ATLAS/ICESat-2 processing begins with two Level-1 products: ATL01, which computes photon times of flight and ATL02 which corrects them for temperature and voltage effects.
The collection then branches out into two, Level-2 scientific data products: one which reports latitude, longitude, and elevation for each photon (ATL03), and a second with atmospheric profiles of normalized relative backscatter.
Beyond that, the ATLAS/ICESat-2 Level-3 products comprise surface-specific data products for land ice, sea ice, the atmosphere, vegetation and land, and oceans and inland water.
The following descriptions provide brief summaries of all the planned ICESat-2 data products. A complete description is available for each product in the corresponding Algorithm Theoretical Basis Document (ATBD) on the NASA Goddard Space Flight Center's ICESat-2 Data Products web page. NASA updates the ATBDs on this page periodically to remain consistent with the latest product versions.
Level 1 ICESat-2 Products
Reformatted Telemetry (ATL01)
Raw ATLAS/ICESat-2 data are decompressed, ordered in time, and reformatted to HDF5. ATL01 consists of all the ICESat-2 data downlinked by the X-band telemetry stream, in a bit-decompressed, time-aligned, product.
ATL01 contains all the pertinent information related to:
- the atmosphere
- the laser reference system
- housekeeping data
- calibration mode
- spacecraft ancillary science data
ATL01 is generated as input for the Level 1B product and for use by researchers who require ATLAS/ICESat-2 data at the lowest possible level.
ATLAS/ICESat-2 Level 1B processing applies instrument corrections to the ATL01 data, such as removing biases in timing and pointing measurements and correcting for the effects of temperature and voltage variations on the ATLAS electronics.
The primary function of ATL02 is to:
- Record radiometric parameters for system–level, quality control analysis
- Support the Precise Pointing Determination (PPD) and the Precise Orbit Determination (POD) processing
- Provide source data for the Level-2 global geolocated photons product (ATL03)
- Provide raw atmospheric profiles for each strong beam to the Level-2 normalized relative backscatter product (ATL04)
Level 2 ICESat-2 Products
ATL03 provides time, latitude, longitude, and ellipsoidal height for each photon event downlinked from ATLAS. Heights are corrected for several geophysical phenomena, such as effects of the atmosphere and solid earth deformation. By design, users can easily remove each geophysical correction, if desired.
Each event is classified as either a likely signal photon or a likely background photon. To distinguish signal photons from the background, ATL03 processing generates histograms of the number of photon events as a function of height and calculates the signal-to-noise ratio for each histogram bin.
ATL03 also contains five surface masks—land, land ice, ocean, sea ice, and inland water—that collectively cover Earth's surface.
ATL04 provides along-track, normalized relative backscatter (NRB) profiles that can be used to identify atmospheric layers and their boundaries, including clouds, aerosols, and blowing snow. The profiles extend from 13.75 km above the local DEM surface to -0.25 km, and as such include the ground.
NRB profiles are generated by subtracting background photon events from raw photon counts, multiplying by the square of the range from the satellite to the return height, and normalizing by the laser energy. The profiles are also passed to and used by higher level products to characterize the atmosphere.
NRB profiles are generated from the strong beam only of each beam pair, and thus each ATL04 granule contains three atmospheric profiles. Strong beam data are downlinked after summing 400 laser pulses—about 280 m of along-track distance—are binned vertically at ~30 m.
Level 3a ICESat-2 Products
ATL06 provides geolocated estimates of land-ice surface heights and ancillary parameters that can be used to interpret the estimates and assess their quality. Photon events are aggregated into overlapping, along-track segments of a fixed length (40 m) whose centers are 20 meters apart, and the algorithm estimates the along-track slope and height at the center of each segment. Across-track slopes are calculated when both beams in a pair obtain height measurements.
ATL08 estimates terrain height, canopy height, and canopy cover at 100 meter, fixed-length steps along the ground track. The surface finding algorithm computes an interpolated ground surface and determines the relative canopy height by differencing the height of canopy photons returns from the height of the interpolated ground surface.
