Lasers in space - NASA's ICESat-2 satellite counts photons from orbit

This week I attended a five day hack-week organised by the University of Washington Polar Science Center and the eScience institute, and taught by members of the NASA ICEsat-2 science team. The hack-week was made up of tutorials on data access and manipulation as well as collaborative projects on polar science.


Light detection and ranging (Lidar) is nothing new in environmental science - it's used to map topography and vegetation in 3D and measure distances with extreme precision, helping scientists monitor systems from volcanic hazards to beach erosion.

But despite the heritage its the technology, NASA’s new ICEsat-2 satellite is a major leap forward, firing laser pulses at earth from low earth orbit and counting individual photons in as they return. By measuring their time of flight, it calculates elevations of the reflecting surfaces with centimetre accuracy. Even better, it does this with six laser beams, scanning a swath of terrain 9km wide. Each beam has a footprint of 17m diameter, offering unprecedented spatial resolution with a measurement every 70cm.

The six beams are divided into three pairs, with paired lasers 90m apart. This allows for measurements of gradient in the and 'across-track' directions, critical for steep and complex surfaces like glaciers.


During the workshop we were quickly encouraged to pitch our ideas for projects, with attendees proposing ocean wave detection, mapping of ice sheet grounding lines and calculation of floe size distribution. I proposed a project to investigate the automatic blowing snow detection algorithm and was very pleased to have five people join my project! I wanted to compare the data to weather data from climate models, but we quickly branched out to mapping the distribution of blowing snow too.

We made a convincing climatology and our comparisons to reanalysis were encouraging, but we limited our scope to land-ice for practical reasons. We're now planning to extend our work to the ICEsat-2 sea ice product, where a climatology of blowing snow has never been made (to our knowledge).

Eric Keenan’s (@EricKeenanCU) plot of blowing snow optical depth over Antarctica. The optical thickness is higher over East Antarctica (figure right) in agreement with published work.

Eric Keenan’s (@EricKeenanCU) plot of blowing snow optical depth over Antarctica. The optical thickness is higher over East Antarctica (figure right) in agreement with published work.

A significant part of the week focussed on software tools like cloud computing in Jupyter Lab and Git, a collaboration and version control tool. It was great to be pushed to use these tools, as I wouldn't have done so otherwise. Git in particular offers our blowing snow team a chance to continue developing our product even now the hack-week is over.

As well as the chance for ongoing collaboration on blowing snow, ICEsat-2 has a lot to bring to my PhD project. I'm currently working on radar altimetry of the sea ice surface and encumbent assumptions about the spatial patterns of snow cover. ICEsat-2 offers the chance to validate radar altimetry, and also to shed light on model-generated snow distributions. During the hackweek I also had a couple of other ideas for novel uses of the data, which I'm going to keep under my hat for now! Perhaps just as valuably, I made some great connections with other PhD students with expertise in connected areas.

It clearly took a lot of time and effort to make this happen; thanks go in particular to the University of Washington eScience institute and Polar Science Center, and Anthony Arendt who brought it all together. I'm looking forward to all the icey science to follow!