For a list of known issues with this product, see ATL12 Ocean Surface Height (v01 and v02) Application Notes and Known Issues.
Data Set ID: 
ATL12

ATLAS/ICESat-2 L3A Ocean Surface Height, Version 2

This data set (ATL12) contains along-track sea surface heights at variable length scales over cloud-free regions. Estimates of height distributions, surface roughness, surface slope, and apparent reflectance are also provided. The data were acquired by the Advanced Topographic Laser Altimeter System (ATLAS) instrument on board the Ice, Cloud and land Elevation Satellite-2 (ICESat-2) observatory.

This is the most recent version of these data.

Version Summary: 

Changes for this version include:

  • The bckgrd_rate parameter (/gt[x]/ssh_segments/stats/) is now read from ATL03 and the mean value for signal photons is written to ATL12 as /gt[x]/ssh_segments/stats/backgr_seg.
  • The input photon arrays used to estimate sea state bias (x, height, time, ocean-segment, and histogram-bin-index/signal-photon-mask) are now sorted by along-track x.
  • All data with heights greater than the shore depth (default = -10 m) are not processed as ocean to ensure ocean processing doesn't occur over land.
  • Changed the processing code and ATL12 template to allow invalid values for significant wave height (/gt[x]/ssh_segments/heights/swh) and sea state bias (/gtx/ssh_segments/heights/bin_ssbias).
  • Fixed errors that caused segments to be skipped after long gaps and along the boundary between two orbits.
  • Updated parameter name "ds_y_bins" to "ds_y_bincenters" in the ATL12 product to be consistent with the ATBD and developmental code.
  • Changed the ocean histogram bin center height edges to eliminate a 0.5 cm ambiguity in histogram statistics.
  • Corrected the order in which surface types are listed on the surf_type_prcnt parameter description.
  • Added an ocean photon selection, boxcar smoothing parameter (/ancillary_data/ocean/pts2bin) and assigned it a default value of 21. This value can be overridden in the control file.
  • Made code updates to modify which residual histogram bins are selected for use. The code now includes a second-pass threshold test based on histogram far tails.
  • Added a tide_equilibrium_seg parameter in /gt[x]/ssh_segments/stats that is the mean of the ATL03 equilibrium tide values for signal photons.
  • Updated the SNRc calculation to use the same process as the deconvolution noise filtering.
  • Shifted the TEP bin center heights to match the fine histogram bin center heights. This allows for better alignment of the surface histogram after Weiner deconvolution of the impulse response.

COMPREHENSIVE Level of Service

Data: Data integrity and usability verified; data customization services available for select data

Documentation: Key metadata and comprehensive user guide available

User Support: Assistance with data access and usage; guidance on use of data in tools and data customization services

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Parameter(s):
  • SEA SURFACE TOPOGRAPHY > SEA SURFACE HEIGHT
Data Format(s):
  • HDF5
Spatial Coverage:
N: 90, 
S: -90, 
E: 180, 
W: -180
Platform(s):ICESat-2
Spatial Resolution:
  • Varies
Sensor(s):ATLAS
Temporal Coverage:
  • 13 October 2018
Version(s):V2
Temporal Resolution91 dayMetadata XML:View Metadata Record
Data Contributor(s):James Morison, David Hancock, Suzanne Dickinson, John Robbins, Leeann Roberts, et al

Geographic Coverage

Other Access Options

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As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Morison, J. H., D. Hancock, S. Dickinson, J. Robbins, L. Roberts, R. Kwok, S. P. Palm, B. Smith, M. F. Jasinski, and the ICESat-2 Science Team. 2019. ATLAS/ICESat-2 L3A Ocean Surface Height, Version 2. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/ATLAS/ATL12.002. [Date Accessed].
Created: 
8 July 2019
Last modified: 
10 February 2020

Data Description

Parameters

Along-track surface height above the WGS 84 ellipsoid (ITRF2014 reference frame), at variable length scales, over the ocean.

File Information

Format

Data are provided as HDF5 formatted files. HDF is a data model, library, and file format designed specifically for storing and managing data. For more information about HDF, visit the HDF Support Portal.

