David Harding

1
ICESat Surface Water Altimetry
David Harding Goddard Space Flight Center NASA
Surface Water Working Group Meeting Irvine,
CA March 23, 2004
2
Outline

• ICESat/GLAS overview
• Lidar waveform measurements of vertical structure
• Repeat orbits, off-nadir pointing revisit
  frequency
• Amazon basin repeat sampling waveforms
• Mississippi River off-nadir target
• Water extent, stage and slope
• Observations through cloud cover
• Future work with ICESat surface water data

3
Ice, Cloud and Land Elevation Satellite

• Measurement Objectives
• Ice sheet elevation and change
• Atmospheric clouds and aerosols
• Land topography
• Vegetation height (low relief areas)

4
Surface Altimetry and Atmospheric Profiling
A first day track across Antarctica showing ice
sheet elevations in red and atmospheric phenomena
in varying colors from light blue for thin clouds
to white for opaque layers Vertical exaggeration
50x, 1064 nm data only shown here, RADARSAT
mosaic image from CSA, visualization courtesy of
GSFC SVS
5
Surface Altimetry and Atmospheric Profiling
A first day track across Antarctica showing ice
sheet elevations in red and atmospheric phenomena
in varying colors from light blue for thin clouds
to white for opaque layers Vertical exaggeration
50x, 1064 nm data only shown here, RADARSAT
mosaic image from CSA, visualization courtesy of
GSFC SVS
6
ICESat Operations Timeline
2004
2003
Laser 2 Laser on February 17 Laser off March
21 33 days 91-day repeat orbit Repeating
Oct-Nov, 03 tracks Subsequent laser 2 and 3 use
depends on laser 2 performance. Nominal goal is
quarterly observations repeating the same 33 day
track set. Next observation planned for late May
- June.
Laser 1 Laser on February 20 Laser failure
March 30 35 days cal/val 8-day repeat orbit
120 million laser fires Laser 2 Laser on
September 25 Laser off November 18 1st 9 days
cal/val 8-day repeat orbit 45 days 91-day repeat
orbit 186 million laser fires
7
ICESat Surface Returns for North America
Laser 2 Operations Sept. 25 - Nov. 18, 2003
Elevations (m)
8
GLAS Ranging Precision

• 2-3 cm range precision demonstrated (10 cm
  designed).

