Author: CSDN @ _Yakult_
This article will introduce the Global Ecosystem Dynamics Survey (GEDI) project background, project tasks, and instrumentation overview.
The Global Ecosystem Dynamics Survey (GEDI) produces high-resolution laser ranging observations of the Earth's three-dimensional structure. GEDI provides precise measurements of forest canopy height, canopy vertical structure, and land surface elevation, greatly improving our ability to characterize important carbon cycle, water cycle processes, biodiversity, and habitats.
GEDI's surface structure data is also of great value for weather forecasting, forest management, glacier and snow cover monitoring, and generating more accurate digital elevation models. GEDI provides a critical piece missing from NASA's observational resources -- three-dimensional structure, which allows us to better understand how Earth behaves as a system and guides the actions we take to maintain critical resources.
The GEDI instrument is a geodetic-grade light detection and ranging (lidar) system consisting of three lasers that generate eight parallel observation tracks. Each laser emits 242 pulses per second and illuminates a 25-meter-sized spot (or footprint) on the surface, through which three-dimensional structure measurements are made. Each footprint is spaced 60 meters apart in the direction of the track, and the lateral distance between each track is about 600 meters. During its scheduled 24-month mission, GEDI is expected to generate about 10 billion observations unaffected by clouds.
The schematic diagram of the GEDI ground sampling mode is as follows,
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1. GEDI specification
GEDI has been optimized to provide measurements of vegetation structure on a global scale. The laser footprint is large enough to measure an entire tree, yet small enough to accurately probe the surface on steep terrain. GEDI measurements are able to penetrate the dense canopy and provide comprehensive sampling.
GEDI contains three Nd:YAG lasers emitting light at 1064 nm. With 242 pulses per second and a power of 10 millijoules, it emits short pulses of 14 nanoseconds long to the surface of the earth, with a beam divergence angle of 56 milliradians, and the average diameter of the footprints formed is 25 meters.
Two of the lasers are at full power and the other is split into two beams, producing a total of four laser beams. Beam swinging units (BDUs) are capable of rapidly changing the deflection angle of the outgoing laser beam by 1.5 milliradians each, allowing it to translate 600 meters above the ground. This produces eight ground traces, four of which are intensity traces and the other four are coverage traces. The footprints are spaced 60 meters apart along the track and 600 meters apart across the track.
GEDI's measurement range covers the Earth's surface between 51.6° north latitude and 51.6° south latitude. On board the Japanese Experimental Aircraft (JEM), GEDI can be rotated by up to 6°, allowing the laser to point to a range of 40 kilometers on either side of the ISS ground track. This capability is used to sample Earth's land surface as completely as possible, filling in the gaps caused by clouds. During GEDI's two-year mission lifetime, it will acquire approximately 10 billion observations of the Earth's surface unaffected by cloud cover. These observations can be segmented into regular coverage areas with different resolutions, such as 1 km grid cells.
An example plot of GEDI track coverage at the equator is shown below,
2. GEDI mission overview
- GEDI produced the first high-resolution laser ranging observations of the Earth's three-dimensional structure.
- GEDI makes precise measurements of forest canopy height, canopy vertical structure, and land surface elevation.
- GEDI has greatly improved our ability to characterize the carbon cycle, water cycle processes, biodiversity and habitats.
- GEDI was deployed to the International Space Station (ISS) in 2018 for a two-year mission that has now been extended until January 2023.
The Global Ecosystem Dynamics Survey (GEDI) produces high-resolution laser ranging observations of the Earth's three-dimensional structure. GEDI provides precise measurements of forest canopy height, canopy vertical structure, and land surface elevation, greatly improving our ability to characterize important carbon cycle, water cycle processes, biodiversity, and habitats.
GEDI's surface structure data is also of great value for weather forecasting, forest management, glacier and snow cover monitoring, and generating more accurate digital elevation models. GEDI provides a critical piece missing from NASA's observational resources -- three-dimensional structure, which allows us to better understand how Earth behaves as a system and guides the actions we take to maintain critical resources.
The GEDI instrument is a geodetic-grade light detection and ranging (lidar) system consisting of three lasers that generate eight parallel observation tracks. Each laser emits 242 pulses per second and illuminates a 25-meter-sized spot (or footprint) on the surface, through which three-dimensional structure measurements are made. Each footprint is spaced 60 meters apart in the direction of the track, and the lateral distance between each track is about 600 meters. During its scheduled 24-month mission, GEDI is expected to generate about 10 billion observations unaffected by clouds.
