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NASA Distributed Active Archive Center (DAAC) at NSIDC

ICESat / GLAS Data

Ice, Cloud, and land Elevation / Geoscience Laser Altimeter System

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Description of Data Releases

Note: Correction to ICESat Data Product Surface Elevations. In January 2013, the ICESat Project Science Team announced that a correction to ICESat surface elevation data products is necessary due to an error in the range determination from transmit-pulse reference selection (centroid vs.Gaussian, G-C). The following products are affected by this finding: GLA06 (global elevation), GLA12 (ice sheet), GLA13 (sea ice), and GLA15 (ocean). GLA14 users who use the range increment for the alternative Gaussian-fitted peaks may want to also use the correction. For details, see the Correction to ICESat Data Product Surface Elevations Web page.

This page summarizes changes and known limitations associated with the most current release of NASA ICESat/GLAS data:

For information about releases prior to the most current one, see the Description of Past Data Releases Web page.

A new release is created when changes occur in the input data or when improvements are made to the processing algorithms. The GLAS Software Development Team determines the release number. Products generated with a new release may be processed forward in time or reprocessed from earlier data. If reprocessed, NSIDC either deletes or hides data granules from previous releases once they have been replaced by the new release. The first data release was Release-12; there were no products with release numbers lower than that. We recommend you work with the latest release of data which is the highest release number and includes improvements to product algorithms. See the following table for more information.

The following table provides links to NSIDC and other Web pages that provide additional information about GLAS data releases and laser campaigns.
 

Web Page Description
Laser Operational Periods Important information about laser operating periods and data releases, including metadata table The Attributes for ICESat Laser Operations Periods
ICESat Science Investigator-led Processing System (I-SIPS) Release Information Table summarizing release information for all ICESat/GLAS data distributed by NSIDC
NASA Wallops Flight Facility's ICESat/GLAS Web site Links to release notes on Wallops Flight Facility's Web site
Algorithm Theoretical Basis Documents (ATBD) A complete description of the physical and mathematical algorithms used in the generation of the data products.
ICESat Laser 1 Summary Description of details concerning the unique nature of the laser 1 period. Laser 1 was turned on 20 February 2003 and failed on 29 March 2003.
ICESat-1 pointing cal/val and obatt file correction summary Summaries of ICESat laser periods, processing, obatt correction, pointing bias correction, and cal/val correction.
Illustration of Gain-Related Saturation Bias in Laser 1 Campaign Data from Lake Vostok Area Crossovers Detailed analysis showing that prior to 13 March 2003 elevation values with gain values below 13 may have reduced precision relative to other Laser 1 and subsequent ICESat data.
Surface-type Mask Global mask to flag surface types for use in creating standard ICESat/GLAS data products.


Release-33

Release 33, which is GSAS release 6.0, is planned to be the final major GLAS Science Algorithm Software (GSAS)release. This release incorporates many changes and enhancements.

Overview

Release 33 includes several important changes.

  1. The saturation correction for Laser 3 has been improved.
  2. The standard waveform fit signal threshold was lowered starting with Laser 3I (L3I, L3J, L3K, L2D, L2E, and L2F). This provides more elevations for the low signal areas. The qualities of the low signal elevations have not been fully evaluated.
  3. A new atmospheric characterization and confidence flag has been added to the products. This should allow more consistency in identifying atmosphere effects on the elevation estimates.
  4. Several changes were made that improve the calibration of the atmospheric product parameters.
  5. Precision Orbit Determination (POD) improvements include updated GPS orbit solutions, reference frame, ground stations, and observational and dynamic and modeling, including the GRACE-derived GGM03C gravity model. See CSR SCF Release Notes for Orbit and Attitude Determination V3 for details.
  6. Precision Attitude Determination (PAD) improvements include updated procedures for handling star tracker data and new estimates and methods for the determination of post-launch biases and attitude hardware motion. See CSR SCF Release Notes for Orbit and Attitude Determination V3. Note that due to the decreasing energy of Laser 2, the onboard Laser Profile Array (LPA) cannot be used to detect individual footprint parameters beyond L2C day 160/2004 (including all of L2D, L2E, and L2F). See also CSR Summary of LPA parameter Estimation v2.

The Y-code for the reprocessed GLA products is incremented to level 6 (633) given the Release-33 changes in POD and PAD. Refer to the YXX Release Numbers Web page for more information about the release number convention.

Note: Use caution when comparing different versions or releases of GLAS data.

Notable changes to the data in Release-33 are summarized on this page. For more detailed information, see the following:

General Changes: Level-1A (GLA01, 02, 03 and 04)

Changes/Improvements

  • Changed the data validation code within ANC32_QA and GLAS_L0p to improve the consistency of the internal quality assessment of the ANC32 GPS time alignment data.

