Algorithm development, cal/val activities of NASA’s ICESat-2 mission and coordination with ESA’s CryoSat-2 and CRISTAL missions

Abstract

CryoSat-2 was launched in 2010, ICESat-2 in 2018, CRISTAL is scheduled for launch in 2027. These missions provide high precision altimetry measurements from space with an emphasis on measuring the heights of sea ice and the ice sheets of Greenland and Antarctica. While CryoSat-2 and CRISTAL are radar altimeters, ICESat-2 is a laser altimeter. We now have 5+ years of overlapping CryoSat-2 and ICESat-2 data and with an expected lifetime of ICESat-2 into the 2030s we likely will have coincident ICESat-2 and CRISTAL data as well. ICESat-2, a photon-counting lidar, provided a new concept for laser altimetry from space. Thus no precursor spaceborne data were available. In order to better understand expected data and to enable pre-launch algorithm development airborne simulators were developed and flown over the wide range of targets. These data were critical for having tested algorithms in place at the time of launch and distribute operational geophysical products (e.g., land ice elevation, sea ice freeboard, tree heights, inland water products etc.) to the community shortly after launch. To ensure proper post-launch calibration we picked Greenland Summit Station as the orbit anchor point as that station conduct routine GPS surveys underneath the ICESat-2 path. Similarly, for the southern hemisphere we have been conducting ground GPS survey along parts of the 88o S line in Antarctica. This is the ICESat-2 orbit inclination and thus the area where all ICESat-2 tracks converge. For post-launch cal/val airborne campaigns, first as part as of Operation IceBridge and later as dedicated flight over specific targets, we coordinated our flights with CryoSat-2 tracks as well and especially aligned flight lines with “Cryo2Ice” lines. Cryo2ice is a collaborative effort between NASA and ESA. ESA is routinely changing CryoSat-2’s orbit to align with ICESat-2’s. This is enabling scientists to fully explore the synergistic and complementary nature of those two missions. For the ESA CRISTAL mission, NASA will be providing a passive microwave radiometer as an additional instrument. While the primary purpose is to correct for potential path delays of the radar signal in the troposphere, the 19 and 34 GHz frequencies of that radiometer have been extensively used for cryospheric science. Thus there is a potential for ground breaking science using coincident radar and laser altimeter data together with the brightness temperatures at those frequencies. Joint airborne campaigns to enable pre-launch exploration are currently in the planning stage.