Timing a space laser with a NASA-style stopwatch

To time how long it takes a pulse of laser light to travel from space to Earth and back, you need a really good stopwatch — one that can measure within a fraction of a billionth of a second. That kind of timer is exactly what engineers have built at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, for the Ice, Cloud and land Elevation Satellite-2.

(via WSJ)