Simple scanning telescopes that measure how much sunlight is reflected by Earth and how much heat the planet emits back to space, each in very wide spectral bands from visible light out into the thermal infrared. By measuring one band that only sees reflected sunlight and another that sees both sunlight and heat, they can subtract the two to work out just the heat component, then use models of how clouds and surfaces reflect and emit in different directions (helped by cloud images from instruments like MODIS, VIIRS or SEVIRI). Those raw measurements are turned into maps of energy flow at the top of the atmosphere. Spatial resolution ranges between 20–50 km.
