icesat brochure

8/7/2019 ICESat Brochure

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Goddard Space Flight CenterGreenbelt, Maryland 20771

FS-2002-9-047-GSFC


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The Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud, and land Elevation Satellite (ICESat) spacecraft immediately following

its initial mechanical integration on June 18th, 2002. Note that ICESats solar arrays have not yet been attached. Left Gordon Casto,NASA/GSFC. Right John Bishop, Mantech. Courtesy of Ball Aerospace & Technologies Corp.


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Possible changes in the mass balance of the Antarcticand Greenland ice sheets are fundamental gaps in our understanding and are crucial to the quantificationand refinement of sea-level forecasts.

Sea-Level Change report, National ResearchCouncil (1990)

I ce, Cloud and lan d Elevat ion Sat ell i t e (I CESat )

Are the ice sheets that still blanket the Earths poles grow ing or sh rinking? Will global sea level rise orfall? NA SAs Earth Science Enterpr ise (ESE) has d eveloped the ICESat m ission to p rovide a nsw ers tothese and other questions to help fulfill NASAs mission to understand and protect our home planet.The primary goal of ICESat is to quantify ice sheet mass balance and understand how changes in the

Earth's atm osph ere and climate affect the polar ice masses and global sea level. ICESat w ill also measureglobal distributions of clouds an d a erosols for stud ies of their effects on atmosp heric processes andglobal change, as well as land topog rap hy, sea ice, and vegetation cover.

Ice sheets are complex and dynamic elements of our climate system. Their evolution has stronglyinfluenced sea level in the past and currently influences the global sea level rise that threatens ourcoasts. Ice streams that sp eed u p, slow d own , and change course illustrate their d ynamic nature.Atmo sph eric factors cause snow fall to vary in space and time across their surfaces. In Antarctica, smallice shelves continue to retreat along th e Antarctic Peninsu la, and large icebergs are released from th elargest ice shelves. In Greenland, the ice sheet margins are thinning and the inland parts of the ice sheetapp ear to be th ickening. Surface meltwater seep s into the ice sheets and a ccelerates their flow. Some of

the factors controlling the mass balance of the ice sheets, and their present and future influences on sealevel, are just beginning to be understood.

The ICESat m ission, p art of N ASAs Earth O bserving System (EOS), is schedu led to launch in December2002. The Geoscience Laser Altimeter System (GLAS) on ICESat will measure ice sheet elevations,changes in elevation through time, height profiles of clouds and aerosols, land elevations and vegetationcover, and approximate sea ice thickness. Future ICESat missions will extend and improve assessmentsfrom the first mission, as w ell as monitor on going chan ges. Together w ith other aspects of NASAs ESEand current an d plann ed EOS satellites, ICESat will enable scientists to stud y the Earths climate and ,ultimately, predict how ice sheets and sea level will respond to future climate change.

In light ofabrupt ice-sheet changes affecting globalclimate and sea level, enhanced emphasis on ice-sheet characterization over time is essential.

Abrupt Climate Change report, National Research Council (2002)

MI SSI ON I NTRODUCTI ON AND SCI ENTI FI C RATI ONALE


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Ar e The Gr een land And

Ant ar ct ic I ce Sheet s Gr owingOr Shr ink in g?

This question lies at the h eart of N ASAs rationale for theICESat mission. The Greenland and Antarctic ice sheets are anaverage of 2.4 km (7900 ft) thick, cover 10 percent of the Earthsland area, and contain 77 percent of the Earths fresh w ater (33million k m 3 or 8 million m i 3). The Antarctic ice sheet has 10 timesmore ice than Greenland because of its greater area and average icethickness. If their collective stored w ater volu me w ere released intothe ocean, global sea level wou ld r ise by abou t 80 m (260 ft). Asmall change of only 0.1% (2.4 m or 7.9 ft) in the average thick-ness of the ice sheets w ould cause a chang e in global sea levelof 8.3 cm (3.3 in). Their vast size and inhosp itable environ-ment make the ice sheets impossible to monitor completelyexcept via satellite.

Presently, new ice accumu lates on the ice sheets at abo ut 17cm (7 in) per year av eraged over Anta rctica and 28 cm (11 in)per year averaged over Greenland. Although most of theAntarctic ice sheet has n et ice accumu lation, about 9% of Greenland has a net ice loss of 1.4 m due to melting in the summer.Together, the volume of water required for this annual accumulationof ice is equ al to th e remov al of 0.8 cm (0.3 in) of water from th e entiresurface of the Earths oceans. Approximately the same amount of water is returnedto the oceans by m elting of ice and snow, by ice flow into the floating ice shelves, and th e directdischarge of icebergs. How ever, the exact amou nts being ga ined or lost vary significantly from region toregion as w ell as through time. The d ifference between the total mass inpu t and total mass outpu t(called the ma ss balance) is imp ortant becau se it might be as mu ch as 20% of the m ass inpu t, which isequivalent to a sea level rise or fall of 0.16 cm (0.06 in) per year.

ICESat is designed to d etect chan ges in ice sheet su rface elevation as small as 1.5 cm (0.6 in) p er yearover areas of 100 km by 100 km (62 mi by 62 mi). Chang es in ice sheet thickness are calculated fromelevation changes by correcting for small vertical motions of the u nd erlying bedro ck. Ice thicknesschanges will enable scientists to assess the ice behavior within individual ice drainage basins and majoroutlet glaciers, as well as th e entire ice sheets. Detection of avera ge thickness chan ges as sm all as 0.3 cm(0.12 in) per year over Greenland and Antarctica will reduce the uncertainty in their contribution to sealevel change to 0.1 cm (0.04 in) every 10 years. Elevation tim e-series constructed from ICESats contin-uous observations throughout its 3 to 5 year mission will detect seasonal and interannual changes in themass balance, caused by short-term changes in ice accumulation and surface melting, as well as thelong-term trends in the net balance between the surface processes and the ice flow.

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How Fast I s Sea Level Rising?

Fifteen thousand years ago, vast ice sheets covered much of No rth Am erica and p arts of Eurasia. In Anta rctica, the icesheet reached to the edge of the continental shelf, as muchas 200 km (125 mi) farther ou t to sea th an tod ay. As theclimate warm ed d uring the end of the last Ice Age, much of the Earth s ice cover m elted. Global sea level rose rap idlyby almost 100 m (330 ft) between 14,000 and 6,000 yearsago. Although the average rise was about 1.25 cm per year(0.5 in), the total rise caused dr ama tic chan ges to coastlinesaround the world.

The Antarctic and Greenland ice sheets, which are the mostsignificant remnants of that period, are currently reactingto present and past climate changes. Global sea level isbelieved to be r ising abou t 2 cm (0.8 in) every 10 years, lessrap idly than at the en d o f the ice age, but still significant.About 25% of the present rise is caused by thermal expan-sion as the oceans w arm, and about 25% by the m elting of small glaciers around the w orld. The remainder could bedue to ice loss from Greenland and Antarctica, but ourun certainty of their actual contributions (plus or minu s) is as large as the total rise in recent d ecades. Sealevel rise is also affected by h um an activities such as p um ping of ground wa ter, filling of reservoirs,

deforestation an d biomass burning, and draining of w etland s. Although sea level changes are small inany given year, sea level increases over the last 50 to 100 years ha ve seriously imp acted low -lyingcoastal regions th rough shoreline retreat, storm erosion, and coastal flooding. Futu re changes associatedwith climate w arming m ay cause an even greater threat to our popu lous coastal environments.

