NASA/TM-2013-217512

June 2013

Charles E. Webb, H. Jay Zwally, and Waleed Abdalati

National Aeronautics and 

Space Administration 

Goddard Space Flight Center

Greenbelt, Maryland 20771

The Ice, Cloud and land Elevation Satellite (ICESat): 

Summary Mission Timeline and Performance Relative to 

Pre-Launch Mission Success Criteria
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NASA/TM-2013-217512

June 2013

Charles E. Webb 

Goddard Space Flight Center, Greenbelt, MD

H. Jay Zwally 

Goddard Space Flight Center, Greenbelt, MD 

Waleed Abdalati

NASA Headquarters, Washington, D.C.

The Ice, Cloud and land Elevation Satellite (ICESat): 

Summary Mission Timeline and Performance Relative to 

Pre-Launch Mission Success Criteria
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The Ice, Cloud and land Elevation Satellite (ICESat): 

Summary Mission Timeline and Performance Relative to

Pre-Launch Mission Success Criteria

Charles E. Webb, Ph.D. 

Principal Scientist, SGT Inc. 

Goddard Space Flight Center 

Cryospheric Sciences Laboratory 

H. Jay Zwally, Ph.D. 

ICESat Project Scientist 

Goddard Space Flight Center 

Cryospheric Sciences Laboratory 

Waleed Abdalati, Ph.D. 

Chief Scientist 

NASA Headquarters 
2

Executive Summary

 On June 22, 2010, NASA initiated the decommissioning of the Ice, Cloud and 

land Elevation Satellite (ICESat) by powering down its on-board instrument, the 

Geoscience Laser Altimeter System (GLAS). This document reviews the scientific 

achievements of the mission, particularly with respect to the Mission Success 

Criteria (MSC) established prior to launch, which stated that ICESat would 

produce: 

(1) A significant improvement in the measurement of the ice sheet surface 

elevation changes over previous radar altimetry estimates to dh/dt < 2 

cm/year (1! value); 

(2) Calibrated profiles of global land and ocean surface elevations, 

especially at high latitudes where there is currently no detailed 

comprehensive data; 

(3) Calibrated profiles of ice sheet surface elevation to better than 25 cm 

accuracy at the ground location of each laser pulse, to serve as a basis 

for comparisons to future elevation observations. 

 Although technical problems were encountered with the lasers on orbit, the 

GLAS instrument team worked closely with the ICESat science team to develop 

a modified operations plan. As a result, ICESat has met or exceeded each of the 

requirements established to define the mission as a success.

 First, elevation change results from Greenland show that !dh/dt is ~1 cm/yr 

for most of the ice sheet, and that larger errors observed along the margins have 

been significantly reduced relative to previous radar remote-sensing missions. 

Second, although the pre-launch changes to the MSC increased the emphasis on 

cryospheric science, calibrated elevation profiles have been routinely produced 

over ocean and land surfaces, throughout the mission, at latitudes up to ±86°. 

Third, elevation differences from intra-campaign crossovers in Antarctica have a 

standard deviation of 25 cm or less for surfaces with slopes less than 2°, which 

encompasses the vast majority of the ice sheet. 

 ICESat data has also been used to make significant scientific contributions 

outside the areas defined by the MSC. In particular, it played a critical role in 

the discovery of new subglacial lakes in Antarctica, enhancing our understanding 

of water transfer beneath the surface. ICESat also contributed important 

information about sea-ice freeboard and thickness at a time of record-low extent 

in the Arctic Ocean. Finally, despite technical problems with the green channel, 

remarkably detailed profiles of the planetary boundary layer and atmospheric 

aerosols were obtained for the first time. 
3

1. Introduction

 The Ice, Cloud and land Elevation Satellite (ICESat) mission was conceived, 

primarily, to quantify the spatial and temporal variations in the topography of 

the Greenland and Antarctic ice sheets. It carried on board the Geoscience Laser 

Altimeter System (GLAS), which measured the round-trip travel time of a laser 

pulse emitted from the satellite to the surface of the Earth and back. Each range 

derived from these measurements was combined with precise, concurrent orbit 

and pointing information to determine the location of the laser spot centroid on 

the Earth. By developing a time series of precise topographic maps for each ice 

sheet, changes in their surface elevations can be used to infer their mass balances. 