ATL07 provides along-track surface height and type (e.g., snow-covered ice, open water) for the ice-covered seas of the northern and southern hemispheres. Sea surface and sea ice height are estimated for segments along each of the six ground tracks. Segment length varies with surface type, determined by the distance over which ~150 signal photons are accumulated. Up to 16 orbit files are provided each day for each hemisphere.
ATL09 reports layer heights and related flags detected from atmospheric backscatter profiles (ATL04). This product also includes two cloud flags, a blowing snow flag, and a multiple scattering warning flag. Heights are available for a maximum of ten layers for each of the three strong beams and a layer attribute flag differentiates clouds from aerosols.
This data product provides estimates of along-track, total freeboard for the ice-covered seas of the northern and southern hemispheres along segments generated in the ATL07 product. Freeboard is determined using a reference sea surface height established from within-segment leads (open ocean). Up to 16 orbit files are provided each day for each hemisphere.
ATL12 will contain estimates of the sea surface height at a given point on the open ocean surface at a given time, plus parameters needed to assess the quality of the height estimates and interpret and aggregate the data over greater distances. The ice-free, ocean surface height, including coastal waters, is provided at length scales between 70 m (100 Hz) and 7 km (1 Hz) for each of three ATLAS strong beams. ATL12 also includes height distribution estimates (decile bins), significant wave height, surface slope, and apparent reflectance.
ATL13 consists of along-track water surface height, slope, and roughness for each ATLAS beam, plus aspect and maximum slope (or gradient) of the planar surface between adjacent strong beams. In addition, the product includes: water surface height statistics; mean quantities such as sub-surface backscatter attenuation coefficient and significant wave height, when applicable; and along track water-body surface maximum slope and aspect with respect to North, derived from adjacent ATLAS strong beams.
Level 3b ICESat-2 Products
ATL11 provides time series of surface heights. It is the lowest-level land-ice product that brings together data from multiple passes over the same points, obviating the need to collect individual ATL06 files for this task. ATL11 contains height estimates both for crossovers and for along-track reference points between crossovers. The product uses consistency checks to cull blunders from the ATL06 record, computes a local-topography correction to remove height variations due to small-scale surface slopes, and contains values that can be directly differenced to obtain height changes through time (dh/dt).
The ATL14 product will use slope-corrected heights from ATL11 to generate Digital Eleveation Models (DEMs) for Antarctica and Greenland posted at 125 meters. ATL15 will synthesize height measurements from ATL06 (Land Ice Height) and the first ICESat mission, to produce maps of height change for different time windows: one between the midpoint of the ICESat mission and the start of ICESat-2; and one for each year of the ICESat-2 mission.
ICESat–ICESat-2 elevation difference maps will be posted at 1 km. ATL14 elevation change surfaces will be posted at 500 m per 3 months.
These two products, which differ only in grid resolution and frequency at which they are produced, will report global cloud fraction, total column optical depth over the oceans, polar cloud fraction, blowing snow frequency, apparent surface reflectivity, and ground detection frequency. The HDF product files will contain gridded 2D arrays plus images of the data fields plotted on a map of the region.
Gridded Land and Vegetation Height (ATL18) - not yet published
This data product consists of gridded maps for terrain and canopy features. The product will be released annually at a coarse resolution, for example, 500 m to 1 km tiles (to be determined). ATL18 will include canopy height, canopy cover, and ground elevation. Higher resolution grids are planned at the end of the mission.
Derived entirely from ATL12 (Ocean Surface Height), ATL19 will contain gridded monthly estimates of sea surface height using all ICESat-2 tracks from the beginning to the end of each month. It will include statistical descriptions of the sea surface—mean, standard deviation, skewness, and kurtosis—within each grid cell.
ATL20 and ATL21 consist of gridded estimates of sea ice freeboard and sea surface height (ice-covered ocean) for both Arctic and Southern Oceans. ATL20 reports sea ice freeboard from all ICESat-2 tracks between the beginning and the end of each month. ATL21 provides daily and monthly averages of sea surface height anomaly from all ICESat-2 tracks between the beginning and the end of each month. Both data products are gridded at 25 km and mapped onto a planimetric grid using the SSM/I Polar Stereographic Projection equations.