ICESat-2 (Ice, Cloud and land Elevation Satellite-2)
ICESat-2 is a NASA follow-up mission to ICESat with the goal to continue measuring and monitoring the impacts of the changing environment. The ICESat-2 observatory contains a single instrument, an improved laser altimeter called ATLAS (Advanced Topographic Laser Altimeter System). ATLAS is designed to measure ice-sheet topography, sea ice freeboard as well as cloud and atmospheric properties and global vegetation. The requirements call for a 5-year operational mission with a goal of 7 years. 1) 2) 3) 4) 5) 6)
Rational and discussion of mission goals: The mass balance of Earth's great ice sheets and their contributions to sea level are key issues in climate variability and change. The relationships between sea level and climate have been identified as critical subjects of study ib the IPCC (Intergovernmental Panel on Climate Change) assessments, the CCSP (Climate Change Science Program) strategy, and the U.S. IEOS (International Earth Observing System). Because much of the behavior of ice sheets is manifested in their shape, accurate observations of ice elevation changes are essential for understanding ice sheets' current and likely contributions to sea-level rise.
ICESat-2, with high altimetric fidelity, will provide high-quality topographic measurements that allow estimates of ice sheet volume change. High-accuracy altimetry will also prove valuable for making long-sought repeat estimates of sea ice freeboard and hence sea ice thickness change, which is used to estimate the flux of low-salinity ice out of the Arctic basin into the marginal seas. Altimetry is best (and perhaps only) technique for change studies, because sea ice areas and extends have been well observed from space since the 1970s and significant trends have been shown, but there is no such record for sea ice thickness.
As climate change proceeds, continuous measurements of both land-ice and sea-ice volume will be needed to observe trends, update assessments, and test climate models. The altimetric measurement made with the lidar instrument, along with a higher precision gravity measurement (such as GRACE-FO), would optimally characterize changes in ice sheet volume and mass and directly enhance understanding of the ice sheet contribution to sea-level rise. Coupled with the interferometric synthetic aperture radar in the DESDynI mission, the instrumentation would provide a comprehensive data set for predicting changes in Earth's ice sheets and sea ice.
In addition to studies of ice, the proposed instrument could be used to study changes in the large pool of carbon stored in terrestrial biomass. In particular, the proposed lidar could be used to measure canopy depth and thus estimate land carbon storage to aid in understanding the responses of biomass to changing climate and land management. 7)
Figure 1: Schematic view of mission goals (image credit: NASA)
Figure 2: Changes in Greenland ice from 1986 to 2006 (image credit: NASA)
• Quantify the polar ice sheet mass balance to determine contributions to current and recent sea level change and impacts on ocean circulation
• Determine the seasonal cycle of ice sheet changes
• Determine topographic character of ice sheet changes to assess mechanisms driving that change and constrain ice sheet models
• Estimate sea ice thickness to examine ice/ocean/atmosphere exchanges of energy, mass and moisture.
• Measuring vegetation canopy height as a basis for estimating large-scale biomass and biomass change
• Enhancing the utility of other Earth observation systems through supporting measurements.
The instrument will use micro-pulse multi-beam photon-counting approach. Science and ancillary data will be collected, stored on-board and subsequently downlinked to ground stations via an X-band communications link. This link will also include stored housekeeping telemetry. The observatory will also receive and store/execute commands and transmit real-time housekeeping telemetry via an S-band link to the NASA Ground Network.
Figure 3: Artist's view of the ICESat-2 spacecraft (image credit: Orbital)
The ICESat-2 mission is assigned to NASA/GSFC. The spacecraft is being procured under the GSFC RSDO (Rapid Spacecraft Development Office). In August 2011, NASA selected OSC (Orbital Science Corporation of Dullas, VA to built and launch the ICESAT-2 spacecraft. The contractor is responsible for the design and fabrication of the ICESat-2 spacecraft bus, integration of the government-furnished instrument, satellite-level testing, on-orbit satellite check-out, and continuing on-orbit engineering support. The ICESat-2 spacecraft is being designed, assembled, and tested at Orbital's satellite manufacturing and test facility in Gilbert, Arizona.
