Image navigation and registration (INR) processing for the Geostationary Lightning Mapper (GLM) assigns geographic coordinates to lightning events using orbit and attitude telemetry and a geometric calibration that matches coastline features from the GLM background scene against a digital map. Required performance is expressed as an optical angle at the aperture of the instrument of 112 μrad (3σ), equivalent to 4 km at the subsatellite point. This is only one-half the linear dimension of the ground footprint of a detector element at the center of the field of view and is challenging to both attain and validate. Our validation approach uses imagery from the Advanced Baseline Imager (ABI) Band 3 (B03), the ABI channel closest to the GLM spectrally, with 1-km pixel resolution at the subsatellite point, as an INR reference. The finer spatial resolution and the high-accuracy INR (<28 μrad, 3σ) of ABI make it well suited for this application. Since both instruments are on the same platform, no parallax correction is required. We measure positions of feature templates extracted from the GLM background scene relative to the ABI reference image to assign spatial coordinates to the center-points of each template. Almost any feature, except clear-sky ocean scenes, can be used for matching, which allows for spatially dense measurement of INR errors in the forward navigation calculation of a GLM pixel. Cloud motion is compensated during the time between the acquisition of the ABI reference image and the GLM background scene, which further improves validation accuracy. We show that GLM on both GOES-16 and -17 spacecraft satisfy their INR requirements. The spatial and temporal density of validation measurements permits examination of smaller systematic errors, including thermally driven alignment offsets, a discontinuity between focal plane hemispheres arising from a readout artifact, the optical distortion of the GLM lens, and a calibration of the telescope focal length.
The GOES-R series is the latest in a long line of American geostationary weather satellites operated by the National Oceanic and Atmospheric Administration (NOAA). The two Geostationary Lightning Mapper (GLM) instruments currently operating on the GOES-16 and GOES-17 satellites give NOAA a unique new capability to map in-cloud and cloud-to-ground lightning flashes across the entire hemisphere within seconds of their occurrence. GLM enables improved warning times for severe weather events, decreased false alarms, persistent coverage over wide geographical areas without sampling bias, and long-term monitoring of trends linked to the changing climate.
Viewed from space, emissions from lightning appear as a series of brief (~500 μs) optical pulses diffused through clouds over scales of tens to thousands of km2. A significant portion of the emitted optical radiation is in the form of emission lines, including a prominent neutral atomic oxygen triplet whose emission lines are near 777 nm. GLM discriminates lightning flashes from the bright sunlit cloud background by taking advantage of the spatial, temporal, and spectral characteristics of the optical signature of lightning.
This paper describes key design drivers in the development of GLM, methods used to calibrate the instrument, and lessons learned from on-orbit testing. We discuss optimization of the entire signal chain, from the telescope optics to the ground processing algorithms.