The HDF Group provides tools for working with HDF5 formatted data. HDFView is free software that allows users to view and edit HDF formatted data files. In addition, the HDF - EOS | Tools and Information Center web page contains code examples in Python (pyhdf/h5py), NCL, MATLAB, and IDL for accessing and visualizing ICESat-2 files.

ATLAS/ICESat-2 Description

The following brief description of the Ice, Cloud and land Elevation Satellite-2 (ICESat-2) observatory and Advanced Topographic Laser Altimeter System (ATLAS) instrument is provided to help users better understand the file naming conventions, internal structure of data files, and other details referenced by this user guide. The ATL12 data product is described in detail in the Ice, Cloud, and land Elevation Satellite-2 Project Algorithm Theoretical Basis Document (ATBD) for Ocean Surface Height (ATBD for ATL12). To obtain the most recent version of this ATBD, visit the NASA Goddard Space Flight Center's ICESat-2 Data Products web page.

The ATLAS instrument and ICESat-2 observatory utilize a photon-counting lidar and ancillary systems (GPS and star cameras) to measure the time a photon takes to travel from ATLAS to Earth and back again and to determine the photon's geodetic latitude and longitude. Laser pulses from ATLAS illuminate three left/right pairs of spots on the surface that trace out six approximately 14 m wide ground tracks as ICESat-2 orbits Earth. Each ground track is numbered according to the laser spot number that generates it, with ground track 1L (GT1L) on the far left and ground track 3R (GT3R) on the far right. Left/right spots within each pair are approximately 90 m apart in the across-track direction and 2.5 km in the along-track direction. The ATL12 data product is organized by ground track, with ground tracks 1L and 1R forming pair one, ground tracks 2L and 2R forming pair two, and ground tracks 3L and 3R forming pair three. Each pair also has a Pair Track—an imaginary line halfway between the actual location of the left and right beams (see figures 1 and 2). Pair tracks are approximately 3 km apart in the across-track direction.

The beams within each pair have different transmit energies—so-called weak and strong beams—with an energy ratio between them of approximately 1:4. The mapping between the strong and weak beams of ATLAS, and their relative position on the ground, depends on the orientation (yaw) of the ICESat-2 observatory, which is changed approximately twice per year to maximize solar illumination of the solar panels. The forward orientation corresponds to ATLAS traveling along the +x coordinate in the ATLAS instrument reference frame (see Figure 1). In this orientation, the weak beams lead the strong beams and a weak beam is on the left edge of the beam pattern. In the backward orientation, ATLAS travels along the -x coordinate, in the instrument reference frame, with the strong beams leading the weak beams and a strong beam on the left edge of the beam pattern (see Figure 2). The first yaw flip was performed on December 28, 2018, placing the spacecraft into the backward orientation. ATL12 reports the spacecraft orientation in the sc_orient parameter stored in the /orbit_info/ data group (see Data Groups).

ATL12 includes data from strong beams only. In general, ICESat-2 only downlinks strong beam data over the ocean. Weak beam data are only downlinked in ocean regions that overlap with ice cover or land buffer zones.


The Reference Ground Track (RGT) refers to the imaginary track on Earth at which a specified unit vector within the observatory is pointed. Onboard software aims the laser beams so that the RGT is always between ground tracks 2L and 2R (i.e. coincident with Pair Track 2). The ICESat-2 mission acquires data along 1,387 different RGTs. Each RGT is targeted in the polar regions once every 91 days (i.e. the satellite has a 91-day repeat cycle) to allow elevation changes to be detected. Cycle numbers track the number of 91-day periods that have elapsed since the ICESat-2 observatory entered the science orbit. RGTs are uniquely identified, for example in ATL08 file names, by appending the two-digit cycle number (cc) to the RGT number, e.g. 0001cc to 1387cc.

Under normal operating conditions, no data are collected along the RGT; however, during spacecraft slews, or off-pointing, some ground tracks may intersect the RGT. Off-pointing refers to a series of plans over the mid-latitudes that have been designed to facilitate a global ground and canopy height data product with approximately 2 km track spacing. Once the ATLAS/ICESat-2 Precision Pointing Determination (PPD) and Precision Orbit Determination (POD) solutions have been adequately resolved and the instrument has pointed directly at the reference ground track for a full 91 days (1387 orbits), the observatory will begin off-pointing data acquisition.

