10/13/0310/13/03 ICESatICESat 11

ICESatICESat: Ice, Cloud and Land: Ice, Cloud and LandElevation SatelliteElevation Satellite

Thomas Herring, 54-618Thomas Herring, 54-618

Graduate Student An NguyenGraduate Student An Nguyenand members of the GLAS Science Teamand members of the GLAS Science Team

tahtah@@mitmit..edueduhttp://www-http://www-gpsggpsg..mitmit..eduedu/~/~tahtah

Working Title:Lasers: The other light data

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OverviewOverview

}} Science objectives of missionScience objectives of mission

}} Mission designMission design

}} Current statusCurrent status

}} Initial resultsInitial results

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Science ObjectivesScience Objectives

}} CRYOSPHERE PURPOSE:CRYOSPHERE PURPOSE:ßß to determine the mass balance of the polar ice sheets and their to determine the mass balance of the polar ice sheets and their

contributions to global sea level changecontributions to global sea level change

ßß to obtain essential data for prediction of future changes in iceto obtain essential data for prediction of future changes in icevolume and sea-level.volume and sea-level.

}} ATMOSPHERE & LAND PURPOSES:ATMOSPHERE & LAND PURPOSES:ßß to measure cloud heights and the vertical structure of clouds andto measure cloud heights and the vertical structure of clouds and

aerosols in the atmosphereaerosols in the atmosphere

ßß to map the topography of land surfacesto map the topography of land surfaces

ßß to measure roughness, reflectivity, vegetation heights, snow-cover,to measure roughness, reflectivity, vegetation heights, snow-cover,and sea-ice surface characteristics.and sea-ice surface characteristics.

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Science MotivationScience Motivation

}} Status of Antarctic and Greenland ice Status of Antarctic and Greenland ice sheetssheetsßß Greenland and Antarctic ice sheets could be growing orGreenland and Antarctic ice sheets could be growing or

shrinkingshrinkingÑÑ Uncertainty is Uncertainty is ±±30% of mass input; 30% of mass input; ±±5 cm/yr5 cm/yr

average ice thickness; average ice thickness; ±±2.3 mm/yr global sea level2.3 mm/yr global sea levelchange).change).

ßß ICESat ICESat will measure < 1 cm/yr average ice thicknesswill measure < 1 cm/yr average ice thicknesschangechangeÑÑ< 5% of mass input and < 0.4 mm/yr global sea level< 5% of mass input and < 0.4 mm/yr global sea level

change).change).

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Science MotivationScience Motivation

}} Future ice Future ice sheets sheets changeschangesßß It is not known whether future changes in mass balanceIt is not known whether future changes in mass balance

associated with climate warming will be positive orassociated with climate warming will be positive ornegativenegative

ÑÑSensitivity perhaps - 10% to + 20% change in massSensitivity perhaps - 10% to + 20% change in massinput/K temperature change = +0.8 mm/yr/K to - 1.6input/K temperature change = +0.8 mm/yr/K to - 1.6mm/yr/K sea level change).mm/yr/K sea level change).

ÑÑ ICESat ICESat should measure changes in mass balanceshould measure changes in mass balanceexpected for 1 K polar warming (depends onexpected for 1 K polar warming (depends onsensitivity estimate)sensitivity estimate)

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Science MotivationScience Motivation

}} Ice sheet and Cloud characteristicsIce sheet and Cloud characteristicsßß Measurement of elevation changes will provide earlyMeasurement of elevation changes will provide early

warning of possible instability of west Antarctic ice sheetwarning of possible instability of west Antarctic ice sheet(e.g. changes in ice-stream flow, critical ice-shelf(e.g. changes in ice-stream flow, critical ice-shelfgrounding lines, ice-shelf rises, inland ice drawdown).grounding lines, ice-shelf rises, inland ice drawdown).

ßß GLAS LIDAR will directly measure cloud heights, forGLAS LIDAR will directly measure cloud heights, forenergy balance calculations, and obtain uniqueenergy balance calculations, and obtain uniqueinformation on polar clouds, especially during the polarinformation on polar clouds, especially during the polarwinter.winter.

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Mission designMission design

}} The primary instrument on The primary instrument on ICESat ICESat is theis theGeoscience Geoscience Laser Altimeter System (GLAS)Laser Altimeter System (GLAS)

}} Instrument is mounted on a Ball Instrument is mounted on a Ball ““imaging platformimaging platform””bus.bus.

