The word sits as a challenge and a tease above the notation “North Pole” on an 1885 map showing Arctic research stations. Gathering the data to measure and map the Poles was an intense undertaking, and included numerous international expeditions. The conditions were brutal and not all expedition members survived. What did survive, in many cases, were the expedition data that began to unlock the secrets of these remote regions.
Modern data and expeditions provide an ever-increasing understanding of the global implications of changes occurring in the cryosphere. The term “cryosphere” refers to Earth’s frozen regions, and includes snow, river and lake ice, sea ice, glaciers, ice shelves and ice sheets, and frozen ground. Data collected by instruments aboard Earth observing satellites have ushered in a new age of cryospheric exploration and are providing measurements with a precision that could not even be imagined by late-19th century researchers.
One example is NASA’s Ice, Cloud, and land Elevation Satellite-2 (ICESat-2). ICESat-2 launched on September 15, 2018, on a planned three-year mission to measure the height of a changing Earth in unprecedented detail. The initial data and data products from ICESat-2 are now publicly available through NASA’s National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC). “The data look fantastic,” says Steve Tanner, the DAAC’s ICESat-2 Data Manager. “In fact, the data are coming in significantly better in a number of areas.”
This is the story of the ICESat-2 data—how these data are collected, processed, and archived, as well as how they can be accessed and used, fully and freely, by anyone anywhere in the world. These data will shed new light on an ever-changing world, all collected, in the words of the ICESat-2 team, “one laser pulse at a time, 10,000 laser pulses a second.”
ICESat-2 data and data products complement a data record that started with NASA’s original ICESat mission (operational 2003 to 2009) and continues with NASA’s ongoing Operation IceBridge airborne series of flights over the Arctic and Antarctic that started in 2009. Data from all three missions are available through the NSIDC DAAC, which is responsible for NASA data and information for snow and ice processes, particularly interactions among snow, ice, atmosphere, and ocean, in support of research related to global change detection and model validation.
The single instrument carried aboard the ICESat-2 spacecraft is the Advanced Topographic Laser Altimeter System (ATLAS). ATLAS has three major tasks: Send pulses of laser light to the ground, collect the returning packets of electromagnetic radiation (called photons), and record the photon travel time. The bright green pulses of the ATLAS laser beam leave the instrument for the ground at a rate of 10,000 pulses per second (compared with 40 pulses per second in the original ICESat laser). This high frequency enables the instrument to measure height about every 2.3 feet along the satellite’s path. As noted in the ICESat-2 mission brochure, if it flew over an American football field, the first ICESat would have taken a measurement outside each end zone; ICESat-2 takes measurements within each yard line.
About 20 trillion photons leave ATLAS with each pulse and about a dozen photons make the 3.3 millisecond return trip for detection by the satellite’s 2.6-foot diameter beryllium telescope. When this travel time is combined with the satellite’s GPS and star trackers, ICESat-2 can measure surface height to a precision of about one inch (about the length of a standard paperclip).
While the primary intent of the ATLAS instrument is to measure cryospheric elevation, the instrument is able to generate elevation data for more than just ice and frozen ground. Much more. Pre-mission experimentation by the science team found that ATLAS does a great job measuring elevation in temperate regions, including measurements of forest cover and vegetation, and is even able to detect water features (such as coral reefs and ocean waves). “These data were not the primary mission concerns, ice was primary, but it looks like these data will be significantly more useful than originally planned,” says Tanner.
Not counting raw telemetry data, 20 unique ICESat-2 data products will be produced (see illustration; note that there is no product designated “ATL05”). The mission is required to produce Level 1 through Level 3 data, based on the data processing levels of NASA’s Earth Observing System Data and Information System (EOSDIS). As Tanner notes, there almost certainly will be Level 4 derived or model data associated with the mission, especially when researchers combine ICESat-2 data with data from Operation IceBridge, the Geoscience Laser Altimetry System (GLAS) instrument aboard the original ICESat, and data from similar missions, such as the European Space Agency’s CryoSat mission.
