Description of Data Releases
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 If reprocessed, NSIDC either deletes or hides data granules from previous releases once they have been replaced by the new release.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.
|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 34 incorporates fixes for several data issues that were determined to exist in the GLAS Release 33 data products.
Release 34 contains fixes for several data issues that were determined to exist in the GLAS Release 33 Altimetry data products:
- Correction to the ICESat Data Product Surface Elevation due to an Error in the Range Determination from Transmit-Pulse Reference-Point Selection (Centroid vs Gaussian).
- Dry Troposphere Correction Jitter.
- The GLAS product high resolution DEM was determined to have issues in the Southern Latitude when the source was SRTM data. It was reported to have wrong values that showed as banding.
- The order of preference for which values are used when the SRTM and CDED overlap was changed. Starting with this release the SRTM is always used in the overlap region.
- Some parameters on GLA14 were invalid when i_elev was valid.
- GLA12, 13, 14 and 15 atmosphere character confidence flag was always zero.
- Occasional mismatch of GLA09 atmosphere characteristic flag value and the value reported on GLA06 and 14.
For further details on the corrections contained in Release 34, see the GLAS/ICESat L1 and L2 Global Altimetry Data, Version 34 documentation, and GSAS V6.1 Release Notes (PDF file from NASA Wallops Flight Facility, 248 KB).
A new variable, GmC, has been added to the data for GLA06, and GLA12-15. The GmC variable is the value applied to correct the G-C elevation problem discovered in Release 33. This variable is defined as the difference in the transmit pulse Gaussian fit and the centroid of the transmit pulse. See the Release 34 Altimetry product documentation for more information.
Fixes and corrections in Release 34 pertain to GLA05, 06, 12, 13, 14, 15 and are summarized above in the Overview section and in detail in the GLAS/ICESat L1 and L2 Global Altimetry Data, Version 34 documentation.
Altimetry Products (GLA05, 06, 12, 13, 14, and 15)
Atmospheric products are not affected by corrections in Release 34. Atmospheric products remain at Release 33.
Release 33, which is NASA GLAS Science Algorithm Software (GSAS) release 6.0, incorporated many changes and enhancements.
Release 33 includes several important changes.
- The saturation correction for Laser 3 has been improved.
- 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.
- 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.
- Several changes were made that improve the calibration of the atmospheric product parameters.
- 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.
- 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:
- Product Format and Definition Changes with Release-33
- GSAS V6.0 Release Notes (PDF file from NASA Wallops Flight Facility, 343 KB)
- CSR SCF Release Notes for Orbit and Attitude Determination V3 (PDF file from NASA Wallops Flight Facility, 272 KB)
- Summary of Laser Profile Array (LPA) Parameter Estimation (PDF file from NASA Wallops Flight Facility, 586 KB)
- Notice Concerning Detection of ICESat/GLAS Inter-Campaign Elevation Bias (PDF file from NASA Wallops Flight Facility, 84 KB)
General Changes: Level-1A (GLA01, 02, 03 and 04)
- 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.
- Fixed the GLA04 LRS Virtual Tracker Centroid Row and Column conversion factor.
Atmospheric Products (GLA07, 08, 09, 10, and 11)
Table 1 summarizes atmospheric products for Release 33.
The atmospheric products are accompanied by the GLAS Atmospheric Products User Guide (pdf, 74KB).
- 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).
- Improved the calibration of some parameters.
- Corrected valid range for d_total_od in the atmospheric transmission calculation.
Release 33 HDF5 contains the 15 GLAS products converted to netCDF-4/HDF5 format.
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.
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.
Digital Object Identifier (DOI)582
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.
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).