Aursee · method

What a satellite fire detection is

How VIIRS finds active fires from orbit, what a 375 m detection means, and when a fire is not seen.

The instrument

Every fire on Aursee starts as a detection from VIIRS, the Visible Infrared Imaging Radiometer Suite. It flies on three polar-orbiting satellites: Suomi NPP, NOAA-20 and NOAA-21. NASA's Fire Information for Resource Management System (FIRMS) runs the active-fire algorithm on each pass and publishes the results; we read the near-real-time product for all three satellites every hour.

A detection is a pixel whose mid-infrared brightness stands out from its surroundings by more than the algorithm's thresholds. The pixel is 375 m across directly below the satellite and grows towards the edge of the 3,000 km swath, so a detection says that something in roughly a 375 m to 800 m square was burning hot enough to be seen.

When the satellites look

Each satellite crosses the equator at about 13:30 and 01:30 local solar time, so most places are seen in an early-afternoon pass and a middle-of-the-night pass, several times a day once the three satellites are combined. Swaths overlap towards the poles, so high latitudes are seen more often than the tropics.

A fire that starts and goes out between passes is not recorded. Neither is one under thick cloud or dense smoke at pass time.

What a detection can be

A pixel can be flagged by any strong heat source: a vegetation fire, a burning field, a gas flare, a volcano, some industrial sites. A fire much smaller than the pixel can still be detected if it is hot enough, especially at night when the background is cool.

Each detection comes with a confidence class (low, nominal, high) and a fire radiative power in megawatts. See fire radiative power.

Detected, derived, inferred

Aursee separates three kinds of statement. Detected: what the instrument recorded, such as the pixel, time and radiative power. Derived: computed from detections by a stated rule, such as grouping pixels into one fire. Inferred: an interpretation, which is always labelled as such on the page.

More on the method

• Fires in the dark: how satellites find them at nightWhy a fire is easier to see from orbit at night, how VIIRS picks a flame of a few dozen square metres out of a 375 m pixel, and what “burning in darkness” means on the map.
• Fire radiative power (MW)What fire radiative power measures, how Aursee adds it up, and how to compare two fires.
• How detections become firesThe rule Aursee uses to turn individual satellite pixels into fires you can open, name and follow.
• The burn archive, 2012 to todayHow Aursee remembers where fire has been detected since 2012, and what the history figures on each page mean.
• Lightning near a fireHow Aursee checks for lightning before a fire was first seen, and where that check is possible.
• Land cover and roads under a fireWhat kind of ground a fire is burning on, and how many roads are around it: sources and resolution.
• Weather at a fireWhere the wind, humidity and temperature on a fire page come from, and at what time and resolution.
• Data sources and licencesEvery dataset Aursee uses, who publishes it, and the licence it is used under.