Table 1: Overview of key spacecraft parameters 12)
Launch: A launch of ICESat-2 is scheduled for 2016 from VAFB, CA on an Atlas V, or Delta-4 or Falcon-9 vehicle.
Orbit: Near polar LEO frozen orbit, altitude =496 km, inclination = 94º, repeat cycle of 91 days with subcycles of 29, 29, and 33 days (Figure 4).
Figure 4: Illustration of monthly subcycles (image credit: NASA)
Project Status (Ref. 3):
• The ICESat-2 mission CDR (Critical Design Review) is planned for March 2013.
• The ICESat-2 mission was assigned Phase C status on December 17, 2012.
• The ICESat-2 project passed instrument PDR (Preliminary Design Review) on Nov. 18, 2011.
• The ICESat-2 team passed the SRR (System Requirements Review) on May 25, 2011.
• The ICESat-2 team passed the ISRE (Instrument System Requirements Review) on December 1, 2010.
• The ICESat-2 team passed the Key Decision Point A (KDP-A) review at HQ on December 11, 2009. Since then the project started officially in Phase A.
Sensor complement: (ATLAS)
ATLAS (Advanced Topographic Laser Altimeter System)
ICESat-2 will use a new type of laser altimeter instrument for measuring elevation, and will acquire far more data. To test the instrument concept, and develop accurate software to process the data, NASA has been flying an instrument called MABEL (Multiple Altimeter Beam Experimental Lidar) on high-altitude aircraft (ER-2) to simulate measurements that the ATLAS (Advanced Topographic Laser Altimeter System ) — GLAS’s successor–will be making from space.
Table 2: ATLAS instrument science measurement requirements (Ref. 16)
MABEL and ATLAS are photon-counting laser altimeters, meaning they measure distance by detecting just a few photons from each laser pulse and timing their round-trip travel from satellite to earth and back extremely accurately. While GLAS used millions of photons to make a single distance measurement, MABEL and ATLAS gather a data set of just a few dozen photons at most, and produce a cloud of points describing the snow or land or vegetation surface structure. Sophisticated software will determine the location of the surface track, the tops of the tree canopy, or the amount of dust or fog in the air.
The original design of the ATLAS instrument for ICESat-2 evolved as a modified version of the ICESat GLAS instrument concept. More specifically, for ICESat-2, the original ATLAS design was a single-beam altimetry system with the laser transmitter operating at a slightly higher repetition rate (50 Hz), lower energy per pulse (50 mJ) and similar 6-7 ns pulse width at the near-infrared (NIR) wavelength of 1064 nm when compared to GLAS. These changes would have provided higher derating on the lasers and potentially longer mission life.
In 2009, the ATLAS instrument on ICESat-2 underwent a complete redesign during the pre-Phase A activities to accommodate more science objectives and incorporate recommendations from the ICESat-2 science workshop (June 2007). For ice sheets, improved pointing will reduce the uncertainty in the ice sheet elevations introduced by the cross-track surface slope. In addition, for land topography and vegetation, improved pointing will provide observations along exact repeat ground tracks, and sampling along uniformly spaced ground tracks will provide well-sampled grids of topography and biomass. Based on this and other recommendations, a new instrument concept was proposed and accepted by the ICESat-2 program. 13) 14) 15) 16) 17)
The new baselined instrument is a high repetition rate (10 kHz), micropulse laser altimeter system. GSFC has begun an in-house program to investigate various potential laser technologies to meet the laser requirements for the ATLAS instrument.
A single laser transmitter having sufficient laser energy will be split into multiple beams using a DOE (Diffractive Optical Element) similar to the one used on LOLA. The current instrument architecture consists of a 9-beam system arranged in a 3 x 3 configuration.
ICESat-2 measurement concept:
In contrast to the first ICESat mission, ICESat-2 will use micro-pulse multi-beam photon counting approach to provide:
- Dense cross-track sampling to resolve surface slopes on an orbit basis
- High repetition rate (10 kHz) generates dense along-track sampling (~70 cm)
- Different beam energies to provide necessary dynamic range (bright / dark surfaces)
The advantages are:
- Improved elevation estimates over high slope areas and very rough (e.g. crevassed) areas
- Improved lead detection for sea ice freeboard.