Users should note that between 14 October 2018 and 30 March 2019 the spacecraft pointing control was not yet optimized. As such, ICESat-2 data acquired during that time do not lie along the nominal RGTs, but are offset at some distance from the RGTs. Although not along the RGT, the geolocation information for these data is not degraded.

ATLAS laser spot conventions, forward orientation
Figure 1. Spot and ground track (GT) naming convention with ATLAS oriented in the forward (instrument coordinate +x) direction.
ATLAS laser spot conventions, backward orientation
Figure 2. Spot and ground track (GT) naming convention with ATLAS oriented in the backward (instrument coordinate -x) direction.
ICESat-2 reference ground tracks with dates and times can be downloaded as KMZ files from NASA's ICESat-2 | Technical Specs page, below the Orbit and Coverage table.

File Contents

ATL12 data represent surface height at a given point on the ocean at a given time. Data files (granules) contain surface heights acquired during four consecutive orbits, plus parameters needed to assess the quality of the height estimates and to interpret and aggregate the estimates over greater distances. ATL12 includes data from strong beams only (ATLAS/ICESat-2 weak beam data are only downlinked in ocean regions that overlap ice cover and land marginal zones).

ATL12 processing is designed for open ocean conditions, and as such includes a sea state bias calculation. However, processing is applied to the world ocean including ice-covered regions reported by the ATLAS/ICESat-2 sea ice height and freeboard products ATL07 and ATL10. In these regions, the statistical parameters reported by ATL12 will be valid for the mixed ocean/ice-covered and ice-free surface, however mean heights will include the average sea ice freeboard.

Data Groups

Within data files, similar variables such as science data, instrument parameters, altimetry data, and metadata are grouped together according to the HDF model. ATL12 data files contain the top-level groups shown in Figure 3:

Image showing ATL12 data groups
Figure 3. ATL12 data groups shown in HDFView.
METADATA

ISO19115 structured summary metadata for the granule, including content that describes the required geospatial information.

ancillary_data

Information that is ancillary to the data product. This may include product and instrument characteristics and/or processing constants.

gt1l – gt3r

Six ground track groups (gt1l – gt3r) that contain the per-beam data parameters (within the /ssh_segments subgroup) for the specified ATLAS ground track. No weak beam data are included over the ocean. Mean latitude, longitude, and time plus duration of the segment are stored at the top level of /ssh_segments. The sea surface height distribution and fit parameters are stored in the /heights subgroup. Quality and corrections to the sea surface heights are stored in /stats. Key parameters include:

  • sea surface height (stored in /heights/h); variance (/heights/h_var); skewness (/heights/h_skewness); kurtosis (/heights/h_kurtosis)
  • significant wave height¹ (/heights/swh)
  • sea state bias¹ (/heights/bin_ssbias)
  • segment length (/heights/length_seg)
  • total number of photons (/stats/n_ttl_photon) and number of surface photons (/stats/n_photons) in the segment 
  • percentage of non-open ocean included in the segment (/stats/surf_type_prcnt. See Spatial Information/Coverage for details.).

¹Not valid for ice-covered water.

For a complete list of output parameters, see "Section 5.5 | Output Parameters" in the ATBD for ATL12. To obtain the most recent version of this ATBD, visit the NASA Goddard Space Flight Center's ICESat-2 Data Products web page.

orbit_info

Orbit parameters that are constant for a granule, such as the RGT number and cycle and the spacecraft orientation (sc_orient).

quality_assessment

Quality assessment data for the granule as a whole, including a pass/fail flag and a failure reason indicator.

Dimension Scales

Two HDF5 dimension scales are stored at the top level alongside the data groups—ds_surf_type and ds_y_bins. These dimension scales index the surface type and y-histogram bin values, respectively.

