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ICESat / GLAS Data

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

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Description of Past Data Releases

This page summarizes changes and known limitations associated with past releases of ICESat/GLAS data. For information about the most current release, see the Description of Current Data Release Web page.

The first data release was Release-12; there were no products with release numbers lower than 12. We recommend you work with the latest release of data which is the highest release number and includes improvements to product algorithms.

Click on a release number below to view a summary of its release notes.

Release-31
Release-29
Release-28
Release-26
Release-24
Release-22
Release-21
Release-19
Release-18
Release-17 Atmospheric Products (GLA02, 07-09)
Release-14 Land Products (GLA14)
Release-13 and -14 Period Data (Oct./Nov. 2003)
Release-12 Cal/Val Period Data


Release-31 [top]

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

Release-29 [top]

This release incorporates significant and extensive changes to the atmosphere processing, additions and corrections to the waveform and elevation processing, and the incorporation of new tide models.

Overview

The format of several products has changed, but those changes were incorporated using unused variables and spares to minimize impact on pre-GSAS 5.4 product code. Significant improvements were made to the data product format documentation, and this data release is accompanied by the release of the GLAS Altimetry Products Usage Guidance (pdf, 260 KB) and the GLAS Atmospheric Products User Guide (pdf, 53 KB).

Release-29 products have a YXX release number of 429 or 529. 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-29 are summarized on this page. For more detailed information, see the following for more information:

Early Campaigns Altimetry Data Problem

A problem with the computation of the saturation index was discovered in the Release-28 data and was corrected in Release-29.  The result of making this correction for Release-29 is that fewer waveforms are considered to be saturated (saturation index >= 2).   The saturation elevation corrections are calculated only when the waveform is considered saturated (saturation index >= 2),  and a zero correction is provided if no saturation is indicated.  Release-29 has been determined to have shots that need a correction applied that are not currently being applied. The consequence is noise in the elevation profiles when the saturation correction is toggling between zero and some non-zero value on a shot by shot basis.  The noise is greatest when the energy is high (such as in Laser 1) when even the gain 13 saturation corrections are in the 10-15 cm range.  The problem is most pronounced in Laser 1, Laser 2B, Laser 3A-C.  Laser 2A appears to have other issues (field of view shadowing) which minimized the effect.  The ICESat science team is working to determine the proper saturation correction and plan on a new release of the elevation data.

Laser 1B Data Gap

There is a period of about 13 hours when the spacecraft was in sun acquisition mode; and for that period, there is no valid data. It spans 2003-03-26 11:41:00 to 2003-03-27 00:57:29 UTC.

Level-1A Products (GLA01, 02, 03, and 04)

Changes/Improvements

  • Changed a GPS-time interval limit on GLAS_L0proc to improve the quality and quantity of ANC32 records produced during GPS boundary condition periods.

Known Problems

None.

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

User's Guide

Release-29 altimetry products are accompanied by the GLAS Altimetry Products Usage Guidance (pdf, 260 KB).

Additions

  • A blowing snow range delay estimate and confidence flag from atmosphere processing has been added to GLA06, GLA12 to 14.
  • 40Hz transmit pulse energy was added to GLA05, GLA06 and GLA012 -15.
  • Long period (equilibrium) tides are now being applied and are available at 2Hz on the
    record

Changes/Improvements

  • The reflectivity estimate is now calibrated for several instrument effects. These values are passed on to GLA06, GLA12 to 15. Previous versions contained errors and were not calibrated.
  • Notice on data quality equivalency of Releases 429 and 529: While ICESat/GLAS data continue to follow the YXX Release Numbers format convention for naming data products, the methods for Precision Attitude Determination (PAD) have evolved since launch in 2003. Procedural details for PAD calibration level 4 (Y-code = 4) have been revised and end results improved. Therefore, campaigns that were processed to that level early in the mission are being re-examined in conjunction with the recent GSAS (XX-code = 29) software change. This change in PAD procedures necessitates an increment in the Y-code (to Y-code = 5) for certain campaigns. In summary: Release 428 products were fully-calibrated using the PAD procedures available at the time of processing. Data from latter ICESat campaigns are being regenerated into Release 429 products without any change in PAD procedures. PAD procedures are being re-examined, and Release 529 data generated, for L2a (Sep-Nov 2003), L2b (Feb-Mar 2004), L3a (Oct-Nov 2004), and L3b (Feb-Mar 2005). Release 429 and 529 data have equivalent PAD calibration.

Bug Fixes

  • An error was fixed in the use of the compression index in waveform processing in the computation of received energy.
  • The saturation range correction for gain equal to 13 was fixed to a maximum of 2.2m. A software bug in v 5.3 had limited it to 1.5m.
  • Solar angle and azimuth is now calculated after a pre-geolocation process in the elevation manager. In earlier versions, it was before the pre-geolocation, and would use passthrough values from GLA05 that were not always updated based on latest pointing
    information.

Known Problems

  • Notice concerning L1a footprints: Campaign L1a data contain periods when users should be cautious about using the 4 footprint parameters (i_tpintensity_avg, i_tpazimuth_avg, i_tpeccentricity_avg, i_tpmajoraxis_avg) found in elevation products GLA06, GLA12-15. This issue stems from an incorrect LPA window that was transmitted to the ground, and therefore ground processing cannot correct the problem. This seriously affects the first 3 days of the laser campaign (50-52) and periodically until day 64. Users should use footprint parameters with caution during these periods.
  • Reflectivity, not corrected for atmospheric effects, is calculated as Refl = R/T, where R is the received energy after it has been scaled for range, and T is the transmitted energy. i_reflctUncorr has also been calibrated for gain non-linearity (only for non-saturated waveforms), ground truth calibration and boresight shift shadowing (BSS). It is not corrected for saturation effects. If the shot is saturated (satindex above 2) then to correct for saturation the reflectivity estimate needs to be multiplied by the ratio of the corrected energy to the uncorrected energy (sat corrected reflectivity = i_reflctUncorr * (i_RecNrgAll + i_satNrgCorr)/i_RecNrgAll)
  • The atmospheric corrected reflectivity may be calculated from this uncorrected reflectivity by multiplying it by d_reflCor_atm.
  • The saturation energy correction has not be applied to the received energy before the computation of reflectance. The saturation energy corrections have not been fully verified and need additional investigation. A few of the concerns are:
  1. If the saturation energy correction is very large compared to the received energy the data is questionable and often a large correction will cause reflectivity values greater than 1 to be computed.
  2. The saturation energy corrections were determined using narrow waveforms and may not be valid for wide over-land waveforms.
  3. For about 20% of land data there is saturation for which the current saturation energy correction does not provide a valid correction.

