GEDI is a Laser Imaging, Detection and Ranging system (LIDAR). As with most ranging systems, it works by pulsing a laser and measuring the time it takes for that light to be reflected. For objects that are further away, the difference in time between the pulse and its return will be greater.
In addition to a single distance metric, LIDAR can also capture density profile characteristics for complex 3D objects. In particular, if a LIDAR beam illuminates a tree canopy, some percentage of the reflected return will come from the tree top, while another portion will come from the ground beneath the canopy. By analyzing the strength of this signal response, we can identify the ground height, the tree canopy density and structure, and the tree canopy height.
Classically, LIDAR systems for forest monitoring have been mounted in an airplane that flies across a region of interest. These scans can be incredibly detailed, but come at a high cost. The GEDI instrument extends the aerial survey concept, but mounts the LIDAR on the International Space Station rather than an aircraft. This allows for continuous measurement of a ground track beneath the orbiting satellite.
To amplify the coverage, GEDI splits the single beam into eight separate components, and takes a measurement from each of these beams. Each of these beams is separted by 600 m. GEDI makes 242 measurements per second, and each point of evaluation has a radius of 25 m. These points are separated by 60 m along the path of the orbit.
The first batch of data was released in January of 2020. Given a couple peculariarities in the way the data is structured, it can be challenging to work with. I have fully documented the process in a Google Colab notebook.