Of par ticular concern is the m arine ice sheet in West Antarctica,much of which is grounded on bedrock and sediment 1500 m (4900 ft)below sea level. The three main components are:1) interior ice that flows relatively slowly (10 m or33 ft p er year ); 2) the fast-moving ice stream sthat flow a hu nd red times faster; and 3) the

floating ice shelves through which most of theice enters the ocean. Some researchers believethat if West Antarcticas ice shelves thinnedsignificantly, they would no longer restrain therate of ice discharge from the ice streams. ICESatsmulti-year elevation-change data, combined with other remotesensing data, as well as atmospheric, oceanic, and ice flow models,should enable more accurate predictions of ice-sheet changes.

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SEA-LEVEL HISTORY NEAR BARBADOS

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Examples of coastal erosion along theeastern seaboard of the United Statesfollowing storms.

Published data on sea level rise with time (Fairbanks, 1989).Current sea level rise is about 2 cm per decade.


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Wil l The I ce Sheet s Th in

Or Thi cken I n A War m erClim at e?

Increased warmth will melt more ice near theedges of the ice sheets. However, a warmeratmosphere holds more m oisture, leading tomore precipitation over the ice sheets and moreice accumulation. Changes in atmosphericcirculation may also change the frequency of storms and the transport of moisture onto theice sheets. A warm er climate should alter thearea and thickness of sea ice surround ing theice sheets, and thereby shorten or lengthen themoisture pathw ay for snowfall. Other impor-tant processes include redistribution of newsnow by wind and sublimation from th e icesheet sur face.

Atmospheric interactions with the ice sheetshave im med iate effects on the ice sheet massbalance at the surface and therefore cause year-to-year effects on both the amount of waterstored an d global sea level. In contrast, the ra teof ice flow tends to respond slowly to climate change, over centuries or longer, as temperature and othereffects slowly p ropag ate d eep into the ice. How ever, the recent d iscovery of accelerations in ice flowdu ring periods of increased sum mer m elting in Greenland d emonstrates a rapid d ynamic response of the ice to climate change. Similarly in An tarctica, increases in su rface and basal melting on ice shelvescauses them to thin, which may lead to an acceleration of grounded ice discharge from the interior.

At present, we do not know which effect will have more impact on the overall ice sheet mass - increasedmelting and ice thinning at the edges or increased snowfall and ice accumulation over the entire icesheet surface. The net change in mass input (snowfall) minus output (melting and icebergs) in the com-ing d ecades could b e as mu ch as -10 % to +10 % for each d egree Celsius of climate w arm ing, resultingin a chan ge of -0.8 to +0.8 cm (-0.3 to +0.3 in) in global sea level per decad e for each 1 C of temp eratu rechange. Over centu ries, the interaction of climate w ith the ice flow could amp lify the chan ges.

ICESat will measure seasonal and interannual variations in ice sheet elevations caused mainly by snow-fall and surface melting, as well as enable estimates of the long-term trend in the overa ll mass balance.Scientists will use these data to answer questions such as which variables (e.g. temperature or precipita-tion) exert control on the ice sheet mass balance, and whether the observed ice changes are caused byrecent or long-term changes in climate. These studies combined with ice models will be used to predictfuture changes in ice volume that would be linked to climate change.

I c e F l o w

Melt andRunoff

W i n d R e d is t r i b u t i o n

Snow

Atmospheric Circulation

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OceanCirculation

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EquilibriumLine

Illustration of key climate variables influencing ice sheet mass balance.Graphic by Deborah McLean.

Surface melt water ponds on the western flank of the Greenland ice sheet.

Courtesy of Konrad Steffen.


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How Wi l l I CESat Measur e Ear t h s I ce, Clou ds, Oceans,

Land, and Veget at ion?The GLAS instrum ent on ICESat w ill d etermine th e distance from the satellite to the Earths surface andto intervening clouds and aerosols. It will do this by precisely measuring the time it takes for a shortpu lse of laser light to tr avel to the reflecting object and return to the satellite. Althou gh su rveyors rou -tinely use laser meth ods, the challenge for ICESat is to perform the m easurem ent 40 times a secondfrom a platform mov ing 26,000 km (16,000 mi) per hou r. In a dd ition, ICESat w ill be 600 km abov e theEarth and the precise locations of the satellite in space and the laser beam on the surface below must bedetermined at th e same time.

The GLAS instrum ent on ICESat w ill measu re precisely how long it takes for ph otons from a laser to

pass through the atmosphere, reflect off the surface or clouds, return through the atmosphere, collect inthe GLAS telescope, and trigger photon detectors. After halving the total travel time and applyingcorrections for the speed of light through the atmosphere, the distance from ICESat to the laser footprinton Earth s surface will be know n. When each pu lse is fired, ICESat w ill collect d ata for calculatingexactly w here it is in space u sing GPS (Global Positioning System) receivers. The an gle at w hich thelaser beam points relative to stars and the center of the Earth will be measured precisely with a star-tracking camera that is integral to GLAS. The data on the distance to the laser footprint on the surface,the p osition of the satellite in space, and the p ointing of the laser are all combin ed to calculate theelevation and position of each p oint measurement on the Earth.

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Schematic illustration of the GLAS instrument making measurement from ICESat while orbiting the Earth.Graphic by Deborah McLean.

OPERATI ONAL OVERVI EW


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GLAS measures continuously along ground tracks defined by the sequence of laser spots as ICESatorbits the Earth. The GLAS laser pu lses are emitted at a rate of 40 per second from th e Earth-facing(nadir) side of ICESat. This prod uces a series of app roximately 70 m (230 ft) diam eter spots on th esurface that are separated by n early 170 m (560 ft) along tra ck. These tracks w ill be repeated every 8days during the initial calibration-validation phase of the mission and every 183 days during the mainpor tion of ICESats mu lti-year mission. Points on the Earth w here these grou nd tracks intersect arecalled crossovers. Crossovers increase in areal density near the North and South Poles. Over the icesheets, the ICESat spacecraft will be controlled to p oint the laser beam precisely tow ard the sam e repeatground tracks to w ithin 35 m. Therefore, elevation changes can be analyzed along repeat groundtracks as w ell as at orbital crossover p oints. In add ition, ICESat ha s the ability to po int GLAS off-nad irto repeatedly measure areas of interest such as an erupting volcano or a collapsing ice shelf.

ICESat will also provid e detailed inform ation on th e global distribution of clouds an d aerosols. To d othis, GLAS emits laser energy a t both 1064 nm an d 532 nm , as this allows co-located elevation an datmospheric data to be obtained simultaneously. This will aid precise determination of the distancebetween ICESat and the Earth, as it is important to know if the laser pulses have traveled through cloudand aerosol layers. If present, these phenomena will diffuse the laser energy and extend the distance thelaser light photons travel by scattering.