 The following section summarizes the Mission Success Criteria (MSC) 

established for the ICESat mission. Section 3 provides an overview of ICESat 

mission operations, from launch through decommissioning, and discusses the onorbit technical challenges faced by the instrument and science teams. Section 4 

assesses the overall performance of the ICESat observatory with respect to the 

MSC, and Section 5 reviews additional scientific contributions made using data 

collected during the mission. Section 6 describes the ICESat data products and 

where they have been archived. 

2. Mission Success Criteria

The Level-1 requirements in the EOS Program Plan (February 2001) 

established an initial set of MSC for the ICESat mission. Prior to launch, 

however, the Single Photon Counting Modules – the detectors for the 

atmospheric green channel – were determined to be unreliable, but they were 

declared to be non-essential for the success of the mission. As a result, the MSC 

were modified, and documented in a separate memorandum, dated December 

2002 (see Appendix A). The official Level-1 requirements were not updated at 

that time, nor at any time since. The revised MSC defined success as “an 

improvement by a significant factor over our current knowledge of ice sheet 

elevation variability and mass balance.” Specifically, they stated that ICESat 

would produce: 

(1) A significant improvement in the measurement of the ice sheet surface 

elevation changes over previous radar altimetry estimates to dh/dt < 2 

cm/year (1! value); 

(2) Calibrated profiles of global land and ocean surface elevations, 

especially at high latitudes where there is currently no detailed 

comprehensive data; 

(3) Calibrated profiles of ice sheet surface elevation to better than 25 cm 

accuracy at the ground location of each laser pulse, to serve as a basis 

for comparisons to future elevation observations. 
4

3. Mission Summary

The ICESat mission began with a launch on January 12, 2003, from 

Vandenberg Air Force Base, on a Boeing Delta-II rocket. The first of the three 

lasers in the on-board GLAS instrument was commanded to begin firing on 

February 20. The initial returns showed strong and well-behaved waveforms, 

yielding high-quality surface and cloud elevation measurements. After only two 

weeks, however, the laser energy began to decline at a much faster rate than 

expected, with some indications suggesting problems in the laser pump diodes. 

This decline continued until March 29, when Laser 1 ceased firing after 36 days 

of on-orbit operation. 

 

 The Independent GLAS Anomaly Review Board (IGARB) was assembled to: 

(1) determine the cause of the premature failure of Laser 1; (2) assess the 

likelihood of similar failures for Lasers 2 and 3; and (3) recommend steps to 

prolong the on-orbit lifetimes of the two remaining lasers in support of the overall 

mission objectives. In its report, the board concluded that an excess of indium 

solder, applied during the manufacture of the laser diode arrays, contributed to 

the failure of Laser 1. The interaction of this solder with gold conducting wires 

led to the formation of gold-indide compounds, which gradually corroded the 

wires. A second contributing factor in the failure was the development of a shunt 

current in a diode array bar, forcing the redistribution of current among the gold 

wires. This accelerated gold-indide formation and increased thermal stresses, 

causing wire fatigue, and eventually, a short to ground, which fused the bond 

wires open. 

 Based on its analysis of the Laser 1 failure, the IGARB recommended that 

Laser 2 be operated at a lower temperature, to slow the growth of the gold-indide 

compounds. Furthermore, in anticipation of similarly shortened lifetimes for the 

two remaining lasers, the mission operations plan was revised to balance spatial 

and temporal coverage requirements in support of the scientific objectives. The 

planned 183-day repeat cycle was replaced with a 91-day repeat cycle, but a 

decision was made to operate GLAS only during a 33-day near-repeat subcycle of 

this new orbit, three times per year: Winter (February/March), Spring 

(May/June) and Fall (September/October). 