Preparing for ICESat-2 data was challenging given the large volume of data expected from the mission. While the volume of data from Operation IceBridge is about 30 terabytes (TB) per year, ICESat-2 is producing just under 1 TB of data per day. Also, ICESat-2 file sizes are very large. “Typically, projects try to limit file sizes to two gigabytes (GB) or less for a given file,” explains Tanner. “Some of the ICESat-2 files were coming in at closer to 10 GB. We needed to make sure our computing systems were robust enough for this volume of data.”
In addition, algorithms had to be developed to process ICESat-2 data. While the GLAS instrument aboard the first ICESat spacecraft looked at the intensity of a returned laser signal and detected groups of returning photons at one time, ICESat-2’s ATLAS instrument detects individual photons. Along with detecting individual photons returned from a laser pulse, the instrument also senses photons from natural sunlight. Algorithms had to be developed to isolate the ATLAS photons to determine the actual elevation of Earth’s surface. Developing these algorithms in advance of the ICESat-2 launch was accomplished using an airborne testbed instrument called the Multiple Altimeter Beam Experimental Lidar (MABEL).
By using MABEL early in the process, ICESat-2 subsetting, reformatting, and reprojection services could be developed at the same time—well before launch. “What this means is that as the science team is tweaking these data products to get them ready for public use, the data services are also being tweaked at the same time,” says Tanner. “It’s just a matter of verifying that the data services can still support the changes from the science team.”
The journey of ICESat-2 data from the ATLAS instrument to the NSIDC DAAC begins as raw, unprocessed data are relayed to a NASA ground station. From there, they are sent to NASA’s Earth Observing System (EOS) Data and Operations System (EDOS) located at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, where they are turned into Level 0 data. Level 0 data are still unprocessed, however communications artifacts, such as synchronization frames, communications headers, and duplicate data, have been removed. These Level 0 data are then sent to the NSIDC DAAC as a back-up and to the ICESat-2 Science Investigator-led Processing System (SIPS) for processing into Level 1, 2, and 3 data products. These SIPS-produced higher-level products are sent to the NSIDC DAAC for archiving and distribution. “We’ve been practicing with the SIPS for several years now making sure that everything is ready for the public release of these data,” Tanner says.
ICESat-2 data can be accessed in several ways. The easiest is through the EOSDIS Earthdata Search application, though there are other means as well. For example, the NSIDC DAAC provides application programming interface (API) access to ICESat-2 data and services, where data users can set up programmatic access or set up a script to constantly get ICESat-2 data for a specific region. Both of these options also offer data services such as subsetting, reformatting, and reprojection, making the data easier to use and reducing the amount of network traffic generated. The ICESat-2 team has put a great deal of energy into making sure these data services meet end-user needs and that they are fully integrated into Earthdata Search and the API access.
Another resource for accessing and working with ICESat-2 data is a project called OpenAltimetry. OpenAltimetry is a cyberinfrastructure platform for ICESat and ICESat-2 data discovery, access, and visualization that is funded through a grant as part of NASA’s Advancing Collaborative Connections for Earth System Science (ACCESS) Program. “You can go to OpenAltimetry to quickly visualize the data to decide if these are files you want to have,” explains Tanner, noting that OpenAltimetry is still in development. “It goes beyond subsetting by letting you look at the data without having to download the data. We’re making OpenAltimetry available to the public to assess the value of it and to decide if we should put it into full production.”
Now that ICESat-2 data are finally getting into the hands of the global user community things are about to become very interesting. As Tanner notes, the ICESat-2 science team is a small community primarily focused on the cryosphere; new applications for ATLAS data continue to be discovered far beyond the mission’s primary objective. “It’s turning out that ICESat-2 is truly a global satellite that can be used to study all sorts of non-cryospheric things,” he observes. “We can look at rain forests, we can look at deserts, we can look at oceans. We want these data to get out there and for people to use these data in ways we haven’t even imagined. To me this would be the coolest thing to come out of the mission.”
Unexplored. A word that now applies to the new voyage of discovery that is being ushered in with the release of ICESat-2 data and the many ways these data will be used.