Earth in the Third Dimension: First GEDI Data Available

From red angelim trees in the Amazon towering hundreds of feet above the ground to clusters of shrubs hugging the surface, terrestrial biomes develop in height and density as well as in length and width. Data depicting this three-dimensional structure, however, are limited. This gap is being filled with several recently-launched Earth observing missions. The first data from one of these missions—NASA’s Global Ecosystem Dynamics Investigation (GEDI)—are now publicly available through NASA’s Land Processes Distributed Active Archive Center (LP DAAC).

Launched on December 5, 2018, and installed on the International Space Station’s Japanese Experiment Module-Exposed Facility (JEM-EF), GEDI is led by a science team at the University of Maryland in collaboration with NASA’s Goddard Space Flight Center in Greenbelt, Maryland. As noted on the GEDI mission website, data are initially transferred to the GEDI Mission Operations Center (MOC) and then processed through the Science Operations Center (SOC), both of which are located at Goddard. Its primary two-year mission is to produce high-resolution laser ranging observations of Earth in order to characterize the effects of climate change and land use on ecosystem structure and dynamics.

GEDI data will be archived and distributed through two discipline-specific NASA Earth Observing System Data and Information System (EOSDIS) Distributed Active Archive Centers (DAACs). Lower-level data (Level 1 and Level 2) are currently available through NASA’s LP DAAC. LP DAAC is a partnership between NASA and the USGS, and provides tools and services for discovering and analyzing EOSDIS data related to land cover and land use.

Higher-level data (Level 3 and Level 4) will be archived and distributed by NASA’s Oak Ridge National Laboratory DAAC (ORNL DAAC). ORNL DAAC is a partnership between NASA and the U.S. Department of Energy, and is responsible for EOSDIS data related to biogeochemical dynamics, ecological data, and environmental processes. Level 3 data are expected to be available in mid-2020, with Level 4 data available in early-2021.

Having lower-level GEDI data at LP DAAC and higher-level GEDI data at ORNL DAAC supports the data needs of different user communities. Researchers interested in land surface and vegetation studies who traditionally acquire data through the LP DAAC will have access to similar GEDI data products along with the tools to subset, reprocess, and reformat these data. Likewise, researchers involved in ecosystem and carbon cycle studies will be able to acquire these focused GEDI products at the ORNL DAAC and have access to specialized tools to use these data.

GEDI data can be searched for and discovered using the EOSDIS Earthdata Search application. In addition, both the LP DAAC and the ORNL DAAC have developed GEDI landing pages on their websites. Lower-level GEDI data also can be discovered using the LP DAAC Data Pool, which provides a direct way to access LP DAAC data product files that can be bulk downloaded using secure HTTPS. The ORNL DAAC will provide direct HTTPS download and will also use their Spatial Data Access Tool (SDAT) to provide subsetting, reformatting, and re-projecting of GEDI higher-level gridded data.

GEDI data are acquired using a light detection and ranging (lidar) laser system. Lidar is a remote sensing technique that uses laser beam pulses to measure the distance of objects from the laser. When paired with a global positioning system (GPS) receiver, lidar can be used to create extremely accurate 3D measurements of Earth. “GEDI’s lidar is particularly useful for measuring things like canopy height, ground elevation, and canopy profiles,” says Tom Maiersperger, the LP DAAC Project Scientist. “GEDI will be the king of vegetation lidar.”

The three lasers comprising the GEDI lidar system produce eight parallel observation tracks. Each laser fires 242 times each second and illuminates a 25-meter spot on the surface over which the surface’s 3D structure is measured. Each 25-meter spot is separated by 60 meters along track, with about 600 meters between each of the eight tracks. Approximately 10 billion observations will be produced over the two-year mission.

The sole GEDI observable is the waveform of the returned laser pulse, and all data products are derived from this measurement. Lidar waveforms quantify the vertical distribution of vegetation by recording the amount of laser energy reflected by plant material (stems, branches, and leaves) at different heights above the ground. Signal processing is used to identify the ground within the waveform, and the distribution of laser energy above the ground can be used to determine the height and density of objects within the 25-meter GEDI footprint. Four types of structure information can be extracted from GEDI waveforms: surface topography, canopy height metrics, canopy cover metrics, and vertical structure metrics.

GEDI data will contribute significantly to research studying the development of terrestrial biomes, and will help further refine our understanding of the carbon cycle and atmospheric concentrations of carbon dioxide (CO₂). Along with helping provide answers to how deforestation has contributed to atmospheric CO₂ concentrations, how much carbon forests will absorb in the future, and how habitat degradation will affect global biodiversity, GEDI data also will help identify and provide a better understanding of how physical disturbances affect ecosystems and carbon storage.

GEDI’s location on the space station means that the instrument does not collect data in a standard data pattern, such as polar-orbiting satellites like NASA’s Terra or Aqua that cross the same point on Earth at the same time every day. “Being on the space station, GEDI has a rather strange, precessing orbit that is not Sun-synchronous, covers about 53 degrees north and south latitude, and is angled across the equator,” explains Maiersperger. “This means it can’t cover the entire Earth and it completes about 12 to 16 orbits per day. If you flatten this orbit out, you end up with these four-kilometer-wide swaths that the GEDI lasers are firing across in these spaghetti-looking patterns.”

The GEDI instrument also can be pointed, which provides further coverage for the GEDI lidar. The ability to rotate the instrument up to six degrees allows the lasers to be pointed as much as 40 km on either side of the space station’s ground track. This feature enables GEDI to sample Earth’s surface as completely as possible given the space station’s orbital track.

Once the GEDI data product generation cadence is set, LP DAAC expects to provide approximately 16 terabytes (TB) of GEDI data per month. “From a data volume standpoint, GEDI is not a big deal for us,” says Cody Hendrix, a systems engineer on the LP DAAC GEDI data team. “Probably the bigger challenge is we’ll have 10 billion individual laser shots [over the two-year mission] that we’ll have to keep track of and try to distribute to users in a cogent way. Because of the space station’s acquisition and orbital characteristics, there’s a lot more education I think we’re going to have to do with the end users to help them understand the very non-standard orbit [of the space station].”

Now that the first GEDI data are available through NASA’s LP DAAC (and soon will be available through NASA’s ORNL DAAC), the global research community is about to embark on an exciting new era of ecosystem studies. “There are a lot of people in a wide range of fields who are just out of their minds excited about getting their hands on GEDI data and using these data,” says Maiersperger. “It will be a very rich dataset.”

Learn more about GEDI and GEDI data

GEDI mission website: https://gedi.umd.edu/

LP DAAC GEDI Landing Pages:

• Level 1B: https://lpdaac.usgs.gov/products/gedi01_bv001/
• Level 2A: https://lpdaac.usgs.gov/products/gedi02_av001/
• Level 2B: https://lpdaac.usgs.gov/products/gedi02_bv001/

ORNL DAAC GEDI Landing Page (Level 3 data expected in mid-2020; Level 4 data expected in early-2021): https://daac.ornl.gov/gedi