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

For Release-29, there are many changes, additions, and bug fixes incorporated into the version 5.4 GSAS atmospheric data products.

User's Guide

The atmospheric products are accompanied by the GLAS Atmospheric Products User Guide (pdf, 53 KB).

Additions

  • Blowing snow has been added to the GLA09 data product. The following are stored at the 5 Hz rate on GLA09: blowing snow layer height, optical depth, range delay and confidence flag. The latter two parameters are also stored on the GLA11 product at the 1 Hz rate.
  • 1064 Total Column Optical Depth (over oceans only at 40 Hz and 1 Hz) has been added to GLA11. This is based on the measured (from altimetry channel) surface reflectance over ocean and the computed surface reflectance value (from the Cox-Munk model) as a function of surface wind speed. The computed (from the surface wind) surface reflectance value is also stored on GLA11.
  • Diurnal Cloud Flag added to GLA09. This new parameter identifies when a given cloud layer detected at night could have been detected during daytime. A space existed in the GLA09 data product for this parameter in prior releases, but its value was not computed until now.
  • Browse Products are now available for GLA07-11.
  • GLA08 now contains a fairly reliable aerosol layer height product. This provides top and bottom height of at most 2 aerosol layers detected from the 1064 channel at a 4 second resolution.

Changes/Improvements

  • Improved 532 calibration during daytime – affects laser 2 and 3 GLA07 532 backscatter profiles
  • Changed (improved) calibration for 1064 laser 3 operation periods – affects GLA07 1064 backscatter profiles
  • Changed the extinction retrieval method from one that operated only within detected cloud and aerosol layers to a method that computes extinction for the entire vertical profile (20 km to the top of the first cloud) – affects GLA10
  • Incorporated GMAO (Goddard Modeling and Assimilation Office) GEOS-5 aerosol transport model calculations of aerosol type as a function of geographic location and time to improve aerosol optical depth calculation – affects GLA11.
  • Improved multiple scattering correction and multiple scattering range delay calculation – affects GLA10 and GLA11
  • Added 532 total column aerosol optical depth and use flag to GLA11 and the bottom height of cloud free troposphere to GLA10
  • No longer use a calculated extinction to backscatter value for cirrus optical depth during daytime (affects some cloud optical depths on GLA11).
  • When the 1064 laser energy is less than 25 mJ, then the following occurs: 1) 40 Hz 1064 cloud search is not executed – the 40 Hz cloud top value is set to 0.0 and the quality flag set to 15. 2) the 1 and 4 second resolution 1064 cloud layer quality flag is set to one to indicate lowest quality
  • When the 532 laser energy falls below 1.5 mJ, 532 processing is halted. All 532 data products will be invalid for both day and night.

Bug Fixes

  • Fixed 532 calibration error induced by altimetry ocean scans and around the world scans.
  • Corrected a bug in the 532 extinction retrieval that caused an incomplete computation of extinction profiles and an error in optical depth.
  • Fixed a bug in cloud/aerosol discrimination that caused a step-decrease in retrieved aerosol amount poleward of 60 degrees.

Utility Changes

  • QA-associated metadata values are now available in the GLAS .MET files. These data will be available for use by customers selecting data from NSIDC.
  • Created a new utility (ANC32_QA) to perform post-L0 processing QA on ANC32 files. This utility provides improved sanity checking on time calibration during the production of Level 1 data products.

Known Problems

  • Background correction for (some) 532 daytime data is still problematic. After being hit with a large amount of background light, the 532 detector has a time dependent response which is a non-linear function of the background level. This will result in the GLA07 attenuated backscatter profiles to be miss-calibrated especially in areas of very bright or rapidly changing background.
  • The 1064 cloud heights (all resolutions, but especially the 40 Hz cloud heights) suffer from false positives (especially) in daytime for operation periods after L3E. This becomes very noticeable when the 1064 laser energy falls below 30 mJ. It also affects the nighttime data as laser energy continues to fall.

Release-28 [top]

This release incorporates significant changes to the Level-1B and Level-2A product parameters. There are now improved corrections for saturation effects and improved flags to aid in data selection.

Overview

Note: The saturation elevation correction needs to be added to the elevations by the user, but the correction is not generally recommended for land surfaces (GLA14).

Many parameters were improved with updated algorithms, and some are now being computed for the first time. The data dictionary and flag descriptions were updated as well. A significant effort was made to improve descriptions of the altimetry products. As a result of applying the updated oscillator frequency to the initial time tag estimates, the record index may have changed from the previous release by one or two counts. All Release-28 products have consistent time tags; the sample times have not changed. The surface slope and roughness are output as invalid values for this release.

A new release number designation was adopted to denote accuracy of orbit and laser pointing knowledge which contribute to elevation accuracy. Refer to the YXX Release Numbers Web page for more information. Release-28 products have a YXX release number of 428 in the new designation.

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

Notable changes to the data in Release-28 are summarized on this page. For more detailed information, see the following for more information:

For this release, file naming has changed. Please see the following for more information about the new file naming convention:

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

Please read the Notes About Saturation Elevation Correction below. The changes to altimetry products include:

  • Corrections are now applied for oscillator frequency as determined by the GLAS engineering team. All elevations are shifted about 2 cm in the horizontal, and the relative elevations between operations periods shift less than 3 to 8 mm. This change means products from version 5.3 (Release-28) and higher should not be used in combination with products from earlier releases in any research.
  • The saturation range correction has been renamed to saturation elevation correction (SatElevCorr) to reflect the proper sign of the correction. More importantly, it is provided when it is believed to be correct and is invalid when a correction is needed but algorithm can not provide a value. Also the flag (satCorrFlg) has been improved for better data usage determination. The saturation correction has not been applied to the elevations, so users should apply this correction when it is valid. When it is not valid, data should be used with care as it may be biased. For this release the percent saturation for the flag to be inconsequential is 2%. This low value was picked so studies can be made of the low saturation correction. Since these are small values this may either improve the data or add a small noise to the results.
  • A correction to the received energy computed from saturated waveforms is provided (satNrgCorr). It has not been applied to the data.
  • The algorithm for setting the saturation index (satNdx) was changed. The index is still the number of samples above a threshold but this threshold is now receiver gain dependent.
  • The fit standard deviations (wfFitSDev) may have been computed from either the raw waveform or a normalized waveform. For this release, the standard fit uses the raw waveform and the alternate fit uses the normalized waveform. The maximum amplitude has been added to most products to allow users to rescale the standard deviations to the base they want to use.
  • The fit to the transmit pulse changed slightly. This may induce a few mm change in its location at times and is reflected in the range and elevations.
  • A new parameter (pctSAT) to express the percent of waveform saturated has been added. The science team is still evaluating this parameter regarding its usefulness in data quality selection.
  • The waveform quality flag word has changed. A bit was added to indicate if waveforms are clipped at maximum values. Also some of the flags were found to be confusing and were removed.
  • The alternate fit algorithm has been tuned to provide fewer peaks in narrow return echoes. These should be more meaningful to true surface features. Most saturated echoes are now fit with a single peak. Also it was found that some fits were not keeping the last peak when they should have been. For this release, the last peak from the initial estimates is kept during the fit process. If its amplitude goes to zero it will be deleted from the output. (In this release, parameters are set so it should never get to zero amplitude, but still may be insufficient.)
  • The sea ice roughness algorithm has been changed (GLA13 RufSeaIce).
  • Some GLA15 algorithms have been updated and additional parameters have been added. Several previously unfilled parameters are now filled appropriately.
  • The high resolution SRTM DEM has been updated to use the “finished” data files The averaging scheme to fill in missing data is no longer being used. Data products now have more complete coverage when not pointing at the reference ground track.
  • The transmit energy algorithm was updated to match the GLAS engineering team calibrated values.
  • An error in the Gaussian filter smoothing was fixed. As a result, the first estimated peak information may be different. This leads to some initial estimates being different and can result in different final peak estimates on the products.
  • Products should be more consistent with corrections being invalid if the parameter is invalid. This may induce changes to editing rules for some users.

Notes About Saturation Elevation Correction

The Release-28 products contain an improved correction to the surface elevation for saturated returns. The corrections are not applied to the elevations on the products. Detector saturation, caused by higher than predicted energy returns, occurred frequently and is most pervasive in the early operation periods of each laser (Laser 1, Laser 2A, Laser 2B, Laser 3A, and Laser 3B). The saturation elevation correction mitigates this problem. Ice sheet crossovers for the Laser 3A and Laser 3B operation periods show a reduction in the ice sheet crossover standard deviation from 20 cm down to below 10 cm. The user is encouraged to apply the correction by adding it to the surface elevation when the correction value is valid. Unsaturated returns will have a correction value of zero.

The saturation elevation correction has two known problems in the Release-28 data.

  • The first is that a cutoff of 1.5 m was used for all gain values. The correct cutoff would have allowed the correction to go as high as 2.25 m for the case when the gain is 13. In this case, the saturation elevation correction is set to invalid and the elevations should be discarded or used with the understanding that a meter-level error may exist.
  • The second possible problem is that determination of which returns are saturated may have discounted some very slightly saturated waveforms.

The effect of both of these issues is that the correction will not be “defined” for all possible saturated returns. It is still recommended that the correction be used when valid. These issues will be corrected in an upcoming release.

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

The changes to atmospheric products include:

  • Boundary Layer Height (GLA08): The PBL height retrieval over oceans was modified such that the result can not exceed 3600 m. Prior versions would produce erroneously high values over ocean due to clouds. For latitudes less than -65 degrees, the boundary layer height quality flag (qf) found in i_LayHgt_Flag (byte offset 301), is set to 1, indicating lowest quality. This was done to flag the PBL height retrievals over Antarctica which are of poor quality.
  • Cloud/Aerosol Discrimination (GLA08 and 09): A problem in prior releases was the misidentification of an aerosol layer with an embedded cloud as a cloud layer. The cloud/aerosol discrimination routine was changed such that it could separate out the two layers into the aerosol and cloud components. However, the identification and removal of false positive cloud and aerosol layers was removing too many valid layers. This was adjusted so as to keep more good layers while eliminating the invalid layers.
  • Aerosol Extinction and Optical Depth (GLA10 and 11): A change to the optical processing routines was made so that it would not process the boundary layer if the quality flag (qf) is set to one. This means that no boundary layer optical depth or extinction retrievals are made for any of the measurements below -65 degrees latitude (where the qf is set to one everywhere). This was done to rectify a problem with high aerosol optical depth over Antarctica being caused by low cirrus clouds being mistaken for boundary layer aerosol. In other areas of the globe where the qf is one (set by the ratio of PBL average signal to the avgas signal above PBL), this should have little effect as retrievals with qf=1 as they consist of low confidence retrievals with weak PBL backscatter (and very low optical depth).

Known Problems with Release-28

  • Notice concerning L1a footprints: Campaign L1a data contain periods when users should be cautious about using the 4 footprint parameters (i_tpintensity_avg, i_tpazimuth_avg, i_tpeccentricity_avg, i_tpmajoraxis_avg) found in elevation products GLA06, GLA12-15. This issue stems from an incorrect LPA window that was transmitted to the ground, and therefore ground processing cannot correct the problem. This seriously affects the first 3 days of the laser campaign (50-52) and periodically until day 64. Users should use footprint parameters with caution during these periods.
  • Notice concerning L3a Release 428 footprints: Campaign L3a Release 428 data contain footprint parameters estimated inconsistently compared to other campaigns. For example, the reported footprint major axis may be too small by a factor of 2 or more. Users should not rely on the 4 footprint parameters (i_tpintensity_avg, i_tpazimuth_avg, i_tpeccentricity_avg, i_tpmajoraxis_avg) found in elevation products GLA06, GLA12-15. Users may wish to substitute the average size (55.8 m) and eccentricity (0.57) as reported in the Attributes for ICESat Laser Operations Periods table. Data will be corrected in the next Release of L3a.
  • Altimetry parameter descriptions are still being improved in the data dictionary.
  • As with prior versions, daytime 532 data from laser 2A and early 2B operating periods suffer from a background problem related to the detectors. Essentially, the background is a function of range and it is difficult to accurately measure and compensate for this effect. Consequently, the daytime 532 data are sometimes poorly calibrated. This problem becomes worse as the laser energy decreases in the laser 2B operating timeframe.
  • Clouds that are embedded within aerosol layers will be detected at the 4 second (low) resolution, but it is possible that under some conditions they will be missed at the higher resolutions. This would usually occur with cumulus clouds embedded within a 3-5 km thick aerosol layer in the lower troposphere.
  • We are still investigating the GLA04 IST time tag alignments.
  • The science team created a new estimated surface slope algorithm for flat surfaces. The new estimated slopes on flat surfaces at low slopes were found to be larger than expected. The science team is investigating the reason why the received and transmit signal widths would be causing this problem. The surface slope and roughness for this release have been changed to output as invalid values.
  • Please read the Notes About Saturation Elevation Correction in the description of changes to altimetry products for Release-28.