Althou gh ICESats d ata on ice sheet surface elevations isof primary imp ortance, land an d ocean elevationdata w ill be obtained around the globe alongICESats grou nd tracks. Data on elevationchanges of the worlds oceans and ice-free land forms is expected to be of great interest to the broaderscientific community. The recentlylaun ched GRACE satellite w illenha nce the ICESat m ission bymap ping the Earths gravita-tional field in unprecedenteddetail. The GRACE da ta, inconjun ction with ICESat resultswill enable a greater u nd er-standing of any changes in thedistribut ion of snow, ice, andwater mass around the globe.

Illustration of ICESats 8 day repeat groundtrack relative to the continental area of Antarctica. Note the convergence around the SouthPole of the ground tracks. ICESats orbit wasdesigned to maximize coverage over the polar ice sheets.The pattern is similar over the northern polar region.Courtesy of Bob Schutz.

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Rap id Thi nn in g Of Gr eenl an d s Coast al I ce

After Antarctica, Greenland s ice sheet contains th e second largest m ass of frozen fresh w ater on Earth.The island of Greenland covers 2,175,000 square kilometers (840,000 square miles) and 85 percent iscovered by ice up to 3370 m (11,050 ft) thick. With its south ern tip protru din g into tem pera te latitudes, itis more sensitive than Antarctica to climate changes experienced in the major population centers of Europe and North America. In contrast to the Antarctic ice sheet, for which most of the surface remainsbelow the freezing point even in summer, 85% of the Greenland ice sheet has melting on the surfaceduring summer. In the zone of net ablation below about 1200 m, all the winter snow plus several metersof ice melts each year. Therefore, the m ass balance of the Greenland ice sheet is very sen sitive to chan gesin temperature and melting, as well as to changes in accumulating snowfall.

A NASA study of Greenlands ice sheet du ring the 1990s revealed rap id thinning at its margins, but also

some thickening across portions of the interior. A NA SA aircraft flew a netw ork of flight lines with alaser altimeter and global positioning satellite receivers in 1993 and 1994 and re-sur veyed the lines in1998 and 1999. From th ese data, scientists reported that th e ice sheet has th inned by m ore than 1 m (3 ft)per y ear on m any coastal outlet glaciers and by as mu ch as 9 m (30 ft) per yea r in a few areas. Scientistsestimated a net loss of ap proximately 50 cubic kilometers (12 cubic miles) of ice per year from the entireice sheet during the 1990s. This volume is sufficient to raise global sea level by 0.13 mm (0.005 inches)per year, or about seven percent of the observed rate of rise in recent decades.

Explaining these observations and the ice andclimate p rocesses causing th em is th e subject of ongoing stud y by the Program for Arctic

Regional Climate Assessment (PARCA). This joint effort by NA SA and affiliated scientistsconsists of combined remote sensing, fieldwork,and mod eling stud ies on the Greenland IceSheet. ICESat w ill contribute to PARCA byobtaining elevation data across Greenlandthroughou t the year in a more consistentfashion than could ever be obtained by thepioneering airborne effort.

Results from a multi-year airborne laser altimetry study of the Greenland ice sheet. The map of observed elevationchanges indicates significant variations in the changesfrom place to place and the complex behavior of the icemass. Blue, green, and gray tones indicate areas of ice lossdue to melting, reduced snowfall, or increased ice dis-charge. The pale-yellow, orange-brown, and red colorsover much of the interior of the ice sheet indicate areaswhere the ice surface is rising. Courtesy of Bill Krabill.

EARTHS DYNAMI C I CE


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Thinn ing of Par t of t he West Ant ar ct ic I ce Sheet

An ad ditional illustra tion of ice sheet elevation changes is prov ided by ana lysis of 7 years of Europ eanRemote Sensing Satellite (ERS-1 and 2) satellite radar altimetry data (1992-1999) over part of the WestAntarctic ice sheet (see pages 10-11). The large area of green an d blue in th e up per par t of the figurecovers mu ch of the ice dr ainage basin th at feeds into tw o major outlet glaciers (Pine Island Glacier andThw aites Glacier). Elevation d ecreases there a re as large as 30 cm (12 in) per y ear. The ice thinn ing islikely to be a continuing response to climate change over manycenturies, and perhaps removal of an ice shelf in front of theglaciers in th e past. Elevation increases up to 20 cm (8 in) per yearare indicated (red and orange) on the ridge between th e PineIsland/ Thwaites drainage basin and the Ross Ice Shelf to thesouthwest (lower right).

The average elevation change for the grounded ice (excluding thefloating ice shelves) over this region is 4.3 cm (1.7 in) per year,and the estimated average ra te of bedrock up lift is 2.5 cm (1 in) peryear. Therefore, the estimated rate of ice thinning is 6.8 cm (2.7 in)per y ear, w hich mean s a ma ss loss of 73 km 3 (17.5 mi 3) of ice p eryear. The resulting contribution to sea level rise is 0.2 mm (0.008in) per year from this p art of West Antarctica. ICESat w ill imp rovethe resolution and accuracy of such measurements, especiallynear the coast where steeper slopes limit radar altimetry, andextend th e measurement inland to 86 S.

Thi nni ng of t he Lar sen I ce Shelf Ant ar ct ic Peni nsula

Analysis of the ERS-1 and 2 radar altimeter da ta on th e north erly part of the Larsen Ice Shelf (LarsenB) for the p eriod 1992 to 1999 (see pages 9 an d 10) showed a lower ing of the elevation of the ice shelf surface by 27 cm (11 in) per year. A 27-cm cha nge in the h eight of the ice shelf sur face above sea levelmean s the total ice thickness chan ged by ab out 1.7 m (5.6 ft), because only 16% of the thickness of float-ing ice shelves is above th e w ater line. Therefore over the 7 years, the sh elf lost abou t 12 m (39 ft) of itstotal thickness of about 170 m (560 ft). The breaku p the n orthern par ts of the Larsen Ice Shelf and otherice shelves in this region has been associated w ithsignificant warming of the Peninsula region in recentdecades. As shown in the figure, the surface elevationof the rema ining p art of the Larsen Ice Shelf (C) hasalso been low ering at a similar rate of 21.7 cm (8.5 in)per year, which im plies ice thinn ing of 1.4 m (4 ft)/ year. Althou gh the Larsen C is abou t 100 m (330 ft)thicker than B wa s, it may face a similar breaku p if warm ing continues.

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Time series of surface elevation change of the Larsen C IceShelf, Antarctic Peninsula. Courtesy of Jay Zwally.

Spatial illustration of surface elevation changeacross a part of West Antarctica from ERS-1 and -2radar altimetry (Zwally et al., 2002).


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Lar sen B I ce Shelf - Aust r al Sum m er 20 02

As pred icted b eforehand by scientists, the rap id thinn ing of the Larsen Ice Shelf led to a series of dr a-matic events captured by satellite images as well as witnessed by Argentine field researchers.

Janu ary 31, 2002 (top left image) Melt pon ds, w ith light blue ton es, are observed across much of theice shelf in surface dep ressions associated with crevasses and flow lines. At th is time, the Larsen B iceshelf extended from Robertson Island (at the upper left) to the Jason Peninsula (along the lower margin),following a significant retreat betw een 1993 and 2001 from its historical observ ed extent earlier in th elast centu ry.

