Artem Moiseev, PhD candidate in our OSIRS group, successfully defended his PhD today! His degree is being awarded by the University of Bergen. Congratulations, Artem!

Ocean surface currents as study object

Moiseev has spent the past three years investigating ways to detect ocean surface currents from satellites. He showed that a certain type of satellite data (Synthetic Aperture Radar, SAR) can be used to reliably monitor ocean surface currents, when applying his new algorithm to clean the data from the influence of the wave motion.

Surface currents dominate the upper layer of oceans. These currents are important for several research fields, such as weather forecasting, climate research, and operational oceanography. Operational oceanography is similar to weather forecasting, but for the ocean and not the atmosphere. Other fields also benefit significantly from knowing how these currents behave over time: They are essential for monitoring life below water, the maritime transport sector, renewable marine energy, maritime safety operations, and for monitoring and tracking marine pollution, such as oil spills.

Shortcomings of tools to observe ocean currents

So, it's easy to understand that knowledge about ocean surface currents is highly relevant for society on several levels. But to obtain this knowledge is not as easy. Observations generally come from in-situ instruments on drifting buoys, shipboard, and anchored moorings in the ocean. Those kinds of observations are very accurate, but they do not cover large areas consistently in time and space. And the oceans are extremely vast. Remote sensing techniques have been developed over the last decades, and they do improve knowledge about ocean surface currents. In coastal zones, land-based radars are being used to monitor ocean currents in real-time. These radar measurements are useful for example for oil spill tracking and search-and-rescue operations close to the coast, but they have a limited spatial range and cannot cover the entire ocean. A tool to investigate currents in the open ocean is radar altimetry provided by satellites. This method mainly works for the open ocean, but does not have a good enough resolution to reliably monitor surface currents, especially in coastal zones.

Synthetic Aperture Radar (SAR) 

SAR technology is used for monitoring several different processes from space, such as wind speed, waves or sea ice changes on Earth. It is even being used on space missions to investigate other planets! When being attached to satellites, monitoring the Earth's surface with SARs is possible with great detail, and this now also applies to ocean surface currents, thanks to Artem Moiseev's work with SAR data from the ESA Sentinel-1 mission!

Math to the rescue

Artem Moiseev is from St. Petersburg in Russia, where he obtained his MSc. degree in applied hydrometeorology from the Russian State Hydrometeorological University in 2016.At the beginning of the mission ocean surface currents could not directly be retrieved from the Sentinel-1 SAR. A major challenge was that there was no reliable way to separate the wave-induced motion in the ocean from the motion induced by the surface currents. These contributions to the signal are different, but when wanting to monitor ocean surface currents, wave-induced motion must be removed to derive reliable data on the surface current.

But Moiseev found a way to separate these two sources! He used machine learning to create an algorithm that can predict and remove the wave component in SAR data. Moiseev compared the "cleaned" datasets (treated with his algorithm) to real-world ocean data, as well as other satellite data from the same area at the same time, to see if his method output is consistent with these other datasets. And it is. His way of "cleaning up" SAR data actually allows the monitoring of ocean surface currents only, without waves interfering with the quality!

Future outlook

Moiseev's findings can become the foundation for future use of SAR data to monitor ocean surface currents in support of the Copernicus Marine Environment Monitoring Service program and the United Nations Decade of Ocean Sciences.

PhD project stats

Moiseev began his PhD project titled “Ocean surface currents derived from Sentinel-1 SAR Doppler shift measurements” in 2017 supported through the uSeaSat project (Research Council of Norway) and the Centre for Integrated Remote Sensing and Forecasting for Arctic Operations (CIRFA). His main supervisor was Johnny A. Johannessen (NERSC), with co-supervision from Morten W. Hansen (MET) and Harald Johnsen (NORCE).

PhD project publications

1. Moiseev, A, Johnsen, H, Hansen, MW, and JA Johannessen. Evaluation of radial ocean surface currents derived from Sentinel-1 IW Doppler shift using coastal radar and Lagrangian surface drifter observations. Journal of Geophysical Research: Oceans 2020. 125(4).  https://doi.org/10.1029/2019JC015743 
2. Moiseev, A, Johnsen, H, Johannessen, JA, Collard, F, and G Guitton. On Removal of Sea State Contribution to Sentinel‐1 Doppler Shift for Retrieving Reliable Ocean Surface Current. Journal of Geophysical Research: Oceans 2020. 125(9). https://doi.org/10.1029/2020JC016288
3. Moiseev, A., J. A. Johannessen, H. Johnsen, (2021), Towards Retrieving Reliable Ocean Surface Currents in the Coastal Zone from the Sentinel-1 Doppler Shift Observations, to be submitted to Journal of Geophysical Research: Oceans.

The thesis as PDF can be found here: Ocean surface currents derived from Sentinel-1 SAR Doppler shift measurements