Release-26 [top]

The data dictionaries and flag descriptions were significantly updated for this release. There were no L1A changes.

Notable changes are summarized here. See a list of product format and definition changes associated with Release-26. Please refer to the NASA Wallops Flight Facility's GSAS v5.2 Release Notes for detailed release notes for each product in Release-26.

Altimetry Products

The changes to altimetry products are:

  • The range correction for saturation is now available for all gains. Calibration testing has shown that only values for saturation correction less then 3 meters should be used. To apply it to the range estimate, subtract the correction from the range estimate. To apply the correction to the elevations, it must be added to the elevation estimates.
  • When computing noise from the echo there was an error in using compressed samples which has been fixed. Past and current processing use the observed noise so this fix will not effect product results.
  • An error in the ordering of the load tide correction was fixed. Minor differences in results are seen.
  • The reference frame for the DEM elevation was changed to the TOPEX/ Poseidon ellipsoid to make it consistent with the GLAS elevations. Previous releases had the SRTM30 elevations referenced to the egm96 geoid.
  • There were no changes to the standard fitting used for the ice sheet elevations. The alternate fitting had changes that for some echoes significantly change the detection of peaks.
  • Alternate fit, when set, now consistently keeps the last peak. It had been found that alternate fit was not keeping the last peak in some cases.
  • An option was added to waveform fitting to not refine initial peak estimations based on the Saturation Index size. This release sets the option such that the initial peaks will never be refined for alternate fits.
  • Alternate fit implemented an adaptive peak selection. When combining close initial peaks, and the number of peaks reaches 6, combining stops. This also means for less then 7 initial peaks it uses the initial peaks. This means the software is determining that when all 6 peaks are close together, it can change the minimum peak separation.
  • Wind speed and direction from met files were added to GLA15.

Atmospheric Products

Release-26 incorporates two significant additions for GLAS 532 nm data products. First, there are improvements to parameter tables used to calculate the aerosol optical depth and cloud multiple scattering factors; these improvements considerably increase accuracy. Second, Release-26 is the first to include data products from the Laser 2B (February 2004) operating period.

  • Aerosol optical depth is primarily calculated by using the 532 nm observed backscatter cross section, cloud clearing and identifying layers, and carrying out a forward integration solution. The solution requires integrating variables, S, of the extinction-to-backscatter cross section, which is aerosol model dependent. Previously, the S values were obtained from a geographical based look-up table of aerosol type. Errors arise since aerosol type varies due to source and transport history. For Release-26, the S value look-up table is modified by output from the Naval Aerosol Analysis and Prediction model. Based on the model output, a daily update of regional aerosol composition is applied to calculate the S value table. Improvements are noticeable in some problem areas, such as low optical thickness over water.
  • Multiple scattering calculation errors were found in the look-up tables for the attenuation correction and cloud pulse stretching used in previous releases. These are largely corrected. Previous tables had the cloud height and thickness indexes reversed. The error was corrected, and the values in Release-26 are substantially more accurate. Cloud forward scattering correction estimates are larger and now appear to agree better with observations. There are still several known issues, including:
    • The 532 nm attenuation correction table has values calculated to the surface for 1064 nm.
    • The statistical noise in values is too large.
    • Calculation of the cloud forward scattering bias to surface ranging is possibly significantly too small for cloud layers at the surface.
  • All data products are provided for the entire Laser 2A period, but for the Laser 2B period, only data from 17 February to 4 March 2004 are provided. An important instrument problem for this time period was the rapid decrease of the laser output energy at 532 nm. While nighttime results are still nominal, the daytime signals were falling into the solar background noise floor. Thus data product accuracy decreases throughout the period, especially for daytime measurements. Therefore, only data through 4 March is released, because daytime products with current processing algorithms are judged too unreliable beyond this date.

Other issues for Release-26 are.

  • Includes the following additional MET parameters: NCEP model PBL height, 2 m specific humidity, 2 m temperature and total cloud cover.
  • The 1064 Medium Resolution Cloud detection had an error that when more than one layer was detected, occasionally (sometimes, but not always) the second or third layer top and bottom height values were reported as invalid, although the number of layers indicated that there should be a valid layer there. This problem was corrected.
  • In previous releases, the availability flags in i_MRir_QAflag were incorrectly packed such that the parameter did not conform to the documentation. They are now packed correctly and are consistent with the documentation.

Known Problems with Release-26

  • The atmospheric correction to range for multiple-scattering (i_erd) is not correct. The problem is the result of an error in the ANC38 file that supplies the data to compute the delay.
  • The estimates of slope and roughness on the elevation products are not correct. The equations in this release do not use the real laser footprint information and do not correct for pointing.
  • The Transmit flag i_TxFlg may not be set correctly
  • The following products parameters are not filled/defined:
    i_satNrgCorr
    i_satPwdCorr
    i_LRg_SourceFt
    i_MRg_SourceFt
    i_HRg_SourceFt
    i_LRir_SourceFt
    i_MRir_SourceFt
    i_cld_ir_OD
    i_Aer_ir_OD
    Aer_ir_ODFlg
    Cld_ir_ODFlg
    FRir_ODFlg
    i_msRngCorr
    i_msCorrFlg
  • The STRM track files will be improved and updated to version 2.
  • The attitude flag on GLA06 and GLA12-15 has been reported to be in error.

Release-24 [top]

This is a major release. Most changes affect the Level-1B and Level-2 products. Only minor changes were made in Level-1A processing (correction made to the EU conversion of GLA04 IST i_VTBoreH and i_VTBoreV). The following products were updated to Release-24: GLA01 - GLA15.