Fixes

  • Fixed the GLA04 LRS Virtual Tracker Centroid Row and Column conversion factor.

Altimetry Products (GLA05, 06, 12, 13, 14, and 15)

User's Guide

Release-33 altimetry products are accompanied by the GLAS Altimetry Products Usage Guidance (pdf, 71 KB).

Additions

  • Added "distance to the reference track" to the elevation products.
  • Added atmospheric characterization flag and removed blowing snow and Estimated Range Delay (ERD). The atmosphere characterization flag (i_atm_char_flag) is new to this release. It is on GLA06, 09 and GLA12 to 15. It is intended to characterize the atmosphere to help aid in the interpretation of altimetry data.
  • Added hi-resolution Canadian DEM. Added elevations from the Canadian DEM to anc51 from 40N to 69.9997N, and to anc54 for areas not already covered by the ICESat Greenland DEM (west of 285E, from 70N through 84N). The elevations are converted from EGM96 before being placed on anc51. Elevations read from the new anc5406 are converted from EGM96 before being placed on the elevation products. The anc51 maker utility has been modified to work on both db1 (HP/Big_Endian) and icesat10 (linux/Little_Endian).
  • Added a WGS84 referenced elevation, and campaign, repeat cycle, and track number.
  • Added the campaign, repeat cycle, and track number to the elevation products at 1Hz.
  • Added global mean pressure and ocean bathymetry at 1Hz to GLA15.
  • Added the Local Solar Time (d_localSolarTime) to the elevation products at 1Hz.
  • Added a 40Hz WGS-84 ellipsoidal surface height to all elevation products. The difference between the elevations with respect toTopex/Poseidon and WGS-84 is now being reported for all GLA06, 12-15 elevation products. They are reported at 40Hz.
  • Increased the precision and frequency of the beam co-elevation and azimuth. Frequency increased to 40 Hz.
  • Added the kurtosis (standard fit) to GLA13 and GLA15.
  • Signal Begin range offset added to GLA12, 13, 15.
  • Added EGM96 geoid to SRTM tracks.

Changes/Improvements

  • Provided greater precision and higher rate beam co-elevation and azimuth.
  • Updated the Laser 3 saturation correction tables.
  • Set reflectance and saturation energy correction to invalid when saturation energy correction is incalculable.
  • Changed to SRTM 90m DEM to 9 values (3x3 box) surrounding the elevation. A 3 x 3 matrix of high resolution DEM values derived from the SRTM track files is now included in the GLAS output files, replacing the single central value previously in use.
  • Updated the attitude flag to indicate spacecraft pointing activities.
  • Removed PAD and POD vectors to make room for additional parameters. Removed i_PADPoint & i_PODFixedPos from GLA06, and GLA12 through GLA15.
  • Provided space on the elevation products for a (yet uncomputed) ocean bias.
  • Changed the ANC32 validation code to improve the consistency of the QA tests within ANC32_QA and GLOP.
  • ANC54 (HiRes DEM) has two new subgranules (High Lat Canada HiRes DEM files) which are additional inputs for GLAS_Alt elevation processing.
  • ANC57 (Bathymetry) is an additional input for the GLAS_Alt elevation processing.
  • ANC58 (Reference Track) is an additional input for the GLAS_Alt elevation processing.
  • ANC59 (Pointing Mode Table) is an additional input for the GLAS_Alt waveform processing.
  • Replaced the Low resolution DEM on GLA15 with the ocean bathymetry. The ocean bathymetry is determined for the GLA15 products at a frequency of 1 per second. The first valid shot with a valid lat/lon in a record is used to determine the bathymetry for that record.
  • i_reflCor_atm on the elevation products is being set with gla11%i_reflCor_atm instead of being calculated in the elevation code.

Fixes

  • Fixed the problem that prevented the high resolution DEM data from anc54 being written to the elevation products.
  • The SRTM high resolution DEM (i_DEM_hires_elv) values have been corrected. In prior releases the EGM96 geoid value was added to the SRTM values to change their reference to the TOPEX/Poseidon ellipsoid to match the GLAS elevations. This was done using the geoid values that were input to the GSAS software. When the GSAS input geoid on the GLAS standard data product was changed to EGM2008 in release 31, this introduced an error. The code that adds the geoid value to the SRTM values has been removed from the GSAS software and the SRTM trackfiles (input ancillary files) have been re-referenced outside of the GSAS system using the EGM96 geoid. Modifications were made such that all elevations with absolute value less than 30000 meters are included in output track files. This allows areas with elevations less than sea level to be included.
  • Fixed a problem that caused the OrbitQuality AdditionalAttribute not to appear in the headers of GLA06,12-15.
  • Fixed an intra-record alignment problem with the atmosphere data on GLA06,12-15.
  • Fixed a problem where writing to ANC06 before ANC06 was opened created an unwanted output file.