February 17, 2002 (top r ight image) Small events resu lted in a n um ber of icebergs with a variety of sizes calving from the ice shelf front. Melt ponds were still evident but appear to be diminished in

extent. The melt ponds became less obvious as the ice shelf began to break apart and water drainedthrou gh th e ice shelf to the ocean below.

March 5, 2002 (bottom left image) Collapse of the majority of the Larsen B ice shelf. Due to w ind andtidal forces, new icebergs now choke the em baym ent from th e ice shelf front to th e end of RobertsonIsland . The light blue color is the result of the exp osure of glacial ice as the ice fragm ents rotate from avertical to a horizontal position in the open water of the bay.

March 17, 2002 (bottom right im age) Disintegrat ion of the ice shelf marg ins continues w ith d ispersalof the ice shelf fragm ents d ue to w ind and tidal forces. The area lost is about 3250 km 2 (1250 mi 2) orapp roximately the area o f Rhode Island, w ith a w eight estimated at 720,000,000,000 tons.

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January 31, 2002

March 17, 2002

Collapse of the Larsen B iceshelf captured by a sequenceof satellite imagery. SeeScambos et al., J. Glaciology,2000, for a history and mecha-nism of ice shelf collapses inAntarctica. The data were col-lected by the ModerateResolution ImagingSpectrometer (MODIS) onNASAs Terra satellite.Courtesy of Ted Scambos.

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Antarctica, thefrozen continent, isa landscape that haslong challenged andintrigued humanity. Thisimage from Radarsat show sspatial d etails with claritynever p reviously achieved by

satellite observa tions. Thisimage serves to provide a contextfor ICESats m easurem ents of su rfaceelevation chang e. To illustrate th e scaleof the ice mass th at m ay influence globalsea level, keep in m ind that theAntarctic continen t is app roximatelytwo thirds the size of North Am ericaor equivalent to the combined areasof the United States and Mexico.From the Anta rctic Peninsula, with

its fringing ice shelves that a re progres-sively collapsing, to West Antarctica, with itsmar ine-based (below sea level) ice sheet andhu ge ice shelves, to East Antarctica w ith its greatthickn ess (> 4,000 m o r 12,000 ft), this continentssurface features are revealed in detail.

This Radarsat Antarctic Mapping Mission mosaicillustra tes the comp lex sur face that ICESat m ust m eas-ure throughout its mission. However, the image illustratesonly th e areal or 2-dim ensional character of th is ice sheet; ICESat

will provide the needed 3rd dimension, surface elevation as well as its changethrough time. Note the huge Ronne-Filchner and Ross Ice Shelves that have produced huge icebergssince this mosaic was obtained. The twisted patterns of ice draining from the interior of the ice sheet outto the ice shelves are extraordina ry. Although not d istinct at th is resolution, ice streams can be id entifiedby the bright radar returns from their crevassed (fractured) margins that reflect radar energy more effi-ciently than surrounding unfractured interior ice. These ice streams are vast rivers of ice that flow up to100 times faster than the ice they chann el through , with sp eeds u p to 1 km (3000 ft) per year. Some icestreams in East Anta rctica extend alm ost 800 km (500 mi). They h ave chan ged their velocity and flowpatterns in response to factors that are just now beginning to be understood through field researchsupported by intermittent satellite data. Ice streams form the most dynamic parts of the Antarctic ice

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RADARSAT Snapshot Of An I ce Sheet

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sheet, and scientists believe that th ey arequite susceptible to rapid change.

ICESat surface elevation tim e series datawill enhance our un derstanding of dy nam ic ice. Because of th eir relativelyfast flow, ice stream s are resp onsiblefor transporting a large percentage of the snow that falls on the continentsinterior to th e ocean. ICESat w ill

monitor the elevation details of thisdy nam ic surface for 3 to 5 years with

far greater accur acy thanRadarsat. Images from sensors

on oth er satellites will providecontext for this missions

elevation change data.

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Radarsat can collect thesespatial data day and night,through cloudy weather orclear. It was this capabilitythat enabled Antarctica to bemapped in just 18 days,compared to the last satel-lite map that requiredimages from five different

satellites spanning a 13-yearperiod from 1980 to 1994.

Following a precise orbitalmaneuver, Radarsat acquired

complete coverage of the continentas part of the first Antarctic

Mapping Mission (AMM-1). Afollow-up mission (MAM) took place

during 2000, and Radarsat-2 is now inorbit. The Radarsat Antarctic Mapping

Project (RAMP) is a joint effort of NASA andthe Canadian Space Agency (CSA). The Byrd

Polar Research Center, Vexcel Corp., the AlaskaSAR Facility, and the Jet Propulsion Laboratory

conducted the project collaboratively, with fundingfrom NASAs Pathfinder and Cryospheric Programs.

The project also received valuable assistance from theNational Science Foundation, the Environmental

Research Institute of Michigan, and the National Imageryand Mapping Agency. For more information on Radarsat

data and this mosaic, contact the National Snow and Ice DataCenter (NSIDC).

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How Do Clou ds and Aer osols Af f ect Cli m at e?

The distribution of atmospheric clouds and aerosolsis one of the most imp ortant factors in global climate.Cloud s can cool the Earth s sur face by reflectingsolar rad iation or w arm th e surface by trapp ing itsrad iated h eat. Being typ ically more reflective thanthe Earth s sur face, low cloud s pr imarily reflect solarradiation and cause relative cooling. High, thinclouds are usually less effective at reflecting solarradiation but effectively trap outgoing infrared heatradiation. Accurate kn owledge of the typ e andheight of clouds is thus very important for climate

stud ies. Like cloud s, aerosols tend to cool the Earthssurface and heat the atmosph ere by scattering an dabsorbing solar radiation. In general terms, aerosolsare distinguished from cloud s by existing at hu miditylevels below saturation and by a particle size which istyp ically 10 to 100 times smaller tha n cloud drop lets.

The laser profiling m easurements from GLAS are a fun dam entally new way to study the atmosph erefrom space. To better understand climate, scientists need to distinguish the multiple cloud and aerosollayers that typically exist in the atmosphere. Other satellite remote-sensing techniques currently in useare limited to passive observations, i.e. the sensor images the Earth at a given wavelength but views all

atmospheric layers simultaneously. Another issue is that such passive instruments cannot measure theheight of layers su fficiently to fullyun derstand the role of cloud s in globalclimate chan ge. The GLAS instru men ton ICESat w ill enable the a ccur ate,multi-year height profiling of atmos-pheric cloud and aerosol layers directlyfrom sp ace for th e first time.

GLAS cloud and aerosol measurementswill also improve studies of other atmos-

pheric phenomena un ique to the polarregions. Polar stratospheric clouds thatform du ring periods of dep leted atmos-ph eric ozone can be easily d etected fromGLAS measurements. Polar d iamonddust, which refers to near-surfacesuspend ed ice crystals from an app ar-ently non -precipitating sky, could also

Illustration of varying cloud optical thicknesses from thehighest thinnest cloud (upper right) to the dense low-lyingclouds (lower left). Courtesy of Art Rangno.