Naming Convention

Example:

  • ATL12_20181013205512_02330101_002_01.h5
  • ATL12_[yyyymmdd][hhmmss]_[ttttccss]_[vvv_rr].h5

The following table describes the file naming convention variables:

Table 1. File Naming Convention Variables and Descriptions
Variable Description
ATL12 ATLAS/ICESat-2 L3A Ocean Surface Height product
yyyymmdd Year, month, and day of data acquisition
hhmmss Data acquisition start time, hour, minute, and second (UTC)
tttt Four digit RGT number of the first of four tracks in the granule. The ICESat-2 mission has 1,387 RGTs, numbered from 0001 to 1387.
cc Cycle Number. The cycle number tracks the number of 91-day periods that have elapsed since ICESat-2 entered the science orbit.
ss Segment number. Not used for ATL12. Always 01.1
vvv_rr Version and revision number.2
1Some ATLAS/ICESat-2 products (e.g. ATL03) are provided as files that span 1/14th of an orbit. As such, these products' file names specify a segment number that ranges from from 01 to 14. Because ATL12 data files span four full orbits, the segment number is always set to 01.
2From time to time, NSIDC receives duplicate, reprocessed granules from our data provider. These granules have the same file name as the original (i.e. date, time, ground track, cycle, and segment number), but the revision number has been incremented. Although NSIDC deletes the superceded granule, the process can take several days. As such, if you encounter multiple granules with the same file name, please use the granule with the highest revision number.


Each data file has a corresponding XML file that contains additional science metadata. XML metadata files have the same name as their corresponding .h5 file, but with .xml appended.

Browse File

Browse files are provided as HDF5 formatted files that contain images designed to quickly assess the location and quality of each granule's data. Browse images are included (for strong beams) that show each segment's: mean sea surface height; standard deviation of the height distribution; significant wave height; skewness of the height distribution; kurtosis of the height distribution; 2-Gaussian fit parameters.

Browse files utilize the same naming convention as their corresponding data file, but with _BRW appended.

File Size

Spatial Information

Coverage

Spatial coverage spans the world ocean surface from approximately 88° N latitude to 88° S. Each granule contains data obtained during four consecutive ATLAS orbit tracks.

Data are processed for areas designated as ocean by the ICESat-2 ocean surface mask. The ocean mask overlaps with all the other surface types in buffer zones up to 20-km wide. As such, ATL12 data may include data collected over low-lying land surfaces which lie close enough to sea level to fall within ±15 m of the geoid. Examples include sea ice surfaces in marginal sea ice zones and low-lying islands. To determine how much non-open ocean is included in any given ocean segment, users can check the gt[x]/ssh_segments/stats/surf_type_prcnt output variable. Future releases are slated to incorporate a bathymetric test designed to ensure ATL12 processing occurs over open ocean only.

Resolution

Heights over the sea surface are defined for ocean segments that vary in both length and spacing along the ground track; this strategy is necessary to ensure photon counts are high enough to adequately characterize sea surface heights given the inherently low and variable reflectance of open ocean surfaces.

In general, one photon reflects back to ATLAS from the ocean surface per laser pulse—0.7 m of along-track distance—or approximately 100 photons in a segment length of about 70 m. In practice, the adaptive algorithm outputs heights at length scales between 70 m and 7 km, depending on conditions and assuming clear skies.

Geolocation

Points on Earth are presented as geodetic latitude, longitude, and height above the ellipsoid using the WGS 84 geographic coordinate system (ITRF2014 reference frame). The following table contains details about WGS 84:

Table 3. Geolocation Details

Geographic coordinate system

WGS 84

Projected coordinate system

WGS 84

Longitude of true origin

Prime Meridian, Greenwich

Latitude of true origin

N/A

Scale factor at longitude of true origin

N/A

Datum

World Geodetic System 1984

Ellipsoid/spheroid

WGS 84

Units

degree

False easting

N/A

False northing

N/A

EPSG code

4326

PROJ4 string

+proj=longlat +datum=WGS84 +no_defs

Reference

https://epsg.io/4326

For information about ITRF2014, see the International Terrestrial Reference Frame | ITRF2014 webpage.