}} GLAS is GLAS is ““self containedself contained”” with its own star trackers with its own star trackersand laser alignment calibration; the bus also hasand laser alignment calibration; the bus also hasstar trackers and gyroscopes for attitudestar trackers and gyroscopes for attitudedetermination and space craft pointing.determination and space craft pointing.

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Design requirementsDesign requirements

}} GLAS specifications:GLAS specifications:ßß Range accuracy: Range accuracy: ±±15 cm (slope <115 cm (slope <1oo); ); ±±22 cm slopes22 cm slopes

between 1-3between 1-3oo..ßß Mean elevations over 100x100 km regions Mean elevations over 100x100 km regions ±±1cm1cmßß Laser is diode pumped, Q-switchedLaser is diode pumped, Q-switched Nd Nd:YAG laser with:YAG laser with

40 Hz pulse repetition; 7540 Hz pulse repetition; 75 mJ mJ at 1064 nm and 35at 1064 nm and 35 mJ mJ atat532 nm532 nmßß Receiver telescope is 1 meter diameterReceiver telescope is 1 meter diameterßß Foot print 70 m diameter, 170 m along track spacingFoot print 70 m diameter, 170 m along track spacingßß GPS determined radial orbit error GPS determined radial orbit error ±±5cm5cmßß Attitude knowledge Attitude knowledge ±±1.5 seconds; pointing ability 301.5 seconds; pointing ability 30

seconds (to allow repeat ground track seconds (to allow repeat ground track ±±60 m).60 m).

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Range Measurement SystemRange Measurement System

}} Return waveform is digitized at 1GHz rate (binsReturn waveform is digitized at 1GHz rate (bins15cm one-way range).15cm one-way range).

}} Over land and ice, 544 waveform samples areOver land and ice, 544 waveform samples arereturned in level 0 data; over oceans 200 samplesreturned in level 0 data; over oceans 200 samplesare returned (based on on-board DEM)are returned (based on on-board DEM)

}} Multiple Gaussians are fit to the returnedMultiple Gaussians are fit to the returnedwaveform with the most distant peak taken aswaveform with the most distant peak taken asground return. (Algorithms still being fine tunedground return. (Algorithms still being fine tunedbased on actual return waveforms).based on actual return waveforms).

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GLAS Laser Pointing SystemGLAS Laser Pointing System

}} Because of the narrow laser beam, pointingBecause of the narrow laser beam, pointingknowledge is critical to GLAS. Specification is 1.5knowledge is critical to GLAS. Specification is 1.5second pointing knowledge.second pointing knowledge.

}} Accuracy is achieved with the star camera (1HzAccuracy is achieved with the star camera (1Hzrate), gyroscopes (interpolation to 40Hz), andrate), gyroscopes (interpolation to 40Hz), andimaging of the laser beam (small pick off ofimaging of the laser beam (small pick off ofoutgoing pulses) in the star camera.outgoing pulses) in the star camera.

}} Bore sight direction was issue before launchBore sight direction was issue before launch(small field of view) and need to align optics in(small field of view) and need to align optics ingravity fieldgravity field

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ICESat ICESat artistartist’’s conceptions conception

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Fully AssembledFully AssembledICESatICESat

}} GLAS InstrumentGLAS Instrumentweight: 300 kgweight: 300 kg

}} Power 330 WPower 330 W

}} Thermal rangeThermal range2020±±5C5C

}} Beam divergenceBeam divergence375 375 mmradrad..

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ICESat ICESat OrbitOrbit

}} Orbit altitude 600 kmOrbit altitude 600 km

}} Inclination 94Inclination 94oo

}} 8-day repeat during Calibration/Validation8-day repeat during Calibration/Validation

}} 183-day repeat during mission (original plan).183-day repeat during mission (original plan).Likely to change to 90-day repeatLikely to change to 90-day repeat

}} 3-yr lifetime, 5-yr desirable3-yr lifetime, 5-yr desirable

}} 3 Lasers to meet lifetime requirements3 Lasers to meet lifetime requirements

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Mission HistoryMission History

}} Launch: Jan 13,2003, Vandenberg AF base, DeltaLaunch: Jan 13,2003, Vandenberg AF base, DeltaII launch vehicle.II launch vehicle.

}} Laser 1: Turn of Feb 20, 2003Laser 1: Turn of Feb 20, 2003

}} Laser 1: Laser 1: Failure March 29, 2003Failure March 29, 2003

}} Originally expected Laser 2 turn on: Mid to lateOriginally expected Laser 2 turn on: Mid to lateAugust 2003.August 2003.

