Spaceborne laser altimetry: 2001 and beyond - Center for Space ...

3ground track repeat to enable several overflights ofverification sites during the 90-120 day verificationperiod. The orbit will be frozen; thus, perigee willremain essentially fixed at 90º for the adopted 94ºinclination. With a mean orbital semimajor axis of6971.5 km and an eccentricity of 0.0013, the groundtrack will repeat in 119 orbital revolutions in 7.989days. The 8-day repeat orbit will be further designedto enable overflights of specific verification sites andprovide near-simultaneous measurements with theMODIS imaging instrument on EOS-AM.The post-verification phase orbit will repeat in 183-days (182.758 days) after 2723 revolutions. Theorbital semimajor axis is 6970.0 km and theeccentricity is 0.0013. After one complete “183-daycycle”, the ground track spacing between samedirectiontracks at the equator will be about 14.7 km.Two subcycles exist within the 183-day cycle whichproduce near-repeat tracks: 8-days and 25-days. Theground track pattern for 25-days will essentiallyrepeat in the subsequent 25-days, but the pattern willbe shifted westward at the equator by 14.7 km. Thenear-repeat 8-day pattern occurs with an eastwardshift of about 100 km. Orbit decay will be controlledwith periodic thrusting to maintain the ground trackrepeat characteristics to within 1 km at the equator.The orbit inclination was chosen to provide coverageof features in the West Antarctic and to providecrossovers to support detection of elevation change.Based on the adopted orbit parameters, the crossovers(formed by differencing the altimeter measurementsat the points where the ascending and descendingtracks intersect) will be approximately orthogonal inthe latitude band from 84º to 85º (North or South).In the 183-day repeat cycle, the orbit geometryproduces a crossover density that rapidly increases asthe maximum latitude allowed by the inclination isapproached, as illustrated in Table 1. In the regionabove 85º (N or S), the crossover angle exceeds 90º,approaching 180º.Table 1. Number of Crossovers in 100 km x 100 kmLatitude Range (N or S) Number of Crossovers70º - 71º ~ 23075º - 76º ~ 55080º - 81º ~ 167584º - 85º ~ 11,500Error Analyses:The single shot error budget for the laser spotlocation is summarized in Table 2. Since theinstrumentation is designed to operate continuously,this error budget applies to cryosphere, land andocean/lake applications. For purposes of this errorbudget, it has been assumed that the laser pointingerror is 1.5 arcsec and the surface slope is 1º. It hasbeen assumed that the dual frequency GPS receiverand ground-based laser ranging data collected duringthe verification phase will be used to improve thegravity model and other parameters used to supportPOD. Current gravity model covariance matrices,such as EGM-96, predict the radial orbit error forICESAT to be > 15 cm, which exceeds the value inthe Table 2. Ongoing efforts to improve the gravityfield, combined with dedicated gravity missions suchas CHAMP and GRACE, should produce a reductionin this predicted value, but significant reduction mayonly occur after the launch of the ICESATObservatory. Nevertheless, simulations have beenconducted by Rim et al. (1996) that demonstratetuning of the gravity field during the verificationphase can be expected to approach the radial orbiterror in Table 2. An additional assumption is that ashift in the centroid of the return pulse produced byatmospheric scattering will be correctable to 10%.Table 2. Single Shot Error BudgetSourceError (cm)Instrument precision 10Radial orbit determination 5Pointing determination 7.5Troposphere delay 2Atmospheric scattering 2Other (c.g. location, etc.) 1RSS 13.8As an example to demonstrate that this error budgetcan meet the science requirements, consider a 100km x 100 km region at 80º latitude. Although 1675possible crossovers will be available within each183-day cycle (inter-cycle crossovers), crossoverscan be formed between different cycle combinations(intra-cycle). With the assumptions that 6 cycles arecompleted (3-yr mission), approximately 250crossovers occur in each cycle, and the individualcrossover error is 20 cm, it can be shown that the 1.5cm/yr requirement can be met. For this example,crossovers between the first and third cycles, the firstand fifth cycles, and the first and sixth cycles, wereformed. More detailed analyses have been conductedwith realistic orbit and attitude errors and with morecomplete simulations of secular and periodicvariations of the surface elevation. These simulationsdemonstrate that the dominant error source on flatsurfaces is the orbit error, whereas on slopedsurfaces, the pointing error dominates, depending onthe slope.Data Products and Validation:The altimeter data products will include the geodeticcoordinates of each surface laser spot centroidexpressed in an appropriate ITRF. These coordinates