 

 With this revised operational scenario in place, Laser 2 began firing on 

September 25, 2003, while in the 8-day repeat cycle of the calibration orbit. As 

with Laser 1, the initial returns provided high-quality surface and cloud elevation 

measurements. The satellite transitioned to the 91-day repeat cycle on October 4, 

and GLAS was scheduled to continue operating for one complete 33-day subcycle. 

Further analysis of the early science data, however, suggested a misalignment of 

the laser beam within the telescope field-of-view. Prior to launch, it had been 

determined that this alignment was sensitive to the temperature of the GLAS 

optical bench, and that, as a result, a minimum temperature of 16.2°C should be 

maintained. Nonetheless, due to difficulties with the control of the component 
5

loop heat pipe that regulated this temperature, the ICESat project decided to 

begin Laser 2 operations with a bench temperature of 14.2°C. 

 After the misalignment was revealed, a command to raise the temperature of 

the component loop heat pipe was uploaded to the satellite, in an attempt to 

steer the laser beam more into the telescope field of view. Unfortunately, because 

the memory configuration of the ground command test bed did not map directly 

to the configuration on board the satellite, this command erroneously raised the 

temperature of the laser loop heat pipe. The error was recognized within a single 

orbit pass, and corrected within another. Subsequent commands successfully 

raised the temperature of the component loop heat pipe, and the beam alignment 

noticeably improved. The ICESat project then extended the period of operations 

for Laser 2 by two weeks, to allow for the collection of a complete 33-day data 

set following the alignment correction. 

 Shortly after the unintended thermal spike to the laser, the transmit energy 

began to decline more rapidly. A newly constituted GLAS Anomaly Review 

Board (GARB), consisting of internal GSFC laser experts, concluded that it was 

likely the result of a photo-darkening process occurring at and near the GLAS 

frequency doubler, which converts a portion of the laser-output near-infrared 

(1064 nm) beam to a green (532 nm) wavelength, for use in the atmospheric 

measurements. The board observed that the electrical power required to heat the 

doubler decreased as the laser energy declined, suggesting that the laser light 

passing through the doubler crystal was being increasingly absorbed. They 

concluded that this was likely due to trace levels of hydrocarbons, outgassed from 

adhesives used in the laser, interacting with the 532-nm photons. The command 

error that led to the increase in the laser temperature probably accelerated this 

process. 

 This operations period, designated the L2a campaign, came to an abrupt end 

on November 19, when the on-board computer was reset after a new basetime 

was uploaded, triggering the satellite to enter a sun-acquisition, safe-hold mode. 

Given the proximity to the planned end date for the campaign, two days later, 

and the initial uncertainty surrounding the cause of the computer reset, the 

ICESat project decided to delay any restart of Laser 2 until the following Spring 

opportunity in 2004. 

 Lasting for a total of 55 days, the L2a campaign consumed approximately 

one-third of the laser’s energy for altimetry. As the Spring 2004 opportunity 

approached, the ICESat project proposed significantly lowering the laser 

temperature, from 27°C to 16°C. Initially, the GARB opposed this because it 

would involve operating the laser at a temperature outside the nominal limits, 

and for which there was little test data. This issue was not resolved in time for 

the Spring opportunity, and the L2b campaign began on February 17, 2004. 

Shortly after, during an around-the-world calibration scan, the satellite entered a 

sun-acquisition, safe-hold mode, when an unfavorable Sun geometry caused limits 
6

in the flight software to be violated. The satellite was reoriented, and science 

data collection resumed within 12 hours. The laser energy output steadily 

declined until March 5, when the transmit gain was increased. The transmit 

energy stabilized for several days, before resuming a gradual decline, and the 

campaign concluded on March 21. 

