in it have been shown to result in temperature retrieval discrepancies of over 125 K BIB001 . Temperature limits: All sensors used for volcanic thermal signal detection have detection limits. Within these limits, the emissions from a volcanic surface can be detected, but if the signal is too low, it will be missed; and if it is too high, it will saturate the individual pixels of the sensor, rendering quantitative analysis impossible (see Figure 6 ). Early in the discipline, saturation was identified as one of the key problems in the remote sensing of thermally-anomalous volcanic activity BIB001 , and such issues persist, although the situation is much improved. Determinants of the detection limits include the dynamic range of the sensor (the range over which radiant signals are detected) and the viewing pixel size (a smaller pixel will saturate at a lower level of radiance when viewing a hot volcanic surface, given that the heated surface components will constitute a greater proportion of the viewing pixel area). In comparing the detection limits of Landsat 8's OLI SWIR bands, with those of Figure 5 . Atmospheric transmissivity in the infrared region of the spectrum. Note the limited transmissivity around 6.0 µm, attributable to absorption by water vapour. These data are based on a 'standard atmosphere'. Source:  . Emissivity: This is the ratio of energy emitted from a particular body, to that radiated from a perfect emitter (a blackbody) at the same temperature and wavelength; in effect, the term quantifies the radiating efficiency of a surface. An object with an emissivity of 1.0 is a perfect emitter, emitting radiation with 100% efficiency; all other objects emit with less efficiency with the effect that the apparent (or radiance) temperature of such objects, if measured remotely and derived using the Planck equation (Equation (1)), is less than the true kinetic temperature of the object. This means that to derive the true temperature of a volcanic, or indeed any surface, remotely, then its emissivity must be known. The emissivity of volcanic surfaces has been shown to vary widely depending on mineralogy, temperature and the waveband of observation (e.g., it has been shown to be 0.92 in the SWIR bands for cooled andesitic lava at Lascar in Chile BIB002 and close to 1.0 for the active lava lake at Erta Ale in Ethiopia  ). Accurate emissivity values are essential, as errors in it have been shown to result in temperature retrieval discrepancies of over 125 K BIB001 . Temperature limits: All sensors used for volcanic thermal signal detection have detection limits. Within these limits, the emissions from a volcanic surface can be detected, but if the signal is too low, it will be missed; and if it is too high, it will saturate the individual pixels of the sensor, rendering quantitative analysis impossible (see Figure 6 ). Early in the discipline, saturation was identified as one of the key problems in the remote sensing of thermally-anomalous volcanic activity BIB001 , and such issues persist, although the situation is much improved. Determinants of the detection limits include the dynamic range of the sensor (the range over which radiant signals are detected) and the viewing pixel size (a smaller pixel will saturate at a lower level of radiance when viewing a hot volcanic surface, given that the heated surface components will constitute a greater proportion of the viewing pixel area). In comparing the detection limits of Landsat 8's OLI SWIR bands, with those of its predecessor, Landsat ETM+, the enhanced dynamic range of the former means that temperatures of up to 747.9 K and 570.4 K can be detected without saturation in Bands 6 (1.56-1.66 µm) and 7 (2.10-2.30 µm), as compared with those for the corresponding ETM+ bands (724.5 and 552.8 K, respectively)  . The problem of saturation was demonstrated in BIB008 , in which it was found that 16% of the volcanic ASTER SWIR scenes analysed displayed saturation, thereby preventing their quantitative analysis. One solution for saturation is the use of varying gain settings to extend the maximum detectable temperatures at the higher end. The SWIR bands of the ASTER sensor, for example, were equipped with low-gain settings for the purpose of volcanic monitoring (a "volcano mode" BIB003 BIB004 ), raising the saturation temperature; in doing so however, it had the effect of reducing sensitivity to cooler surface phenomena BIB008 . its predecessor, Landsat ETM+, the enhanced dynamic range of the former means that temperatures of up to 747.9 K and 570.4 K can be detected without saturation in Bands 6 (1.56-1.66 µm) and 7 (2.10-2.30 µm), as compared with those for the corresponding ETM+ bands (724.5 and 552.8 K, respectively)  . The problem of saturation was demonstrated in BIB008 , in which it was found that 16% of the volcanic ASTER SWIR scenes analysed displayed saturation, thereby preventing their quantitative analysis. One solution for saturation is the use of varying gain settings to extend the maximum detectable temperatures at the higher end. The SWIR bands of the ASTER sensor, for example, were equipped with low-gain settings for the purpose of volcanic monitoring (a "volcano mode" BIB003 BIB004 ), raising the saturation temperature; in doing so however, it had the effect of reducing sensitivity to cooler surface phenomena BIB008 . 