The ICESat-2 Mission: Laser altimetry of ice, clouds and land elevation T. Markus, T. Neumann NASA Goddard Space Flight Center W. Abdalati Earth Science and Observation Center, Univ. of Colorado Boulder

Draft Level 1 Baseline Science Requirements ICESat-2 shall produce an ice surface elevation product that enables determination of ice-sheet elevation changes to 0.2 cm/yr accuracy on an annual basis. ICESat-2 shall produce an ice surface elevation product that enables determination of annual surface elevation change rates on outlet glaciers to an accuracy of better than 0.25 m/yr over areas of 100 km 2 for year-to-year averages, and along linear distances of 1 km along-track. ICESat-2 shall produce an ice surface elevation product that enables resolution of winter (accumulation) and summer (ablation) ice-sheet elevation change to 2.5 cm at 25 km x 25 km spatial scales. ICESAT-2 shall provide monthly near-repeat surface elevation measurements that enable determination of sea-ice freeboard to an uncertainty of 3 cm at 25 km x 25 km spatial scales for the Arctic Ocean and Southern Oceans. ICESat-2 shall make elevation measurements for a minimum five-year duration. ICESat-2 shall produce elevation measurements that enable determination of global vegetation height to 3-m accuracy at 1-km resolution Development of a vegetation height model with 1-m accuracy at 1-ha resolution has been identified as a science requirement by the ecosystem structure research community. ICESat-2 will not achieve the vegetation science objectives, but rather will support them to the extent possible without compromising ice science objectives. Results from the ICESat mission suggest that ICESat-2 will be capable of producing a height surface with 3-m accuracy at 1-km spatial resolution, assuming that off-nadir pointing can be used to increase the spatial distribution of observations over terrestrial surfaces.

ICESat-2 Mission Concept In contrast to ICESat design, ICESat-2 will use micro-pulse multi-beam photon counting approach Provides: Dense cross-track sampling to resolve surface slope on an orbit basis (different sampling geometries are currently discussed with the SDT). High repetition rate (10 kHz) generates dense along-track sampling (~70 cm). Advantages: Improved elevation estimates over high slope areas and very rough (e.g. crevassed) areas Improved lead detection for sea ice freeboard.

Ice Sheets and Sea Ice aSDt and SDT analyzed suitability of micro-pulse multi-beam approach for ice sheets Sampling provided by multi-beam is significantly preferable to analog approaches Amount of improvement directly related to number of beams Using nominal 15 cm range accuracy, configuration meets 25 cm/yr over 100 km 2 and 2.5 cm over 25 km x 25 km areas. Testing various approaches to extract freeboard has given biases of < 2.5 cm in freeboard. Farrell, Markus, Miller, GSFC Use ATM data (black) over sea ice to simulate ICESat-2 data product (magenta).

Vegetation Assessment: Preliminary Analysis Approach Study Area: SERC – a 140m x 900m section of Sigma Space photon-counting data 14 – 10m x 900m profiles extracted. 10 kHz simulation: 10 m footprint moved 70 cm along-track ICESat-2 vegetation simulations Concerns with vegetation data: Low energy beams may not penetrate closed canopy; vegetation has low reflectivity. 10 kHz results – n = 115: s Mean (m) Std Dev (m) A B C Reference A B C Results: Plots and data: Ross Nelson, GSFC

Phase A studies: 1. Engineering design cases (already underway) Level 1 science requirements do not provide engineering design metrics. Established ~8 cases as engineering design tool. Specify environmental conditions (roughness, cloudiness, reflectivity) for instrument performance modeling. 2 ice sheet interior, 2 outlet glacier, 2 sea ice, 2 vegetation. 2. Instrument configuration studies (underway; to be completed early 2010) Select optimal footprint geometry and laser energy distribution Select optimal pulse repetition rate for optimal along-track sampling, data volume 4. High Altitude Aircraft (to be ready by 10/2010) ICESat-2 simulator Address engineering and science questions Verification of design models 3. Vegetation Assessment Rigorous assessment of existing aircraft vegetation data Vegetation modeling using ICESat-2 design Identify design choices to improve the prospects for veg. product Current ICESat-2 footprint pattern

The ICESat-2 Mission Timeline Feb. 2009: Mission Concept Review Nov. 2009: δ-Mission Concept Review 2009, 2010: Phase A Trade Studies May 2012: Mission Critical Design Review Oct. 2015: Launch Readiness Date 2016: ICESat-2 data collection begins