 After additional deliberation, the GARB unanimously recommended lowering 

the laser temperature to 16°C, and the necessary commands were uploaded early 

in the L2c campaign, which began on May 18, 2004. Once this temperature 

change was implemented, the rate of laser energy decline slowed significantly, 

leaving approximately 5 mJ available for altimetry by the end of the Summer 

2004 opportunity. By this point, Laser 2 had operated on orbit for nearly three 

times as long as Laser 1, a reflection of the lessons learned by the instrument 

team, and establishing a solid basis for Laser 3 operations. As a result, a decision 

was made to switch to Laser 3 for future campaigns, and to return to Laser 2 

only when the former’s energy had been depleted. 

 On October 3, 2004, Laser 3 was commanded to begin firing at the start of 

the Fall opportunity. Table 1 summarizes all of the campaigns conducted with 

this laser. The 532-nm (green) energy was significantly lower than expected, and 

the GARB later concluded that this was likely due to a mechanical shift of the 

doubler crystal, caused by shrinkage of the polymer used in its mount. The nearinfrared energy for altimetry, however, declined at a much slower rate than for 

either of the other lasers. This has been attributed to the lower operating 

temperature of the laser, a longer outgassing time, and lower green energy, all of 

which contribute to reduced levels of photo-darkening. Laser 3 ceased firing on 

October 19, 2008, after a total of 354 days on on-orbit operation. 

2004 L3a 03 Oct " 08 Nov

L3b 17 Feb " 24 Mar

2005 L3c 20 May " 23 Jun

L3d 21 Oct " 24 Nov

L3e 22 Feb " 28 Mar

2006 L3f 24 May " 26 Jun

L3g 25 Oct " 27 Nov

L3h 12 Mar " 14 Apr 2007 L3i 02 Oct " 05 Nov

L3j 17 Feb " 21 Mar 2008 L3k 04 Oct " 19 Oct

Table 1. ICESat Laser 3 Campaigns 
7

 After several failed attempts to restart Laser 3, the ICESat project decided 

to return to Laser 2. The orbit was adjusted to shift the satellite back to the 

reference ground tracks that were covered at the end of the L3k campaign, and 

Laser 2 was restarted on November 25, 2008. After falling below 3 mJ, the laser 

transmit energy was boosted by increasing the laser temperature on December 9, 

in accordance with earlier GARB recommendations. This produced a much 

larger-than-expected jump in the output energy, to more than 8 mJ. At the 

conclusion of the L2d campaign, on December 17, the laser retained nearly 5 mJ, 

about what it had at the end of the L2c campaign, more than four years earlier. 

 With the advent of the Spring 2009 opportunity, Laser 2 was restarted, on 

March 9. During the L2e campaign, an impressive number of returns were 

obtained at unprecedented low-energy levels. The laser transmit energy was 

allowed to decline to nearly 1 mJ. Another, although smaller, increase in the laser 

temperature again boosted the output energy, which remained stable at 2 mJ 

until the campaign concluded on April 11. On September 30, the L2f campaign 

commenced. The transmit laser energy declined gradually, until the laser stopped 

firing on October 11. After several failed attempts to restart Laser 2, restarts of 

the other two lasers were attempted, but none succeeded. Decommissioning of the 

ICESat spacecraft began with powering off the GLAS instrument on June 22, 

2010. 

4. On-Orbit Performance

 Despite the challenges posed by reduced laser lifetimes and limited 

operational periods, the ICESat mission has demonstrated a remarkable advance 

in satellite remote sensing of the Earth. The GLAS instrument team worked 

closely with the science team to understand the technical problems encountered 

with the lasers. Together, they developed a modified operations plan that 

extended the lifetimes of the remaining lasers well beyond what was expected 

after the failure of Laser 1. 