Active profiling of the atmosphere by GLAS (model data shown here)allows individual cloud and aerosol layers to be seen separately, whereastraditional satellite imaging techniques combine the radiation from alllayers into one measurement. Thicker clouds with higher optical densities(brighter colors) are contrasted with thin clouds, aerosols, and clear skies(progressively darker colors). Courtesy of Jim Spinhirne.

12

CLOUDS, AEROSOLS, LAND ELEVATI ON, AND VEGETATI ON


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be d etected by GLAS. Finally, GLAS w ill overcome an end urin glimitation of high-latitud e research, the inaccessible and un inhabitable nature of mu ch of the polar regions, and make d ata

available from regions that have previously not been well studied.

Cloud s play an esp ecially critical role in the climate of the p olarregions. The polar temperature balance is depend ent on cloud spreventing heat loss to space. Passive observations of clouds arecan be inaccurate in these areas because th e visible and infraredradiative signatures of cloud s are hard to distinguish from that of the bright, cold snow and ice covered surfaces below them. Imagetechniques to distinguish between snow-covered surfaces andtypical cirrus clouds in these regions are complex, and requireconsiderable data analysis. The GLAS instrument will enableunam biguous d etection of all but the thinnest atmosph eric layersand this will permit the development of consistent climatologiesof the Arctic and An tarctic. GLAS measu rements will alsoimprove studies of polar stratospheric clouds that facilitatedep leted atm ospheric ozone.

Und erstanding the role of hazeaerosols is also an issue for climatechange researchers. Atmosphericaerosols globally impa ct radiative

transfer of energy and also influ-ences cloud formation. In addition,the fertilization for biologicalactivity of large p arts of th e oceanand parts of the tropical rain forestoccur by aerosol transport.Significantly, aerosol distributionand characteristics are highly vari-able and complex. Aerosols aretransported around the world bywind s that can change d ramatically

with altitude, but no existingobservation can reliably give theheight of aerosol layers. The GLASaerosol profiles will be a uniqu eand critical contribu tion to earthscience and global change research.

13

Image of clouds and sea ice around north-ern Greenland from NASAs Aqua MODISsensor. The 1 km resolution image wastaken July 13, 2002. Courtesy of the Aqua

MODIS Science Team.

A true-color image acquired May 4, 2001, by the Sea-viewing Wide Field-of-view Sensor reveals a large plume of aerosols blowing eastward over theNorth Atlantic Ocean. Courtesy of the SeaWiFS Project.


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Measur ing Ear t hs Land Sur f ace And Veget at ion

The topography and vegetation cover of the Earths land surface form a complex mosaic that is theprod uct of a d iverse set of solid Earth, glacial, hyd rologic, ecological, atmosp heric, anthrop ogenic, andother processes. The landscape we see today is the cumulative result of the interaction of thoseprocesses through time. Measurement of landscape properties, including elevation, slope, roughness,and vegetation height and density, is a necessary step tow ard u nd erstanding the interplay betweenformative p rocesses and thus toward more accurate m odeling of futu re changes. Knowledge of theseproperties and their changes with time is important for resource management, land use, infrastructuredevelopment, navigation, and forecasting the occurrence and impact of natural hazards such as volcaniceruptions, landslides, floods, and wild fires.

The ICESat p rofiles will provide a global samp ling of the elevation of the Earth s land surface with

un preceden ted accuracy. This globally-consistent grid of high-accura cy elevation d ata w ill be used as areference framework to evaluate and improve the accuracy of topographic maps acquired by other air-borne and space-based m ethods su ch as conventional stereo-photogrammetry an d radar interferometry.In particular, ICESat profiles will be combined with the near-global mapping accomplished by theShuttle Radar Topography Mission to greatly improve our knowledge of the Earths topography. Also,using ICESats consistent global framework, topographic maps acquired through time using a variety of methods can be better co-registered, enabling long-term observations of topographic changes.

14

Dramatic topographic and vegetation cover change can be causedby natural hazards. This is illustrated here by two views of Mt. St.Helens, Washington, from Spirit Lake before (upper) and after(lower) the May 18, 1980 eruption. The upper picture is courtesy of Jim Nieland and the lower picture is courtesy of the USGSCascades Volcano Observatory. The satellite view of Mt. St. Helens(right image) shows the ejected debris field outside the crater and iscourtesy of the Landsat-7 Project.


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ICESat profiles repeated through time across dynamic landforms will also enable direct observation of topog rap hic change. By p ointing th e ICESat sp acecraft off nad ir, features can be targeted for profilingevery 12 days on average near the equator, and even more frequently at higher latitudes. This targetingcapability will be used to monitor selected phenomena such as volcanic eruptions, changes in river andlake levels, seasonal snow-p ack dyn amics, glacier surges and retreats, soil erosion, and migration of desert san d sheets. The global access of ICESats elevation profiling w ill add the h eight d imension toimages of the dynamic Earth acquired by other orbital remote sensing instruments.

In addition to acquiring elevation data, ICESats measurement of the laser pulse return shape providesunique information about the height distribution of the surface features within each laser footprint. Inareas lacking veg etation cover, this is a measure of relief (grou nd slope and rough ness), an indication of the intensity of geomorphic processes. In vegetated areas of low relief, the elevation of the ground andthe height and d ensityof the vegetation covercan be inferred fromthe return p ulse. Thevegetation observationsenable estimation of above-ground biomassand its loss due todeforestation, animportant componentof the carbon cycle.

15

Photograph of Yosemite Valley,California, showing complexlandscape elements. Courtesy of David Harding.

Photograph of the Brazilianrain forest canopy. Courtesyof the Large Scale Biosphere-Atmosphere Experiment inAmazonia.


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What I s The Geosci ence Laser

Al t im et er Syst em ?The Geoscience Laser Altimeter System (GLAS) is a next-generation space lidar. It is the sole science pay load forNASAs ICESat Mission. The GLAS design combines a 15cm (6 in) precision su rface lidar w ith a sensitive du alwavelength cloud and aerosol lidar. GLAS operates withinfrared an d v isible laser light p ulses at 532 nm and 1064nm w avelengths a t eye-safe signal levels. These laser lightpulses illuminate the Earth and will enable GLAS to measurethe surface elevation of th e p olar ice sheets accurately, establish

a netw ork of height data on the Earths land top ography, andprofile the vertical distribution of cloud s and aerosols on aglobal scale. GLAS is integrated onto the ICESat spacecraftbuilt by Ball Aerospace. ICESat will be launched into a n ear-polar or bit that is slightly inclined relative to the equ ator, sothat it will cover the Earth from 86 N to 86 S.

GLAS will measure th e vertical distance from orbit to th eEarths sur face 40 times a second w ith p ulses from a N D:YAGlaser. Each GLAS laser pulse at 1064 nm can yield a singledistance measurement as w ell as other information abou t the

character of the Earths surface. On Earth, the laser footpr intshave a d iameter of ap proximately 70 m (230 ft) and 170 m (560ft) center-to-center spacing. The GLAS receiver uses a 1 m (3 ft)diam eter telescope to collect th e reflected 1064 nm laser lightand a detector that precisely times the outgoing and reflectedlaser pulses. A digitizer records each transm itted and reflectedlaser pulse with 1 nano-second (ns) resolution.