Known Problems

  • i_ElevBiasCorr is currently not computed.

Atmospheric Products (GLA07, 08, 09, 10, and 11)

Table 1 summarizes atmospheric products for Release 33.


Table 1. Release 33 Atmospheric Products
Laser Obs GLA07 GLA08 GLA09 GLA10 GLA011
L1 x   x   x
L2A x x x x x
L2B x x x x x
L2C x   x   x
L2D x       x
L2E x       x
L2F x       x
L3A x x x x x
L3B x x x x x
L3C x x x x x
L3D x x x x x
L3E x   x   x
L3F x   x   x
L3G x   x   x
L3H x   x   x
L3I x   x   x
L3J x       x
L3K x       x

User's Guide

The atmospheric products are accompanied by the GLAS Atmospheric Products User Guide (pdf, 74KB).

Additions

  • Added atmospheric characterization flag and removed blowing snow and ERD. The atmosphere characterization flag (i_atm_char_flag) is new to this release. It is on GLA06, 09 and GLA12 to 15. It is intended to characterize the atmosphere to help aid in the interpretation of altimetry data.
  • Two new parameters are added to the GLA11 product and two others that currently exist on GLA10 are added to GLA11. All have to do with characterization of detected aerosol layers. The first parameter (i_aer4_sval_ratio) is the ratio of the 532 nm extinction to backscatter ratio (S532) to the 1064 nm extinction to backscatter ratio (S1064) for each detected aerosol layer. The second new parameter (i_aer4_aod_ratio) is the ratio of 532 nm aerosol optical depth to 1064 nm aerosol optical depth for each detected aerosol layer. This provides a way for the user to obtain the 1064 optical depth if the 532 optical depth is known. These parameters were computed from an aerosol transport model at the University of Arizona. Also added to GLA11 are the true 532 extinction to backscatter ratio (i_aer4_sval1) and Aerosol true S Values use flag (i_aer4_sval_uf), both also on GLA10.
  • Atmosphere attenuation correction factor: ID i_reflCor_atm is an atmospheric attenuation correction factor to be applied to the surface reflectance value to account for the loss of energy of the laser pulse from the satellite to and from the surface. In prior releases, this factor was too large as it did not properly account for multiple scattering. This version applies multiple scattering correction to the computed attenuation. The calculation of atmospheric attenuation is from the 532 channel. This means that when the 532 laser energy drops below 5.5 mJ and it is daytime, or when 532 energy drops below 1.5 mJ (day and night), the attenuation calculation cannot be done and i_reflCor_atm is set to invalid (gi_invalid_i2b). When the 532 laser energy is between 1.5 and 5.5 mJ, the attenuation calculation is only done for night data, and the daytime values of i_reflCor_atm will be invalid.
  • This version incorporates an adjustment to the Cox-Munk model, producing less transmission loss and lower column optical depth at 1064 nm. The Cox-Munk model relates surface wind speed to the reflectivity of water. Given surface wind speed, the reflectivity of water surfaces is obtained from the Cox-Munk model. Since surface wind speed is known at any location over the ocean from the NCEP model, total atmospheric transmission loss (at 1064 nm) can be obtained by comparing the measured ocean reflectance from the altimetry channel to the Cox-Munk calculated ocean reflectance. Careful analysis of the results of this retrieval has revealed that the calculated atmospheric transmission loss was too great, especially in low surface wind conditions. It was determined through an analysis of clear atmosphere only data that a small adjustment to the Cox-Munk model coefficients would alleviate this problem.
  • This version also computes the 1064 nm optical depth over ice sheets. The assumption is made that over ice sheets, the true surface reflectance is a constant 0.82. It should be noted, that while 0.82 is a measured mean value of the surface reflectance over ice sheets based on an analysis of many ICESat observation periods, the actual surface reflectance at 1064 9 nm over ice sheets varies considerably. Thus, the retrieved optical depth from this approach will have an error bar of roughly +/- 20% (over ice sheets).

Changes/Improvements

  • Improved the calibration of some parameters.

Fixes

  • Corrected valid range for d_total_od in the atmospheric transmission calculation.

Utility Changes

  • None

Known Problems

  • None

Release-33 HDF5

Release 33 HDF5 contains the 15 GLAS products converted to netCDF-4/HDF5 format.

Overview

Original GLAS products were created in an integer-binary format. The binary products have been converted to HDF5 to promote interoperability between GLAS data products, products from other earth science missions and future ICESat-2 data products, and to provide products in standards-compliant format.

The ICESat GLAS Release 33 HDF5 data are converted directly from the Release 33 binary data. Data files were not re-processed during conversion. All science data values remain the same as the binary Release 33 products. There are no value-added science parameters. However, parameters in the HDF5 files are re-ordered, re-named, and logically grouped to take advantage of the HDF5 file structure.