 This section discusses the results obtained in pursuit of the primary scientific 

objectives of the mission, as expressed by the revised MSC listed in Section 2. 

For each criterion, ICESat has met or exceeded the requirement established to 

define the mission as a success. 

Criterion 1

Produce a significant improvement in the measurement of the ice sheet surface 

elevation changes over previous radar altimetry estimates to dh/dt < 2 cm/year 

(1!).

 Prior to the launch of ICESat, Zwally et al. (2002) outlined a procedure for 

assessing the error in the derived dh/dt estimates using crossover analysis. At 
8

points where ascending and descending tracks cross, the elevations interpolated 

from the two profiles are differenced and divided by the time between the 

observations, yielding dh/dt. Averaging these crossover dh/dt values over a large 

enough area (e.g., a typical 100 km x 100 km drainage basin) reduces the error of 

the spatially averaged dh/dt to less than 2 cm/year. 

After the failure of Laser 1, the adoption of the seasonal campaign strategy 

shifted the emphasis for dh/dt recovery from crossovers to repeat-track analysis. 

The science data, however, revealed that the elevation profiles were typically 

offset horizontally from the targeted reference track, due to pointing control 

errors, with offsets varying between 56 and 111 meters (RMS) for different 

campaigns (Webb et al., 2010).

 Figure 1 illustrates the 1! errors associated with the dh/dt estimates 

obtained for Greenland, from ICESat data (2003-2007) and from ERS data (1992-

2002). The ICESat results show a clear and significant improvement in accuracy, 

particularly in the interior of the ice sheet. In this region, which constitutes the 

vast majority of its area, 1! errors are below 1 cm/year. Furthermore, along the 

Figure 1. [Left] Map of !<dH/dt> for ICESat (2003-2007), computed 

from cell-averaged <dH/dt> solutions at individual reference 

points (Zwally et al., submitted). [Right] Map of !dH/dt for ERS 

(1992-2002), computed from time-series analysis of crossover 

differences (Zwally et al., 2005)
9

margins of the ice sheets, where the rates of elevation change are much higher, 

the bands of larger errors have been dramatically narrowed. 

Criterion 2

Produce calibrated profiles of global land and ocean surface elevations, especially 

at high latitudes where there is currently no detailed comprehensive data. 

 The change to the ICESat MSC, in December 2002, increased the emphasis 

on cryosphere measurements for the mission. Despite this, and the limitations 

posed by the spatial and temporal coverage, the ICESat data products constitute 

a substantial, consistently referenced land elevation data set, with unprecedented 

accuracy and quantified measurement errors. With support from the NASA 

Earth Surface and Interior program, ICESat data is being used to build a global 

geodetic ground control point (GCP) database to improve existing digital 

elevation models (DEMs), such as those from the Shuttle Radar Topography 

Mission (SRTM) and the Advanced Spaceborne Thermal Emission and Reflection 

Radiometer (ASTER). Each ICESat waveform provides a unique sampling of the 

elevations within a particular laser footprint (Harding and Carabajal, 2005). In 

forested areas, this has enabled the determination of the height of the SRTM 

elevations, measured at the radar phase center, within the vegetation canopy. 

 Figure 2 shows an example of this application for Australia, where differences 

between the ICESat elevation profiles and the SRTM DEM revealed 5-meter 

amplitude, undulating errors in the latter, over hundreds of kilometers (Carabajal 

et al., 2010). The continental-scale coverage afforded by ICESat can be used to 

remove these long-wavelength errors from the DEM. Figure 2 also highlights the 

consistency in the mean elevation differences, across various ICESat campaigns, 

suggesting very stable performance of the GLAS instrument. 

 In addition, the United States Geological Survey has led a collaborative 

effort to assess the accuracy of five ASTER Global DEM (GDEM) tiles, using 

various sources of ground control, including ICESat-derived geodetic GCPs 

(Carabajal et al., 2009). Figure 3 illustrates this work, and similar evaluations of 

an improved version of the ASTER GDEM will be made before it is released. 