The ATLAS instrument is a multi-beam micropulse laser altimeter with the following features:
• Single laser beam split into 9 beams
• 10 m ground footprints
• 10 kHz repetition rate laser (~1 mJ)
• Multiple detector pixels per spot
• On-board boresight alignment system
• LRS (Laser Reference System) gives absolute laser pointing knowledge.
Figure 5: Illustration of the ATLAS instrument (image credit: NASA)
ATLAS will employ a micropulse laser transmitter frequency doubled to 532 nm (visible green) with a 1 ns FWHM pulse width and operating at a 10 kHz repetition rate (0.7 m along-profile footprint sampling). A narrow 20 µrad beam divergence from a 500 km orbit altitude will yield 10 m diameter footprints. To improve spatial sampling ATLAS will employ a DOE (Diffractive Optic Element) that will split the single transmit beam into 6 beams, creating a pattern consisting of 3 sets of 2 closely spaced (< 100 m) beams. The closely-spaced beam pairs will resolve local slope, enabling determination of real elevation change from a single repeat of a reference track (Ref. 4).
With ICESat-2 operating in a 91 day repeat orbit and ATLAS operating continuously, seasonal observations of inter-annual ice sheet elevation change will be possible. The beam pairs will be separated cross-track by 3 km, providing improved spatial coverage as compared to that of ICESat. Over land, rather than repeating reference tracks, spacecraft pointing will be used to systematically displace the profiles cross-track through time in order to build up dense global sampling of topography and vegetation over the course of the mission.
In the traditional analog Si:APD detection approach used by GLAS of ICESat-1, thousands of photons reflected from the Earth’s surface were acquired per laser fire (for clear atmospheric conditions) in order to obtain waveforms with sufficient SNR to achieve the 3 cm ranging precision. In the ATLAS micropulse approach the transmit pulse energy will be significantly lower such that only a few to ~10 photons will be detected per footprint per laser fire using a 0.8 m diameter telescope and photon-sensitive Photomultiplier Tube (PMT) detector arrays. Laser fire times and the arrival time of each photon, those reflected from the surface as well as from solar background noise, will be time tagged with 0.15 ns precision yielding < 20 cm single-photon range precision. Post-processing on the ground will yield “point clouds” of geolocated single photon surface returns. An advantage of this approach is that the combination of small, oversampled footprints, narrow pulse width and high-precision timing can yield elevation data of higher spatial and vertical resolution. In addition the geospatial information content of the point cloud is amenable to a greater diversity of analysis approaches than afforded by analog waveforms, opening up possibilities for new ways to characterize the vertical structure of the Earth’s surface (Ref. 4).
Figure 6: Schematic view of the ATLAS measurement concept (image credit: NASA)
Legend to Figure 6: Single laser pulse, split into 6 beams. Redundant lasers, redundant detectors.
Figure 7: Snapshot of the 3x3 laser spot pattern currently conceived for ICESat-2 (image credit: NASA, Ref. 13)
Figure 8: ICESat-1 observation spacing at Jakobshavn Isbra (left) and planned ICESat-2 spacing overlay (right), (image credit: NASA, Ref. 6)
Figure 9: Schematic of analog versus photon counting (image credit: NASA)
Legend to Figure 9: It is important to note that the integrated photon-counting sample (“histogram”) looks like the analog wave - but it is not - the information content is different, and the method of analyzing the data is different.