16

Three views of the GLAS instrument following integration with the ICESat satellite at Ball Aerospace & Technologies Corp. inBoulder, Colorado. Courtesy of Ball Aerospace.

The GLAS instrument following final assem-bly and testing at NASA Goddard SpaceFlight Center. Courtesy of the GLASInstrument Team.

Illustration of the GLAS instrument from thezenith-facing side. Courtesy of the GLASInstrument Team.

I NSTRUMENT AND MI SSI ON SPECI FI CS


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GLAS will also measu re the v ertical distribution s of cloud s and aerosols by recording ver tical profiles of laser backscatter at both 1064 nm and 532 nm. A 1064 nm detector w ill be used to measu re the heightand echo p ulse shape from thicker clouds. The lidar receiver at 532 nm u ses a narrow band width etalonfilter and highly sensitive ph oton coun ting d etectors. The 532 nm ba ckscatter profiles will be used tomeasure the vertical extent of thinner clouds an d the height of the atm ospheric boundary layer.

A precision Blackjack GPS receiver carried on the ICESat spacecraft w ill measure GLASs location inorbit. Accurate knowledge of the lasers pointing angle relative to inertial space is needed to minimizeuncertainty when measuring over sloping surfaces such as ice sheet margins. On its zenith or star-facing side, GLAS uses a stellar reference system (SRS) to m easure th e p ointing an gle of each laser p ulserelative to selected bright stars tha t are effectively static reference points in sp ace. GLAS uses redu nd anthigh precision star cameras and gyroscopes to determine the orientation of its measurement baseline onthe instruments optical bench. Each laser pulse is measured relative to the star camera with a laserreference system (LRS). Analysis of the reflected 1064 nm an d 532 nm pu lse data, along with the GPSand LRS data, enables final d etermination of the distance to the reflecting sur face, the degree of pu lsespreading due to atmospheric conditions, and vertical distribution of any surface vegetation.

Graphical illustration of the GLAS instrument (with itstelescope sun shade and main optical bench removed)showing the 1 m telescope mirror receiving a reflectedlaser pulse (in green) from Earth into its detectors.Courtesy of the GLAS Instrument Team.

Graphical illustration of the GLAS instrument showingLaser-1 firing a laser pulse (in green) toward Earth.Courtesy of the GLAS Instrument Team.

Telescope Sun Shade

Laser 1

12

3

Radiator

Radiator

Main Optical Bench

Stellar Reference SystemInstrumentStar Tracker

Altimeter Detectors (2)

Single PhotonCounting Modules

Telescope Bench

1 m Telescope Mirror Secondary Mirror Tower

17

Outgoing Laser Light Pulse

Incoming LaLight Pulse


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The I CESat Mission Over vi ew

The overall mission is composed of the GLAS instru men t, the ICESat sp acecraft, the laun ch vehicle,mission operations, and the science team. Goddard Space Flight Center (GSFC) staff developed theGLAS instrument in partnership with university and aerospace industry personnel. The ICESat space-craft was dev eloped by Ball Aerospace & Technologies Corp . (Ball Aerospace) in Bould er, Colorad o. BallAerospace will also sup por t ICESat wh en it is on orbit. NA SA Kenned y Spa ce Center is providing th eexpend able Boeing Corp oration Delta II launch v ehicle. The science team is comp osed of researchersfrom universities, GSFC staff, and supporting industry personnel. The science team is developing thescience algorithms a nd is responsible for all science data p rocessing, as w ell as the generation of sciencedata products.

A Delta II laun ch vehicle will carry ICESat, as well as a second p ayload called CH IPSat, into a near-

polar low Earth orbit of app roximately 600 kilometers altitud e. The Delta II will be laun ched from SpaceLaunch Complex-Two (SLC-2), Western Test Range, Vandenberg Air Force Base, California in mid-December, 2002. While on orbit, ICESat achieves 10 arcsec pointing accuracy and ~2 arcsec pointingknow ledge. To accomp lish th is, a three-axis stabilized attitud e control system comp osed of tw o startrackers, four reaction w heels, a hemisph erical resonator gy roscope on th e GLAS instrum ent, 15 coarsesun sensors, three magnetometers, and three torque rods is u sed. Orbital position is derived from aredu nd ant set of NA SA JPL Blackjack GPS receivers and a global netwo rk of groun d receivers,provided by the International GPS Service. Additional orbit position information is obtained from theInternational Laser Ranging Service, a netw ork of groun d laser stations that w ill use a laser retroreflec-tor m oun ted on the na dir (earth-facing) side of ICESat. While on orbit, the spacecraft will commu nicatewith the Earth Observing System (EOS) Polar Grou nd Stations (EPGS) four times a d ay over X-Band

and S-Band radio frequency channels.

Solid RocketMotors Impact

Liftoff

Solid Rocket Motor Burnout (3)t = 1.1 minutesAlt. = 16.3 km (10.1 mi)Vel. = 2340 kph

(1450 mph)

Solid Rocket Motor Jettison (3)t = 1.7 minutesAlt. = 31.2 km (19.5 mi)Vel. = 2820 kph

(1750 mph)

Main Engine Cut Offt = 4.4 minutesAlt. = 104.7 km

(65.1 mi)Vel. = 17430 kph

(10830 mph) Second Engine Cut Off It = 11.2 minutesAlt. = 188.2 km

(117.2 mi)Vel. = 28470 kph

(17690 mph)

Dual Payload AttachmentFitting Separationt = 96.7 minutesAlt. = 592.3 km

(368.6 mi)Vel. = 27240 kph

(16930 mph)

ICESat Separationt = 64.0 minutesAlt. = 590.4 km

(367.4 mi)Vel. = 27230 kph

(16920 mph)

CHIPSat Separationt = 103.3 minutesAlt. = 585.2 km

(364.2 mi)Vel. = 27250 kph

(16930 mph)

Depletion Burn:removes Stage 2 fromvicinity of spacecraft,

while lowering Stage 2orbit perigee altitude

Second Engine Cut Off II(restarted inflight)t = 59.8 minutesAlt. = 589.1 km

(366.6 mi)Vel. = 27230 kph

(16920 mph)

I

C

KEY (nominal values)t = Time (since launch)Alt. = AltitudeVel. = Velocitykm = kilometersmi = mileskph = kilometers per hourmph = miles per hour

Second Stage Ignitiont = 4.6 minutesAlt. = 115.6 km

(71.9 mi)Vel. = 17400 kph

(10810 mph)

Fairing Jettisont = 4.9 minutesAlt. = 129.6 km

(80.7 mi)Vel. = 17620 kph

(10950 mph)

18

Illustration of theICESat launch.Graphic by DeborahMcLean and BoeingCorp.


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Mission Oper at ions And Cali br at ion/ Vali dat ion

Once ICESat is on orbit, mission operation s will be cond ucted b y tw o organizations. The Earth ScienceData and Information System (ESDIS) Project at GSFC will provide space and ground network support.The University of Colorad os Laboratory for Atm osph eric and Space Ph ysics (LASP) teamed w ith BallAerospace will provide mission operations and flight dynamics support. The GLAS data are recordedby the on-board Solid State Recorder an d are played back du ring scheduled contacts via X-band dow n-link communications. The ICESat Science Investigator Processing System (I-SIPS) at GSFC will conductdata processing and generation of products with support from the Center for Space Research (CSR) atthe Un iversity of Texas, Au stin. The Nationa l Snow and Ice Data Cen ter (NSIDC), located at theUniversity of Colorado in Boulder, will archive and distribute ICESat d ata p rodu cts to the scientificcommunity and other users. The interactions of these organizations are summarized in the chart below.

