Polar-orbiting satellites carry infrared sensors (MODIS on Terra/Aqua, VIIRS on Suomi-NPP and NOAA-20) that measure thermal energy. When a pixel is far hotter than its surroundings, the algorithm flags it as an active-fire detection — a “hot spot”. FireWatch plots each detection as an event.
Fire Radiative Power (FRP), in megawatts, estimates how much energy the fire is radiating in that pixel — a proxy for fire intensity and, roughly, how much is burning. FireWatch maps FRP onto its color scale, so a fierce crown fire stands out from a low, smoldering ground fire.
Detections arrive in passes: a satellite only sees a given spot a few times a day, so hot spots appear in batches rather than continuously. Clouds, thick smoke and small or cool fires can be missed, and a hot industrial source can occasionally trigger a false detection.
A detection means “heat was here at this overpass,” not “the fire is exactly this shape.” Use clusters of detections and named incidents — not a single pixel — to judge where a fire is and where it is heading.