}} Inquiry board to study failure of laser 1.Inquiry board to study failure of laser 1.

}} Turn of laser #2: 13:11 today, data available 1Turn of laser #2: 13:11 today, data available 1hour later (over Norway at the time of turn on).hour later (over Norway at the time of turn on).

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Preliminary ResultsPreliminary Results

}} Mission still in Calibration/Validation Phase and soMission still in Calibration/Validation Phase and soall results are preliminary.all results are preliminary.

}} Laser failure occurred before green channel Laser failure occurred before green channel lidarlidardetectors had out-gassed so detectors had out-gassed so lidar lidar results will beresults will bebased on the lower SNR 1064 nm channel.based on the lower SNR 1064 nm channel.

}} Full attitude calibration has not yet beenFull attitude calibration has not yet beencompleted (i.e., pointing angles between laser andcompleted (i.e., pointing angles between laser andoptical bench).optical bench).

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Calibration/ValidationCalibration/Validation

}} Primary CAL/VAL target is White Sands, NMPrimary CAL/VAL target is White Sands, NMßß Array of diode detectors to detect laser pulseArray of diode detectors to detect laser pulseßß Array of corner cube reflectors (at different heights) thatArray of corner cube reflectors (at different heights) that

can be seen in return pulsescan be seen in return pulsesßß Photography of laser spots on groundPhotography of laser spots on groundßß Due to laser failure, CAL/VAL has not been completedDue to laser failure, CAL/VAL has not been completed

herehere

}} Other CAL/VAL:Other CAL/VAL:ßß Ocean sweeps (off nadir circular pattern on oceanOcean sweeps (off nadir circular pattern on ocean

surface)surface)ßß Areas of calibrated DEMS for detailed comparisonAreas of calibrated DEMS for detailed comparison

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ExamplesExamples

}} GLAS Data releases 9 and 11 (changes as systemGLAS Data releases 9 and 11 (changes as systembetter understood). Also results from release 12better understood). Also results from release 12(currently being distributed)(currently being distributed)

}} Attitude determination using Attitude determination using ““Ocean ScansOcean Scans””..Orientation angles, range and timing biases areOrientation angles, range and timing biases aredetermined from circular scan over the ocean (3-5determined from circular scan over the ocean (3-5degrees off nadir).degrees off nadir).

}} Waveforms and altimetry over ice sheets.Waveforms and altimetry over ice sheets.

}} Signal return strength and surface smoothnessSignal return strength and surface smoothness

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High Freq. Attitude Error ImprovementHigh Freq. Attitude Error Improvement

0 200 400 600 800 1000

Seconds from 2003 DOY 75, 11-hr. 25-min.

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OS-3 Preliminary Pre- and Post-Calibration Range Residuals

Post-CalibrationPre-Calibration

Release 9

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OS-4 Preliminary Pre- and Post-Calibration Range Residuals

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OS-3 Release 11 Post-Calibration Range Residuals

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Post-Calibration

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Post-Cal. Release 11 Crossover Residuals (Cycle 3.5)Post-Cal. Release 11 Crossover Residuals (Cycle 3.5)

-2.5 -2.1 -1.7 -1.3 -0.9 -0.5 -0.1 0.3 0.7 1.1 1.5 1.9 2.3

meters

0

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400

600

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Release 11 Post-Calibration Global Crossover Residuals (Cycle 3.5)

RMS = 53.6 cmMean = -0.2 cmNo. Obs. = 7366

-2.5 -2.1 -1.7 -1.3 -0.9 -0.5 -0.1 0.3 0.7 1.1 1.5 1.9 2.3

meters

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Release 11 Post-Calibration High Lats. Crossover Residuals (Cycle 3.5)

RMS = 51.2 cmMean = 3.8 cmNo. Obs. = 6403

-2.5 -2.1 -1.7 -1.3 -0.9 -0.5 -0.1 0.3 0.7 1.1 1.5 1.9 2.3

meters

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Release 11 Post-Calibration Mid. Lats. Crossover Residuals (Cycle 3.5)

RMS = 57.6 cmMean = 0.1 cmNo. Obs. = 978

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Release 9

Release 11

Elevation profilesshowing that raising the signal threshold and fixing codingerror to not use data for which Gaussian fitting was unsuccessfuledited out bad data.

