Surface Water Working Group

1
ICESAT LASER ALTIMETRY

• B. E. Schutz
• Center for Space Research
• University of Texas at Austin
• schutz_at_csr.utexas.edu

2
ICESat

• Ice, Cloud and land Elevation Satellite (ICESat)
  launched January 13, 2003 0045 UTC from
  Vandenberg (CA) on Delta-II
• Primary instrument on ICESat is GLAS (Geoscience
  Laser Altimeter System)
• NASA Earth Science Enterprise mission
• GLAS is a NASA Goddard instrument
• Spacecraft built by Ball Aerospace
• Mission operations at LASP/University of Colorado
• Science Team

3
ICESat

• Science goals
• Measure changes in polar ice mass balance (1.5
  cm/yr over 100 km x 100 km regions)
• Map land topography and sea ice, and vegetation
  canopy information
• Measure distribution of clouds and aerosols
• Science Requirements Document (1997) provided the
  science justification, error budget, requirements
  on instrument and mission to meet the goals,
  including mission lifetime, etc.

4
GLAS Design

• Science Requirements flow down into instrument
  design
• Laser instrumentation
• Three lasers to meet mission lifetime
• Interaction between instrument design and science
  requirements
• Example Pulse repetition rate
• High PRR reduces lifetime
• Wavelength of surface features
• Design 40 Hz -gt 170 m surface sampling
• Other laser divergence results in illuminated
  surface spot of 70 meters

5
Mission Design

• GLAS data analysis requirements to detect
  temporal change
• Crossovers (dictates that inclination is not
  exactly polar) ICESat 94?
• Repeat tracks (orbit altitude requirements)
• ICESat 8day repeat and 91 day repeat (altitude
  difference 1.5 km)
• Flow down
• Orbit control (1 km at equator)
• Off nadir pointing with accuracy ? 50-100 m

6
Measurement Concept

• Surface profile obtained from determination of
  laser spot location on Earths surface
• Laser spot geodetic coordinates inferred from
• POD (GPSSLR) gives position vector of GLAS
  reference point (Req lt5 cm radial achieved 2-3
  cm)
• PAD (star trackers/gyros) plus laser time of
  flight gives altitude vector of GLAS reference
  point (Pointing Req 1.5 arc-sec -gt 4.5 m on
  Earth surface achieved 5 arcsec, but expect to
  achieve requirement)

7
Amazon Profiles
8
2003 September 29 Track
Landsat image showing confluence of Rio Tapajos
with Amazon
Prepared by Tim Urban, CSR
9
Waveforms GLA06 and GLA14
10
Confluence of Rio Tapajos with Amazon

• Closer examination of flat area shows
  differences between GLA06 and GLA14 products
  (different analysis of digitized echo waveform)
• GLA06 based on max peak
• GLA14 based on entire pulse
• Indicative of vegetation
• Track crosses Amazon and upstream Rio Tapajos
• Region from 2.4? to -3.2? appears to be along
  Rio Tapajos

11
GLAS Precision Estimate

• Residuals to low degree polynomial fit to
  elevation on preceding chart represent GLAS
  precision
• Both GLAS data products give similar result (echo
  waveform is Gaussian)
• 40 Hz points shown (no averaging)
• Over this water surface, the precision is lt 3 cm
• May be remaining decimeter level altitude bias,
  but elevation slope is very accurate

12
GLAS Accuracy

• Data product used in previous example
• Time geodetic latitude, longitude of laser
  illuminated surface spot geodetic height above
  reference ellipsoid
• Current accuracy estimate
• Horizontal location of illuminated spot
• 5 arcsec -gt 15 meters (dependent on temporal
  changes and laser pointing biases)
• Requirement is 1.5 arcsec have not exhausted
  analysis so optimistic that requirement will be
  reached for most of the data
• Elevation (geodetic height above reference
  ellipsoid)
• On flat surfaces, precision is few cm, but
  remaining decimeter level range bias

13
ICESat Released Data

• Portions of ICESat data released, available from
  NSIDC for all global data
• Data not yet fully calibration
• Calibration analysis continues new data release
  expected
• 8 day period of laser 1 in March 2003 (8 day
  repeat orbit)
• Accuracy estimate horizontal geolocation 30-45
  meters
• 33 day period of laser 2 in October-November
  2003 (33 day subcycle of 91 day repeat orbit)
• Accuracy estimate horizontal geolocation 15
  meters
• Data not yet released pending further calibration
  analysis
• 28 days of 8 day repeat cycle, laser 1
• Laser 2
• 8 day repeat cycle in September 2003
• 14 additional days in September-October 2003
• 33 days in February-March 2004

14
Surface Sampling and Orbit Evolution
15
9-day Orbit Evolution in 91-day Repeat
16
Orbit and Sampling

• ICESat uses two orbits
• 8-day repeat
• 91-day repeat (with 30 day near-repeat
  subcycle)
• Two orbits are 1.5 km difference in altitude
• Sampling
• 8-day repeat produces 300 km equatorial
  separation between tracks in same direction
• 91-day repeat produces 30 km equatorial
  separation between tracks in same direction
• 8-day near-repeat subcycle
• In 30 days orbit evolves into nearly uniform
  track spacing with 90 km track separation
• Visualize evolution as 30 day pattern, shift it
  eastward by 30 km for next 30 days, and again
  eastward by 30 km for next 30 days

17
Off Nadir Pointing

• ICESat design and operations allows for pointing
  off nadir up to 5? (50 km)
• Lay down a series of GLAS illuminated spots up
  to 50 km parallel or nearly parallel to the
  natural track
• Off nadir pointing accuracy is 100 m
• Enables creating tracks across a region that
  would not naturally be sampled

18
Considerations

• No single orbit design that will meet the needs
  of all science interests
• Can find an orbit that would repeat every day, or
  two days, etc. (determine mean orbit altitude)
• What sampling is required?
• A 1-day repeat would give daily sampling of
  points along or near (with off-nadir pointing)
  near the design track
• Very limited sampling would require a
  constellation of satellites (multiple satellites)
• Longer repeat cycles give denser track spacing,
  but less frequent temporal sampling of locations
  on or near the reference track
• What is highest latitude of interest? (determine
  orbit inclination)
• If optical imager included, need daylight which
  implies inclination of 98?? (but even
  Sun-synchronous can be eclipsed, so some periods
  will not have image data),

19
Laser

• GLAS carries 3 lasers
• Laser 1 failed after 36 days
• Laser 2 has been operated for 90 days just
  powered off March 21, will be restarted
  May/June
• Laser 3 not yet used
• Future laser altimeters need to address laser
  lifetime
• Require extensive testing of off the shelf
  components
• Advanced ICESat under discussion in various
  versions