A laser-based instrument being developed for the International Space Station will provide a unique 3-D view of Earth’s forests, helping to fill in missing information about their role in the carbon cycle.

对于森林碳循环研究，该仪器可以弥补大量信息不足。

Called the Global Ecosystem Dynamics Investigation (GEDI) lidar, the instrument will be the first to systematically probe the depths of the forests from space. The system is one of two instrument proposals recently selected for NASA’s Earth Venture Instrument program and is being led by the University of Maryland, College Park. The instrument will be built at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

仪器的名字叫做 GEDI，属于首次系统探测森林深度的仪器。

“As a global leader in research and discovery related to environmental sustainability, the University of Maryland is extraordinarily proud to be a part of this new venture with our partners from NASA,” said University of Maryland Vice President and Chief Research Officer Patrick O'Shea. “GEDI lidar will have a tremendous impact on our ability to monitor forest degradation, adding to the critical data needed to mitigate the effects of climate change.”

对森林退化监测有用，进而助力全球变化研究。

“GEDI will be a tremendous new resource for studying Earth’s vegetation,” said Piers Sellers, deputy director of Goddard’s Sciences and Exploration Directorate. “In particular, the GEDI data will provide us with global-scale insights into how much carbon is being stored in the forest biomass. This information will be particularly powerful when combined with the historical record of changes captured by the U.S.’s long-standing program of Earth-orbiting satellites, such as Landsat and MODIS.” The MODIS, or Moderate Resolution Imaging Spectroradiometer, is an instrument that flies aboard NASA’s Terra and Aqua satellites.

By revealing the 3-D architecture of forests in unprecedented detail, GEDI will provide crucial information about the impact that trees have on the amount of carbon in the atmosphere. Although it is well-established that trees absorb carbon and store it long-term, scientists have not quantified exactly how much carbon forests contain. As a result, it’s not possible to determine how much carbon would be released if a forest were destroyed, nor how well emissions could be countered by planting new trees.

对评估树木对大气中碳储量的影响至关重要。尽管树木吸收碳并长期存储已经广为人知，科学家却很难准确量化森林究竟固定了多少碳。自然也就很难判断森林毁坏或者种植究竟释放或者固定了多少碳。

"One of the most poorly quantified components of the carbon cycle is the net balance between forest disturbance and regrowth,” said Ralph Dubayah, the GEDI principal investigator at the University of Maryland. “GEDI will help scientists fill in this missing piece by revealing the vertical structure of the forest, which is information we really can’t get with sufficient accuracy any other way.”

GEDI项目主要领导者马里兰大学的Ralph Dubayah称，“碳循环中质量最差的量化数据就是森林再生数量与被破坏的森林数量之间的净差额，通过GEDI将会通过精确测绘森林的垂直结构，补充这方面的森林信息。”

GEDI can do this because it’s a laser-based system, called a lidar, that can measure the distance from the space-based instrument to Earth’s surface with enough accuracy to detect subtle variations, including the tops of trees, the ground, and the vertical distribution of aboveground biomass in forests. Its immediate predecessors are Goddard’s Ice, Cloud, and land Elevation Satellite (ICESat) and airborne Land, Vegetation and Ice Sensor, known as LVIS, which is flown on high-altitude aircraft to measure forests, land topography, ice sheets, glaciers and sea ice.

“Lidar has the unique ability to peer into the tree canopy to precisely measure the height and internal structure of the forest at the fine scale required to accurately estimate their carbon content,” said Bryan Blair, the deputy principal investigator for GEDI at Goddard.

GEDI will carry a trio of specialized lasers, developed in-house at Goddard, and will use sophisticated optics to divide the three beams out into 14 tracks on the ground. Together, these tracks will be spaced 1,640 feet (500 meters) apart on the surface creating a total swath width of about 4 miles (6.5 kilometers). GEDI will sample all of the land between 50 degrees north latitude and 50 degrees south latitude this way, covering nearly all tropical and temperate forests.

GEDI携带三个独立的激光发射器，通过复杂的光学设备，将三束光转换到地面上的14个轨道。这些轨道间隔500米，扫描幅宽6.4公里。扫描范围为地球纬度50度以内，包括几乎所有的热带雨林和温带森林。

The lasers will illuminate the surface with brief pulses of light that are optimized to pass through the canopy of even very dense forests without causing harm. (The lasers are eye-safe.) The team estimates that the instrument will send out 16 billion pulses in one year.

仍然采用脉冲技术，可以穿透密集森林。每年GEDI系统将发出160亿束脉冲。

A small fraction of each pulse – the return pulse – is reflected back to a detector on the orbiting instrument. The amount of time it takes to complete this round trip is measured precisely and converted into a distance. In addition, the materials that a pulse encounters along the way will modify the signal slightly, resulting in a different fingerprint or vertical profile when a pulse interacts with leafy tree tops versus woody branches and trunks or the ground.

发出的激光脉冲至地球表面，一旦这些脉冲抵达地球，脉冲将反馈至GEDI系统。测量出距离，

These fingerprints will provide enough detail to measure the height of the trees and where the tree canopy begins with an accuracy of about 3-1/3 feet (1 meter). From this information, scientists will be able to estimate how much biomass the trees contain and, in turn, how much carbon they are storing.

树冠的高度精度大概1米，可以用于估算生物量，并转化为碳储量。

By combining these findings with spatially comprehensive maps from other satellites showing where development and deforestation are taking place, or with studies that reveal the composition of forests, scientists will have a more powerful tool set for addressing questions about land use, habitat diversity and climate effects. For example, researchers will be able to relate forest architecture with habitat quality and the biodiversity of certain birds. They also may be able to estimate the age of trees in specific forests. The ultimate goal, Dubayah said, is to be able to monitor these and other changes in forests over time.

GEDI is scheduled to be completed in 2018. NASA’s Earth Venture Instrument program is part of the Earth System Science Pathfinder program, managed by NASA’s Langley Research Center in Hampton, Virginia, for NASA’s Science Mission Directorate. The GEDI team includes co-investigators from Goddard; Woods Hole Research Center, Woods Hole, Massachusetts; the U.S. Forest Service, Ogden, Utah; and Brown University, Providence, Rhode Island.