An Indepth Look at ICESat and GLAS

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An In-depth Look at ICESat and GLAS

• By Vishana Ramdeen

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Abstract

• The ICESat (Ice, Cloud, and land Elevation
  Satellite) spacecraft was launched on January 12,
  2003 into a near-circular, low Earth,
  Sun-Synchronous Orbit with a 94? inclination.
  Its objective is to quantify the mass balance of
  the polar ice sheets and observe the changes with
  time. The only scientific observation instrument
  onboard is the GLAS (Geoscience Laser Altimeter
  System) instrument. It consists of three lasers
  (Laser 1, 2, and 3) which operate one at a time
  for three 33-day campaigns per year. GLASs
  Lasers 1 and 2 experienced a rapid decrease in
  laser energy which was attributed to issues with
  the frequency doubler, a manufacture defect of
  the laser diode pump array, and trace levels of
  outgassing. One of the major shortcomings of
  ICESat is systematic pointing errors. Two of the
  calibration methods used to compensate for these
  errors are ocean scans and round the world
  scans. The ICESat has provided scientists with
  information that is unsurpassed by any existing
  satellite and expectations for the future are
  high.

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Agenda

• Introduction
• Overview of the physical instruments
• ICESat
• GLAS
• General and Orbital Characteristics
• GLAS Lasers
• Shortcomings
• solutions
• Conclusion

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Introduction

• ICESat Ice, Cloud, and land Elevation Satellite
• Launched into orbit on January 12, 2003 on a
  Boeing Delta II rocket from the Vandenberg Air
  Force Base in California

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Introduction

• Inclination 94?
• Retrograde Orbit
• Near circular, low Earth orbit
• Sun-Synchronous Orbit
• Altitude 590 km initially
• currently at 600 km
• Orbital period 96.6 minutes
• Two reference tracks 8-day exact repeat (for
  calibration purposes) and 91-day exact repeat
  (for denser track coverage)

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ICESat
Schutz et al. ICESat Overview
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ICESats Mission Objectives

• The primary goal of ICESat is to quantify ice
  sheet mass balance and understand how changes in
  the Earths atmosphere and climate affect polar
  ice masses and global sea level.
• Measures height profiles of clouds, aerosols
  distribution, vegetation cover, and land
  elevations.

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ICESat/GLAS

• ICESat (zenith view)
  
• 
  GLAS

www.csr.utexas.edu/glas and glas.gsfc.nasa.gov
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GLAS

• GLAS Geoscience Laser Altimeter System
• First LIDAR used for continuous global
  observations of the Earth
• A combination of a precision surface LIDAR with a
  sensitive dual-wavelength cloud and aerosol LIDAR
• Only scientific observational instrument on the
  ICESat spacecraft

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GLAS

• Consists of three lasers
• referred to as Lasers 1, 2, and 3
• Mounted on a stationary optical bench
• Laser measurement direction is nadir, but the
  spacecraft can perform roll maneuvers to point up
  to 5 off nadir
• Only one laser is in operation at a time, pulsing
  at 40 Hz
• Pulse width is 5 nanometers

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GLAS Lasers
Abshire et al. Geoscience Laser Altimeter System
(GLAS)

• Each pulse has an averaged 65 m footprint
• successive laser pulses spaced 172 m along-track
• Area on the Earths surface covered by each pulse
  is not equal
• Lasers far field patterns are hypothesized to
  change due to the change of spatial distribution
  of light from the laser diode pump arrays over
  time more generally with laser energy and time
• laser diode pumps are used to produce very high
  power outputs such as that used for GLASs
  continuous pulses

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GLAS Lasers
Abshire et al. Geoscience Laser Altimeter System
(GLAS)

• 2 out of the 3 lasers experienced more rapid
  energy loss than planned
• After the first 10 days Laser 1s transmitted
  energy began to rapidly decline
• by day 38 it completely failed
• caused by manufactures defect of laser diode
  pump array
• To counter the effects of having less lasers,
  ICESats duty cycle was reduced from 100 to 25
• Lasers 2 and 3 are operating for three 33-day
  campaigns per year (Campaigns a, b, and c)
• Laser 2s decline is attributed to a process
  related to the frequency doubler and trace levels
  of outgassing.
• To slow this fate, Laser 3 has been operating at
  a lower energy level

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Coverage?

• http//en.wikipedia.org/wiki/ICESat

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Hows it done?

• GLAS emits a 1064 nm (for surface and cloud tops)
  and 532 nm (for atmospheric aerosols) laser pulse
  toward Earths surface.
• time emitted is recorded
• Upon impact to a surface, the photons are
  scattered back to the satellite.
• When the photons come into contact with the
  receiver another timestamp is taken.
• measurements of time of flight, emission angle
  relative to inertial space, transmitted pulse
  waveform, and echo pulse waveform are taken
• the time of flight is divided by 2 and multiplied
  by the speed of light giving us the distance

http//www.nasa.gov/vision/earth/lookingatearth/ic
esat_billion.html
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Backscatter

• Strong echo pulses indicate a region of flat,
  bright ice
• 532 nm profiles (B) have better signal-to-noise
  ratios for weak aerosol backscatter because of
  their more sensitive photon counting detectors

A
B
Abshire et al. Geoscience Laser Altimeter System
(GLAS)
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An Example

• http//earthobservatory.nasa.gov/Library/ICESat/Im
  ages/greenland_icesheet.jpg

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Shortcomings

• Systematic pointing errors are preventing ICESat
  data from meeting its full potential
• long-term trend bias
• systematic variations with satellite orbital
  period revolution
• These errors create elevation errors in the data
  and change with time
• They pose problems since they are indiscernible
  from actual surface elevation change

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Can we fix it?

• Ocean scans
• performed twice a day over the mid-Pacific
• errors are estimated through the reduction of the
  ocean surface range deviations from each ocean
  scan
• Round the world scans
• performed every 8 days
• estimates the systematic pointing errors as a
  function of orbital angle

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ICESat Video
http//www.nasa.gov/mission_pages/icesat/
Note To see this video, go to the above website
and click on the second video from the top.
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Conclusion

• The ICESat has provided scientists with
  unprecedentedly accurate data of the globes
  polar regions. With the addition of a few
  calibrations the usefulness of its outputs would
  grow increasingly.

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References

• Abshire, J., et al. (2005), Geoscience Laser
  Altimeter System (GLAS) on the ICESat Mission
  On-orbit measurement performance, Geophysical
  Research Letter, doi10.1029/2005GL024028,
  1534-1536.
• ICESat (2008). In Wikipedia, The Free
  Encyclopedia. Retrieved 0537, May 7, 2008, from
  http//en.wikipedia.org/w/index.php?titleICESato
  ldid182396442.
• Luthcke, S. B., D. D. Rowlands, T. A. Williams,
  and M. Sirota (2005), Reduction of ICESat
  systematic geolocation errors and the impact on
  ice sheet elevation change detection, Geophysical
  Research Letters, 32, L21S05, doi10.1029/2005GL02
  3689.
• Schutz, B. E., H. J. Zwally, C. A. Shuman, D.
  Hancock, and J. P. DiMarzio (2005), Overview of
  the ICESat Mission, Geophysical Research Letters,
  32, L21S01, doi10.1029/2005GL024009.