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Release 9

Release 11

Reduction from 6 to 2 peaks smoothes out some of the elevationProfile noise – saturation error still not accounted for

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Effects of changes for saturated returns with forward scattering

Release 9 – 6 peaks – fit looksGood – physical significance lost

Release 11 – 2 peaks with last peak always kept and no sample weighting – missesmain ground return

Release 11+ - weighting forces better fit to leading edge; mostsignificant peaks kept so onefits main ground return – stillcontains saturation error

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Crossover AnalysesCrossover Analyses

}} Crossovers: Interpolation to the same point for twoCrossovers: Interpolation to the same point for twocrossing tracks.crossing tracks. Initially useful for assessing data quality.Initially useful for assessing data quality.Later crossovers can be Later crossovers can be ““plottedplotted”” as a function of time as a function of timeseparation of the tracks and rates of change of surfaceseparation of the tracks and rates of change of surfaceheight deduced.height deduced.

}} Because of the laser failure the current data sets are notBecause of the laser failure the current data sets are notfully calibrated. In particular, pointing knowledge of thefully calibrated. In particular, pointing knowledge of thelaser is not as good as it should belaser is not as good as it should be

}} Examine:Examine:ßß Histograms of crossovers as functions of:Histograms of crossovers as functions of:

ÑÑ Angle separation of tracksAngle separation of tracksÑÑ Surface slope differences (20 pts along track around crossover)Surface slope differences (20 pts along track around crossover)ÑÑ Surface RMS scatter (form 20 points)Surface RMS scatter (form 20 points)

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Histograms over AntarcticaHistograms over AntarcticaRelease 11 and 12 of dataRelease 11 and 12 of data

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Angle-separation binningAngle-separation binning

}} Notice as tracks become parallel RMS differences inNotice as tracks become parallel RMS differences incrossovers decrease; suggesting pointing errorscrossovers decrease; suggesting pointing errors

}} Although near 180 degree crossover are at 86 degreeAlthough near 180 degree crossover are at 86 degreelatitude, smaller crossover angles are near the edges oflatitude, smaller crossover angles are near the edges ofthe ice sheetthe ice sheet

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Slope-difference binningSlope-difference binning(slopes from 20-points in each track)(slopes from 20-points in each track)

Dslope (deg)

}} Notice linear trend as function of slope difference:Notice linear trend as function of slope difference:suggests pointing error (~23 meters on ground)suggests pointing error (~23 meters on ground)

}} <60 cm RMS at near zero degrees slope<60 cm RMS at near zero degrees slopedifference giving indication of data qualitydifference giving indication of data quality

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20-point linear-fit RMS binning20-point linear-fit RMS binning

}} Indicates precision of instrument: larger number ofIndicates precision of instrument: larger number ofpoints with <20 cm RMS scatter about linear trendpoints with <20 cm RMS scatter about linear trendalong track for 3.4 kmalong track for 3.4 km

}} Lowest RMS values: 2.5 cmLowest RMS values: 2.5 cm

Binning changes here

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Signal Strength StudiesSignal Strength Studies}} Three class of studies carried outThree class of studies carried out}} Peak received voltage. Large percentage are reaching limitPeak received voltage. Large percentage are reaching limit

of 1.5 volts, peaks in Gaussians have similar (and oftenof 1.5 volts, peaks in Gaussians have similar (and oftenlarger values).larger values).ßß Area under the received waveform.Area under the received waveform.ßß Three basic classes of large areas:Three basic classes of large areas:ÑÑStrong sharp pulseStrong sharp pulseÑÑLong duration pulses with relatively low amplitude but integrate toLong duration pulses with relatively low amplitude but integrate to

large area (clouds)large area (clouds)ÑÑNot clear cases: Large area, by short pulse with moderateNot clear cases: Large area, by short pulse with moderate

amplitudeamplitudeßß Slopes of smooth terrain (defined by small RMS fit to linear trendSlopes of smooth terrain (defined by small RMS fit to linear trend

over 1-sec).over 1-sec).

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Peak signal VoltagePeak signal Voltage

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Histogram of Max Received SignalHistogram of Max Received Signal

Fully Saturated pulses

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Signal Strength and surface shapeSignal Strength and surface shape

}} The peak return voltage is clearly being clipped and theThe peak return voltage is clearly being clipped and theoften even over clouds we are seeing very strong returnsoften even over clouds we are seeing very strong returns(although often with long return pulses)(although often with long return pulses)

}} Integrated power under waveform is also large althoughIntegrated power under waveform is also large althoughthe distribution in deceasing with increasing sizethe distribution in deceasing with increasing size

}} No re-normalization for receiver gain yet. (Scaling of gainNo re-normalization for receiver gain yet. (Scaling of gaincounts?)counts?)

