The Geostationary Lightning Mapper (GLM) is a single-channel optical instrument on the GOES-R series satellites that detects the light emitted by lightning at cloud tops. It watches most of the Western Hemisphere continuously, day and night.
What the instrument does
GLM is a narrow-band near-infrared optical transient detector. It watches a single wavelength band at 777.4 nanometers, where excited atomic oxygen from lightning discharges emits strongly, and it looks for brief brightening within its field of view that indicates a flash.
The frame rate is roughly 2 milliseconds, and spatial resolution runs about 8 kilometers at nadir and 14 kilometers near the edge of the disk. Coverage extends to about 52 degrees north and south latitude. Detections are organized into a hierarchy of events, groups, and flashes so that both fine-scale and storm-scale activity can be tracked.
How forecasters use it
GLM measures total lightning, meaning in-cloud, cloud-to-cloud, and cloud-to-ground flashes all contribute. That is important operationally because in-cloud activity tends to rise before cloud-to-ground activity and before severe surface reports, so total flash trends can lead other severe-weather cues by minutes.
The instrument is especially valuable over oceans, mountains, and remote areas where ground-based lightning networks are sparse. It helps warning forecasters rank storms by intensity, watch for rapid intensification, and maintain situational awareness during radar outages.
Important limits
A single-band optical detector cannot recover the full physics of a discharge. It cannot separate cloud-to-ground from in-cloud flashes, cannot measure peak current, and cannot resolve polarity. Ground-based networks that sense radio-frequency emissions carry information GLM cannot.
Detection efficiency depends on cloud-top optical thickness and viewing geometry. A very deep, cold anvil can attenuate the light reaching space, so an apparent lull in flash rate may reflect the storm's structure rather than its electrification.