NOTE: References in this documentation to GLAS binary product names GLA01 to GLA15 refer to original GLAS binary data, and are retained for informational and provenance purposes. Access to GLAS binary data was removed 01 August, 2017. All GLAS data are available in HDF5 format, products GLAH01 to GLAH15.
This document covers seven data sets as listed in Table 1.
Table 1. GLAS Data Sets Described in this Document
Short Name |
Long Name |
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GLAH01 |
GLAS/ICESat L1A Global Altimetry Data (HDF5), V33.2 |
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GLAH05 |
GLAS/ICESat L1B Global Waveform-based Range Corrections Data (HDF5) |
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GLAH06 |
GLAS/ICESat L1B Global Elevation Data (HDF5) |
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GLAH12 |
GLAS/ICESat L2 Antarctic and Greenland Ice Sheet Altimetry Data (HDF5) |
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GLAH13 |
GLAS/ICESat L2 Sea Ice Altimetry Data (HDF5) |
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GLAH14 |
GLAS/ICEsat L2 Global Land Surface Altimetry Data (HDF5) |
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GLAH15 |
GLAS/ICESat L2 Ocean Altimetry Data (HDF5) |
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Note: GLAH01 Release 33 did not require Surface Elevation reprocessing, so was not reprocessed into Release 34, and remains as Release 33.2. The 33.2 minor version revises the format of the d_4nsBgMean parameter from counts to volts. GLAH01 contains all altimetry information transmitted from the spacecraft, including the long and short waveforms. The number of received samples is either 200 or 544. These change at the frame boundary and are nominally set by the onboard surface-type mask. In normal operations, GLAS receives 200 samples over sea ice and ocean, and 544 samples over ice sheet and land. The transmit pulse, received echo samples, and associated digitizer addresses are transferred from Level-0 telemetry without calibration or unit changes. This is the only product that contains the altimeter transmitted and received waveforms, which may be required by altimetric scientists investigating the instrument health. This product is not intended for use by the general science community.
GLAH05 is an intermediate product that contains important information calculated from the waveform. Level-1B waveform parameterization data include output parameters from the waveform characterization procedure and other parameters required to calculate surface slope and relief characteristics. The binary GLA05 was used for creating GLA06 and Level-2 elevation products. The higher products contain scientific parameters derived from algorithms that specifically used GLA05 as input. GLAH05 contains parameterizations of both the transmitted and received pulses and other characteristics from which elevation and footprint-scale roughness and slope are calculated. The received pulse characterization uses two implementations of the retracking algorithms: one tuned for ice sheets, called the standard parameterization, used to calculate surface elevation for ice sheets, oceans, and sea ice; and another for land (the alternative parameterization).
GLAH06 is a product that is analogous to the geodetic data records distributed for radar altimetry missions. It contains elevations previously corrected for tides, atmospheric delays, and surface characteristics within the footprint. Elevation is calculated using the ice sheet parameterization. Additional information allows the user to calculate an elevation based on land, sea ice, or ocean algorithms. Level-1B elevation data include surface elevation, surface roughness assuming no slope, surface slope assuming no roughness, and geodetic and atmospheric corrections for range measurements. Both Level-1B products are geolocated to the center of the laser footprint.
GLAH12 to GLAH15. GLAH06 is used in conjunction with GLAH05 to create the Level-2 altimetry products. Level-2 altimetry data provide surface elevations for ice sheets (GLAH12), sea ice (GLAH13), land (GLAH14), and oceans (GLAH15). Data also include the laser footprint geolocation and reflectance, as well as geodetic, instrument, and atmospheric corrections for range measurements. The Level-2 elevation products, are regional products archived at 14 orbits per granule, starting and stopping at the same demarcation (± 50° latitude) as GLAH05 and GLAH06.
Surface Type Mask
Each regional product is processed with algorithms specific to that surface type. Surface type masks define which data are written to each of the products. If any data within a given record fall within a specific mask, the entire record is written to the product. Masks can overlap: for example, non-land data in the sea ice region may be written to the sea ice and ocean products. This means that an algorithm may write the same data to more than one Level-2 product. In this case, different algorithms calculate the elevations in their respective products. The surface type masks are versioned and archived at NSIDC, so users can tell which data to expect in each product.