ICESat and ICESat-2: Preparing to assess decadal-scale elevation change over the ice sheets

One of the first, and most dramatic, assessments of elevation change to occur after the Ice, Cloud and land Elevation Satellite-2 (ICESat-2) enters its science orbit in early 2018 will be to compare the altimetry data being collected to the baseline established by the original ICESat mission between 2003 and 2009. Both missions use altimeters that send laser pulses from the satellite to the Earth. By measuring the travel time, the range to the surface can be inferred, and then combined with the position of the satellite and the pointing direction of the laser to determine where the pulse landed on the surface and its height there. The first ICESat mission employed a single-beam, full-waveform altimeter with a near-infrared (1064-nm wavelength) laser pulsed at 40 Hz. This produced surface heights at 170-meter intervals along reference tracks that extended to +/- 86 degrees latitude. ICESat-2 will carry an altimeter that sends a green (532-nm wavelength) laser through a diffractive optical element to be split into six beams, and pulsed at 10kHz. This will yield overlapping footprints every 70 cm along each beam track, extending to +/-88 degrees latitude. Rather than capturing the full returned waveform, however, the altimeter will use photon-counting detectors to measure the travel times of individual photons. Once on the ground, these data will be used in the science data processing system to produce a latitude, longitude and ellipsoidal height, marking the location from which each photon returned from the surface. Higher-level data products will characterize the surface more precisely by aggregating photons to reduce noise along each of the six beam tracks. Here, we describe the ICESat and ICESat-2 measurements and ice sheet data products, and discuss possible methods for comparing them to assess elevation change over the Greenland and Antarctic ice sheets in the nine years between the two missions.