A laser-based photon-counting technique will enable the measurement of Earth’s height from space, allowing researchers to track the melt and growth of its frozen regions. NASA conducted testing using the MABEL (Multiple Altimeter Beam Experimental Lidar) instrument on board aircraft over the Arctic Ocean and snowy terrain of Greenland in 2012. Greenland and the Arctic Ocean, captured with NASA's MABEL instrument. Images courtesy of NASA. Both MABEL, an airborne test bed device for NASA’s ICESat-2 satellite, and ICESat-2’s ATLAS instrument are photon counters, sending out pulses of green laser light while timing how long it takes individual photons to bounce off Earth’s surface and return. This timing, along with ATLAS’ exact position from an onboard GPS, is used to determine the elevation of Earth’s surface, measuring changes to as little as the width of a pencil. “Using the individual photons to measure surface elevation is a really new thing,” said Ron Kwok, a senior research scientist at NASA’s Jet Propulsion Laboratory. “It’s never been done from orbiting satellites, and it hasn’t really been done much with airborne instruments either.” The original ICESat-1 employed a single laser, which made it more difficult to measure the elevation of an ice sheet. With a single beam, when the instrument flew over a spot a second time, researchers couldn’t tell if the snowpack had melted or if the laser was slightly off and pointed down a hill, said Kelly Brunt, a NASA research scientist. The ICESat-2 addresses this problem by splitting the laser into six beams. The flights over the ocean near Greenland allowed researchers to demonstrate that they can measure the height difference between open water and sea ice, which is key to determining ice thickness. MABEL (Multiple Altimeter Beam Experimental Lidar) instrument, an airborne test bed device for NASA's new ICESat-2 satellite. “Part of what we’re doing with MABEL is to demonstrate ICESat-2’s instrument is going to have the right sensitivity to do the measurements,” said Bill Cook, MABEL’s lead scientist. ICESat-2 will launch in 2017 to measure elevation across Earth’s entire surface, including vegetation and oceans. Its focus will be on change in the frozen areas of the planet, including ice sheets and sea ice, where scientists have observed dramatic impacts from climate change. This summer, NASA’s MABEL team will again fly over glaciers and ice sheets, this time to observe changes in warmer weather. For more information, visit www.nasa.gov.