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Oceanographers need precise measurements of waves and currents when studying the ocean. To collect this data, researchers turned to buoys equipped with sensors that operate as floating field stations. These buoys are designed to measure the movement of water across the ocean surface and the speed and size of waves. To operate on the high seas, these buoys use GPS and other satellite data in place of land-based reference points.
However, despite their usefulness, only a few wave buoys have been deployed. This is because they are expensive and require complex calculations to process the data, which are done via expensive onboard computing equipment and can take days to complete.
To solve these problems, a group of researchers in Qingdao, China, built a new type of buoy. Like other wave buoys, their technology uses Global Navigation Satellite System (GNSS) technology. But instead of collecting current standard dual-frequency satellite data, their technology enables lower cost, higher accuracy and instantaneous calculations through single-frequency analysis.Researchers describe their methods A study published on March 19 IEEE Transactions on Geoscience and Remote Sensing.
Dual-frequency satellite data collects data in two independent frequency bands. It is often used to achieve more accurate measurements than using data from a single frequency band; adding a second frequency band can correct for interference in the atmosphere, but takes longer to process.
Although satellite data itself can provide some information about ocean dynamics from a distance through remote sensing, it lacks the accuracy of direct observation. In fact, the GNSS buoys themselves were originally developed as calibration tools to improve satellite measurements of tides and long-term changes in global sea level, said study author Xu Yongsheng, a researcher at the Institute of Oceanography of the Chinese Academy of Sciences. The researchers then realized they could use buoys to measure wave period, height and direction directly from the ocean. The discovery that high-frequency GNSS data also contains important wave motion information … has made these buoys useful beyond their original purpose, Xu said.
Only with a sufficient number of observations can we better understand and predict waves. Xu Yongsheng, academician of the Chinese Academy of Sciences
Current GNSS buoy designs use a number of different methods to process data. Some buoys calculate the buoy vertical velocity based on the Doppler shift, as measured by overhead satellites.Other ways to check acceleration on a buoyPrecise measurements are achieved by including ground-based reference stations or downloading information about satellite orbits and timing from their onboard clocks, Xu said. However, this means that for the reference station, the buoy must be located within 50 kilometers of the coast. Downloading additional satellite data increases the burden on the buoy bandwidth and computing resources.
The researchers tested their buoy off the coast of Qingdao, China.Xu Yongsheng
The Qingdao researchers’ buoy uses satellite-based principles but uses single-frequency GNSS chips that are cheaper than the dual-frequency counterparts currently used in buoys. Switching to single frequency also showed slightly higher accuracy. Xu explained that the dual-frequency method involves additional calculations that may inadvertently amplify measurement errors.
In the researchers’ approach, an algorithm uses a single-frequency GPS signal to measure the buoy’s speed in three dimensions to determine its displacement and therefore the size of the wave. While other methods require post-processing, which can take days and sometimes up to a week, this method can be done instantly and does not require the downloading of external materials. In the researchers’ tests, the single-frequency method was five times more accurate than the Doppler-shifted method, the only other method that could be done on the fly.
The single-frequency chip also reduces costs; Xu estimates their buoys cost about one-tenth the price of other wave buoys. Lower costs mean scientists can deploy large networks of buoys, including near the coasts of low-income countries. Expanding the numbers will allow for better monitoring of global waves, in line with the United Nations’ goal of making the 2020s a global wave. Ocean Decade and increase focus on marine science.
Xu said waves vary greatly in space and time. Only with a sufficient number of observations can we better understand and predict waves. Reducing the cost of wave buoys could make this possible.
Researchers are currently working to solve a key challenge: maintaining satellite links. Xu said rough sea conditions and stormy weather can affect the buoy’s ability to maintain a direct line of sight to receive satellite radio signals, which is critical to the buoy’s function. The researchers aim to improve the mechanical design of the buoys to address this issue in the future.
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