Active sensors for visible light (laser altimeters)
“Altimeter missions” are used to measure the height of the Earth’s surface, including the sea-ice surface. In the visible light range, the NASA satellites ICESat (Ice, Cloud and Land Elevation Satellite; in operation from 2003 to 2009) and ICESat-2 (in operation since 2018) are a frequently used space mission. The “Advanced Topographic Laser Altimeter System” (ATLAS) active sensor on board ICESat-2 emits a laser beam, records the time it takes to receive the reflected signal, and uses this data to calculate the distance to the target area. In this way, ATLAS delivers readings that can be used to determine height differences in sea ice, changes in the thickness of ice sheets, and the heights of surface topography and vegetation, or to create vertical profiles of clouds and aerosols. ATLAS stands out for its excellent precision, which allows it to measure height down to the nearest 4 mm. Its laser has a wavelength of 532 nm – which equates to a vivid green in the optical range.