Notable changes are summarized here. See a list of product format and definition changes associated with Release-24. Please refer to the NASA Wallops Flight Facility's GSAS v5.0 Release Notes for detailed release notes for each product in Release-24.

Altimetry Products

The key change made to the altimetry products is the lengthening of all of the Level-1B and Level-2 products allowing space for new variables and additional growth. These new variables include:

  • A high-resolution DEM value from the SRTM V1 90 m resolution DEM for latitudes between 60° N and 60° S.
  • Gain and received energy placed on all of the Level-1B and Level-2 products.
  • Surface pressure, temperature, and relative humidity on all Level-2 products.
  • Saturation range correction for low gain (13 counts) returns.
  • 1064 nm cloud top and integrated signal from the lidar team (40 Hz).

Changes have been made to both the transmitted and received energy calculations, as suggested by the instrument team. For details of the energy computation, see the waveform ATBD.

Two minor changes have been made to the standard parameterization waveform fitting. The standard fit now uses the absolute peak location change (instead of a percentage change) as the convergence criteria in the Gaussian fitting. An error in the computation of the standard fit sigma has also been corrected. Neither of these changes has significantly affected the elevations on the data.

In the alternate waveform fitting, an option to normalize the waveform based on the peak amplitude was implemented. An error in reporting the alternate fit standard deviation was corrected and the precision was changed on the product to accommodate the smaller numbers. The alternate fit parameters were changed to perform normalization by peak amplitude before fitting, the minimum distance between selected peaks was set to be greater than 15 ns, and the weight of the sigma widths was tuned to provide the best alternate fits on selected land waveforms.

Atmospheric Products

Through I-SIPS Release-23, cloud detection using the GLAS 1064 nm atmospheric channel was limited to 4 and 1 second resolution. This was due mainly to the low inherent signal-to-noise of the 1064 data. Over approximately the last 6 months, we have been striving to produce 1064 cloud top height on a shot-to-shot basis (40 Hz). The new algorithms have been tested and are included in this release of GSAS. The limited amount of testing done to date has shown that we can definitely detect clouds at this resolution, though the exact limits of what can be detected (in terms of optical depth) have not yet been quantified. An assessment will be performed after the laser 2A data has been reprocessed using this release of the GSAS software, by comparing the 1064 nm cloud detection with that of the 532 nm channel. A part of the cloud detection algorithm is based on the integrated 1064 nm signal, which is also stored as a separate quantity on the Release-24 GLA09 product. Testing has shown that when the magnitude of the integrated signal exceeds a certain threshold, it is highly likely (as corroborated by the 532 channel) that there is a cloud present even if the threshold algorithm did not detect a cloud. In this case, the cloud height is set to 10 km and a flag is set to indicate this fact. The algorithm also interrogates the ground return signal width in an effort to detect very low clouds. Over flat terrain such as ice sheets, the 1064 ground return signal has a characteristic width. When this signal is unusually wide, and clouds have not been detected higher up, the cloud top height is set to 100 m and a flag is set to indicate the likely presence of low clouds. While these approaches improve the overall cloud detection, they also have their limits. Extensive testing is needed to determine those limits.

Known Problems with Release-24

  • More improvements to waveform processing are forthcoming, including better normalization, alternate fitting, and saturation algorithms.
  • New parameters i_Surface_wind and i_Surface_wdir are not yet filled.
  • New parameters i_LRg_SourceFt, i_MRg_SourceFt, i_HRg_SourceFt, i_LRir_SourceFt, i_MRir_SourceFt, i_cld_ir_OD, i_Aer_ir_OD, i_Aer_b20_prop, i_PBL_prop, Aer_ir_ODFlg, Cld_ir_ODFlg and FRir_ODFlg are not yet filled.
  • GLA01%main%i_TxFlg is not always correctly set.
  • The STRM track files will be improved and updated to V2.
  • The 1064 medium resolution cloud layers on the GLA09 product has a problem. When more than one layer is detected, sometimes, but not always, the second or third layer top and bottom height values are reported as invalid, but the number of layers indicate that there should be a valid layer there. This problem happens very infrequently when only one layer is detected, but it does happen occasionally. It is usually associated with multiple layers. The low resolution 1064 cloud heights do not seem to have this problem.
  • Another problem was found in the release 24 data. This is the value of the i_LRcld_grd, i_MRcld_grd, i_HRcld_grd, and i_FRcld_grd parameters on GLA09. In prior releases, when no ground is detected, they had a value of -88. Now they have a value of -13. The documentation for these parameters is: "The height from the reference ellipsoid of the ground as detected by the low resolution cloud processing algorithms. A value of -880 indicates that the ground was searched for, but not detected."

Release-22 [top]

The following products were updated to Release-22: GLA01, GLA02, GLA03, GLA04, GLA05, GLA07, GLA08, and GLA09. This release contains significant improvements to the waveform alternate fitting process, to energy/reflectance calculations, and to atmospheric products. Notable changes are as follows:

Altimetry Products

The energy calculation improved for GLA01. For GLA05, energy/reflectance calculations improved and a saturation index was added. The alternate fit uses the normalized waveform based on area before functional fit. (Standard fit uses raw voltage waveform.) The alternate fit now also uses up to five largest peaks plus the peak nearest the ground. The algorithm keeps all six peaks, but amplitudes are allowed to go to zero. The alternate fit least-squares exit criteria changed; maximum iterations are now significantly less frequent. If the maximum iteration flag is set, data should not be used (or used with caution). See a list of product format and definition changes associated with release-22.

Known Problems with Release-22 Altimetry Products

  • A test in the received energy calculation based on the noise signal standard deviation is not being performed.
  • An investigation is needed for the alternate fit related to tuning parameters and always keeping six peaks.
  • 40 Hz cloud heights based on the 1064 nm channel in atmosphere processing are forthcoming.
  • Shuttle Radar Topography Mission (SRTM) DEM improvements are forthcoming.
  • Standard fit is not performing according to the Algorithm Theoretical Basis Document (PDF file, 3.2 MB), but it is currently not being changed.
  • Fit standard deviation for alternate is based on a normalized waveform, but for standard, it is based on the raw waveform.
  • Significant product format changes (including record size changes) are forthcoming in the next major release.

