How a Satellite 22,000 Miles Up Sees Every Lightning Flash

The lightning on Lightning Tracker's map does not come from a network of ground sensors. It comes from a single instrument on a weather satellite parked over the equator at 75.2° west, about 22,000 miles above the Atlantic, watching the entire hemisphere at once.

The instrument is the Geostationary Lightning Mapper, GLM for short, and it rides on NOAA's GOES-19. Here is what it actually does.

Staring, not scanning

Most satellite instruments sweep. They build an image by sampling a scene piece by piece, which means any given spot is only observed periodically. That is fine for cloud cover, which changes over minutes. It is useless for lightning, which lasts a few hundred milliseconds.

GLM does not sweep. It holds a fixed stare at the full disc of the hemisphere and reads its detector hundreds of times per second, continuously, day and night. Nothing has to be pointed at a storm, because everything in view is always being watched.

Because the satellite is geostationary, it orbits at exactly the rate the Earth turns and stays over the same longitude permanently. The view never changes and there are no gaps between passes.

Finding a flash against daylight

The hard problem is contrast. Lightning is bright, but a sunlit cloud top is far brighter. Picking a flash out of that is the central trick.

GLM solves it two ways. First, it looks at a very narrow slice of the spectrum, around 777.4 nanometers, a near-infrared wavelength where excited oxygen emits strongly during a lightning discharge. Filtering to that band throws away most of the reflected sunlight while keeping most of the lightning signal.

Second, it looks for change rather than brightness. The instrument compares each frame against a running background estimate of the scene, and a lightning flash registers as a sudden departure from that background. A bright cloud is constantly bright. A flash is a transient. That distinction is what makes daytime detection possible at all.

Events, groups, and flashes

Raw detections arrive as individual pixel-level triggers, and a single lightning flash lights up more than one pixel across more than one frame. The data is therefore assembled in three tiers before anyone sees it:

• Events are single pixel triggers in a single frame.
• Groups are events that occur in the same frame in adjacent pixels, which correspond roughly to one stroke.
• Flashes are groups clustered together in space and time, which correspond to what a person standing outside would call one bolt.

Lightning Tracker plots flashes. That is the level that matches human intuition, and it is what NOAA distributes in the product called GLM-L2-LCFA.

Every 20 seconds

GOES-19 writes a new file of detections every 20 seconds and publishes it to a public bucket on AWS. That is about 4,320 files per day from this one satellite, arriving on a metronome.

Our ingest service subscribes to a notification the moment each file lands, downloads it, parses the flashes out, and pushes the updated map within seconds. From the flash actually happening to it appearing on your screen is typically 30 to 60 seconds, most of which is NOAA's processing and publishing rather than ours. If a strike falls inside a radius somebody is watching, the push notification goes out in roughly the same window.

There is a pleasant operational side effect of a metronomic feed: silence is diagnostic. If no file arrives for three minutes, something is broken, and we get paged.

What it sees, and where

Total lightning. GLM detects in-cloud flashes as well as cloud-to-ground strikes. Most lightning in a storm never reaches the ground, and in-cloud activity frequently intensifies before the first ground strike, so watching total lightning gives an earlier read on a cell than watching ground strikes alone.

Position to within about 5 to 10 km. The instrument sees light escaping the top of a cloud, and that light scatters on its way out, so the apparent center of the glow sits near but not exactly above the channel. For deciding whether to leave an exposed field, that is comfortably precise enough.

The Americas, with a latitude limit. From 75.2° west, GOES-19 covers the continental United States, southern Canada, Mexico, Central America, the Caribbean, and most of South America. Coverage degrades past roughly 52 to 54 degrees of latitude, where the viewing angle gets too oblique to be reliable, which puts far-northern Canada and most of Alaska outside usable range. The satellite cannot see Europe, Africa, Asia, or Australia at all.

There is a sister satellite, GOES-18, holding the western position with an identical instrument, and a European equivalent on Meteosat-12. Both are adapters we can add to the same pipeline when coverage demands it.

The honest limitations

GLM does not catch every flash. Light from a discharge buried deep inside a very thick cloud may not reach the top with enough intensity to register, and detection performance falls off somewhat toward the edges of the disc where the viewing angle is shallow.

So the map is extremely good evidence and not a guarantee. If the map is empty but you can hear thunder, trust the thunder.

What the instrument does deliver is genuinely remarkable: continuous, hemisphere-wide, day-and-night lightning detection, updated three times a minute, published openly by NOAA for anyone to build on. Lightning Tracker is a thin layer of software on top of a very good public instrument.