https://www.nasa.gov/feature/goddard/icesat-2-laser-fires-for-1st-time-measures-antarctic-height

Please ignore the asinine video depicting ‘How LIDAR works’…

The laser instrument that launched into orbit last month aboard NASA’s Ice, Cloud and land Elevation Satellite-2 (ICESat-2) fired for the first time Sept. 30. With each of its 10,000 pulses per second, the instrument is sending 300 trillion green photons of light to the ground and measuring the travel time of the few that return: the method behind ICESat-2’s mission to monitor Earth’s changing ice. By the morning of Oct. 3, the satellite returned its first height measurements across the Antarctic ice sheet.

“We were all waiting with bated breath for the lasers to turn on and to see those first photons return,” said Donya Douglas-Bradshaw, the project manager for ICESat-2’s sole instrument, called the Advanced Topographic Laser Altimeter System, or ATLAS. “Seeing everything work together in concert is incredibly exciting. There are a lot of moving parts and this is the demonstration that it’s all working together.”

ICESat-2 launched on Sept. 15 to precisely measure heights and how they change over time. It does this by timing how long it takes individual photons to leave the satellite, reflect off the surface, and return to receiver telescope on the satellite. The ATLAS instrument can time photons with a precision of less than a billionth of a second, which allows the mission to detect small changes in the planet’s ice sheets, glaciers and sea ice.