Accurate measurements of global lightning are essential for understanding present and future atmospheric electricity, composition, and climate. The latest space-based lightning detector, the Geostationary Lightning Mapper (GLM), was the first to be placed in geostationary orbit, with a continuous view of most of the American continents. Prior to the GLM, the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite collected lightning measurements from which numerous lightning climatologies have been developed, including those used in global models. However, this study finds that both the GLM and a second, similar LIS placed on the International Space Station (ISS) in 2017 detect lightning at similar rates and are undercounting lightning compared to ground-based Lightning Mapping Arrays (LMAs). The GLM undercounts lightning by an average factor of 7.0, reaching a maximum over 120 as a function of satellite zenith angle, radar reflectivity at a height where the temperature is -10 degrees C, flash height, and thunderstorm polarity. The LIS is estimated to undercount lightning by an average factor of 5.6, reaching a maximum of 75.0 as a function of radar reflectivity at the -10 degrees C level, flash height, and thunderstorm polarity. Preliminary predictive equations for the GLM and LIS lightning undercount factor, or scaling factor (SF), use ice-water content, equilibrium level, flash height, and satellite zenith angle, all of which can be derived in models. These equations are developed to encourage updating lightning parameterizations within global models and will likely increase modeled lightning's effects on atmospheric electrical circuits, composition, chemistry, and climate change. Lightning plays an important role in the atmosphere's electrical circuit, chemical composition, and climate. Defining that role requires accurate global measurements of lightning, which are included in models to determine current and potential future effects. The global lightning measurements used in most models were collected by the orbiting Lightning Imaging Sensor (LIS). However, models plan to update to lightning measurements collected by a newer lightning detector, the Geostationary Lightning Mapper (GLM). To test the accuracy of these two space-based lightning detectors, they are compared to ground-based Lightning Mapping Arrays (LMAs) that measure lightning's radio static. The LMAs detected 7.0 times more lightning than the GLM and an estimated 5.6 times more lightning than the LIS on average. For both comparisons, radar reflectivity at a high elevation, flash height, and thunderstorm polarity are contributors to these undercounts. For the GLM, satellite zenith angle at the lightning's location is also a factor. Satellite measurements must be corrected for these lightning undercounts before they are used in global models for atmospheric electricity, composition, and climate to better understand lightning's current and potential future impacts on these topics. The Geostationary Lightning Mapper (GLM) and Lightning Imaging Sensor (LIS) undercount lightning by average factors of 7.0 and 5.6Lightning undercounting is a function of satellite zenith angle, radar reflectivity, flash height, and thunderstorm polarityPredictive equations of lightning undercounts are defined to encourage updating lightning parameterizations within global models
