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Prof. Kaoru Ichikawa
Research Institute for Applied Mechanics, Kyushu University, Japan Area of Interest: Satellite altimetry, Kuroshio Mesoscale Eddies, HF radar, GNSS reflectometry |
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Presentation topic:
Ferry-mounted GNSS to monitor wider sea surface dynamic height and wave field
Ferry-mounted GNSS to monitor wider sea surface dynamic height and wave field
The Global Navigation Satellite System (GNSS) is possibly one of the most widespread remote sensing techniques. Currently, its receivers are integrated into various products, including automobiles, smartphones, smartwatches, and even pet tags. Ocean observations also greatly benefit from GNSS, as it can determine geolocation at any place and time, which is particularly crucial in the open ocean where no fixed landmarks are visible. Examples include the positions of observational casts, velocity measurements from surface drifter trajectories, and fish habitat tracking via biotags. Nevertheless, those geolocations generally refer only to horizontal positions, since accurate centimeter-level height measurement requires precise positioning techniques, such as real-time kinematic (RTK) and precise point positioning (PPP), using multi-frequency GNSS receivers. Moreover, extra height measurements between the antenna and the sea surface are necessary to convert the antenna height to the sea surface height (SSH).
Anchored surface GNSS buoys presume constant antenna heights above the sea surface, enabling variations in antenna height to be translated into heave motions induced by surface waves, tsunamis, and tides at specific sites. However, obtaining spatial distributions of the SSH demands the deployment of many buoys, which is practically unfeasible. Satellite altimeters have contributed to obtaining the SSH field and revealed active mesoscale eddies in open oceans and significant fluctuations of the Kuroshio current. Furthermore, the recent introduction of the wide-swath SWOT altimeter has expanded the spatial resolution of the SSH maps. Yet, the temporal resolution of the SSH field is insufficient for fast-moving small-scale events, including coastal changes and the propagation of Kuroshio meanders. Because spatial expansion with reasonable temporal resolutions can be achieved by the use of repeated ferry cruises, ferry-mounted GNSS could be supplementary SSH observation methods together with satellite altimeters.
However, the ship's draft will vary due to payload and speed, so a constant antenna height with respect to the sea surface is no longer expected. By utilizing GNSS Reflectometry (GNSS-R) to determine the distance between the antenna and the sea surface, where GNSS signals are reflected toward the antenna with a longer travel path than the signals directly received by the antenna, the antenna's elevation above the sea surface can be determined, hence enabling the calculation of the SSH. Upon eliminating the geoid and tidal heights, the sea surface dynamic height (SSDH), whose gradient correlates with geostrophic velocity, can be derived along the ship tracks. The SSDH profiles along repeated ferry cruises across the Kuroshio are explained in this talk.
Anchored surface GNSS buoys presume constant antenna heights above the sea surface, enabling variations in antenna height to be translated into heave motions induced by surface waves, tsunamis, and tides at specific sites. However, obtaining spatial distributions of the SSH demands the deployment of many buoys, which is practically unfeasible. Satellite altimeters have contributed to obtaining the SSH field and revealed active mesoscale eddies in open oceans and significant fluctuations of the Kuroshio current. Furthermore, the recent introduction of the wide-swath SWOT altimeter has expanded the spatial resolution of the SSH maps. Yet, the temporal resolution of the SSH field is insufficient for fast-moving small-scale events, including coastal changes and the propagation of Kuroshio meanders. Because spatial expansion with reasonable temporal resolutions can be achieved by the use of repeated ferry cruises, ferry-mounted GNSS could be supplementary SSH observation methods together with satellite altimeters.
However, the ship's draft will vary due to payload and speed, so a constant antenna height with respect to the sea surface is no longer expected. By utilizing GNSS Reflectometry (GNSS-R) to determine the distance between the antenna and the sea surface, where GNSS signals are reflected toward the antenna with a longer travel path than the signals directly received by the antenna, the antenna's elevation above the sea surface can be determined, hence enabling the calculation of the SSH. Upon eliminating the geoid and tidal heights, the sea surface dynamic height (SSDH), whose gradient correlates with geostrophic velocity, can be derived along the ship tracks. The SSDH profiles along repeated ferry cruises across the Kuroshio are explained in this talk.