Atmospheric Products

The 532 nm cloud layer detection improved. The 40 Hz cloud search is now executed for every shot, independent of the results of cloud searches at lower resolutions. Also, the starting height for the cloud search is now 10 km, which means that clouds are now reported when they are found up to 10 km altitude. (In prior releases, 4 km was the maximum altitude). These changes mean that a cloud could be reported for one or more 40 Hz shots of a given second, while no cloud was reported at 1 second or 5 Hz resolution.

The major problems with these data pertain to the 532 channel only. The 1064 backscatter data (GLA07) appear well-calibrated and the 1064 cloud heights are robust both day and night.

The 532 channel had only 5 mJ of energy and a fractured far-field laser beam pattern that reduced the signal strength considerably. As a result, the calibration for daytime 532 data is often poor. The calibration of nighttime data is generally acceptable. The 532 channel signal levels are very low, which severely affects the daytime measurements.

Because of the poor 532 channel signal, elevated aerosol and PBL height data (GLA08) will be unusable during day and only marginal at night. Thicker aerosol and PBL layers can still be detected, but only at night. Cloud height data (GLA09) are good during night, but only thick clouds can be detected during the day.

Optical depth and extinction data (GLA10 and GLA11) are not included in this release but attempts will be made to release nighttime only data in the future.


Release-21 [top]

This release contains the following improvements to the elevation products only (GLA06, GLA12-15). No product format changes were implemented.

  • Fixes a problem that caused the GLA06 Precision Attitude Determination (PAD) data to pass directly from GLA05, mostly bypassing any PAD input files.
  • Fixes a data synchronization issue that occurs when GLA09 and GLA11 are input to the elevation processing, which caused the ANC09 data to be unavailable for a large percentage of the time.
  • Fixes an unrelated problem where a large number of instrument state changes caused an array overflow error.

Known Problems with Release-21

Known problems are the same as Release-19, plus flagging does not correctly indicate when GLA05 pass-through PAD is used in place of data from ANC09.


Release-19 [top]

Notable improvements include the following:

Altimetry products

  • Several product format and definition changes.
  • Implemented a 1-shot gain shift algorithm that should improve energy-related parameters.
  • Added a new flag (i_GainShiftFlg) to GLA01. Each of the 40 flags indicates if the corresponding gain measurement was shifted.
  • Fixed multiple problems involving the energy computation.
  • Improved alternate fit for a specific case of waveforms. Waveform alternate fits were changed to always keep the six most significant peaks for fitting. See Revised Alternate Waveform Fitting Release Notes for Release-19 for more specific changes regarding relevant improvements in this release.
  • Added new Laser Profile Array (LPA) orientation parameters to waveform and elevation products.
  • Improved calculation of sea ice roughness parameters and added new sea ice parameters to elevation products. See Attachment A, Level-2 Sea Ice Product - GLA13 at the end of GSAS v4.1 Release Notes (PDF, 300 KB) for more specific changes regarding relevant improvements in this release.
  • Added parameters and modified parameter units in several products. See GSAS v4.1 Release Notes (PDF, 300 KB) for details.
  • Release-19 fixes a problem where the incorrect EGM96 geoid undulations were used as input on the previous release.

Atmospheric products

  • Several product format and definition changes.
  • Added along-track QA and browse capability, and significantly improved QA processing and browse products.
  • Added surface type flag (i_surfType) to atmospheric products.
  • Improved aerosol/cloud discrimination and layer detection.
  • Added parameters and modified parameter units in several products. See GSAS v4.1 Release Notes (PDF, 300 KB) for details.

Known Problems with Release-19

  • More improvements may be needed in sea ice roughness calculations. Validity of the additional parameter needs to be verified.
  • 532 nm background in daylight conditions is not computed properly.
  • Area fit of saturated waveforms is still being investigated.
  • i_reflCor_atm is corrected only for the molecular optical depth, not for cloud and aerosol effects.
  • Laser transmitted energy calculation is incorrect.
  • 40 Hz cloud layer height generation needs improvement.
  • 40 GHz cloud heights from the 1064 nm channel are not implemented.
  • Need to replace estimated atmospheric transmissivity with actual in reflectivity calculation.
  • GLA02 browse product is not available.
  • Duplicate images in GLA05 browse products cause a mismatch when viewed in the GLAS Visualizer.

Please refer to the NASA Wallops Flight Facility's GSAS v4.1 Release Notes for detailed release notes for each product in Release-19.


Release-18 [top]

Improvements include the following:

  • All products: The ScienceQualityFlag is set to InferredPassed.
  • Waveform product (GLA05): Fixes error that discarded some standard and alternate fits that were flagged as poor fits. Corrects condition where alternate fits were not attempted on some waveforms that had a standard fit, and where there was no fit for some transmitted pulses.
  • Elevation products (GLA06, 12-15):
    • Release-18 data were supposed to contain correct geoid height calculations after errors were discovered in previous data releases, but geoid height errors are still apparent in Release-18. Release-19 data should contain corrected geoid heights./LI> Corrects geoid heights in elevation products. (Read more about this issue.) Geoid heights are now properly referenced to the GLAS ellipsoid in a mean-tide system.
    • Five new atmospheric flags were added: i_atm_avail, i_erd, i_rdu, i_cld1_mswf, and i_MRC_af.
    • Corrects bit 40 of i_rng_UQF.
  • Atmospheric products:
    • Improves calculations of i_erd, i_lat, i_lon, and multiple scattering correction to extinction and optical depth in GLA10 and GLA11.
    • Corrects bugs in i_cld1_mswf, i_erd, and i_rdu when no layers are sensed.
    • Modifies the threshold for ground detection in GLA09 to reduce occurrence of false positives.
    • Changes the 1064 nm calibration value for laser 1 data from 42 to 53. Calibration for the 1064 nm channel is very good now, and can be verified by independent coincident measurements.
    • Corrects bug in code that prevented the 1064 nm cloud layer detection routine from executing when the SPCMs (532 nm channels) were turned off.

Known Problems in Release-18 Altimetry Products

ICESat/GLAS Release-18 and earlier data products error in received energy computation:

The 40 Hz received gain values i_gainSet1064 reported on GLA01, and i_gval_rcv reported on GLA05, are offset from the shot to which they actually pertain. The gain used onboard for shot N+1 is contained in the telemetry for shot N. Ground processing should therefore use the gain reported with for shot N in the energy computations for shot N+1.

