# An Overview of Infrared Remote Sensing of Volcanic Activity
## 3 Specific Considerations for the Infrared Remote Sensing of Volcanic Activity
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. Figure 3: The swath width of an Earth Observing satellite sensor. Figure 2: Infrared imagery of Palu’e Island, and Paluweh Volcano, Indonesia. (**a**) Landsat-8 infrared imagery, with a spatial resolution of 30 m, captured 0200 UTC on 29 April 2013; (**b**) MODIS infrared imagery (Calibrated Earth View data), with a spatial resolution of 1000 m, captured 0210 UTC on 30 April 2013. Note the greater level of detail discernible in (a) compared to (b). 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 [26]), 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 [27; 28]. 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. 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 [29]), 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.