Temporal Information

Coverage

13 October 2018 to present

Resolution

Each of ICESat-2's 1387 RGTs is targeted once every 91 days (i.e. the satellite has a 91-day repeat cycle).

Data Acquisition and Processing

Background

The ATL12 product contains sea surface heights at varying length scales, plus other descriptive parameters. These data are derived from geolocated, time-tagged photon heights passed to ATL12, along with other parameters, from the ATLAS/ICESat-2 L2A Global Geolocated Photon Data (ATL03) product. The following figure illustrates the family of ICESat-2 data products and the connections between them:

Schematic showing Icesat-2 data processing flow
Figure 4. ICESat-2 data processing flow. The ATL01 algorithm reformats and unpacks the Level 0 data and converts it into engineering units. ATL02 processing converts the ATL01 data to science units and applies instrument corrections. The Precision Pointing Determination (PPD) and Precision Orbit Determination (POD) solutions compute the pointing vector and position of the ICESat-2 observatory as a function of time. ATL03 acts as the bridge between the lower level, instrumentation-specific products and the higher-level, surface-specific products.
The following sections briefly describe the approach used to generate the sea surface heights and descriptive statistics reported by the ATL12 data product. The approach is detailed in "Section 4 | Algorithm Theory" and "Section 5.0 | Algorithm Implementation" in the ATBD for ATL12. To obtain the most recent version of this ATBD, visit the NASA Goddard Space Flight Center's ICESat-2 Data Products web page.

Acquisition

Sea surface heights are determined primarily from ATL03 geolocated photon heights. These inputs consist of low-, medium-, and high-confidence signal photons over open ocean combined with a buffer of photons that lie within ±15 m of the EGM2008 geoid. For a complete list of inputs to ATL12, see "Section 5.2 | Input Parameters" in the ATBD for ATL12.

Processing

Because the sea surface is in constant motion, surface finding requires searching ATL03 photon heights for a representative sea surface height distribution. The algorithm treats every ATL03 photon height passed as input to ATL12 as an individual surface-height point measurement, spaced on average less than one meter apart, but with an x-y location uncertainty on the order of 10 m. Figure 5 shows ATL03 photon heights collected over 7-km of ocean surface. Note that the dense cloud of surface-reflected photons clearly stands out from the noise photons above and below.

figure showing how ATL03 photon sea surface heights stand out from noise photons
Figure 5. ATL03 photon heights (magenta) from Oct. 16, 2018 over the Pacific Ocean. Waves in the dense photon cloud are apparent as well as subsurface and atmospheric noise photons. Image from ATL12 Ocean Surface Height Release 001 Application Notes and Known Issues.

From the population of ATL03 input heights, the ATL12 algorithm accumulates photons returns that likely fall within the sea surface height distribution, until it traverses 7 km of along-track distance or tallies 8000 photons. The accumulated photons over the resulting ocean segment are then filtered to select heights in the histogram at levels above the background rate. After one iteration, a linear trend and average height are removed and then the data are filtered a second time (the trend and average height are retained and output with the ocean segment statistics).

The trimmed, detrended surface photon histogram is then: 1) deconvolved with an ATLAS instrument impulse response histogram¹ to produce a surface height distribution; and 2) fit with a 2-Gaussian mixture model to produce the first four moments of the surface height distribution (see Figure 6).

Figure showing 4 sample plots of ATL12 segment statistics
Figure 6. ATL12 ocean segment statistics. Upper left: Mean dynamic ocean topography (DOT) = SSH (moment 1) - geoid (EGM2008); upper right: significant wave height (SWH) = 4 x standard deviation (moment 2); lower left: skewness of sea surface height (moment 3); lower right: kurtosis of sea surface height (moment 4). Yellow DOTS are ATL12, Version 1 (revision 1) and blue dots are from ATL12 Science Team MATLAB developmental code applied to ATL03, which generally does not segment the data the same as ATL12. The agreement is better where segments are more closely matched.

¹The impulse response function represents the height uncertainty associated with the lidar transmit pulse width and other instrumental factors.