Antarctic 5 km DEM shaded relief map from ICESat
profiles.
9
ICESat Radar Altimeter Elevation Accuracy
Antarctica Crossovers
1193
Radar accuracy
3500
2403
1517
592
732
500
Radar precision
400
Crossover Residual Standard Deviation (cm)
300
ICESat Laser 1 precision (as of 01/04). 3 to 4x
decrease expected from improving pointing
calibration
200
100
0
lt 0.05
gt .1 lt .2
gt .2 lt .3
gt .3 lt .4
gt .4 lt .5
gt 0.05lt.1
gt .5 lt .75
gt .75 lt 1.0
gt 1.0 lt 1.5
gt 1.5 lt 2.0
Surface Slope from GSFC Radar Altimeter Mosaic
(deg)
ICESat - ICEsat
ERS-2 - ERS-2
ICESat - ERS-2
10
GLAS Waveforms for Vegetated Landscape
Reflected Energy Height Distribution 15 cm
Sampling, 75 cm Resolution
1064 nm Laser Pulse
Threshold
ICESat Elevation Products
Signal Start highest detected elevation
Gaussian Fit to Largest Peak standard ice
sheet elevation
Travel Time
Centroid of Signal Start to End average
detected elevation
Gaussian Fit to Last Peak ground elevation in
flat areas
Signal End lowest detected elevation
100 x 50 m diameter footprint spaced 175 m
along profile
Return Amplitude
11
Track Separation vs. Latitude Repeat Time
ascending to ascending or descending to
descending track separation 96 inclination orbit
50 km off-nadir access by 5
pointing from 600 km
25 day
33 day
45 day
91 day
183 day
12
Targeting Frequency for Equatorial River Site
ICESat 96 Inclination, 183-day Repeat Orbit with
25-day Near Repeat
Frequency of observations increases at higher
latitudes, tripling for latitude of 70.
13
8 Day Near-Repeat Tracks in 91-day Orbit
Amazon River Drainage Basin
Day 1
Day 2
Day 3
Day 4
Day 5
Day 6
Day 7
Day 8
The ICESat 91-day repeat orbit has an 8-day
near-repeat period. During 8-days, the tracks on
succeeding days shift westward 330 km at the
equator.
14
8 Day Near-Repeat Tracks in 91-day Orbit
Amazon River Drainage Basin
Cross-track width accessed by 5 off-nadir
pointing
Cycle 1
Cycle 2
This pattern is repeated, shifted westward 100
km every 8 days. Any location at the equator can
be observed twice with 5 pointing during the
course of three 8-day cycles.
15
Amazon March, 04 Early Inundation Season
16
Amazonia ICESat Flood Plain Profile
17
Amazonia ICESat Flood Plain Waveforms
18
Amazonia ICESat Flood Plain Waveforms
Is last return reflection beneath vegetation from
water or non-inundated flat ground? The pulse
width is narrow, comparable to the transmit pulse
(blue), indicating it is an impulse response from
a flat, smooth surface.
19
Next Laser Ops Late May - June Melt Season
Lena River Drainage Basin
Cross-track width accessed by 5 off-nadir
pointing achieves full coverage at 70
Lena River long-term mean daily discharge at
Kusur station basin outlet
8-days of 91-day orbit tracks
Figures from Yang et al., JGR, 2002.
20
ICESat Pointing Accuracy to Reference Track
Cycle 28 Laser 2
Cross-track accuracy of spacecraft pointing to
reference track is currently 150 m (3
sigma). Specification of 90 m (3 sigma) likely
can be achieved with improved calibration of
spacecraft star camera temporal biases.
Cycle 29 Laser 2
21
ICESat Targeting of Lower Mississippi River
targeted path mode track 2.5 off-nadir
targeted path coincident w/ river reach
8-day reference track
22 km
22
Lower Mississippi River Extent, Stage Slope
2.5 Off- Nadir
23
Lower Mississippi River Extent, Stage Slope
2.5 Off- Nadir
24
U.S. 6 Conical Off-Nadir Calibration Scan
25
Off-Nadir Loss of Signal from Water
River Crossing 5.9 Off-Nadir
26
Ambiguity Between Water and Flat Flood Plain
River Crossing 5.9 Off-Nadir
27
Surface Water Returns Under Clouds
Lake Superior Under Clouds 5.8 Off-Nadir
28
Cloud-free Viewing of Selected River
Basins (those examined by Birkett using
TOPEX/Poseidon radar altimeter, Water Resources
Research, 1998)
Note 1) these are worst-case cloud-cover
estimates because they are based on ground
observer and satellite radiometer measurements of
total cloud cover which overestimate cover that
would prevent laser altimeter operation. 2) these
are low-latitude river basins frequency of
ICESat observations increase at higher latitudes
(e.g., tripling for latitude of 70).
29
Future Work with ICESat Surface Water Data

• Quantify signal return amplitude from surface
  water as a function of off-nadir angle,
  atmospheric cloud/aerosol optical depth, and
  surface water roughness
• Examine frequency-of-surface-return global
  climatology
• spatial and temporal variability of observing
  through clouds/aerosols
• Evaluate surface water retrieval capability for
  inundated forests
• Quantify accuracy of slope measurements for along
  channel profiles, multiple crossings of river
  meanders, and areas of inundation
• Establish absolute accuracy of stage retrievals
  by comparison to in-situ gauges
• Demonstrate stage change retrieval from repeat
  profiles