4will be geodetic latitude, longitude and ellipsoidalheight. In addition, the digitized waveform, statisticalinformation, and corrections applied to the rawmeasurements will be available.GLAS data collected during the first 90 to 120 dayswill be used to verify the instrument performanceand, if necessary, generate calibration corrections. Inaddition, the data products generated during theverification period will be subjected to intensescrutiny to assess their validity.One planned approach to verification/validation is touse selected sites that enable a direct determination ofr spot . This directly determined vector will becompared with the inferred vector, which is thestandard data product. The directly-determined spotlocation will be accomplished by capturing an imagecontaining the GLAS-illuminated spots at verificationsites. The primary site is currently planned to beWhite Sands, New Mexico, at the Shuttle landingfacility. The surface is very flat and smooth. Theimage of the spots will contain fiducial referencemarks whose coordinates will be determined in theITRF using GPS. With the fiducial marks in theimage, the ITRF coordinates of the laser spot will bedetermined in the ITRF as well. This verificationprocess will characterize the performance of theLRS/SRS and provide data for the validation of thePOD, PAD, and the corrections applied to thealtimeter measurement.Fundamental changes in system configuration, suchas changing lasers and/or detectors, will warrantinitiation of special verification activities. If theseconfiguration changes occur during the 183-dayrepeat cycle, use of off-nadir pointing provided by thespacecraft will be used to facilitate pointing the laseraltimeter at the verification sites.Other techniques for validation will be used. Forexample, a spacecraft roll maneuver over the oceanswill enable determination of attitude corrections, aswill operation of the instrument over undulating, butwell-characterized, surface topography.project success. The ICESAT Project Manager is JoeDezio, the Instrument Team is represented by RonFollas (Instrument Manager) and James Abshire(Instrument Scientist). The Project Scientist is JayZwally. The contributions of the entire ScienceTeam, Instrument Team, Project Office andcontractors are gratefully acknowledged. The supportof NASA through NAS5-33021 is herebyacknowledged for the preparation of this paper. Formore information about the project, includingpointers to other sites:http://www.csr.utexas.edu/glas/http://icesat.gsfc.nasa.govReferences:GLAS Science Team, Geoscience Laser AltimeterSystem Science Requirements, Version 2.01, October1997 (available in pdf-format on web sites notedunder Acknowledgements).GLAS Science Team, GLAS Validation Plan,Version 0.1, April 1998.Houghton, J. T., L. G. Meira Filho, B. Callander, N.Harris, A. Kattenberg and A. Maskell, ClimateChange 1995: The Science of Climate Change,Cambridge University Press, 1996.Nerem, R. S., B. J. Haines, J. Hendricks, J. F.Minster, G. T. Mitchum, and W. B. White, Improveddetermination of global mean sea level variationsusing TOPEX/POSEIDON altimeter data, Geophys.Res. Letters, v. 24, no. 11, 1331-1334, June 1, 1997.Oerlemans, J., A projection of future sea level, Clim.Change, 15, 151-174, 1989.Rim, H. J., G. Davis, B. Schutz, Dynamic orbitdetermination for the EOS laser altimeter satellite(EOS ALT/GLAS) using GPS measurements, Jour.Of Astronautical Sci., Vol. 44, No. 3, 409-424, July-September 1996.The data products from ICESAT will be distributedthrough the NASA EOSDIS. Except for the initialverification period, these products will be available tothe international community within 2-3 weeks afterthe measurements have been collected.Acknowledgments:The exceptional contributions and dedication ofnumerous individuals should be acknowledged. Theteam leads are noted here, but the individualcontributions of the team members are crucial to the