GEDI Overview


Introduction

The Global Ecosystem Dynamics Investigation (GEDI) instrument is aboard the International Space Station (ISS) and it's mission aims to characterize ecosystem structure and dynamics to enable radically improved quantification and understanding of the Earth’s carbon cycle and biodiversity. The GEDI instrument produces high-resolution laser ranging observations of the 3-dimensional structure of the Earth. GEDI was launched on December 5, 2018 and is attached to the International Space Station (ISS). GEDI collects data globally between 51.6° N and 51.6° S latitudes at the highest resolution and densest sampling of any light detection and ranging (lidar) instrument in orbit to date. The GEDI instrument consists of 3 lasers producing a total of 8 beam ground transects, which consist of ~30 meter (m) footprint samples spaced approximately every 60 m along-track. The GEDI beam transects are spaced approximately 600 m apart on the Earth’s surface in the cross-track direction, for an across-track width of ~4.2 kilometers (km). GEDI measurements of forest canopy height, canopy vertical structure, and surface elevation can greatly advance our ability to characterize carbon and water cycling processes, biodiversity, and habitat. GEDI data can also be of immense value for weather forecasting, forest management, snow and glacier monitoring, and the generation of digital elevation models.

Back to top

GEDI Naming Conventions

GEDI Filenames

GEDI filenames (i.e., the local granule ID) follow a naming convention which gives useful information regarding the specific product.

In this example of a Level 01B product, the filename GEDI01_B_2019110110221_O01997_T03335_02_002_01.h5 indicates:

  • GEDI01_B = Product Short Name
  • 2019110 = Julian Date of Acquisition in YYYYDDD
  • 110221 = Hours, Minutes, and Seconds of Acquisition (HHMMSS)
  • O01997 = O = Orbit, 01997 = Orbit Number
  • T03335 = T = Track, 033335 = Track Number
  • 02 = Positioning and Pointing Determination System (PPDS) type (00 is predict, 01 rapid, 02 and higher is final)
  • 002 = GOC SDS (software) release number
  • 01 = Granule Production Version
  • .h5 - Data Format

In this example of a Level 02A product, the filename GEDI02_A_2019110014613_O01991_T04905_02_001_01.h5 indicates:

  • GEDI02_A = Product Short Name
  • 2019110 = Julian Date of Acquisition in YYYYDDD
  • 014613 = Hours, Minutes, and Seconds of Acquisition (HHMMSS)
  • O01991 = O = Orbit, 01991 = Orbit Number
  • T04905 = T = Track, 04905 = Track Number
  • 02 = Positioning and Pointing Determination System (PPDS) type (00 is predict, 01 rapid, 02 and higher is final)
  • 001 = GOC SDS (software) release number
  • 01 = Granule Production Version
  • .h5 - Data Format

In this example of a Level 02B product, the filename GEDI02_B_2019110045136_O01993_T02061_02_001_01.h5 indicates:

  • GEDI02_B = Product Short Name
  • 2019110 = Julian Date of Acquisition in YYYYDDD
  • 045136 = Hours, Minutes, and Seconds of Acquisition (HHMMSS)
  • O01993 = O = Orbit, 01993 = Orbit Number
  • T02061 = T = Track, 02061 = Track Number
  • 02 = Positioning and Pointing Determination System (PPDS) type (00 is predict, 01 rapid, 02 and higher is final)
  • 001 = GOC SDS (software) release number
  • 01 = Granule Production Version
  • .h5 - Data Format

GEDI Product Long Name

The GEDI Product Long Name (i.e., Collection-Level) convention provides useful information regarding all GEDI products.

Below is an example of a GEDI Level 1B product. All GEDI Product Long Names follow the same structure.

The GEDI L1B Geolocated Waveform Data Global Footprint Level V001 collection has the following characteristics:

  • GEDI – Instrument
  • L1B – Processing Level
  • Geolocated Waveform Data – Geophysical Parameter
  • Global – Global Coverage
  • V001 – Version
Back to top

GEDI Temporal and Spatial Resolution

GEDI Temporal Resolution

All GEDI products distributed by the LP DAAC are produced at nominally daily temporal resolution. The true revisit period for a given location is variable based on the instrument’s orbital cycle aboard the ISS.

GEDI Spatial Resolution

The sole GEDI observable is the waveform from which all other data products are derived. Each waveform is captured with a nominal ~30 m diameter.

Back to top

GEDI Geolocation

The location of the ranging point (also referred to in this document as a bounce point) can be commonly described in terms of geodetic latitude, longitude, and height above a reference ellipsoid, or it can be represented in Cartesian coordinates in an Earth Centered Inertial (ECI) or Earth Centered Fixed (ECF) reference frame.

Several reference frames are used in the precise positioning and precise pointing required for geolocation of GEDI footprints. The Space Station Analysis Coordinate System (SSACS) and GEDI frame are used to orient instruments relative to one another. The Optical Bench Frame (OBF) provides a connection to the GEDI frame and a reference system for targeting calculations. Star tracker frames connect these to the ECI frame allowing conversion to geodetic coordinates.

Back to top

GEDI Data Processing

GEDI Processing Levels

LP DAAC distributes GEDI land data processed to Level-1 and Level-2:

  • Level-1A: Raw Waveforms (not publicly available)
  • Level-1B: Geolocated Waveforms
  • Level-2A: Ground Elevation, Canopy Top Height, Relative Height Metrics
  • Level-2B: Canopy Cover Fraction (CCF), CCF profile, Leaf Area Index (LAI), LAI profile

GEDI Processing Description

GEDI science data products include footprint and gridded data products that describe the 3-dimensional features of the Earth. These data products are assigned different levels, which indicate the amount of processing that the data has undergone after collection. All products are publicly available, with the lower level products (L1 & L2) from NASA’s Land Processes Distributed Active Archive Center (LP DAAC) and the higher level (L3 & L4) products from the Oak Ridge National Laboratory DAAC. Data are initially transferred to the GEDI Mission Operations Center (MOC) at the Goddard Space Flight Center, which deploys weekly acquisition planning, and then processed through the Science Operations Center (SOC) to distribute science data products to the above DAACs.

The physical theories, mathematical procedures, and model assumptions that are used in the creation of these data products are described in the Algorithm Theoretical Basis Documents (ATBDs).

Back to top

GEDI Metadata

GEDI products have two sources of metadata: the embedded HDF5 metadata and the external ECS metadata. The HDF5 metadata contains valuable information, including global attributes and dataset specific attributes pertaining to the granule. The ECS (generated by the EOSDIS Core System) .met file is the external metadata file in XML format, which is delivered to the user along with the GEDI product. It provides a subset of the HDF metadata.

The dataset attributes contain specific SDS information, such as the data range and applicable scaling factors for the data. The LP DAAC data products page provides these details within a concise document for each of the products. An HDF5 file also contains core metadata essential for search services. It should be noted that it is difficult for a standard HDF call to interpret HDF5 geolocation or temporal information without further knowledge of the file structure.

Back to top

GEDI Tools and Services

Data access options such as Data Pool and Earthdata Search Client can be found under Tools.

An HDF5 file contains information essential for NASA data access tools and services. Unlike less complex formats, users cannot read the HDF5 files directly without using the HDF5 software library. Additionally, it is difficult for a standard HDF call to interpret HDF5 geolocation or temporal information without further knowledge of the file structure.

Third party tools with HDF5 support include IDL and Matlab. The following list of open source tools is also applicable to GEDI HDF5 files:

Open Source Tools

Back to top