January 29, 2014
NASA’s New Laser Technology Tracks Arctic Ice With Precision
April Flowers for redOrbit.com - Your Universe Online
Flying over Greenland and the Arctic Ocean in April 2012, a high-altitude aircraft completed the first polar test of a new laser-based technology designed to measure the height of Earth from space. That instrument was the Multiple Altimeter Beam Experiment Lidar (MABEL), an airborne test bed instrument designed for NASA's ICESat-2 mission, which is slated for launch in 2017.
MABEL has returned new results demonstrating that a photon-counting technique will give researchers the ability to track the melt or growth of Earth's frozen regions with more precision than ever before.
MABEL and ICESat-2's ATLAS instrument are both photo counters. Both instruments send out pulses of green laser light, then time how long it takes individual light photons to bounce from the planet's surface and return. Computer programs combine this time with ATLAS's exact position from an on-board GPS to tell researchers the elevation of the planet's surface. They are able to measure change to as little as the width of a pencil with this method.
This type of photon-counting technology is new for satellites. From 2003 to 2009, ICESat-1's instrument examined the intensity of a returned laser signal. This intensity included many photons, so the individual photon data from MABEL provides new insights for researchers, preparing them for the vast amounts of elevation data they will receive from ICESat-2.
"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 mission of ICESat-2 is to measure elevation across Earth's entire surface, including vegetation and oceans. The mission is supposed to focus especially on changes in the frozen regions of the planet, where scientists have observed dramatic impacts from climate change. Ice sheets and sea ice, the two types of ice in these regions, reflect light photons in different patterns. Found on land in places such as Greenland and Antarctica, ice sheets and glaciers are formed as snow and rain accumulates. On the other hand, sea ice forms from frozen seawater floating in the Arctic Ocean and off the shore of Antarctica.
Bill Cook, MABEL's lead scientist at NASA's Goddard Space Flight Center, said that MABEL's 2012 Greenland campaign was designed to observe a range of interesting icy features. The photon counts from varying surfaces allowed other scientists to start analyzing the data to determine which methods of analysis would allow them to best measure the Earth's elevation.
"We wanted to get a wide variety of target types, so that the science team would have a lot of data to develop algorithms," Cook said. "This was our first real dedicated science mission."
For example, the flights over the ocean near Greenland demonstrated that the researchers can measure the height difference between open water and sea ice. This is key to determining ice thickness. Cook said that MABEL can detect enough of the laser light photons. This data is plugged into programs that make necessary elevation calculations.
"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," Cook said. "You can do this photon counting if you have enough photons."
Kwok's team demonstrated how to calculate elevation from MABEL data in a recent article published in the Journal of Atmospheric and Oceanic Technology. Their results included how to calculate the elevation over different types of ice—from open water, to thin, glassy ice, to snow-covered ice.
"We were pretty happy with the precision," Kwok said. "The flat areas are flat to centimeter level, and the rough areas are rough."
The researchers were also able to tell the type of ice the instrument was flying over by the density of the photons detected.
When monitoring ice sheets and glaciers, the contours of the icy surface are also important. ICESat-1's original mission used a single laser, making it more difficult to measure the loss or gain of elevation. With just a single laser, the scientists were unable to tell if the snowpack had melted or if the laser was slightly off, making it necessary for the satellite to make 10 passes over the same area to determine if the ice sheet was changing.
"ICESat-1 was fantastic, but it was a single beam instrument," said Kelly Brunt, a research scientist at NASA Goddard. "We're more interested in repeating tracks to monitor change – that's hard to do."
The laser on ICESat-2 is split into six beams to address this problem. The six beams are arranged into three pairs, and within each pair, the beams are spaced 295 feet apart. Scientists compare the height of one site to the height of its neighbor to determine the terrain's general slope.
MABEL data from the 2012 Greenland campaign was used by Brunt and her colleagues to detect slopes as shallow as four percent incline. In order to simulate the weaker laser beams aboard ICESat-2, the team only counted a portion of the photons to complete their findings, which will be published in an upcoming issue of Geoscience and Remote Sensing Letters. The researchers used computer programs to determine the slope, then verified their results against results of earlier missions.
"The precision is great," Brunt said. "We're very confident that with ICESat-2's beam pair, we can see slope."
There are more missions ahead for MABEL. A summer campaign is planned for 2014 to fly over glaciers and ice sheets in warmer weather. "We want to see what the effects of the melt is," Cook said. "How do glaciers look if they're warmer, rather than colder?"