Furthermore, the inclusion of ICESat data during the production of DEMs from 

other sensors, such as Interferometric Synthetic Aperture Radar (InSAR) 

enhances the quality of regional models, enabling better process modeling and 

interpretation (Atwood et al., 2007). 

ICESat has measured elevation change in Greenland with an accuracy 

of < 1 cm/year over the majority of the ice sheet
10

Figure 2. [Left] Mean ICESat centroid – SRTM differences for 

the L3e observation period. [Center] MODIS land-cover 

classification. [Right] Histograms of elevation differences for 

highest, centroid and lowest ICESat elevations (top) and means 

and standard deviations for centroid elevation differences 

during Laser 3 campaigns (bottom). (Carabajal et al., 2010) 

 

Figure 3. Evaluation of ASTER DEM tiles, using ICESat altimetry: [Left] 

ASTER topography with ICESat elevation profiles (black) as geodetic 

GCPs. [Right] Summary statistics for elevation differences for each tile. 

(Carabajal et al., 2009) 

 
11

 One of the most important, and challenging, land applications for ICESat 

data concerns the characterization and monitoring of vegetative cover. The 

Center for Ecological Applications of Lidar (CEAL) at Colorado State University 

is investigating the use of lidar remote sensing to study three-dimensional 

ecosystem spatial structure. They have used ICESat data, in conjunction with 

multi-spectral Moderate Resolution Imaging Spectroradiometer (MODIS) data, to 

create a global map of forest canopy height, which is shown in Figure 4. 

 

 Global ocean surface profiles have been produced routinely with ICESat 

data. Spatial coverage of the oceans extends to ±86°, well beyond the ±66° 

latitude limits of traditional, radar-based oceanographic missions, such as 

TOPEX/Poseidon and Jason 1/2. Urban and Schutz (2005) compared global 

ocean elevations from ICESat, collected during the L2a campaign, to concurrent 

measurements made by TOPEX/Poseidon. Figure 5 illustrates a 10-cm bias 

between the two satellites, which they found by computing the sea-surface 

anomaly (SSA, measured elevation minus mean sea surface) for each. 

Furthermore, they noted that, because of its inherently higher resolution, ICESat 

“measures small-scale ocean variability caused by waves, wind effects and swell.” 

Nonetheless, after removing this bias, the agreement between the ocean states 

observed by ICESat and TOPEX/Posedion (Figures 5a and 5b) is pronounced. 

Rather than being constant, however, this bias was subsequently found to vary 

with each ICESat campaign, suggesting a possible link to laser energy levels. The 

analysis of ocean data continues to play an important role in assessing residual 

inter-campaign biases, which have also been observed over ice sheets (Gunter et 

al., 2009). 

Figure 4. Global forest height map. Heights are the 90th percentile of GLAS 

height observations within a MODIS patch (Lefsky, 2010) 
12

L2a campaign (Sep-Nov 2003). 

Figure 6. Ross Ice Shelf, Antarctica: (a) blue line outlines model 

domain, red line delimits southern limit of TOPEX/Poseidon orbit 

at 66°S, and green line delineates ice front from MODIS Mosaic of 

Antarctica (MOA); (b) red dots mark ICESat crossovers included 

in the study, and additional symbols mark validation data sets, 

including gravimeter records (+), GPS receivers (o), and tide 

gauges (!). (Padman et al., 2008) 

Figure 5. Global sea surface 

anomaly (SSA), Sep-Nov 

2003: (a) ICESat, -10 cm 

bias removed; (b) TOPEX; 

(c) daily averages (Urban 

and Schutz, 2005)

(c)
13

 The expanded latitude coverage afforded by ICESat, relative to 

TOPEX/Poseidon, for example, has allowed for significant improvements to be 

made to ocean tide models in the polar regions. Notably, Padman et al. (2008) 

introduced ICESat elevation measurements from the Ross Ice Shelf to improve 

existing models for ocean tides near Antarctica. They validated their inverse 

solution using independent in situ data sets, including GPS receivers deployed on 

the ice shelf during the austral summers 2003/04 and 2004/05. Figure 6 

illustrates the region and data sets included in their analysis. 