}} Now examine surface slope and RMS fit to 1-sec data.Now examine surface slope and RMS fit to 1-sec data.Looking for areas of specula reflections.Looking for areas of specula reflections.

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Ground tracks March 22, 2003Ground tracks March 22, 2003

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GLA06_030316 Ground TrackGLA06_030316 Ground Track

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GLA06 elevation plotGLA06 elevation plot

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GLA06 Slope and RMS AnalysisGLA06 Slope and RMS Analysis

}} Analysis looks at the slope and RMS of GLA06 elevationAnalysis looks at the slope and RMS of GLA06 elevationvalues by taking the data in 1-sec samples and fit a linearvalues by taking the data in 1-sec samples and fit a lineartrend and computing the RMS about the trend.trend and computing the RMS about the trend.

}} Most of the RMS fits < 20 cm are over the oceans and soMost of the RMS fits < 20 cm are over the oceans and sofar all the slopes have been small (<10 far all the slopes have been small (<10 millimilli-deg). RMS-deg). RMScan be as low as 25 mm over calm oceans.can be as low as 25 mm over calm oceans.

}} Slopes comparable to the Slopes comparable to the ICESat ICESat off-nadir pointing angleoff-nadir pointing angleand small RMS seen on ice sheets and some continentaland small RMS seen on ice sheets and some continentalareas. If these surfaces are capable of specula reflectionsareas. If these surfaces are capable of specula reflectionsthen slopes are a problem (Damage to detectors possible).then slopes are a problem (Damage to detectors possible).

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RMS and SlopesRMS and Slopes

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Zoom ice areaZoom ice area

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High slope area of ice sheetHigh slope area of ice sheet

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Signal Strength SummarySignal Strength Summary

}} Signal StrengthsSignal Strengthsßß 9-10% of pulse reach the 1.5Volt limit on return signal9-10% of pulse reach the 1.5Volt limit on return signal

strengthstrengthßß Issue with maximum possible signal at detectors:Issue with maximum possible signal at detectors:ÑÑGain changes not accounted for yet (need to knowGain changes not accounted for yet (need to know

scaling): Concern if gain high and strong returnsscaling): Concern if gain high and strong returnscomes incomes inÑÑMany areas of 2-20cm elevation RMS; many inMany areas of 2-20cm elevation RMS; many in

oceans where slopes seems too small to causeoceans where slopes seems too small to causespecula reflection problems but some on ice sheetsspecula reflection problems but some on ice sheetswhere slopes can be similar to current off-nadirwhere slopes can be similar to current off-nadirpointing angle.pointing angle.

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Current IssuesCurrent Issues

}} Link budget better than expected: End result is saturationLink budget better than expected: End result is saturationof pulses:of pulses:ßß Fitting of Gaussians can detect Fitting of Gaussians can detect ““ringing in detectorsringing in detectors”” and yield a and yield a

return from below the surfacereturn from below the surface

ßß Forward scattering in the atmosphere leads to non-symmetricForward scattering in the atmosphere leads to non-symmetricpulses that can bias Gaussian fitspulses that can bias Gaussian fits

}} Jitter in the attitude system; 0.6 Hz oscillation that may beJitter in the attitude system; 0.6 Hz oscillation that may bedue to loose solar panel connection (not clear yet)due to loose solar panel connection (not clear yet)

}} Ability to point to Ability to point to ±±60 m on the ground not yet confirmed60 m on the ground not yet confirmed(need laser running to be certain)(need laser running to be certain)

}} Lidar Lidar green channel detectors not turned on yet (needgreen channel detectors not turned on yet (needlaser)laser)

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ConclusionsConclusions

}} So far data collected with So far data collected with ICESat ICESat has been of very highhas been of very highquality but because of laser failure absolute calibration notquality but because of laser failure absolute calibration notyet completed.yet completed.

}} High signal strengths are causing saturation on about 10%High signal strengths are causing saturation on about 10%of pulses: Next laser turn on will be at lower temperatureof pulses: Next laser turn on will be at lower temperatureand less power. Waveform processing is being modified toand less power. Waveform processing is being modified toaccount for signal strength.account for signal strength.

}} Confident that lasers 2 and 3 will run at least as long asConfident that lasers 2 and 3 will run at least as long aslaser 1; final assessments are being made to ensure longerlaser 1; final assessments are being made to ensure longerlifetimes of these lasers.lifetimes of these lasers.

}} Hopefully laser 2 fired successfully during this talk.Hopefully laser 2 fired successfully during this talk.