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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 If reprocessed, NSIDC either deletes or hides data granules from previous releases once they have been replaced by the new release.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.
 

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.

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-31

This release includes several important changes to the GSAS code affecting the rec_ndx scale factor and the waveform Gaussian fit solution matrix. Several changes have been made to the elevation products.

Overview

Release 31 includes several important changes to the GSAS code. The record index (rec_ndx) scale factor has been changed to avoid an October 2009 rollover. This means that the record index now increments by 5 rather than 10. An error fixed in the waveform Gaussian fit solution matrix resulted in minor changes to many fit parameters. The fit control parameters have been adjusted to recover more low signals for standard fit and less false signals for alternate fit. Several changes have been made to the elevation products including replacement of selected portions of the DEM and an update to the geoid.

Release-31 products have a YXX release number of 531. 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-31 are summarized on this page. For more detailed information, see the following for more information:

General Changes

The rec_ndx scale factor has been changed to avoid an October 2009 rollover. This means that the rec_ndx increments by approximately 5 instead of approximately 10. The rec_ndx is designed as a unique record identifier across products within a release and not a time counter, so caution should be used when using rec_ndx for things it was not necessarily intended. This extends the range of rec_ndx for another 7 years.

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

User's Guide

Release-31 altimetry products are accompanied by the GLAS Altimetry Products Usage Guidance (pdf, 260 KB).

Additions

  • Pole tide (i_polTide) had been added to GLA06, 12-15 over ocean.
  • The boresight calibration factor table has been supplied for L2D and L2E.

Changes/Improvements

  • The DEMs over Greenland and Antarctica have been replaced with the ICESat-derived DEM. The GTOPO30 values over Greenland and Antarctica have been replaced with the GLAS-derived 1k and 500m (respectively) DEMS on GLA06, 12-14. The Icesat DEM on GLA06, 12-14 has meters as units.
  • On GLA15, the DEM has been replaced with the Danish National Space Center (DNSC08) Mean Sea Surface. The reference frame for the MSS values is the TOPEX/Poseidon ellipsoid. i_DEM_elv was changed to i_MSS_elv.
  • The Geoid has been updated to Earth Gravitational Model EGM2008. The new geoid file contains the Earth Gravitational Model EGM2008 referenced to the Topex-Poseidon ellipsoid, with mean tides applied.
  • The signal fit width has been adjusted to improve performance with low transmit power.
  • i_msRngCorr and i_msCorrFlg have been changed to spares on GLA06, 12-15.
  • The description on i_cntRngOff on GLA06, 12-15 has been changed.
  • ANC5401-04 (Icesat-DEM) are new inputs to elev processing.
  • ANC55 (MSS) is a new input to elev processing.
  • ANC56 (Pole Tide) is a new input to elev processing.

Bug Fixes

  • An error in the waveform fit solution matrix has been fixed. This results in minor changes to many fit parameters.
  • Corrected the calculation of i_garea on GLA14.

Known Problems

  • The dry troposphere correction is extremely elevation-sensitive. The geoid is the source of the elevation used in the tropospheric refraction correction. The update from EGM96 (old geoid) to EGM2008 (new geoid) causes a 1 to 2 cm shift in elevation due to the dry trop correction. This means that Release 31 data cannot be used with prior releases. Use only Release 31 later data for comparisons.
  • The Icesat DEM on GLA06,12-14 is has meters as units. There is more precision available on the DEM, but it was not put on the product. This may be fixed in a newer release.

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

For Release-31, there are minimal changes, additions, and bug fixes incorporated into the version 5.6 GSAS atmospheric data products.

Due to certain issues with the ICESat lasers since the start of the mission, delivery of the atmospheric products to the user community has been sporadic. With algorithm improvements included in Release 31 reprocessing, some atmospheric products that were not available in the past are now available, while others that were available are not. Table 1 summarizes atmospheric products for Release 31.

The GLA08 and GLA10 products require sufficient 532 laser energy and do not contain any of the 1064 channel derived products. As a result of the decline of 532 laser energy, there are a limited number of useful observation periods for Release 31 GLA08, GLA09 and GLA10 products, as shown in Table 1. GLA08 relies totally on good 532 data (L2A, L2B and nighttime L3A – L3D) to obtain boundary layer height. GLA09 contains both 532 nm and 1064 nm derived cloud heights and blowing snow detection. The 532 derived parameters are good for observation periods L2A and L2B for both day and night. They are only good at night for L2C and for L3A through L3E. After L3E, the 532 laser energy is too low for even nighttime retrieval of cloud height or blowing snow.

The 1064 nm cloud heights on GLA09 are good both day and night for all the observation periods marked in Table 1. Beyond observation period L3I, the 1064 laser energy is too low for reliable cloud detection. Though substantially dependent on the 532 channel, GLA11 contains some very useful information obtained from the 1064 channel that does not depend on laser energy. This includes 1064 column optical depth over oceans and blowing snow detection. The 532 cloud optical depth and column optical depth provided on GLA11 is limited to those periods with sufficient 532 laser energy. This includes L2A, L2B (day and night) and the nighttime data from L3A through L3D. Even though there are 532 nm parameter retrievals on both GLA10 and GLA11 during nighttime past the L3D observation period, their quality is severely degraded and should not be used. During daytime, the 532 parameters on GLA11 and GLA10 are set to invalid for L2C and all laser 3 observation periods. Please see the Release Schedules page for currently available campaigns.


Table 1. Release 31 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, 53 KB).

Additions

  • None

Changes/Improvements

  • Full Resolution Ground Return Signal at 532 nm is now properly filled on GLA09.

Bug Fixes

  • None

Utility Changes

  • None

Known Problems

  • None

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