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GLAS Instrument (Geoscience Laser Altimeter System)

The GLAS (Geoscience Laser Altimeter System) instrument aboard the ICESat spacecraft plays a crucial role in measuring polar ice-sheet topography and monitoring temporal changes. It also evaluates cloud heights, planetary boundary layers, and aerosol vertical structures, as well as land and water topography. Utilizing advanced STOP (Structural-Thermal-Optical) analysis with NASTRAN modeling, the GLAS instrument ensures accurate predictions of optical performance despite thermal distortions. Laser interferometry is employed to measure thermal properties at a microscopic level, enabling precise tracking of deformation and motion.

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GLAS Instrument (Geoscience Laser Altimeter System)

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  1. GLAS Instrument(Geoscience Laser Altimeter System) • Measures polar ice-sheet topography and temporal changes in topography; cloud heights, planetary boundary heights, and aerosol vertical structure; and land and water topography GLAS Instrument (ICESat Spacecraft) Picture Courtesy of Ball Aerospace

  2. GLAS Instrument Structure w/ Mass Simulators Telescope Bench 2” Honeycomb Core 0.06” Composite Facesheets Optical Bench 4” Honeycomb Core 0.08” Composite Facesheets

  3. STOP Analysis(Structural-Thermal-OPtical) • Used to predict optical performance of a system degraded by thermal distortions • NASTRAN used to predict motion and deformation of optical components Accurate Material Thermal Properties Required for STOP Analysis GLAS NASTRAN Thermal Model

  4. Laser Interferometer Used to Measure GLAS Bench CTE • Laser Interferometer suitable for real time displacement of components with CTE values as low as 10-9 /K • Temperature can be accurately cycled over wide range • 8” x 8” Samples Information Provided By Precision Measurements and Instruments Corporation (PMIC)

  5. Michelson Laser Interferometer • Helium-neon laser provides stable frequency beam • Laser is split by beam splitter (BS) • First beam is reflected off mirror PZT and passes into vacuum chamber • Second beam passes though BS and enters vacuum chamber • Beams are reflected off sample mounted mirrors and return along their respective paths and interfere, forming a fringe pattern • Beam splitter SP separates finge pattern light and directs them to photo-detectors PD1 and PD2 • Each shift in the fringe pattern corresponds to a change in sample length equal to 1/2 the wavelength of the laser light (12.456 min) Information Provided By Precision Measurements and Instruments Corporation (PMIC)

  6. Optical Bench Thermal Expansion in Ribbon Direction

  7. Optical Bench Thermal Expansion in Cross-Ribbon Direction

  8. Composite Face Sheet (Plate Element) CTE = 0.21 min/in*C Aluminum Honeycomb Core (Solid Element) CTE = 0.21 min/in*C (L and W Directions) GLAS Optical Bench NASTRAN Model

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