In the 60 da ys followin g laun ch, ICESat will und ergo a series of tests to establish th at all systems arefunctioning w ithin specifications in the orbital environm ent. This comm issioning p hase w ill be follow edby a p eriod of intense activity to verify the p erforman ce of GLAS and all related systems. The objectiveof this intense period (calibration/ validation or cal/ val), is to help insure that geop hysical interpreta-tions can be d rawn from the d ata prod ucts. Cal/ val includ es evaluation of the GLAS measurementsagainst ground truth observations. A variety of instrumented and precisely map ped ground -truth siteswill be used, includ ing d ry lakebeds, land scapes w ith un du lating su rface topograp hy, and the ocean, allof which will be periodically scanned.

Commandsand Telemetry

EPGSEOS Polar Ground System

EDOSEOS Data and Operations System

NSIDCNational Snow and Ice

Data Center

StandardProducts

Level 0Products

GPS andPrecision Rateand AttitudePacket Data

I-SIPSICESat Science Investigator-Led

Processing System

UTUniversityof Texas

SFCsScience

ComputingFacilities

OrbitalTrack.Info.System

CentralStd.Auto.FileSystem

MOCMission Operations Center

MMFDFMulti-Mission

Flight DynamicsFacility

Acquisition ofData Products

Science Data Playback

Schedules Playback H/K

Commands,Real-Time H/K

Real-TimeH/K

GLAS OperationsRequests

GLAS StatusLevel 0

Level 1 Products

Science Plan

Orbit and Attitude Data

Status

ESDIS

Level 1 POD and PAD StandardProducts

ISFInstrument

SupportFacility

GroundStations

ICESat

Science

Level 0Products

H/K = Housekeeping

19

Schematic diagram of ICESat mission operations. Graphic by Deborah McLean.


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Acronyms

Ref er ences

ICESats Laser Measur emen ts of Polar Ice, Atmo sph ere, Ocean, and Land , H. J. Zw ally, B. Schu tz, W. Abd alati, J.

Abshire, C. Bentley, A. Brenner, J. Bufton, J. Dezio, D. Hancock, D. Harding, T. Herring, B. Minster, K. Quinn, S.Palm, J. Spinhirn e, and R. Thomas, Jour nal o f Geody nam ics, 34, 3-4, pp . 405-445, 2002.

Enhan ced Geolocation of Spaceborne Laser Altimeter Surface Return s: Para meter Calibration from theSimultaneous Reduction of Altimeter Range and Navigation Tracking Data, S.B. Luthcke, C.C. Carabajal, and D.D.Rowlands, Journal of Geodynamics, 34, 3-4, pp. 447-475, 2002.

PARCA: Mass Balance of the Gr eenland Ice Sheet, Special Section of Journ al of Geoph ysical Research -Atm osp her es, 106, D24, pp . 33,689-34,058, 2001.

Laser Altimeter Canopy Height Profiles: Methods and Validation for Deciduous, Broadleaf Forests, D.J.Harding,M.A. Lefsky, G.G. Park er, and J.B. Blair, Rem. Sens . Environm ent , 76, 3, pp . 283-297, 2001.

System to A ttain Accurat e Pointin g Know ledge o f the Geoscience Laser Altimet er, J.M. Sirota, P. Millar, E. Ketchum ,B.E. Schutz, a nd S. Bae, pp . 39-48 in AAS 01-003, Guid ance an d Control 2001, Adva nces in th e Astrona uticalSciences, R. D.Culp and C. N. Schira (eds.), 107, 2001.

Greenland Ice Sheet: High-Elevation Balance and Periph eral Thinn ing, W. Krabill, W. Abd alati, E. Freder ick, S.Manizade, C. Martin, J. Sonntag, R. Swift, R. Thomas, W. Wright, and J. Yungel, Science, 289, pp. 428-430, 2000.

A Method of Combinin g ICESat an d GRACE Satellite Data to Con strain An tarctic Mass Balance, J. Wahr, D.Wingham, and C.R. Bentley Journal of Geophysical Research 105, B7, pp. 16,279-16,294, 2000.

Geoscience Laser Altimeter System (GLAS), Algorithm Theoretical Basis Docum ents, (Version 3.0), A. C. Brenn er, H.J. Zwally, C. R. Bentley, B. M. Csath, D. J. Harding, M. A. Hofton, J.-B. Minster, L. Roberts, J. L. Saba, R. H. Thomas,

and D. Yi, NASA Technical pap er, 306p., 1999.

Monitorin g Ice Sheet Behavior from Space, R. Bindscha dler, Reviews o f Geoph ysics 36, 1, pp . 79-104, 1998

Observations of the Earth's Topography from the Shuttle Laser Altimeter (SLA): Laser-Pulse Echo-RecoveryMeasu rem ents of Terrestr ial Surfaces, J. Garvin , J. Bufton , J. Blair, D. Hard ing, S. Luthcke, J. Frawley, and D.Rowland s, Physics an d Chem istry of th e Earth , 23, pp . 1053-1068, 1998.

Space Based At mosp heric Measu remen ts by GLAS, J.D. Spinh irne a nd S.P. Palm, p p. 213-217, in Ad vances inAtm osph eric Remote Sensing with Lidar, A. Ansm ann (ed.), Spr inger, Berlin, 1996

More information on th e ICESat mission can be found at: icesat.gsfc.nasa.gov as w ell as links to m anyother supporting sites related to this and other EOS satellite missions.

ATBD Algorithm Theoretical Basis DocumentBall Ball Aerospace & Technologies Corp.CHIPSat Cosmic Hot Interstellar Plasma SpectrometerCSR Center for Space Research

University of Texas, AustinEOS Earth Observing SystemEOSDIS EOS Data and Information SystemEPGS EOS Polar Ground System (Alaska and Svalbard)

ESDIS Earth Science Data and Information SystemFOV Field Of ViewGLAS Geoscience Laser Altimeter SystemGPS Global Positioning SystemGRACE Gravity Recovery and Climate ExperimentGSFC Goddard Space Flight CenterICESat Ice, Cloud, and land Elevation SatelliteI-SIPS ICESat Science Investigator-led Processing SystemIST Instrument Star Tracker

JPL Jet Propulsion LaboratoryKSC Kennedy Space CenterLASP Laboratory for Atmospheric and Space Physics

University of Colorado, BoulderLEO Low Earth Orbitlidar LIght Detection And RangingLRS Laser Reference SystemLSM Laser Select Mechanism

NASA National Aeronautics and Space AdministrationND:YAG Neodymium:Yttrium Aluminum GarnetNSIDC National Snow and Ice Data CenterPAD Precision Attitude DeterminationPOD Precision Orbit DeterminationSAR Synthetic Aperture RadarSCF Science Computing FacilitySLR Satellite Laser RangingSPCM Single Photon Counting Module

