Assessment of NASA’s ICESat-2 photon counting laser altimeter in the Baltic Sea to support ice mapping estimates of ridging and local bathymetry

Abstract

The Ice, Cloud and land Elevation Satellite (ICESat-2) is the first Earth-orbiting photon counting laser altimeter. The novel measurements strategy allows for unprecedented global elevation measurements useful for measuring sea ice and shallow water bathymetry at high resolution. This thesis will assess how well ICESat-2 photon observations compare with ice charts from the Finnish Ice Service (FIS) and estimate ridging features in the Baltic, and asses to what extent shallow water bathymetry in the Baltic can be derived.

With sea ice being a main concern of a navigator in ice-covered regions, it is necessary to provide a deformation numeral of degree of ice ridging (DIR) to aid navigation, which provides guidance on whether a ship can safely pass through an area. The FIS ice charts provide DIR information based on ice breaker observations and SAR images. With the high-resolution and dense surface elevation observations from ICESat-2, this thesis has shown it possible to differentiate and estimate DIR. Aggregating segments of 150 photons and estimating relative elevation measurements on ridging proved able separate the observations into different levels of deformation. Future work includes additional filtration schemes to remove erroneous photons from low-laying clouds, fog and/or background sun events. These results are the first to showcase ICESat-2’s ability to measure small-scale roughness structures and encourages scientists to explore ICESat-2 photons for sea ice pressure ridge detection.

Shallow water bathymetry is notoriously difficult to map, but of great interest to science applications, ship navigation and engineering applications. Presented here are the first results of shallow water bathymetry in the murky and eutrophicated waters of the Baltic. Maximum depth down to 16~m depth and average maximum depth of 8.58~m was identified and based on monthly maximum Secchi depths (1979-2018), ICESat-2 proved able to retrieve depths up to 0.87 of the maximum Secchi depth. ICESat-2 also identified specific submerged features at high resolution. Overall, these results present the capability of ICESat-2 to estimate shallow water bathymetry in more turbid waters than previously studied. Future work includes applying refraction corrections, which is estimated to be 0-5~m for depths of 0-20~m.