User Guides

Release-33 HDF5 altimetry products are accompanied by the GLAS Altimetry HDF5 Product Usage Guide (pdf, 553 KB).

Release-33 HDF5 atmospheric products are accompanied by the GLAS Atmospheric HDF5 Products User Guide (pdf, 213 KB).

General Changes - Products and Parameters

For each GLA product (1-15) there is a corresponding HDF5 product, with the related parameters in each product arranged in groups. Groups are utilized to logically organize parameters. The 15 original GLAS products contain a total of more than 2000 parameters. In the GLAS HDF5 products data are grouped together by rate (1Hz or 40Hz). Each rate group has a time parameter and corresponding latitude/longitude that correspond 1-to-1 with other data parameters within that rate group. Within each rate, parameters are further organized by discipline. Most of the parameter names in GLAS HDF5 are the same as the GLAS binary data products. Attributes are added to each parameter making the products self-documenting.

Differences between the parameters in the original binary products and the HDF5 products include:

  • GLAS binary flags were unpacked and placed on the GLAS HDF5 products as individual parameters.
  • The GLA04 multi-file granules were combined into a single GLAS HDF5 granule. GLAH04 is Combined LPA, LRS, GYRO, IST, BST, SPCA Data File 1A.
  • GLAS binary parameters defined as spares are not present in GLAS HDF5 products.
  • Parameters that were not implemented in the original GLAS products are not present on the GLAS HDF5 products.
  • Some GLAS parameters that corresponded to the first and last shot have been interpolated across all shots.
  • A shot counter, i_shot_count, was added to the GLAS HDF5 products if one did not already exist. When combined with the rec_ndx parameter, the shot_count enables unique identification of each pulse generated by the laser.

Standards Compliant Format

Universally Unique Identifier (UUID)

Each GLAS HDF5 granule has been assigned a Universally Unique Identifier (UUID) which can be used to identify each GLAS HDF5 granule. Shown below are the UUIDs included in the data dictionaries for GLAH05 and GLAH06.

GLAH05 Attribute Value
identifier_file_uuid D65E7C2A-7BC1-444F-AE6F-991DAD0B45FF
GLAH06 Attribute Value
identifier_file_uuid 04D42283-3EE8-40A8-A488-AE95192F7F20

Digital Object Identifier (DOI)

Each GLAS HDF5 product type has a unique DOI registered with the International DOI Foundation. The DOI is embedded in each HDF5 data file, and is included in the data dictionary. Examples:

  • The DOI for GLAS/ICESat L1A Global Altimetry Data (HDF5) is 10.5067/ICESat/GLAS/DATA105
  • The DOI for GLAS/ICESat L1A Global Atmosphere Data (HDF5) is 10.5067/ICESAT/GLAS/DATA106

Climate and Forecast (CF) Metadata Conventions

The climate and forecast CF Metadata Conventions are intended to promote interoperability among data providers, data users, and data services by providing a clear and unambiguous standard for representing geolocations and times of earth-science data, physical quantities that the data represent, and other ancillary information useful in interpreting the data or comparing it with data from other sources. The conventions define metadata that provide a definitive description of what the data in each variable represents, and the spatial and temporal properties of the data (NASA EOSDIS List of Standards, Tech Notes, and RFCs - ESDS-RFC-021).

The GLAS Release 33 HDF5 products follow CF metadata conventions at the granule and parameter levels to have consistent and adequately defined metadata across products and missions. Each parameter written to an HDF5 file includes CF attributes that describe the parameter and provide information both on the HDF5 file and for the generated data dictionary.

netCDF-4/HDF5

The GLAS HDF5 products are designed to be netCDF-4/HDF5 compliant by using appropriate metadata and dimension scales.

The netCDF-4/HDF5 file format enables the expansion of the netCDF model, libraries, and machine-independent data format for geoscience data. Together the netCDF interfaces, libraries, and formats support the creation, access, and sharing of scientific data. Use of the HDF5 storage layer in netCDF-4 software provides features for improved performance, such as compression, parallel I/O, relaxed size limits, and the performance benefits of chunking and endianness control. NetCDF-4 implements the netCDF classic and enhanced data models using HDF5 as the storage layer. (NASA EOSDIS List of Standards, Tech Notes, and RFCs - ESDS-RFC-022).

ISO 19115:2003 - Geographic Information - Metadata

GLAS HDF5 products are aligned with ISO 19115.

ISO 19115:2003 defines the schema required for describing geographic information and services. It provides information about the identification, the extent, the quality, the spatial and temporal schema, spatial reference, and distribution of digital geographic data (ISO 19115:2003 Geographic Information - Metadata).