20


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Acknowledgement s

ICESat ScientistsProject Scientist: Jay Zw ally - NASA/ G SFCD ep u ty Project Scien tist: C hr istop h er Sh u ma n - N A SA / G SFCProgram Scientist: Waleed Abd alati - N ASA H ead quarters

GLAS Science Team

Team Leader: Bob Schutz - University of Texas-AustinTeam Mem bers: Charles Bentley - University of Wisconsin-Mad isonJack Bufton * - N ASA/ GSFCDavid H ardin g (ex officio) - NASA/ GSFCThomas H erring - Massachusetts Institute of TechnologyJean-Bernard Minster - Scripps Institution of Oceanogr aph yJames Spinhirne - N ASA/ GSFCRobert Thomas - EG&G Corp orationJay Zw ally - N ASA/ GSFC

GLAS Instru men t TeamInstrument Scientist: Jim Abshire - N ASA/ GSFCInstrument Manager: Ron Follas - NASA/ GSFC

Instrument System Engineer: Eleanor Ketchum - NASA/ GSFC

Lead Engineers:Laser Lead : Robert Afzal* - NASA/ GSFC, Joe Dallas* - SSAISRS Lead : Pam ela Millar - N ASA/ GSFC, Marcos Sirota - Sigm aOptical Lead : Marzouk Marzouk - Sigma, Lu is Ramos-Izquierdo - OSCLidar Lead : Michael Krainak - N ASA/ GSFC;Detector Lead : Xiaoli Sun - NASA/ GSFCElectrical Lead : Gregory Henegar, Steve Meyer - NASA/ GSFCFligh t Softw are Lead : Man uel Mald on ad o, Jan McGarry - N ASA / GSFCMechanical Lead : Cheryl Salerno, Gord on Casto - N ASA/ GSFCTherm al Lead : Eric Grob, Charles Baker, Walter

Ancarrow* - N ASA/ GSFCIntegration and Test Lead: Tom Feild - NASA/ GSFC, Juli Landers - OSCBench Check Equ ip . Lead : Haris Rir is - Sigma

Project Managers Joe Dezio, Jim Watzin, - NASA/ GSFCDepu ty P ro ject Manager Linda Greenslade, Greg Smith - NASA/ GSFCObservatory Manager Bill Anselm - N ASA/ GSFCMiss ion Systems Engineer Tim Trenk le - NASA/ GSFC

Brochure PreparationBrochure Writers: Jay Zw ally and Christop her Shum an - NA SA/ GSFCBroch u re Ed itor s: Jim Ab sh ire, Bill A nselm , Bob Sch u tz , Ja mes Sp in hir ne, M ich ael Kin g,

Claire Parkinson, David Harding, Charlotte Griner, David Herring,Jack Saba, Anita Brenner

Brochure Design: Winnie Humberson - SSAICover Graphic Robert Simmon - SSAIICESat and GLAS Graphics Jason Budinoff - NASA/ GSFCICESat Logo Design: H ailey Kin g - Kn ott Loboratory, In c.

NA SA/GSFC N ASA Goddard Space Flight Center OSC Orbital Sciences CorporationSSAI Science Systems Applications, Inc.Sigma Sigma Research and Engineering Corporation* Changed affiliation


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Science Measurement Requirements: Measure ice sheet elevations to 1.5 cm/yr over 100x100 km areas Measure all radiatively significant clouds and aerosols globally Measure global land topography Three year operational life (5 year goal)

Specifications:Surface 1 Atmosphere

Wavelengths 1064 nm 532 nmLaser Pulse Energy 74 mJ 30 mJLaser Pulse Rate 40 Hz 40 HzLaser Pulse Width 5 nsec 5 nsecTelescope Diameter 1.0 m 1.0 mReceiver FOV 0.5 mrad 0.16 mradReceiver Optical Bandwidth 0.8 nm 0.03 nmDetector Quantum Efficiency 30% 60%Detection Scheme Analog Photon CountingVertical Sampling Resolution 0.15 m 75 mSurface Ranging Accuracy (single pulse) 5 cmLaser Pulse Pointing Knowledge < 2 arcsec 1 Also measures backscatter from optically thick clouds

Measurement Approach:GLAS has 3 lasers, a single laser operates at any given time

Each Nd:YAG laser has 3 stages and emits pulses at 1064 and 532 nmcontinuously at 40 Hz

Precison orbital and pointing knowledge will be obtained via redundantGPS units, a gyro system, a laser reference sensor, and instrument-mounted star trackers, and ground-based laser ranging

Thermal control by heat pipes and radiators supplemented by heaters

Measurement Characteristics:GLAS operates at 40 pulses per second (Hz); ground track is ~70 m "spots"separated along-track by ~170 m; cross-track resolution is a function ofthe 183-day ground-track repeat cycle (15-km spacing at equator and 2.5km spacing at 80 latitude); orbit inclination is 94

Altimetry measurements are determined from the round-trip travel time ofthe 1064 nm pulse

Cloud and aerosol data are extracted from the 532 nm pulse signal withoptically thick cloud tops extracted from the 1064 nm pulse signal

GEOSCIENCE LASER ALTIMETER SYSTEM (GLAS)


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I n t r oduct ion To I ce

Ice exists in the natu ral environm ent in m any form s. The figure below illustrates the Earths dyn amic ice features.At high elevations and / or high latitud es, snow that falls to the groun d can grad ually build u p to form thick consol-idated ice masses called glaciers. Glaciers flow downhill under the force of gravity and can extend into areas thatare too warm to support year-round snow cover. The snow line, called the equilibrium line on a glacier or ice sheet,separates the ice areas that melt on the surface and become snow free in summer (net ablation zone) from the iceareas that remain snow covered during the entire year (net accumulation zone). Snow near the surface of aglacier tha t is grad ually being com pressed into solid ice is called firn.

Ice sheets, which are th e largest form s of glaciers in the w orld, cover mu ch of Greenland a nd A ntarctica. Smaller icecaps are located in Iceland, Canada, Alaska, Patagonia, and mountainous regions of central Asia. These types of large ice mass h ave sma ller ou tlet glaciers or ice streams nea r their m argins. Moun tain glaciers, smaller than icesheets or ice caps, flow from high mountain areas and are present on all continents except Australia. In some placesw here the ice sheets reach the ocean, large floating ice shelves or floating glacier tongu es are formed . Icebergs arefloating ice masses that have broken away from ice shelves, glacier tongues, or directly from the grounded ice sheetin some locations. Sea ice, which is produced when saline ocean water is cooled below its freezing temperature of app roximately -2C or 29F, extends on a seasonal basis over great areas of the ocean.

Sea ice and icebergs are both carried by winds and currents into warmer waters. Melt water from sea ice, iceshelves, ice tongues, and icebergs does n ot contribute to sea level rise, because these ice masses already d isplace anequivalent a mou nt of sea w ater. Ho wev er, sea level rise is caused by the flow of grou nd ed g lacial ice into the oceanand by surface or subsurface melt water discharged from the glacier, if the sum of those amounts exceeds theamount of ice accumulated from snowfall on the glacier or ice sheet.

icesat brochure

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