In space, things happen very slowly and very quickly. Slow events are timed with technology that we've had for centuries - calendars and clocks. But timing fast events - which happen on the scale of billionths of a second - is more difficult.
That was a problem that Nasa engineers faced when they designed the Ice, Cloud and land Elevation Satellite-2, known as ICESat-2 for short. It's a spacecraft designed to measure the height of different objects - sea ice, glaciers, ice sheets, forests, clouds and more, on the Earth's surface.
Lasers are fired
To obtain accurate measurements, it uses six green laser beams in an instrument called the Advanced Topographic Laser Altimeter System (ATLAS). Those lasers are fired at the surface of the Earth, bounce off, and return - and the time it takes to return changes depending on the height of that surface.
But that came with a problem.
"Light moves really, really fast, and if you’re going to use it to measure something to a couple of centimeters, you’d better have a really, really good clock," Tom Neumann, ICESat-2’s deputy project scientist.
So they built a really, really good clock - capable of an accuracy of a fraction of a billionth of a second. It's triggered by redirecting a few of the photons of the laser into a pulse detector, which then tells the satellite to log exactly where it is in space.
With that information, it then calculates a rough estimate of how long it should take for the laser pulse to return, using already-existing information about the height of the terrain below. Over the Himalayas, for example, the laser pulse should return faster than over the Netherlands.
When the pulse does return, it's passed through a filter that removes all photons except the ones exactly matching the green of the laser.
They're then counted, and the time it took them to return is measured using an - a tiny crystal that 'ticks' 100 million times a second. Those ticks are then subdivided further with electronics on the photon counters - resulting in an accuracy level of hundreds of picoseconds.
Repeating this process hundreds of times a second, and measuring the same piece of land multiple times, you're able to get a highly-accurate reading of exactly how high the terrain is below.
"If you know where the spacecraft is, and you know the time of flight so you know the distance to the ground, now you have the elevation of the ice," said Phil Luers, deputy instrument system engineer with the ATLAS instrument.
ICESat-2, which will be carrying ATLAS, is due to launch in 2018.