# Geoscience Laser Altimeter System (GLAS)
*sensor . products*

Full-waveform laser altimeter on NASA ICESat (2003-2010), 1064 nm and 532 nm, 40 Hz PRF, 66 m footprint, 170 m along-track spacing.

## Specifications
- **manufacturer**: nasa
- **product status**: retired
- **deployment context**: orbital
- **sensor**: {"instrument_family":"lidar","measurement_principle":"active","wavelength_nm":1064,"pulse_energy_mj":75,"pulse_repetition_freq_hz":40,"beam_count":1,"footprint_diameter_m":66,"along_track_spacing_m":170,"return_type":"full-waveform","beam_divergence_mrad":0.11,"notes":"Three redundant Nd:YAG lasers (one active at a time); 1064 nm altimetry + 532 nm cloud/aerosol channel; 70 mJ initial pulse energy declining over mission; Si APD + 1 GHz digitizer waveform recorder; 5 cm vertical precision flat surfaces; built NASA GSFC, spacecraft Ball Aerospace."}
- **entity type**: sensor
- **last verified date**: 2026-06-02
- **verified by**: agency-doc
- **claim status**: agency-sourced
- **provider**: nasa
- **attributes**: {"kind":"sensor","manufacturer":"nasa","product_status":"retired","deployment_context":"orbital","description":"Full-waveform laser altimeter on NASA ICESat (2003-2010), 1064 nm and 532 nm, 40 Hz PRF, 66 m footprint, 170 m along-track spacing.","sensor":"{\"instrument_family\":\"lidar\",\"measurement_principle\":\"active\",\"wavelength_nm\":1064,\"pulse_energy_mj\":75,\"pulse_repetition_freq_hz\":40,\"beam_count\":1,\"footprint_diameter_m\":66,\"along_track_spacing_m\":170,\"return_type\":\"full-waveform\",\"beam_divergence_mrad\":0.11,\"notes\":\"Three redundant Nd:YAG lasers (one active at a time); 1064 nm altimetry + 532 nm cloud/aerosol channel; 70 mJ initial pulse energy declining over mission; Si APD + 1 GHz digitizer waveform recorder; 5 cm vertical precision flat surfaces; built NASA GSFC, spacecraft Ball Aerospace.\"}"}
- **technology**: lidar

## Editorial
The Geoscience Laser Altimeter System (GLAS) was a full-waveform lidar altimeter that flew aboard NASA's ICESat satellite from January 2003 until the mission's retirement in August 2009. GLAS was built at NASA Goddard Space Flight Center; the ICESat spacecraft was built by Ball Aerospace.

GLAS carried three independent Nd:YAG lasers operating at 1064 nm for surface altimetry, with a 532 nm channel for cloud and aerosol profiling. Only one laser was active at a time, with the others held in reserve as the operational laser degraded. Pulse repetition rate was 40 Hz. Pulse energy figures in the literature span a range: 75 mJ [^eoportal-icesat], 70 mJ on-orbit initial [^pmc-glas-altimetry], and 100 mJ in early design documentation [^gsfc-glas-page], reflecting design, on-orbit initial, and nominal values at different phases of the mission. Energy declined progressively over each laser's operational period. Footprint diameter was 66 m, along-track shot spacing 170 m, and beam divergence approximately 0.11 mrad [^eoportal-icesat] [^pmc-glas-altimetry]. Waveforms were digitised by a 1 GHz analogue-to-digital converter, yielding vertical precision better than 5 cm on flat surfaces [^gsfc-glas-page].

The three-laser configuration addressed the laser reliability constraints of the era; each unit carried a limited pulse-life budget. Laser 1 failed after 37 days; Laser 2 operated across multiple campaign periods through 2007; Laser 3 carried the mission to its end in 2009. Campaigns were time-limited and non-continuous, so the global coverage pattern differs from continuously operating altimeters.

GLAS demonstrated spaceborne lidar for ice-sheet elevation change, sea-ice freeboard, vegetation canopy height, and cloud-top height retrieval, establishing the scientific and technical foundation that ICESat-2 ATLAS continued with its photon-counting architecture.

**Supported methodologies**

| Methodology | Evidence class | Mission context |
|---|---|---|
| Spaceborne LiDAR Altimetry and Canopy Structure | Demonstrated | ICESat [^gsfc-glas-page] [^pmc-glas-altimetry] [^eoportal-icesat] |

## Compositional position
- icesat (missions) --[mission_payloads]--> Geoscience Laser Altimeter System (GLAS)

## Sources
- [gsfc-glas-page] | GLAS Instrument Overview, NASA GSFC ICESat Project | https://icesat.gsfc.nasa.gov/icesat/glas.php | tier=agency-doc | accessed=2026-06-02
- [eoportal-icesat] | ICESat (GLAS) Mission Overview, ESA eoPortal | https://www.eoportal.org/satellite-missions/icesat | tier=community | accessed=2026-06-02
- [pmc-glas-altimetry] | ICESat/GLAS Altimetry Measurements: Received Signal Dynamic Range and Saturation Correction, PMC | https://pmc.ncbi.nlm.nih.gov/articles/PMC6110114/ | tier=peer-reviewed | accessed=2026-06-02

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Source: https://eo-atlas.org/products/sensor/icesat-glas
Maintainer: SpectraWorks B.V. (CC-BY 4.0)