In this particular historical period, it is necessary to pay attention to the consequences of man's work on the entire planet Earth. Biologists, meteorologists, geologists, all agree in recognizing that man, with his uncontrolled activity and little respect for other terrestrial ecosystems, has managed to unbalance the slight balance that has governed our planet for millions of years.
There NASA's GEDI mission deals with monitoring mutations in the various natural habitats: deforestation, changes in the water and carbon cycle, changes in biodiversity and much more. All this, of course, from Space.
GEDI philosophy
We said the GEDI, acronym for Global Ecosystem Dynamics Investigation (and which is pronounced just like the Jedi of the universe of Star Wars), allows you to monitor a wide variety of natural phenomena. How was the project born? GEDI has been selected by the NASA in July 2014 under the Earth Venture Instrument-2 program. The development of the system and instruments began in November 2014. The December 5, 2018 and subsequently installed on the International Space Station.
It successfully passed the commissioning period on March 25, 2019 and will remain on the ISS for two years, constantly collecting data, which we will discuss shortly. These data are processed by the scientific team behind the GEDI project and the first results were made public just a few days ago.
There mapping of the structure of the ecosystem is important to understand many factors that characterize the terrestrial system, including the carbon cycle, the quality and biodiversity of the habitat, the state of health and productivity of the forests, the hydrological cycle and the effects negative caused by man on nature. But in what way?
GEDI makes high resolution observations of the Earth, analyzing its conformation and elaborating a 3D model of the surface. Measurements made by GEDI they concentrate, for example, on the height of the forests and on the extension in height of a large crown forest.
Using these data, it is possible to define with high precision mechanisms such as that of the water and carbon cycle, or deforestation (in the image that follows the operations of deforestation annual from 2000 to 2016, mapped through Landsat. The red colors indicate deforestations close to 2016).
GEDI data relating to the surface structure are also very useful for forecasting weather, for forest management, monitoring of glaciers and snow cover. This is because the work of GEDI, i.e. the creation of a 3D model of the earth's surface, is accompanied by data obtained from other missions currently active in the NASA, and from this collaboration it is possible to understand more clearly how the Earth behaves regarding man's activity, and what actions man can take to stem this great natural imbalance.
Data on the vertical structure of forests are a key element for biomass and biodiversity studies. With a global map of the mass contained in the trunks, branches and leaves of forest trees, researchers can estimate the amount of carbon contained in the different forest areas. By combining GEDI data with other satellite data, it is possible to estimate how forest biomass has changed in the past and how it could change in the future: the atmospheric concentration of carbon dioxide is the main protagonist of climate change and the growing vegetation absorbs CO2 which then it is put back into the atmosphere in less quantity. Considering that half of the plant biomass is carbon, the quantification of the plant biomass will allow scientists to estimate the amount of total carbon stored by the vegetation.
How do you do it?
The main tool of GEDI is a laser system installed on the International Space Station. It is made up of 3 lasers that produce 8 tracks sampling parallels, which we can imagine as 8 lanes of a highway. Each laser turns on 242 times per second and illuminates a portion 25 meters wide (called footprint) on the surface you want to rebuild in 3D. Once the scan of the first impression is complete, the lasers move to the next: each impression is about 60 meters from the next, while each track is about 600 meters from the other. An example of what GEDI collects? Here:
This graph shows the height of a portion of vegetation: on the horizontal axis there is the entire path followed by the lasers (we could say that 0m is the beginning of the measurement and 2500m is the end of the path) and on the vertical axis it is present the height recorded by the laser. The following image, on the other hand, shows a generic track followed by GEDI: it is a track about 4 kilometers long, performed on a portion of Amazon rainforest.
When using a LIDAR
The laser system introduced earlier is actually a LIDAR. What is it about? Lidar means a remote sensing technology (like a sort of laser version of the radar) that uses pulses of laser light to measure a certain three-dimensional object, in this case a portion of the Earth. The light sent by impulses comes reflected from the soil and vegetation that he encounters during his journey, and so he returns to space until he reaches the instrumentation of GEDI.
These newly collected photons are then directed towards optical instruments which convert the brightness of the light into one electric voltage, thus managing to measure the arrival time of each photon.
The arrival time is converted into distance by multiplying everything by the light speed (being Velocity =Metri / Secondi, to get the meters traveled it is necessary to calculate Velocity X Seconds = Meter). Here is the tool that collects the reflected light:
A integrated GPS system in the instrumentation it allows NASA scientists to know where GEDI is in its orbit, and its position relative to the surface of the Earth, while the trackers aligned with the fixed stars allow to keep the instrument well oriented. GEDI's LIDAR uses light, energy-efficient lasers and new generation optics that divide and scan lasers into multiple beams, increasing sampling without adding weight to the structure.