For Release-18 and earlier ICESat/GLAS data products, this shift was not made; therefore, all parameters that used the gain on GLA01, 5, 6, 12, 13, 14, and 15 have some error. The energy parameters are in error by an amount proportional to the shot-to-shot change in the received gain setting. No error is induced in the elevation measurements from this wrong application of the gain setting.

Parameters that are known to be affected are:

i_RecNrgAll_EU
i_RecNrgLast_EU
i_reflctUncorr
i_reflctuncmxpk

Gain is also used to determine the waveform saturations flags:

i_WFqual (gwi_satFlagLo)
i_WFqual (gwi_satFlagHi)
i_WFqual (gwi_satFlagHiF).

The gain on GLA01 will be shifted to the correct shot in a future release of the data. This will fix all the problems in the ground processing and correct the parameters on the data products.

Known Problems in Release-18 Atmospheric Products

GLA08 and GLA09: Cloud aerosol discrimination usually results in layers misidentified as aerosol when they should in fact be cloud. The problem is most severe in the high-latitude regions and over the poles, and in regions of multiple cloud layers where the signal attenuation from the upper layer makes the lower layer "look" like aerosol.

GLA07: Background computation problem in areas of very bright light cases signal to be poorly calibrated in some instances where the background is high.


Please refer to the NASA Wallops Flight Facility's GSAS v4.0 Release Notes for detailed release notes for each product in Release-18.


Release-17 Atmospheric Products (GLA02, 07-09) [top]

Release-17 data include protection for floating point exceptions, corrections to science software documentation, and improved Quality Assurance Products (QAPs) and browse products. Release-17 products include data from GLAS laser #2. Atmospheric products now have substantially improved PBL height retrieval (GLA08), improved cloud layer detection from 1064 channel, and a new time-dependent calibration for 1064 nm.

The GLAS data product Release-17 is the initial production run of data products for the GLAS 532 nm atmospheric channel. Preliminary data quality and validation activities have been preformed, but additional testing and improvements will be incorporated in further releases.

GLAS is a two channel atmospheric lidar and was operated for two periods in 2003. The instrument was run with only the less sensitive 1064nm channel from February 21 to March 29. GLAS was first fully turned on at the end of September. There is data from both 1064 and 532 nm channels from Sept. 25 to Nov. 18. From Sept. 25 to 28 the filter and alignment for the 532 nm channel were not yet set, and there were also subsequent short periods where the 532 nm data quality was variable when the alignments were reset. For most of the fall period of operation, we obtained very good data.

The GLAS instrument was launched with a number of hardware level problems that effect atmospheric data quality. These include some signal non-linearity and variation of offset under certain conditions. The large majority of errors have been corrected or mitigated in the data processing, but some known issues require further attention.

GLA02 Lidar Signal

The 1064 nm detector circuits introduce “signal droop” resulting in too low values following strong signals. The effect is corrected in the GLA02 software for low and medium strength signals, but due to saturation and other effects, is not fully corrected for strong return signals.

GLA07 Calibrated Observed (Attenuated) Backscatter Cross Section

The 532 backscatter cross section calibration is very good at night and with daytime low to medium backgrounds. At higher background levels (> 10 photons/bin), the detector response varies somewhat with range, causing problems in the background subtraction. which in turn causes the 532 profile to not follow Rayleigh with high accuracy. The problem is estimated to be in about 10% of the daytime data. The 1064 calibration must be based on comparison to the 532 nm data where signals overlap and ground data intercomparisons and is not as accurate as the 532 nm calibration. The 1064 nm calibration in release 17 are now thought to give backscatter values about 5-10% low (calibration constant about 5-10% high), based on further analysis and results from 532 calibration transfers.

GLA08 Planetary Boundary Layer Heights

Planetary boundary layer heights are in general very good. Some problems are:

  1. Over polar regions some low cirrus type clouds may at times be identified as PBL.
  2. In regions where the gradient of scattering at PBL top is very small, a PBL height may be assigned even though it is likely in error. This problem can be mitigated by checking the value of the PBL quality flag and eliminating those PBL heights with a low quality flag.
  3. A few of the elevated aerosol layers reported in GLA08 are actually clouds. This is especially a problem in polar regions and multi layer clouds where attenuation has made the lower layers appear as lower cross sections.

GLA09 Cloud/Aerosol Heights Including Multi Layers

  1. Under certain attenuation conditions in geometrically thick but optically rarefied layers, the analyzed layer bottom is too high.
  2. Some false bottoms are generated in the daytime signal which sometimes leads to multiple layer results where only one layer exists.
  3. PBL heights sometimes supercede elevated aerosol layer results and cloud results.
  4. Layer cloud/aerosol determination depends upon magnitude of backscatter coefficient and vertical gradient of magnitude. Two known situations where clouds are designated to be aerosol layers are: a) when a vertically extensive but tenuous cloud layer exists at a low altitude, as those found over Antarctica; b) when attenuation from an overlying layers reduces the signal of a lower layer to the aerosol category. Another known situation where aerosol is designated as cloud is some cases of smoke injected into higher regions in the troposphere. (These false results may be remedied by using a color ratio parameter in the discrimination algorithm to be completed.)
  5. The results of the layer boundary detection function for nighttime (constant threshold) have not been quality checked.
  6. Diminishing of ground signal caused by steep terrestrial slopes can cause false negative ground signal detection results and false positive lower level layer results.
  7. In cases where no layers are detected at 1 Hz resolution, the ground location detected and stored in the GLA09 product fails to get transferred to the GLA10 and GLA11 products.

GLA10 Extinction and Attenuation Corrected Backscatter Cross Sections
GLA11 Thin Cloud and Aerosol Optical Depth

Due to a coding error in the latitudes and longitude which was found after release 17 processing, these products will not be available until Release-18.

Please refer to the NASA Wallops Flight Facility's GSAS v3.9 Release Notes for detailed release notes for each product in Release-17.


Release-13 and -14 Period Data (Oct/Nov 2003) [top]

Release-13 and Release-14 use data from GLAS laser #2, and include improved waveform fitting and precision-pointing data for altimetry products (GLA05, GLA06, GLA12-15).