In addition, the algorithm analyzes the spatial series of surface photon heights to characterize surface waves and to calculate the photon return rate and surface height correlation that constitutes the electromagnetic (EM) sea state bias (SSB) in the mean sea surface height. Thus, the ATL12 ocean product includes histograms and statistics of sea surface height over variable lengths.

Quality, Errors, and Limitations

Data quality metrics provided with the ATL12 product are listed in "Section 7.0 | Data Quality" in the ATBD for ATL12. In addition, users should consult "Section 9.0 | Constraints, Limitations, and Assumptions" in the ATBD for ATL12. This section summarizes the important factors that impact ATL12 coverage and quality and determine how the data should be interpreted and compared with other open ocean topography data sets.

ATL12 aims to achieve standard errors in mean sea surface height of 1 cm or better, under typical sea states. However, due its utilization of 532 nm, visible light, the ATLAS sensor requires mostly clear skies to sample the ocean surface. Furthermore, the relatively low reflectance of the ocean surface requires greater along-track lengths to accumulate enough height measurements to meet the desired statistical significance.

Limitations in the current understanding of altimetric returns from the sea surface will necessarily carry through to the ATL12 product. In particular, subsurface returns represent a significant concern because the ATLAS 532 nm laser penetrates water, and the higher subsurface density of photons apparent in Figure 5 may be due in part to subsurface scattering in the ocean. However, the ATL12 Science Team has noted similarly enhanced subsurface densities over clear, deep ocean waters (and even over land) where subsurface backscatter does not apply. At this time, the ICESat-2 Science Team believes that the subsurface noise level stems from forward scattering delays in the atmosphere. To eliminate subsurface returns masked by the true surface distribution, future releases will adopt a scheme that selects photons based on a histogram of height anomalies about a moving, 11 photon average of high confidence photon heights. This histogram is much narrower than the surface height histogram and has a more distinct negative-side noise tail that will facillitate deletion of subsurface returns under the surface wave crests.

In addition, the ATL12 Science Team has found larger ocean segment-to-segment variation in average DOT = SSH - Geoid (e.g., see Fig. 6 upper left, ±0.1 to 0.3 m in ) than expected from instrumental factors. If every one of the approximately 8,000 photon heights in an ocean segment were independent, even with a SWH = 2.5 m (th = .625 m), the uncertainty in estimates of the mean should be less than 1 cm. However, the height measurements over long ocean waves are not independent, and the degrees of freedom can approach the number of wavelengths in the ocean segment. To address this problem, future ATL12 releases will include harmonic analyses of surface height for each ocean segment and the option to use a zero wave-number amplitude to represent SSH. In addition, the spectral analysis will be used to derive measures of the effective number of degrees of freedom for each ocean segment. These measures will be output as an ATL12 parameter in order to optimally combine ATL12 ocean segment heights and produce the lowest uncertainties in the planned gridded SSH product (ATL19).

As this sea surface height product is being developed and evaluated on an ongoing basis, users should consult the most recent version of "ATL12 Ocean Surface Height Application Notes and Known Issues." This document is located on the Technical References tab of the ATL12 data set landing page. This document will be updated with each new release.

Version History

Version 2 (October 2019)

Contacts and Acknowledgments

James H. Morison
Applied Physics Laboratory
University of Washington
Seattle, WA 98105

David Hancock
NASA Goddard Space Flight Center
Mail Code: 615
Greenbelt, MD 20771

Suzanne Dickinson
Applied Physics Laboratory
University of Washington
Seattle, WA 98105

John Robbins
NASA Goddard Space Flight Center
Mail Code: 615
Greenbelt, MD 20771

Leeanne Roberts
KBR, Inc
Greenbelt, MD 20770

Ron Kwok
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109

Steve Palm
Science Systems and Applications, Inc.
NASA Goddard Space Flight Center
Greenbelt, MD 20771

Ben Smith
Applied Physics Lab
Polar Science Center
University of Washington
Seattle, WA 98105

Mike Jasinski
NASA Goddard Space Flight Center
Mail Code: 617
Greenbelt, MD 20771

Bill Plant
Applied Physics Laboratory
University of Washington
Seattle, WA 98105

Timothy Urban
Center for Space Research
University of Texas at Austin
Austin, TX 78705

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