Scientists lack critical observations from the ocean beneath the sea ice in the Arctic Ocean, and without them, key questions about climate change, ocean currents, and ecosystems remain unanswered. The Nansen Center, the U.S. Scripps Institution of Oceanography, and partners in the EU project High Arctic Ocean Observation System (HiAOOS) are now developing a groundbreaking network of acoustic sound sources. This network enables underwater vehicles and autonomous floats to determine their position beneath the ice using sound.
Measurements from beneath the sea ice in the Arctic are essential for gaining a comprehensive understanding of how climate change affects ocean temperatures, currents, and ecosystems. At the same time, the processes occurring under the ice have a significant impact on the climate itself. To fill these critical knowledge gaps, large amounts of direct observations from the ocean are needed, and this requires technology capable of operating in one of the world’s most inaccessible environments.
The challenge: GPS does not work under ice
There are several ways to measure conditions in the ocean. Unmanned underwater vehicles, such as gliders and drifting floats, can carry a variety of sensors to collect data on temperature, salinity, ocean currents, and more. The most widely used observational platform is the Argo float. These floats drift with deep ocean currents and, every ten days, rise to the surface to take measurements before transmitting the data via satellite. The observations are provided with time and position from satellite-based geopositioning systems, such as GPS.
Today, there are around 4,000 Argo floats in the world’s oceans. More are being deployed in the Nordic Seas, with some following the northward Atlantic current past the coast of Svalbard and under the sea ice in the Arctic Ocean. When the floats are beneath the ice, they continue measuring, but if the surface is covered by ice, they cannot surface to transmit data, determine their position, or update the timing of their instruments. Satellite signals cannot be used to position the floats either, as radio waves do not propagate through water or ice. The same limitation applies to other underwater vehicles that may remain under the ice for extended periods without satellite communication. Therefore, alternative methods for positioning in ice-covered areas are necessary.
The solution: sound as a positioning tool
Sound waves travel at approximately 1,500 meters per second, and the signals used by NERSC can be detected at distances of up to 1,000 km from the source. By placing acoustic sources at strategic points in the Arctic Ocean, it is possible to calculate the positions of gliders and Argo floats with high precision. The underlying principle is called trilateration (see illustration). The idea is the same as using GPS over the ocean surface, but with sound instead of radio waves. Without a comparable system, gliders and Argo floats operating in the ice-covered Arctic Ocean would not be able to determine where the data was collected or their own positions.
