Every active-fire satellite product rests on the same physical principle: a burning fire emits far more energy in the mid-infrared band than the ground around it does, even through smoke that would obscure a visible-light image. Sensors tuned to that band can flag a pixel as "hot" the moment a satellite passes overhead, without needing daylight or clear skies. The practical differences between fire-detection systems come down to which sensor is doing the looking, and how often.
The physics of thermal fire detection
Detection algorithms compare the brightness temperature of a candidate pixel, measured in the mid-infrared, against its immediate surroundings and against expected background temperature for that time of day and land cover. A sufficiently large contrast triggers a detection. From that same signal, sensors also derive Fire Radiative Power (FRP), an estimate of the rate of energy release from the fire, in megawatts. FRP is not just a diagnostic curiosity: it correlates with fuel consumption rate and gives OrbiVigil's fusion engine a quantitative signal to rank detections by intensity, not just presence.
Polar-orbiting vs geostationary trade-offs
VIIRS, flown on the Suomi NPP and NOAA-20/21 satellites, resolves fire pixels to roughly 375 metres, small enough to catch fires while they are still a few hectares in size, but each satellite only passes over a given location a small number of times per day. MODIS, an older instrument with a longer historical record, offers coarser 1 kilometre resolution. Meteosat Third Generation's Flexible Combined Imager takes the opposite trade: roughly 1–2 kilometre resolution, but a full-disk scan every ten minutes from geostationary orbit, which can reveal a fast-growing ignition hours before the next polar overpass. Sentinel-3's SLSTR instrument adds a dedicated fire radiative power product at intermediate resolution. No single sensor dominates on both axes; a serious detection system needs all of them.
From hotspot pixel to fire radiative power estimate
A single detection, from a single sensor, at a single pass, is a hypothesis, not a fact. Sun glint, industrial flaring, steel and cement plants, agricultural burning and even certain rooftop materials can all produce a thermal signature that superficially resembles a small fire. Robust systems require a candidate to be corroborated — either by a second, independent sensor observing the same location within a short window, or by persistence and growth across successive observations, or by exclusion against a registry of known non-fire heat sources.
How OrbiVigil does this
OrbiVigil's Tier 1 detection layer ingests VIIRS and MODIS data through NASA FIRMS, Meteosat Third Generation's active-fire product directly from EUMETSAT, and Sentinel-3 SLSTR fire radiative power, treating each as an independent witness to the same physical event rather than a single source of truth. A candidate detection is checked against the persistent-hotspot registry — refineries, steel and cement works, flaring sites, solar farms — built from a rolling twelve months of prior detections at the same location, and its fire radiative power is tracked over successive passes to distinguish genuine growth from a static industrial signature. Only detections that survive this cross-checking are promoted toward a confirmed event.
To see how this thermal layer fits into the full detection pipeline, read wildfire detection. For how OrbiVigil tracks a fire once confirmed, see satellite wildfire monitoring. For the platform's approach across all hazard types, see natural disaster early warning.