 

Criterion 3

Produce calibrated profiles of ice sheet surface elevation to better than 25 cm 

accuracy at the ground location of each laser pulse, to serve as a basis for 

comparisons to future elevation observations. 

 Although dual crossovers — in which the two elevation measurements are 

from different ICESat campaigns — have been used only on a limited basis in the 

determination of dh/dt, intra-campaign crossovers still provide significant insight 

into elevation accuracy. In this approach, elevations at locations where ascending 

and descending tracks cross within a single ICESat campaign are compared. The 

resulting elevation differences are assumed to represent the combined effects of 

orbit, ranging and pointing errors, along with any short-term elevation change. 

Figure 7 summarizes the standard deviation of all intra-campaign crossovers in 

Antarctica for each of eleven ICESat campaigns, as a function of slope. Nearly all 

of the campaigns yield ! values below 25 cm for slopes less than 2°, which 

encompasses the majority of the ice sheet. The one campaign that exceeds this 

goal is L2a, which suffered from high saturation in the return signal. 

Improvements in the saturation correction are expected to bring these crossover 

results in line with other campaigns. 

ICESat has provided high-quality elevation data over oceans and 

land, increasing coverage significantly in the polar regions 

ICESat has measured elevations in Antarctica with an accuracy of 

< 25 cm over the majority of the ice sheet
14

Figure 7. Standard deviations of all Antarctic intra-campaign 

crossovers versus slope 

5. Additional Scientific Contributions

 In addition to achieving its primary scientific objectives, ICESat data have 

been used to make significant contributions outside the areas defined by the 

MSC. Many of these were published in a 2005 series of three special issues of 

Geophysical Research Letters (Volume 32, Issues 21-23). They have also been 

detailed in recent ICESat Senior Review proposals (see, for example, GSFC, 

2009). 

 Notably, Fricker et al. (2007) identified specific sites in Antarctica where 

changes in surface elevation suggested repeated filling and emptying of subglacial 

lakes, as shown in Figure 8. The volumes of water and their rates of transfer were 

determined to be much larger than those previously thought possible. Smith et 

al. (2009) subsequently used ICESat data to produce the first comprehensive 

survey of subglacial lakes in Antarctica. Figure 9 shows a map of their results. 
15

Figure 8. [Left] Locations of elevation change events detected by ICESat on the 

Whillans and Mercer Ice Streams, with colors indicating the magnitude of the 

changes. [Right] Repeat ICESat elevation profiles across oscillating (upper), rising 

(middle) and falling (lower) regions. (Fricker et al., 2007) 
16

Figure 9. Locations and volume-range estimates for 124 active lakes 

under the Antarctic ice sheet, with colors indicating volume range. 

Background shading shows a combination of satellite-radar-derived 

surface velocities (Joughin et al., 2006) and balance velocities 

(Bamber et al., 2009). (Smith et al., 2009) 

 The nature of the ICESat data has also lent itself to the application of 

techniques from other disciplines. For example, Pritchard et al. (2009) applied 

triangular irregular networks to ICESat elevations to determine dh/dt in both 

Greenland and Antarctica. Their method allows them to distinguish between 

dynamically induced elevation changes, and those associated with accumulation 

or melt. Their results, shown in Figure 10, led them to conclude that “the most 

profound changes in the ice sheets currently result from glacier dynamics at 

ocean margins.” 
17

Figure 10. Rates of elevation change for Greenland (left) and 

Antarctica (right). The white dashed line delineates the southern 

latitude limit for radar altimetry measurements in Antarctica, and 

the black lines identify drainage basins (Pritchard et al., 2009) 

 One of the most important contributions made by ICESat has been its 

monitoring of sea ice. Farrell et al. (2009) developed a new method for seasurface height retrieval to compute the first-ever time series of sea ice freeboard 

in the Arctic Ocean, spanning five years, between 2003 and 2008. They conclude 

that this trend is “due to thinning of the sea ice pack, rather than changes in 

snow cover.” Figure 11 illustrates the dramatic decline in freeboard that they 

observed between the start and end of this period. 