The instrument can also rotate on itself, allowing it to ensure greater coverage of the earth's surface to be scanned. This ability is used to sample the Earth's surface as completely as possible, filling in the blanks due to clouds. The observations can then be grouped into regular covers of different resolution, such as 1 km grid cells. Here is an image to better understand how the coverage is carried out:
When GEDI "sees" a forest, he actually sees a collection of waveforms that show the tops of the trees, the ground, the branches, the leaves and the empty space in the middle. Overall, waveform collections begin to show the structure of the forest, not only vertically, but also horizontally. This allows scientists to calculate the amount of biomass and carbon contained in a certain area of the forest.
The image above shows the waveform that reaches the GEDI tool (left). The light brown area underlying the curve represents the energy returning from crown of trees, while the dark brown area indicates the energy from the light reflected from the ground.
In short, the whole vertical axis includes both the effects due to the presence of the soil and the effects due to the presence of the crowns of the trees. The diagram on the right shows the distribution of the trees that produced the waveform on the left, with the "track" of 25m we told you about earlier.
The first published data
The images you have just seen come from the first data published by NASA. These data cover the first eight weeks of GEDI: hundreds of millions of "footprints", which analyze the globe between the south of Canada and the tip South America. At the end of the second year of activity, GEDI will have collected approximately 10 billion laser observations, or the most complete satellite data set on the forest structure ever produced in the human history.
"GEDI collects 6 million laser observations every day. So, beyond the tropics, we have already collected about two orders of magnitude more data than what was the" state of the art "before", he has declared Ralph Dubayah, principal curator of the GEDI project and professor of geographic sciences at the University of Maryland.
From the first data processed, it can be seen that while the forests in the Pacific Northwest are as high and dense as those in the Amazon, taking a closer look at the GEDI data, structural variations between the two forests can be observed, with ad example much taller trees in the Pacific Northwest. GEDI can "see" in the forest better than any other tool, providing researchers with clues to how the conformations of terrestrial forests differ.
"The amount of carbon that is retained in the earth's surface by trees, and how it has changed over time due to natural imbalances, is the least understood aspect of the global carbon cycle"Dubayah said. "Until we know the amount of carbon currently present in the terrestrial forests and how this amount has changed in the past 20-30 years, we will have difficulties to predict how much carbon forests will continue to absorb in the future and what role we will play in helping to mitigate or accelerate atmospheric concentrations of carbon dioxide. "
Beyond the worries, many words of praise were spent on the mission success; Dubayah himself congratulates: "For me, this is an extraordinary achievement and reflects the excellence of our engineering partners from NASA Goddard and the entire GEDI scientific team".
The researchers, he added, will also be able to make further significant progress in the accuracy of their maps and in modeling biodiversity worldwide and over the years. In fact, GEDI collaborates with public and private organizations who are dedicated to the research and protection of wildlife all over the world, therefore it can count on a not indifferent workforce.
"Having data on vertical structures everywhere, it will be possible to identify relationships between a given structure and diversity, the abundance of species and the quality of the habitat"Dubayah said. The team will continue to calibrate the LIDAR system to get more and more accurate data and make the data more accurate GEDI algorithms, in many cases with the help of researchers around the world.
The real advantage of GEDI technology is a matter of timing: the ability to look at the forest structure in a global way allows you to see a wide variety of conditions of the forest mantle, and this is well established. But the real difference with the past is that now you can observe areas never mapped before and in extremely short times. There is even talk of mappings in 90 minutes.
As Michelle Hofton, co-investigator of the GEDI project and research professor at the University of Maryland said. Hofton explains that they often make detections "in places where we have never been able to go with aerial instruments. Now we can see different parts of these areas every 90 minutes."
GEDI responds directly to the observation priorities established by the National Academy of Sciences and NASA's Directorate of Scientific Missions, which emphasize the need for measurements of vertical structures through LIDAR to understand the key factors of the carbon cycle and answer questions inherent in biodiversity .
Information from GEDI lasers will be the focus of observations for the next generation of NASA missions, including the NASA ISRO Synthetic Aperture Radar (NISAR) and Ice Cloud and Elevation Satellite-2 (ICESat-2, we talked about some time ago) , among many. In addition, GEDI collaborates with the German aerospace center (DLR) to exchange your data with the mission team TanDEM-X SAR (pictured above), with whom it shares the same goal.
Another important objective of the GEDI mission is linked to a far from secondary issue: providing space for the life of other organisms. With the information on the structure of vegetation provided by GEDI, in the future it will be possible to characterize the quality of the habitat for a large number of different organisms. In this way, GEDI not only answers the questions on the carbon present in the vegetation, but contributes to the conservation of biodiversity by improving knowledge on the quality of a given habitat.
These are the results of investments and efforts born with the aim of improving man's understanding of the environment in which he lives. Can this promising project help to find a solution to huge damage that man has procured for the whole of Nature? It is not for us to say it, but one thing can be said: the effort must be collective, avoiding to behave and speak lightly and without knowledge of the facts, because the problem we are trying to stem concerns all of us, nobody excluded.