Release-13 (GLA01) and Release-14 (GLA05, GLA06, GLA12-15) elevation data products covering October/November 2003 have not been fully calibrated. The fundamental elevation product contains: time tag, geodetic latitude, longitude and height (elevation) above the TOPEX reference ellipsoid. These products result from a combination of round-trip laser pulse travel time, precision orbit determination (POD), and precision attitude determination (PAD). Contributions from various sources lead to the following absolute accuracy assessment of the fundamental elevation products.

  1. The time tag, when converted to laser pulse ground bounce time, has been validated with an accuracy of about one microsecond.
  2. The geodetic latitude and longitude provide the horizontal location of the laser spot on the Earth's surface. The accuracy of this location is assessed to be 15 meters (1-sigma), resulting from about 5 arc-sec accuracy in the laser-pointing knowledge.
  3. The geodetic height may be biased by approximately 10 cm, but this bias is also dependent on the surface slope. For example, on 1 degree slopes with the 5 arc sec horizontal position accuracy, there will be an apparent 15 cm bias in range and hence in elevation. The precision of the height will also be dependent on surface characteristics, with smooth flat surfaces producing 2-3 cm precision, while rough surfaces will be decimeters.

In some occasional cases, off-nadir pointing was used at some locations, which introduces an apparent surface slope of up to 5 degrees, depending on the angle used.

Comments on Release-14 Land Products (GLA14)

ICESat products for land areas (all continents including Antarctica, and ocean islands, including Greenland) are distributed in the GLA14 data product. The GLA14 results are produced using processing results designated as alternate in the GLA05 waveform processing data product, in contrast to the standard processing results used to create the GLA12 ice sheet products. ICESat products have been produced in a number of releases, with improving data acquisition, calibration, and processing techniques. Data covering an 8-day period in March, 2003 were distributed as Release 12. Data covering a 33-day period in October and November, 2003 are distributed as Release 14.

The primary difference between the Release 12 and Release 14 versions of the GLA14 land products is a substantially reduced frequency of saturated returns, achieved by modifying the onboard gain algorithm to better accommodate rapid changes in received signal amplitude caused by changes in atmospheric transmissivity and surface reflectance. The modified gain algorithm was implemented for the fall, 2003 operation period.

An additional difference between the Release 12 and 14 data sets is the threshold level used in ground processing to detect the presence of a return signal. A threshold 5 times the standard deviation of the background noise level was used in Release 12. This threshold detected some weak returns from clouds that were thus included in the GLA14 product. To reduce the number of cloud false-alarm returns, a higher threshold, 7.5 times the background noise standard deviation, was used in Release 14. However, the higher threshold also causes some weak returns from the land surface beneath clouds to be excluded from the GLA14 product.

In addition to detecting the presence of returns, the threshold is used to define waveform extent based on the start and end of the signal corresponding to the first and last crossing of the threshold. The higher 7.5x threshold used in Release 14 underestimates waveform extent as compared to the 5x threshold used in Release 12. The Release 12 designations of signal start and end are a more accurate measure of the highest and lowest detected surfaces within the laser footprint. Furthermore, the elevation reported in the GLA14 product corresponds to the centroid of the received signal between signal start and end. The elevation is therefore sensitive to the threshold used.

Because of the above considerations, future releases of the GLA14 products will revert to the lower 5x threshold in order to detect weak surface returns and more accurately define waveform extent.

Future releases of GLA14 products will include several additional improvements. Fitting of Gaussian distributions to waveform peaks will be refined to more fully characterize multi-peaked distributions. Waveform fitting results in Release 14 are known to have some deficiencies and should be used only as a provisional product. Additionally, Releases 12 and 14 of GLA14 include the location of the signal centroid and end, but not signal start, as defined by the threshold above noise. The signal start currently included in GLA14 is based on a different threshold defined as a percentage of the maximum signal amplitude. This signal start will be replaced by the noise threshold signal start in future releases, so as to be compatible with the centroid and signal end designations. The position of the noise threshold signal start is available in the Release 12 and 14 data sets in the GLA05 waveform processing product.

Finally, for some returns in the data acquired during 2003 the signal extent exceeds the maximum width of the waveform and the upper end of signal is truncated. This occurs for footprint locations with large relief caused by steep topographic slopes and/or tall forest canopies. In mountainous regions, approximately 10% of the waveforms are truncated in this way. During 2003 operations the sampled waveform covers an extent of 81.6 m (544 waveform bins x 0.15 m per bin). Where the signal exceeds this maximum extent, signal start is designated as the first (highest) waveform bin. For operations in 2004 an onboard compression algorithm has been implemented whereby the waveform extent is increased to 150 m, thus minimizing occurrences of signal truncation.


Please refer to the NASA Wallops Flight Facility's GSAS v3.7 Release Notes for detailed release notes for each product in Releases 13 and 14.


Release-12 Cal/Val Period Data [top]

Release-12 is the original, at-launch version of all products, using data from GLAS laser #1.

The data calibration and validation activities by the ICESat/GLAS Science Team were interrupted by the premature end of laser-1 operation after about 36 days on March 28, 2003, and were resumed with operation of laser-2 beginning on September 25. The results from ICESat demonstrate the unique capabilities of the laser measurements with unprecedented accuracy. However, the cal/val period data, in particular, should be regarded as not fully calibrated and validated (i.e. with some uncertain errors). Some limitations may be described by flags on the data products. One important limitation (to be corrected in future releases) is the laser-attitude accuracy (at the 10 to 15 arcsec level), which affects the accuracy of the surface elevations (cms to meters depending on surface slope) and geolocations of the measurements (tens of meters).

Another limitation is the distortion of the received-pulse shape due to saturation of the detector amplifier. Type-1 saturation, which non-linearly distorts the top and trailing edge of the waveform, affects much of the data over ice and other strong reflectors (tens of cm range effect) and may be correctable with future algorithms. Type-2 saturation, for which the gain goes to a maximum and stays there awhile, causes an effect in the meters range. Type 2 has been corrected for laser 2 data by changing the instrument gain algorithm. Lidar data on clouds and aerosols from the 1064 nm channel is included, but the backscatter cross-section magnitudes have a large offset that will be corrected. The more-sensitive 532 nm lidar channel was not yet activated, so there are no optical-depth or extinction cross-section products. Other limitations may be discovered as analysis and calibration/validation continues.

Please refer to the NASA Wallops Flight Facility's GSAS v3.6 Release Notes for detailed release notes for each product in Release-12.