Figure 10: Overview of the ATLAS instrument (image credit: NASA)
Table 3: Current laser transmitter performance requirements for the ICESat-2 micropulse laser altimeter system
Figure 11: Block diagram of the ATLAS instrument (image credit: NASA)
Figure 12: ATLAS functional block diagram (image credit: NASA)
1) Waleed Abdalati, H. Jay Zwally, Robert Bindschadler, Bea Csatho, Sinead Louise Farrell, Helen Amanda Fricker, David Harding, Ronald Kwok, Michael Lefsky, Thorsten Markus, Allexander Marshak, Thomas Neumann, Stephen Palm, Bob Schutz, Ben Smith, James Spinhirne, Charles Webb, “The ICESat-2 Laser Altimetry Mission,” Proceedings of the IEEE. Vol 98, No. 5, May 2010. Pages 735-751., URL: http://icesat.gsfc.nasa.gov/icesat2/publications/pubs_2010/abdalati_et_al_2010.pdf
2) Douglas D. McLennan, “Ice, Clouds and Land Elevation (ICESat-2) Mission,” Proceedings of the SPIE Remote Sensing Conference, Toulouse, France, Vol. 7826, Sept. 20-23, 2010, paper: 7826-32, 'Sensors, Systems, and Next-Generation Satellites XIV,' edited by Roland Meynart, Steven P. Neeck, Haruhisa Shimoda, doi: 10.1117/12.865200
4) David J. Harding, “NASA's Lidar measurements of the Earth's surface from space,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012
5) Molly Brown, Mark Carroll, Vanessa Escobar, “ICESat-2 Applications Workshop Report, NASA Goddard Space Flight Center, April 12, 2012, URL: http://icesat.gsfc.nasa.gov/icesat2/applications/ICESat2_Applications_Workshop1_Report_final.pdf
6) Kelly M. Brunt, Sinéad L. Farrell, Vanessa M. Escobar, “ICESat-2: A next generation laser altimeter for space-borne determination of surface elevation,” 93rd American Meteorological Society Annual Meeting, Austin, TX, USA, Jan. 6-10, 2013, URL: https://ams.confex.com/ams/93Annual/webprogram/Handout/Paper224015/BruntAMS2013.pdf
8) S. Volz, “NASA Earth Sciience New Miissiion Concepts for the Future,” 5th SORCE Science Meeting, Santa Fee, NM, Feb. 7, 2008, URL: http://lasp.colorado.edu/sorce/news/2008ScienceMeeting/doc/Session4/S4_08_Volz.pdf
9) John Loiacono, ICESat-2 Progress Report for FY08 and FY09 Plans,” Feb. 11, 2009, URL: http://decadal.gsfc.nasa.gov/documents/03_ICESat_II.pdf
10) “Orbital Wins ICESat-2 Earth Science Satellite Program Contract,” Space Daily, Sept. 2, 2011, URL: http://www.spacedaily.com/reports/Orbital_Wins_ICESat_2_Earth_Science_Satellite__999.html
11) “NASA Selects Contractor For Icesat-2 Spacecraft,” NASA, Aug. 31, 2011, URL: http://www.nasa.gov/home/hqnews/2011/aug/HQ_C11-037_ICESat2.html
12) “ICESat-2,” Orbital Fact Sheet, URL: http://www.orbital.com/NewsInfo/Publications/ICESat-2_Fact.pdf
13) Anthony W. Yu, Mark A. Stephen, Steve X. Li, George B. Shaw, Antonios Seas, Edward Dowdye, Elisavet Troupaki, Peter Liiva, Demetrios Poulios, Kathy Mascetti, “Space Laser Transmitter Development for ICESat-2 Mission,” Proceedings of SPIE, Vol. 7578, LASE 2010, San Francisco, CA, USA, Feb. 15-18, 2010, 'Solid State Lasers XIX: Technology and Devices,' edited by W. Andrew Clarkson, doi: 10.1117/12.843342, URL: http://icesat.gsfc.nasa.gov/icesat2/publications/pubs_2010/Yu_et_al_2010.pdf
15) Douglas D. McLennan, Thorsten Markus, Thomas Neumann, “ URL: http://www.scribd.com/doc/76177455/ICESat-2-ND-Presentation-4-18-11
16) Charon Birkett, T. Markus, T. Neumann, “The ICESat¿2 Mission - Laser altimetry of ice, clouds and land elevation ....and also ocean, coastal, and continental waters,” OSTM SWT (Science Working Team), San Diego, CA, USA, October 19-21, 2011, URL: http://www.aviso.oceanobs.com/fileadmin/documents/OSTST/2011/oral/03_Friday/Birkett.ICESat2.pdf
17) Douglas D. McLennan, Thorsten Markus, Thomas Neumann, “The Vital Role of ICESat Data Products,” 2011, URL: http://realmedia.aero.und.edu/spst/2011_0418_McLennan_The_Vital_Role_of_ICESat_Data_Products.ppt
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.