GEDI laser
GEDI laser fires laser light and returns brightness
Waveform data acquired by GEDI laser
The laser echo energy along the track shows the vertical distribution of vegetation as follows,
GEDI responds directly to observational priorities set by the National Academies of Sciences and NASA's Science Mission Directorate, highlighting the need for LiDAR vertical structure measurements to address key challenges in the carbon cycle and biodiversity. Information from GEDI's advanced lasers further advances observations by NASA's next-generation missions, including NASA ISRO Synthetic Aperture Radar (NISAR) and Ice Cloud Elevation Satellite-2 (ICESat-2). In addition, GEDI has a formal collaboration with the German Aerospace Center (DLR) to combine its lidar data with the DLR TanDEM-X SAR interferometry mission to generate full-coverage maps of canopy height and other structural indicators.
GEDI was competitively selected in 2014 for NASA's Earth Exploration Instrument (EVI) program. The project, limited in cost to $94 million, is being led by the University of Maryland in partnership with NASA's Goddard Space Flight Center. GEDI's scientific data algorithms and products are created by the GEDI Scientific Definition team.
3. Overview of GEDI Instruments
GEDI has the highest resolution and most densely sampled lidar ever deployed in orbit. This required a series of innovative technologies developed at NASA Goddard Space Flight Center.
GEDI is a full-beam lidar instrument that makes detailed measurements of the three-dimensional structure of the Earth's surface. Lidar is an active remote sensing technology (similar to the laser version of radar) that uses pulses of laser light to measure structure in three dimensions. After the laser beam illuminates the ground, vegetation and clouds, it is collected by GEDI's telescope. These photons are then directed to a detector, which converts the brightness of the light into a voltage signal and records it as a function of time interval (1 nanosecond or 15 centimeters). Convert time to distance (a distance) by multiplying by the speed of light. The voltage recorded as a function of distance is the full beam.
GEDI instrument showing schematic diagram of laser, optical path, detector and digitizing equipment,
GEDI's 80 cm telescope was used to collect the received light, as shown below,
GEDI is deployed on the Japan Experimental Extravehicular Facility (JEM-EF). Highlighted boxes indicate where GEDI is located on JEM-EF.
GEDI is installed on the International Space Station (top) and the Japanese Experimental Extravehicular Facility (bottom), highlighted in gold. The GPS system provides information on the GEDI instrument's position in orbit relative to the Earth's surface, while the star sensor provides the instrument's orientation information. This allows the position of the GEDI waveform on the Earth's surface to be calculated with horizontal accuracy to within plus or minus 9 meters. GEDI minimizes its non-orthogonal pointing to less than about 6° to avoid inaccurate distance measurements.
GEDI's only observables are waveforms, from which all other data products are derived. Signal processing is used to identify ground in the waveform. The distribution of laser energy over the waveform can be used to determine the height and density of objects within the footprint. Lidar's viewing angle and active use of light allow the ground to be identified through small gaps in the tree canopy, enabling non-saturated measurements of denser forests than passive optics (such as space cameras) or shortwave radar systems. Furthermore, in contrast to other satellite remote sensing, height and vertical distributions are measured directly and can be compared with in situ observations.
4. Product overview
GEDI's data products are listed in the table below.
GEDI's Tier 1 data products include two separate products, Tier 1A (L1A) and Tier 1B (L1B) products.
- GEDI L1A data products contain basic instrument engineering and device data, as well as raw waveform and geolocation information, used to calculate higher-level data products.
- GEDI L1B geolocation waveform data products, similar to L1A data products, contain specific data to support calculations for higher level 2A and 2B data products. The L1B data includes corrected received waveforms as well as geolocation information for the received waveforms. L1B data products provide end users with background information on higher level L2 products and enable end users to apply their own waveform interpretation algorithms.
- The L2 product contains information from the GEDI echo waveform, including elevation, height and structure measurements of the ground, and other waveform measurements describing the imaged Q-plane.
In particular, it is important to note that the final product of many US satellites will be presented in the h5 file format, which has become the standard format for US satellites.