 

 Kwok and Rothrock (2009) used their ICESat-derived freeboard calculations 

to generate estimates of sea ice thickness in the Arctic Ocean. They noted a large 

decline over the same five-year period, from 2003 to 2008. In addition, they 

examined submarine ice draft data, extending back to 1958, to place the ICESat 

trends into a broader historical context. Figure 12 shows this overall trend, 

revealing long-term thinning of Arctic sea ice, beginning in the early 1980s. 
18

Figure 11. Polar stereographic maps of Arctic sea ice freeboard in 

February/March 2003 (left) and February/March 2008 (right) 

(Farrell et al., 2009) 

Figure 12. Interannual changes in winter (blue) and summer (red) 

ice thickness from regression analysis (RA) of submarine data and 

10 ICESat campaigns, with error bars shown in blue (Kwok and 

Rothrock, 2009) 
19

 Finally, despite the technical limitations of the 532-nm (green) channel, 

ICESat has provided remarkable results in atmospheric science. Palm et al. 

(2005) used GLAS attenuated backscatter data to produce the first-ever global 

mapping of the depth and structure of the planetary boundary layer (PBL), 

shown in Figure 13. 

 

Figure 13. [Top] GLAS attenuated backscatter (yellow) 

showing the PBL, along with the ECMWF model forecast 

(black) for Track 52 over the Pacific Ocean on October 1, 

2003; [Bottom] Global map of the GLAS-derived PBL for 

October 2003 (Palm et al., 2005) 

 Spinhirne et al. (2005) also provided the first global measurements of the 

true height distribution of aerosol layers in the atmosphere. They found that the 

532-nm (green) channel in GLAS provided much better resolution than the 1064-

nm (near-infrared) channel. In Figure 14, for example, they noted the finer detail 

visible in the upper plot, showing aerosols emanating from convective clouds, 

during a pass over Western Australia. 
20

Figure 14. GLAS 532-nm (top) and 1064-nm (bottom) atmospheric 

data over track shown at right, from Western Australia to 

southern China (Spinhirne et al., 2005)

6. ICESat Data Products

 The National Snow and Ice Data Center (NSIDC), at the University of 

Colorado at Boulder, archives and distributes 15 ICESat data products. The four 

Level-1A products, designated GLA01 through GLA04, contain global altimetry, 

atmosphere, engineering and laser-pointing data, respectively. In addition, global 

waveform parameters, elevation and backscatter data are available in three 

Level-1B products: GLA05 through GLA07, respectively. Four of the Level-2 

products, GLA08 through GLA11, pertain to atmospheric measurements, 

providing global data related to the planetary boundary layer, cloud heights, 

aerosol vertical structure, and optical depth data, respectively. Altimetry data 

from the Antarctic and Greenland ice sheets, sea ice, land surface, and oceans are 

available in the final four Level-2 products: GLA12 through GLA15, respectively. 

 As of this writing, these data products and detailed descriptions of their 

parameters can be found at: http://nsidc.org/data/icesat/data.html. 

7. Acknowledgments

 The authors would like to thank the ICESat Science Team, and its leader, 

Prof. Bob Schutz, of the University of Texas at Austin, for their review of the 

final manuscript. They would also like to acknowledge the support of the NASA 

Cryospheric Sciences Program in compiling this assessment. 
21

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39

Appendix A