In order to measure the time taken by a laser light pulse to travel from space to Earth and back, an exceptionally good stopwatch with the ability to measure within a fraction of a billionth of a second is required.
Engineers at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, have built a similar timer for the Ice, Cloud and land Elevation Satellite-2 (ICESat-2). Scheduled to be launched in 2018, ICESat-2 will use six green laser beams to measure height.
Researchers can use its unbelievably accurate time measurements to compute the distance between the satellite and the Earth below, and from there, they can record accurate height measurements of ice sheets, sea ice, forests, glaciers, and the remaining surfaces of the planet.
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, Deputy Project Scientist of ICESat-2
If the ICESat-2’s stopwatch kept time even to an exceptionally precise millionth of a second, it can only measure elevation within about 500 feet. Researchers would not be able to measure the top of a five-story building from the bottom, which is not adequate when the aim is to record even minute alterations such as melting of ice sheets or sea ice thins.
In order to achieve the required accuracy of a fraction of a billionth of a second, Goddard engineers had to create and construct their own series of clocks on the instrument of the satellite, the Advanced Topographic Laser Altimeter System (ATLAS). The timing precision will ensure that the engineers can measure heights within about 2″.
“Calculating the elevation of the ice is all about time of flight,” stated Phil Luers, deputy instrument system engineer of the ATLAS instrument. ATLAS sends beams of laser light pulses to the ground and subsequently records the time taken by each photon to return.
The researchers can calculate the distance traveled by the laser light by combining the time taken with the speed of light. The height of Earth’s surface below can be calculated by the researchers by combining this flight distance with the accurate knowledge of the location of the satellite in space.
The stopwatch measuring the flight time begins with each laser pulse from the ATLAS. According to Luers, when billions of photons are streamed down toward Earth, a few are directed toward a start pulse detector triggering the timer.
Simultaneously, the satellite records its location in space as well as the object that it orbits over. The ATLAS uses this information to approximately calculate the time taken for the photons to return to the satellite. The time taken by photons over Mount Everest to return is less than the time taken by those over the Death Valley because of the reduction in distance.
The photons that return to the instrument by means of the telescope receiver system pass through filters that block anything that is not equivalent to the accurate shade of the green pulse from the laser, specifically sunlight. The green pulses reach a photon-counting electronics card, stopping the timer.
A major portion of the photons stopping the timer will be reflected sunlight that is the same green. However, when the laser is fired 10,000 times per second, the “true” laser photon returns get coalesced and provide data on surface elevation to the researchers.
“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,” stated Luers.
The timing clock in itself comprises of various parts to maintain track of time in a better manner. It includes a GPS receiver (i.e. a coarse clock indicating the time for the satellite) ticking off every second. There is another clock in the ATLAS: an ultrastable oscillator counting off every 10 ns within the GPS-derived seconds.
Between each pulse from the GPS, you get 100 million ticks from the ultrastable oscillator. And it resets itself with the GPS every second.
Tom Neumann, Deputy Project Scientist of ICESat-2
However, 10 ns are not adequate. In order to achieve more accurate timing, researchers have fixed a fine-scale clock within each photon-counting electronic card. The fine-scale clock further subdivides the 10-ns ticks, enabling the return time to be measured in hundreds of picoseconds.
Specific alterations to this travel time ought to be performed on the ground. Computer programs can not only merge various photon travel-times to enhance the accuracy but can also compensate for the time taken to move through the wires and fibers of the ATLAS instrument, the effects of changes in temperature on electronics, and so on.
“We correct for all of those things to get to the best time of flight we possibly can calculate,” stated Neumann, enabling scientists to have a detailed view of the third dimension of Earth.
Deputy Systems Engineer Phil Luers explains how ICESat-2’s ATLAS instrument transmitter and receiver subsystems come together to calculate the timing of photons, which, in turn, measure the elevation of ice. Credits: NASA's Goddard Space Flight Center/Ryan Fitzgibbons