Nasa's ICESat-2 to have on board a stopwatch that can measure a fraction of a billionth of a second

Nasa engineers have built a stopwatch that can measure accurately fraction of a billionth of a second. The timer is to be used for the Ice, Cloud and land Elevation Satellite-2 (ICESat-2) scheduled for launch in 2018, Nasa said. ICESat-2 will use six green laser beams to measure height. With its incredibly precise time measurements, scientists can calculate the distance between the satellite and the Earth below, and from there record precise height measurements of sea ice, glaciers, ice sheets, forests and the rest of the planet’s surfaces. “Light moves really, really fast, and if you’re going to use it to measure something to a couple of centimetres, you’d better have a really, really good clock,” said Tom Neumann, ICESat-2’s Deputy Project Scientist. If its stopwatch kept time even to a highly accurate millionth of a second, ICESat-2 could only measure elevation to within about 500 feet. Scientists would not be able to tell the top of a five-storey building from the bottom. That does not cut it when the goal is to record even subtle changes as ice sheets melt or sea ice thins. To reach the needed precision of a fraction of a billionth of a second, engineers at Nasa’s Goddard Space Flight Center in Greenbelt, Maryland, had to develop and build their own series of clocks on the satellite’s instrument – the Advanced Topographic Laser Altimeter System, or ATLAS. This timing accuracy will allow researchers to measure heights to within about two inches, Nasa said in a statement on Monday.

ATLAS pulses beams of laser light to the ground and then records how long it takes each photon to return. This time, when combined with the speed of light, tells researchers how far the laser light travelled. This flight distance, combined with the knowledge of exactly where the satellite is in space, tells researchers the height of the Earth’s surface below. The stopwatch that measures flight time starts with each pulse of ATLAS’s laser. As billions of photons stream down to Earth, a few are directed to a start pulse detector that triggers the timer, said Phil Luers, Deputy Instrument System Engineer with the ATLAS instrument.